1 OH-PLIF MEASUREMENTS AND ACCURACY INVESTIGATION IN HIGH PRESSURE GH 2 /GO 2 COMBUSTION By ARAVIND VAIDYANATHAN A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2008
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1
OH-PLIF MEASUREMENTS AND ACCURACY INVESTIGATION IN HIGH PRESSURE GH2/GO2 COMBUSTION
By
ARAVIND VAIDYANATHAN
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
Hydroxyl Radical (OH) in Non-premixed Flames .................................................................25 Motivation for the Current Work............................................................................................28
2 OH PLANAR LASER INDUCED FLUORESCENCE - THEORY AND REVIEW ...........29
Fluorescence Modeling...........................................................................................................29 Fluorescence and Interference Signals ............................................................................37 Laser ................................................................................................................................38 Absorption and Excitation, Line Shape and Fluorescence Efficiency ............................38 Experimental Constants...................................................................................................38
Review of OH PLIF Diagnostic Studies.................................................................................39 Fluorescence Strategy and Interference Signals..............................................................64 Laser ................................................................................................................................64 Absorption & Excitation, Line Shape and Fluorescence Efficiency...............................65 Experimental Constants...................................................................................................66
3 EXPERIMENTAL FACILITY AND DIAGNOSTICS METHODS.....................................68
Experimental Test Facility and Operating Conditions ...........................................................68 OH-PLIF Diagnostics .............................................................................................................72 Wall Boundary Conditions .....................................................................................................75
4 OH-PLIF IMAGE PROCESSING AND QUANTITATIVE ANALYSIS ............................77
Fluorescence and Interference Signals ...................................................................................77 Laser .......................................................................................................................................82 Absorption and Excitation, Line Shape, and Fluorescence Efficiency ..................................84 Experimental Constants ..........................................................................................................86
6
5 RESULTS AND UNCERTAINTY ANALYSIS ...................................................................89
Chamber Pressure Measurements...........................................................................................89 OH-PLIF Measurements.........................................................................................................92 Quantification of OH Concentration and Uncertainty at 10, 27, 37 and 53 bar ...................100
A MATLAB® SCRIPTS USED FOR DATA PROCESSING.................................................125
B PROPOSED NEW METHODOLOGY FOR PHOTON CALIBRATION..........................154
C OH ABSORPTION PROFILES...........................................................................................160
OH Absorption Profiles at 10 bar and 2500–3500 K Temperature Range...........................160 OH Absorption Profiles at 27 bar and 2500–3500 K Temperature Range...........................163 OH Absorption Profiles at 37 bar and 2500–3500 K Temperature Range...........................166 OH Absorption Profiles at 53 bar and 2500–3500 K Temperature Range...........................169
D OH NUMBER DENSITY CONTOURS..............................................................................172
Thirteen Instantaneous OH Number Density Contours at 10 bar.........................................172 Thirteen instantaneous OH Number Density Contours at 27 bar .........................................176 Thirteen Instantaneous OH Number Density Contours at 37 bar.........................................180 Thirteen Instantaneous OH Number Density Contours at 53 bar.........................................185
E TEMPERTURE MEASUREMENTS AND BOUNDARY CONDITIONS........................190
LIST OF REFERENCES.............................................................................................................196
3-4 Laser spectral profile measured using Burleigh Wavemeter before doubling to 283 nm ......................................................................................................................................73
4-1 Average of 13 instantaneous images obtained at near steady state for chamber pressure of 10 bar...............................................................................................................78
4-2 Average of 13 instantaneous images obtained at near steady state for chamber pressure of 27 bar...............................................................................................................79
4-3 Average of 13 instantaneous images obtained at near steady state for chamber pressure of 37 bar...............................................................................................................80
4-4 Average of 13 instantaneous images obtained at near steady state for chamber pressure of 53 bar...............................................................................................................81
4-6 Camera calibration corresponding to the detection strategy employed in the OH-PLIF measurements and region of interest ........................................................................87
5-1 Chamber pressure versus time for GH2/GO2 combustion for 10 bar and O/F mass flow of 3.7..........................................................................................................................90
5-2 Chamber pressure versus time for GH2/GO2 combustion for 27 bar and O/F mass flow of 3.7..........................................................................................................................90
9
5-3 Chamber pressure versus time for GH2/GO2 combustion for 37 bar and O/F mass flow of 3.7..........................................................................................................................91
5-4 Chamber pressure versus time for GH2/GO2 combustion for 53 bar and O/F mass flow of 3.7..........................................................................................................................91
5-5 Instantaneous image-processed OH-PLIF images at near steady state chamber pressure of (a) 10, (b) 27, (c) 37 and (d) 53 bar.................................................................93
5-6 Average of thirteen instantaneous image-processed OH-PLIF images at near steady state chamber pressure of (a) 10, (b) 27, (c) 37 and (d) 53 bar..........................................94
5-7 Average of thirteen instantaneous image-processed OH-PLIF images at near steady state chamber pressure of (a) 35, (b) 36, and (c) 37 bar indicating the repeatability and reliability of OH-PLIF measurements for determination of OH concentration..........95
5-8 Mean position of reaction zone determined from the average OH-PLIF images at (a) 10, (b) 27, (c) 37 and (d) 53 bar.........................................................................................97
5-9 Temperature and specie mole fraction variation based on equilibrium calculations with equivalence ratios of 0.5–3 at (a) 10, (b) 27, (c) 37 and (d) 53 bar .........................102
5-10 Absorption coefficient (12
9'
1
BBf∑ ) variation with equivalence ratio and temperature
(2500–3500 K) at (a) 10, (b) 27, (c) 37 and (d) 53 bar showing that the variation with respect to mean is 12.4, 14.6, 14.5 and 15.1% respectively ............................................104
5-11 Absorption profile of OH at (a) 3017 K and 10 bar, (b) 3085 K and 27 bar, (c) 3103 K and 37 bar, and (d) 3125 K and 53 bar simulated using LIFBASE showing a complete overlap with the laser spectral profile at all pressures .....................................106
5-12 Overlap integral laser abs dνΦ Φ∫ variation at (a) 10, (b) 27, (c) 37 and (d) 53 bar with temperature corresponding to equivalence ratio of 0.5–3, indicating that the variation with respect to mean is 1.3, 1, 0.8 and 0.5% respectively and can be assumed negligible..........................................................................................................................109
5-13 Collisional quench rate Q21 variation at (a) 10, (b) 27, (c) 37 and (d) 53 bar with temperature and colliding species mole fraction corresponding to equivalence ratio of 0.5–3 indicating that the variation with respect to mean is 4.1, 3.9, 3.8 and 3.7 % respectively ......................................................................................................................112
5-14 Instantaneous OH number density contours at near steady state chamber pressure of (a) 10, (b) 27, (c) 37 and (d) 53 bar .................................................................................113
5-15 Average of thirteen instantaneous OH number density contours at near steady state chamber pressure of (a) 10, (b) 27, (c) 37 and (d) 53 bar. ...............................................114
10
5-16 OH-PLIF measurement uncertainties at (a) 10, (b) 27, (c) 37 and (d) 53 bar .................117
B-1 Calibration set-up for photon calibration.........................................................................154
B-2 A series of 900 images of 32x32 pixel size was obtained at each exposure....................156
B-3 A series of 900 images of 32x32 pixel size was obtained each exposure........................157
B-4 Counts vs exposure time at 532 nm .................................................................................158
B-5 Photons vs counts at 310 nm............................................................................................158
C-1 Absorption profile of OH simulated using LIFBASE at equivalence ratio of (a) 0.5, (b) 1, (c) 1.5, (d) 2, (e) 2.5 and (f) 3 corresponding to temperatures of 2500–3500 K for gaseous H2-O2 flame at 10 bar. ..................................................................................162
C-2 Absorption profile of OH simulated using LIFBASE at equivalence ratio of (a) 0.5, (b) 1, (c) 1.5, (d) 2, (e) 2.5 and (f) 3 corresponding to temperatures of 2500–3500 K for gaseous H2-O2 flame at 27 bar. ..................................................................................165
C-3 Absorption profile of OH simulated using LIFBASE at equivalence ratio of (a) 0.5, (b) 1, (c) 1.5, (d) 2, (e) 2.5 and (f) 3 corresponding to temperatures of 2500–3500 K for gaseous H2-O2 flame at 37 bar. ........................................................................................168
C-4 Absorption profile of OH simulated using LIFBASE at equivalence ratio of (a) 0.5, (b) 1, (c) 1.5, (d) 2, (e) 2.5 and (f) 3 corresponding to temperatures of 2500–3500 K for gaseous H2-O2 flame at 53 bar. ..................................................................................171
D-1 Thirteen instantaneous OH number density contours at near steady state chamber pressure of 10 bar.............................................................................................................176
D-2 Thirteen instantaneous OH number density contours at near steady state chamber pressure of 27 bar.............................................................................................................180
D-3 Thirteen instantaneous OH number density contours at near steady state chamber pressure of 37 bar.............................................................................................................184
D-4 Thirteen instantaneous OH number density contours at near steady state chamber pressure of 53 bar.............................................................................................................189
E-1 Chamber wall temperatures vs time at inner locations of 37, 47, 58, 70, 89 and 102 mm from the injector face................................................................................................190
E-2 Chamber wall temperatures vs time at middle locations of 37, 47, 58, 70, 89 and 102 mm from the injector face................................................................................................190
E-3 Chamber wall temperatures at inner and middle locations along the chamber wall at end of the 8 s ....................................................................................................................191
11
E-4 Exponential function assumed for heat flux evolution with time ....................................191
E-5 Experimental and computational temperatures at 37 mm axial location.........................192
E-6 Experimental and computational temperatures at 47 mm axial location.........................192
E-7 Experimental and computational temperatures at 58 mm axial location.........................193
E-8 Experimental and computational temperatures at 70 mm axial location.........................193
E-9 Experimental and computational temperatures at 89 mm axial location.........................194
E-10 Experimental and computational temperatures at 102 mm axial location.......................194
E-11 Chamber wall heat fluxes calculated based on 3D computations and linear + unsteady assumption at 37 bar .........................................................................................195
E-12 Computational and Experimental Temperatures for 37 bar at the end of 8s. ..................195
12
NOMENCLATURE
A Electronic Excited State
Alaser Cross sectional area of the laser beam or sheet (cm2)
Pixel ProjectionA Pixel projection area (cm2)
21A Spontaneous emission rate (s-1)
12B Einstein B coefficient for absorption (cm3J-1s-2)
21B Einstein B coefficient for emission (cm3J-1s-2)
12
'B 212B c (cm J-1)
c Speed of light (cms-1)
C Heat capacity (J kg-1 K-1)
E Laser energy per pulse (J)
E( )v Laser spectral energy per pulse (Jcm)
1g Degeneracy in the ground electronic state
2g Degeneracy in the upper excited electronic state
Tinner Temperature at 3.2 mm from inner wall (K, oC)
Tmiddle Temperature at 9.5 mm from inner wall (K, oC)
U velocity (m/s)
V Volume probed by the laser (cm3)
VET Vibrational energy transfer
12W Stimulated absorption rate (s-1)
21W Stimulated emission rate (s-1)
14
X Electronic ground state
ΔT Temperature difference (K, oC)
Δt Time difference (s)
Δx Distance between temperature measurement locations
cvΔ Collisional width (cm-1)
DvΔ Doppler width (cm-1)
shiftCvΔ Collision induced shift (cm-1)
D shiftvΔ Doppler induced shift (cm-1)
ν Wavenumber (cm-1)
im Reduced mass of OH and the colliding species
is Colliding species cross section
4πΩ Fraction of solid angle
lτ Laser pulse duration (ns)
Bf Boltzmann factor
( )c vf Normalized collisional line shape function (cm)
( )D vf Normalized Doppler line shape function (cm)
( )abs vΦ Absorption line shape function (cm)
( )laser vΦ Laser spectral profile (cm)
F ( )( )
2 2
2 2
O H actual
O H stoichiometric
m mm m , equivalence ratio
F Fluorescence yield
15
ρ Density (kg m-3)
ic Colliding species mole fraction
16
Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
OH-PLIF MEASUREMENTS AND ACCURACY INVESTIGATION IN HIGH PRESSURE
GH2/GO2 COMBUSTION
By
Aravind Vaidyanathan
August 2008 Chair: Corin Segal Major: Aerospace Engineering
In-flow species concentration measurements in reacting flows at high pressures are needed
both to improve the current understanding of the physical processes taking place and to validate
predictive tools that are under development, for application to the design and optimization of a
range of power plants from diesel to rocket engines. To date, non intrusive measurements have
been based on calibrations determined from assumptions that were not sufficiently quantified to
provide a clear understanding of the range of uncertainty associated with these measurements.
The purpose of this work is to quantify the uncertainties associated with OH measurement
in a oxygen-hydrogen system produced by a shear, coaxial injector typical of those used in
rocket engines. Planar OH distributions are obtained providing instantaneous and averaged
distribution that are required for both LES and RANS codes currently under development. This
study has evaluated the uncertainties associated with OH measurement at 10, 27, 37 and 53 bar
respectively. The total rms error for OH-PLIF measurements from eighteen different parameters
was quantified and found as 21.9, 22.8, 22.5, and 22.9 % at 10, 27, 37 and 53 bar respectively.
These results are used by collaborators at Georgia Institute of Technology (LES), Pennsylvania
State University (LES), University of Michigan (RANS) and NASA Marshall (RANS).
17
CHAPTER 1 INTRODUCTION
Over the past several decades, considerable effort has been dedicated for the development
of rocket engine technology including the space shuttle main engine (SSME) which operates at
pressures of 350 bar and a range of upper stage engines which operate with pressure ranges from
several bars to fewer than 100 bar. Yet, considerable difficulties remain to develop a design tool
that will adequately describe the physical processes occurring in the rocket engines. These
predictive tools require validation through accurate experiments.
An example of a current area of concern is illustrated by the photograph of the SSME
injector face shown in Figure1-1 The cracks and blanching in the chamber wall near the outer
row of the injectors is due to local uneven heating and must be corrected in future design.
Figure 1-1. Chamber wall cracks due to local heating. Blanching indicates regions of insufficient wall cooling [Courtesy: Mr.Kevin Tucker, NASA Marshall Space Flight Center, Huntsville, AL]
The consequences can be viewed as increased flight risk and maintenance costs and
indicates that that there is still a need to better understand the combustion chamber dynamics.
The most reliable method to accomplish this task is by the experimental study of the full scale
18
engines; however despite their reliability and robustness these experiments are costly. Hence,
Computational Fluid Dynamics (CFD) is continuously being developed for future designs.
The capabilities and limitations of CFD as a rocket injector design tool were addressed by
Tucker et al. [1]. The major challenges currently faced in CFD are due to lack of adequate date
base for the CFD validation. The expected performance of the CFD is such that the physical
description of the problem will develop from a small scale simulation to near full prototype with
continuously increased complexity and confidence [1, 2].
An example of the current status of the predictive capability is shown in Figure 1-2.
Figure 1-2. Comparison of CFD predicted wall heat flux measurements with experimental results
[Source: 3rd International Workshop on Rocket Combustion and Modeling, Paris, March 2006]. The CFD predicted results of the six different groups are inconsistent with each other and quite inaccurate when compared to experiment.
The plots in Figure 1-2 show the comparison of wall heat fluxes results obtained from
various CFD groups with the experiments. The CFD predicted results of the six different groups
19
are inconsistent with each other and quite inaccurate when compared to experiments. This shows
that considerable improvements need to be made in the predictive capabilities of the CFD tool.
Tucker et al. [1] indicated the necessity to obtain experimental database for a single
element gas-gas injector for code validation and optimization of the injector performance.
According to the authors [1] the single element design, referred to as the baseline design, can be
used to model performance and environmental indicators as function of the geometric variables
like orifice sizes, post tip thickness and cup details of the injector. Moreover the simplicity to run
a CFD code for a single element injector for code validation and subsequent improvement in the
code before validating more complex configurations were also addressed in detail.
In the study conducted by Calhoon et al. [3] a systematic approach to investigate and
characterize high performance injectors are explained in detail. The importance of single element
injector small scale testing, which gradually paved ways to multi element full scale testing of
rocket engines was also emphasized.
The importance and relevance of gas-gas injector for the development of gas-liquid
injector technology was further discussed by Schley et al. [4] who indicated that the accurate
prediction of gas-gas system using the CFD codes is necessary before applying the CFD codes to
predict gas-liquid system. Clearly, the accurate prediction of the gas-gas system is not a
sufficient condition to predict gas-liquid system but is a necessary preliminary step before the
inclusion of additional complexities like accurate treatment of atomization and spray
combustion.
The gas-gas single element dataset consists of
• inflow measurements of species concentration, temperature and velocity; • temperature boundary conditions at inlet and exit of the combustion chamber; • wall heat transfer boundary conditions;
20
A brief review of the existing experimental data focused on the inflow species
measurements for coaxial injector studies is tabulated in Table 1-1 and covers rocket injector
studies in the past 10–15 years. The reviews clearly indicate the lack of adequate inflow
quantitative species measurement with a thorough uncertainty analysis. Furthermore, when
evaluated, the uncertainties shown in Table 1-1 indicate that considerable work remains to be
done to improve the existing accuracy so that the database may be useful to support code
validation.
21
Table 1-1. Previous Experimental Studies on Rocket Injectors Uncertainty Ref. Injector type Chamber
Pressure (bar)
Parameters Experimental Method
Species Quantification Source
(% error) Rms error (%)
Foust et al. [5]
Single element shear (GH2/GO2)
13 Inflow velocity and species concentration (H2O, H2, O2)
LDV for velocity and Raman spectroscopy for species
Mole fraction of H2O,H2 and O2
(i) Non-linear temperature dependence of Stoke band factor (40)
40
Foust et al. [6]
Single element shear, swirl (GH2/GO2)
13–69 Inflow species concentration (H2O, H2, O2)
Raman spectroscopy
Mole fraction of H2O,H2 and O2
(i) Laser pulse energy fluctuation(5), (ii) Non-linear temperature dependence of Stoke band factor (45)
Table 1-1. Continued. Uncertainty Ref. Injector type Chamber
Pressure (bar)
Parameters Experimental Method
Species Quantification Source
(% error) Rms error (%)
Mayer et al. [17]
Single element shear (LOx/GH2)
63 Jet and flame visualization
Shadowgraph, OH emissions
Signal intensity (qualitative)
- -
Kalitan et al. [18]
Single element swirl (LOx/CH4)
41 Inflow species (OH, CO2) and jet visualization
OH visualization by PLIF and emission images, CO2 by emission images and jet visualization by shadowgraph and laser light scattering
Signal intensity (qualitative)
- -
Singla et al. [19]
Single element shear (LOx/CH4)
1–70 Inflow species visualization (OH, CH)
OH and CH emissions
Signal intensity (qualitative)
- -
Singla et al. [20]
Single element shear (LOx/GH2)
63 Inflow species concentration (OH) and visualization (OH)
PLIF for OH concentration and OH emissions for flame visualization
Signal intensity ( semi quantitative)*
(i) Boltzmann fraction variation in 2000–2500 K temperature range (10), (ii) laser beam absorption by OH(10–30) and (iii)Variation in quench rate due to species and temperature variation
32
24
Table 1-1. Continued. Uncertainty Ref. Injector type Chamber
Pressure (bar)
Parameters Experimental Method
Species Quantification Source
(% error) Rms error (%)
Singla et al. [21]
Single element shear (LOx/CH4)
25–30 Inflow species visualization (OH)
PLIF for OH visualization
Signal intensity (qualitative)
(i) UV PAH fluorescence and OH fluorescence are of same intensity at 25–30 bar
-
Smith et al. [22]
Single element shear (LOx/GH2)
40–60 Inflow species (OH) and jet visualization
Shadowgraph, OH emissions
Signal intensity (qualitative)
- -
Vaidyan-athan et al. [23]
Single element shear (GO2/GH2)
10–37 Inflow species concentration (OH)
PLIF for OH concentration
Mole fraction of OH
(i) Boltzmann fraction variation in 2500–3000 K temperature range (15), (ii) laser beam absorption by OH over a distance of 3 mm(8)
17
#PLIF – Planar Laser Induced Fluorescence *Singla et al. [20] provided semi-quantitative OH distribution in signal intensities without converting them to the actual number densities. Additional error sources which typically originate from photon calibration, shot noise, spatial variation of camera sensitivity and spatial variation in laser sheet intensity profiles were not addressed. One of the main objectives of the study carried out by Singla et al. was to provide OH distribution for CFD validation.
25
From the previous experimental studies tabulated in Table 1-1, it can be seen that only one
third of them addressed the uncertainties associated with the measurements and only a limited
number of factors have been included. A comprehensive and thorough investigation of the
uncertainties associated with the inflow measurements is clearly needed. This is the primary
motivation of the present work.
Before discussing the motivation of the current work, the importance of hydroxyl radical
measurement in non premixed flames is reviewed.
Hydroxyl Radical (OH) in Non-premixed Flames
In the injector vicinity of a non-premixed flame the OH radical is present in the reaction
zone of the fuel-oxidizer shear layer jets and is, therefore, a good flame marker [24–32].
Seitzman et al. [25] characterized OH structures in turbulent non-premixed hydrogen
flames and found that the OH was confined to the flame as a thin structure at the base of the
flame and was also found in the diffuse regions near the tip of the flame where the hot product
gases existed.
According to Barlow et al. [27] OH concentration peaks near the stoichiometric condition
in hydrogen flames. In this study [27] the equivalence ratio in the shear layer of supersonic and
subsonic jets varied between 0.8–1. The authors opined that since the stoichiometric contour is
often separated from the centre of the shear layer in turbulent diffusion flames, the OH
fluorescence can be a good reaction zone marker. In this study [27] the growth and relative
widths of shear layer for both compressible and incompressible flow were determined based on
the OH measurements.
Clemens and Paul [28] also discussed the use of OH as reaction zone marker. According to
the authors [28] the OH can also appear as a product in lower temperature regions due to its
relatively slow three-body recombination reaction, H+OH+M → H2O + M, M being the third
26
body. However, these regions appear as distributed and diffused OH zones when compared to the
thin laminar like filament structures in the primary reaction zone. Thus the appearance of OH in
the shear reaction zones represents the flame front and could be used to mark the reaction zones
in the GH2/GO2 combustion carried out in the current study. Similarly Ivancic et al. [13] in the
study of time and length scales in LOx/GH2 rocket combustors found out that the OH emissions
present on the symmetry line in the near injector regions come from the OH radicals produced
within the reaction zone.
Donbar et al. [30] identified the reaction zone structures in a turbulent non-premixed
methane jet flames based on CH-OH PLIF images. According to the authors [30] if the wrinkling
in the flame is not severe, the fuel rich boundary of the OH zone can be identified and used as
the stoichiometric contour. The stoichiometric contour in this study was identified as existing in
a thin zone in the gap between CH and OH regions. The stoichiometric contours were used to
determine the flame surface density and degree of flame wrinkling.
The visualization of reaction zone from OH-PLIF images is mentioned in the work done
by Pickett et al. [31]. According to these authors in non-premixed flames OH is consumed in the
fuel rich region and hence the OH zone is confined to the flame whereas in the case of premixed
flames, OH continues to exist in high temperature product regions. Singla et al. [21, 22] cites the
importance of OH radical in high pressure cryogenic flames as representing the characteristics of
combustion reactions, presence in high temperature stoichiometric regions and flame-front
marker.
Experimental investigation of the effects of heat release in a subsonic turbulent planer H2
jet was done by Theron et al. [32]. In this study H2 was injected through the central rectangular
slot whereas air was supplied from the upper and lower channels above and below the slot
27
respectively. The OH radical was tracked by fluorescence technique and the mean position of the
reaction zone was identified as the position of maximum OH fluorescence signal intensity from
the centre line along the test section height. The axial evolution of the mean position of the
reaction zone was represented as the stoichiometric contour of maximum temperature.
These studies clearly identified the usefulness of tracking OH in non-premixed flames as a
marker of the flame zone that is close to stoichiometric region; hence the continuous use of OH
measurement for combustion applications.
28
Motivation for the Current Work
Based on the existing information the present work is focused on providing OH
measurement with a detailed uncertainty analysis. The flow field is generated by a shear coaxial
H2/O2 flame.
This study was aimed at obtaining quantitative OH concentration at chamber pressures of
10–50 bar range and oxygen/fuel (O/F) mass flow ratio of four using OH-PLIF diagnostic. The
uncertainty sources and their respective contributions to the OH concentration measurements
will be addressed and discussed in detail in Chapters 4 and 5.
The data obtained here includes OH-PLIF measurements at pressures of 10, 27, 37 and 53
bar. Temperature measurements for boundary conditions are also included to compliment the
information provided to the CFD modelers. The data corresponding to chamber pressure of 10,
27, 37 and 53 bar were post-processed in this work and the uncertainties associated with the OH
measurements were identified and evaluated.
Thus, the rest of the document includes the following discussions:
• theory and review of OH planar laser induced fluorescence • experimental facility and diagnostic methods employed • OH-PLIF image processing and quantitative analysis • results and uncertainty analysis • conclusions • future work
Equation Section 2
29
CHAPTER 2 OH PLANAR LASER INDUCED FLUORESCENCE - THEORY AND REVIEW
A brief discussion of laser induced fluorescence (LIF) application to obtain the number
density of the species being probed, in this case, OH is given below followed by a review of
existing studies.
Fluorescence Modeling
Fluorescence modeling is based on a two level excitation / detection strategy within the
linear regime. Detailed explanations are given in Eckberth [33] and others [34–39].
λ = 283 nm
λ = 306-320 nm
ν’’
ν’ ν’
ν’’
Laser Excitation
01
23
01
23
01
23
01
23
A A
X X
Fluorescence Emission
Ground State
Excited State
Step 1 Step 2Vibrational level
Rotational level
λ = 283 nm
λ = 306-320 nm
ν’’
ν’ ν’
ν’’
Laser Excitation
01
23
01
23
01
23
01
23
A A
X X
Fluorescence Emission
Ground State
Excited State
Step 1 Step 2λ = 283 nm
λ = 306-320 nm
ν’’
ν’ ν’
ν’’
Laser Excitation
01
23
01
23
01
23
01
23
A A
X X
Fluorescence Emissionλ = 283 nm
λ = 306-320 nm
ν’’
ν’ ν’
ν’’
Laser Excitation
01
23
01
23
01
23
01
23
A A
X X
Fluorescence Emission
Ground State
Excited State
Step 1 Step 2Vibrational level
Rotational level
Figure 2-1. Two-State Quasi-Steady Two-Step Modeling of Fluorescence
The laser induced fluorescence process is illustrated in Figure 2-1. It consists of a two step
process: the first step is the excitation of the molecule/radical from the ground state (X) to the
upper excited state (A) by laser absorption; the second step is the spontaneous emissions of
photons when the molecule relaxes from the upper excited state to their ground states. Given the
certain energy loss associated with the process, emission is at longer wavelength than the
excitation. Emission occurs very close after absorption and is of the order of less than 10 ns in
the case of OH in an atmospheric flame [38]. The quantification of the number of photons
collected in this process can be used to determine the number density of the molecule/radical in
30
the region of interest provided all the processes involved in the fluorescence are properly
accounted for and modeled.
The processes involved in fluorescence can be more specifically termed as stimulated
absorption-W12, stimulated emission-W21, spontaneous emission-A21 and collisional quenching-
Q21. These four processes of energy transfer take place between the electronic states, in this case,
the ground state (X) and the upper excited state (A). In the upper excited state the two processes
of interest are the rotational energy transfer -RET and the vibrational energy transfer -VET.
The excitation is provided by a monochromatic source from a pulse laser with short
duration of less than 10 ns. This permits fluorescence detection time of less than 500 ns which
helps in avoiding the interference from other background emissions during diagnostics.
The rate of absorption by the molecule/radical is given by
1212 2
BW I( )
cv⎛ ⎞= ⎜ ⎟
⎝ ⎠ (2-1)
Here 12W (s-1) is the stimulated absorption rate, 12B is the Einstein B coefficient for
absorption (cm3J-1s-2), c is the speed of light (cms-1), I( )v is the laser spectral fluence (Wcm-2 cm)
given bylaser
E( )A l
vτ
, where E( )v is the laser spectral energy per pulse (Jcm), Alaser (cm2) is the cross
sectional area of the laser beam or sheet and lτ (s) is the laser pulse duration. Since the
absorption process involves laser/molecule interaction it is called stimulated absorption rate.
The molecule/radical will relax from the upper state to the ground state by the following
three processes as described below.
The first path constitutes of stimulated emission, in which the molecule/radical interacts
with the laser and returns to the ground state. The stimulated emission rate, W21 (s-1) is given by
31
2121 2
BW I( )
cv⎛ ⎞= ⎜ ⎟
⎝ ⎠ (2-2)
where 21B is the Einstein B coefficient for emission (cm3J-1s-2). The absorption and emission
rates are related by
1 12 2 21g W =g W (2-3)
Here 1g and 2g are the degeneracies of the ground and the upper electronic states respectively.
The second path constitutes of the spontaneous emission in which the molecules relax from
the upper excited state to the ground state by emitting fluorescence. This is the main mechanism
for LIF signal production. The spontaneous emission rate is dictated by Einstein coefficient for
spontaneous emission 21A (s-1). The spontaneous emission rate and the stimulated absorption
rate are related by
321
12
A8
Bhπ ν= (2-4)
where h (J.s) is the Planck’s constant and ν is the wave number of the particular individual
transition (cm-1).
In the third process, the molecules in the upper excited electronic state can relax to the
ground state by collisions with other molecules called collisional quenching. The quenching rate
is modeled as,
12
218
Q Bi i
iB i
k TPk T π
⎛ ⎞⎛ ⎞= ⎜ ⎟⎜ ⎟⎝ ⎠ ⎝ ⎠
∑c sm
(2-5)
where P is the pressure, Bk is the Boltzman constant, T is the temperature, ic represents the
colliding species mole fraction, is , the colliding species cross section and im is the reduced
mass of excited molecule/radical ,in this case, OH and the colliding species. Quenching
32
represents the rate of non-radiative decay of the excited state molecule to the ground state. It can
be noticed from Equation 2-5 that quenching linearly increases with pressure and hence at high
pressures the fluorescence signal intensity due to spontaneous emission can be significantly
reduced due to quenching. This is one of the major challenges in applying LIF techniques at high
pressures.
In RET the molecules in the upper excited rovibrational state can move to neighboring
rotational levels in the same excited electronic state due to collisions with other molecules.
Similarly in VET the molecules migrate to neighboring vibrational levels of the same upper
excited state. The collisional quench model in Equation 2-5 needs to be modified to take into
account the effect due to RET and VET. The modified model for collisional quench rate of OH
which also takes into account the effect of RET and VET is discussed in Chapter 4 in detail.
Other mechanisms involved in the energy transfer processes are predissociation and
photoionization [33]. Predissociation is the process in which the excited molecule dissociates
prior to the emission of the photon. In photoionization, the excited molecule gets ionized prior to
the emission of the photon.
Based on the two state two step model as shown in Figure 2-1 a mathematical formulation
of all the processes involved in fluorescence is made to infer target species number density. The
population density in the ground state, n1 (cm-3) and in the excited state n2 (cm-3) constitute the
total population density of n = n1 + n2 (cm-3) for the specific robvibrational transition being
excited. The rate of change of molecules in the upper excited state (A) per unit volume is then
given by
( )21 12 2 21 21 21W W Q A
dnn n
dt= − + + (2-6)
33
In the current study fluorescence in the linear regime is considered, thus the fluorescence
signal is linearly proportional to the input laser irradiance. In other words, the number of
fluorescence signal photons collected is linearly proportional to the number of input laser
photons supplied during the duration of the pulse. In contrast to linear regime, fluorescence
signal photons become independent of both laser irradiance and collisional quenching in the case
of saturation regime. The laser irradiance used in the current study which is 0.445 x 106 W/cm2 is
nearly four-five orders of magnitude less than the laser irradiance employed for saturation LIF
studies by Carter et al. [40]. Hence for the current study the pumping is weak and the
fluorescence can be considered to be in the linear regime.
At steady state, 2dndt
is zero and in the linear regime, as 12W is negligible [33], n2 is
expressed as
1 122
21 21
W(Q A )
nn =
+ (2-7)
The fluorescence signal or the number of photons, pN can then be expressed as
p 2 21N A V4 ln τπΩ
= (2-8)
where, V (cm3) is the volume probed by the laser and4πΩ is the fraction of the solid angle
detected.
Substituting the expression of n2 from Equation 2-7 and rearranging Equation 2-8
( )
21p 1 12
21 21
AN W V
Q A 4 ln τπΩ
=+
(2-9)
For weak pumping, n2<< n1 and total population density n ~ n1. The population density n1
(cm-3) in the ground state rovibrational energy level is related to the total number density of the
34
molecule/ radical by n1 = noBf . Here no is the total number density and Bf is the Boltzmann
fraction of the specific rovibrational energy level in the ground state. Thus, pN in Equation 2-9
can be rewritten as
( )
21p 12
21 21
AN W V
Q A 4o
B ln f τπΩ
=+
(2-10)
Substituting the expression of 12W from Equation 2-1 into Equation 2-10
( )
12 21p 2
21 21
B AN I( ) V
Q A 4co
B ln f v τπΩ⎛ ⎞= ⎜ ⎟ +⎝ ⎠
(2-11)
Emitted and absorbed light has a finite bandwidth which is called the line broadening [33,
35]. This means that in reality, the energy of a dipole transition which is well defined by the
energy difference between two quantum states is not monochromatic and has a certain spectral
width and shape. The line broadening in a typical combustion environment is due to three main
reasons, namely natural broadening, collisional/pressure broadening and Doppler broadening.
Each is briefly discussed below.
Natural broadening is due to the finite lifetime of the molecule/radicals in the excited
state. If the molecule were to radiate energy for an infinite period, the line shape is a delta
function. Since the lifetime is finite it represents a Lorentzian function [35]. In general the effect
of natural broadening is much smaller compared to collisional and Doppler broadening; hence, it
is often neglected [33]. Similarly in the case of collisional broadening, the lifetime of the
molecule in radiating the energy is reduced if it collides with other molecules. The Doppler
broadening occurs due to the Doppler shift caused by the relative motion of the molecule and the
laser beam propagation.
The collisional broadening represented by a Lorentzian function [35] is
35
2
1(2
( )2
) c
co
cv
vv v
vπ
Δ
Δ⎛ ⎞− + ⎜ ⎟⎝ ⎠
=f (2-12)
where ( )c vf is the normalized line shape function, cvΔ is the spectral width associated with
collisional broadening, ov is the central frequency of the transition involved. For OH the
collisional width could be calculated from the empirical model provided by Davidson et al [41]
based on spectroscopic measurements carried out in a shock tube at conditions of 60 bar and
1735 K.
0.75
-13000.140 cmCo
PvP T
⎛ ⎞⎛ ⎞ ⎡ ⎤Δ = ⎜ ⎟⎜ ⎟ ⎣ ⎦⎝ ⎠⎝ ⎠ (2-13)
Similarly the Doppler broadening represented by Gaussian profile [35] is
20.52 ln 2( exp 4ln 2) o
DD D
v vv v
vπ
⎡ ⎤⎛ ⎞−⎛ ⎞ ⎢ ⎥− ⎜ ⎟⎜ ⎟Δ Δ⎝ ⎠ ⎢ ⎥⎝ ⎠⎣ ⎦=f (2-14)
where ( )D vf is the normalized line shape function, DvΔ is the spectral width associated with
collisional broadening and ov is the central frequency of the transition involved. The Doppler
width [35] is
2
-7 -12
8 ln (2)7.16 x 10 cmB o
D ok Tv Tv v
Mmc⎡ ⎤Δ = = ⎣ ⎦ (2-15)
where T is the temperature, kB, the Boltzmann factor, m, the mass of the molecule/radical and M
is the molecular weight of the molecule/radical which is OH in the current study.
The spectral distribution due to the line broadening is expressed as a normalized line shape
function, ( )abs vΦ and is defined as ( ) 1abs v dv+∞
−∞
=Φ∫ . The absorbing species line shape function,
36
( )abs vΦ is obtained as the convolution of collisional and Doppler line shape functions which is
generally referred to as the Voigt profile [33, 35].
Moreover, the central frequency of the absorption profile gets shifted due to the collision
with neighboring molecules and/or due to the Doppler effect [20, 37, 41]. The collision induced
shift for OH is given [20, 41] by
0.45 0.08
-1 shift
3000.0305 cmCo
PvP T
±⎛ ⎞⎛ ⎞ ⎡ ⎤Δ = − ⎜ ⎟⎜ ⎟ ⎣ ⎦⎝ ⎠⎝ ⎠ (2-16)
and the Doppler shift [37] is given by
-1D shift
v cmcov v ⎛ ⎞ ⎡ ⎤Δ = − ⎜ ⎟ ⎣ ⎦⎝ ⎠
(2-17)
Here shiftCvΔ and D shiftvΔ represent the collisional and Doppler shifts respectively, ov is the
central frequency of the specific rovibrational transition, v, the velocity of the molecules and c, is
the speed of light.
In the current study, the absorption profile for OH is simulated using the commercially
available software LIFBASE [37]. The laser profile used in this study is assumed to be well
represented by the Gaussian profile. The laser line profiles and the absorption line profiles
relevant to the current study will be discussed later in Chapters 3 to 5.
Thus, to account for the spectral distribution of the laser profile and the absorption profile
of the target species, the fluorescence signal in Equation 2-11 is modified as
( )
12 21p 2
21 21
B AN I( ) ( ) V
Q A 4c abs lo
B v v dvn f τπΩ⎛ ⎞= Φ⎜ ⎟ +⎝ ⎠∫ (2-18)
Substituting for laser l
E( )I( )A
vvτ
= and E( ) E ( )laserv v= Φ where E is the laser energy per pulse and
( )laser vΦ is the laser line shape, into Equation 2-18 and rearranging,
2 mJ per pulse (i) Measured time decay of the fluorescence with pressure was used to obtain quenching rate constants. (ii) The measured cross sections had 15% accuracy
- Attractive forces between the molecules need to be properly taken into account in the case of quenching models for accurate prediction of quenching cross sections.
Garland & Crosley [47]
- - - Temperature and species dependent quenching cross section of OH was predicted using a model based on attractive forces
- The predicted quenching cross sections of NH3, H2, NO, O2, H2O, N2O,CH4, CO and CO2 agreed within +30 % of the experimental values
42
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
Excitation P2(7), A-X(1,0) Detection 312 nm vidicon camera
(i) Laser attenuation was negligible. (ii) radiative trapping was < 5%
+5 % variation in initial state population in 1000–2000 K temperature range
10 nm bandwidth filter centered at 312 nm
(i)OH concentration was obtained from flat flame calibration. (ii) +10 % error from calibration measurements, 7% due to photon statistics (iii) OH concentration was five times higher than equilibrium values in reaction zones
Seitzman et al. [14]
OH-PLIF in a turbulent non-premixed H2/air jet atmospheric flame
Excitation Q1(3), A-X(0,0) Detection A-X(0,0),(1,1) CCD camera
(i) 50–120 mJ per pulse (ii) laser absorption is 3–20 %
+40 % variation in initial state population in 1000–2000 K temperature range
- Spatial autocorrelation was used to determine flame angle and correlation lengths
44
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
(i) There was loss of fluorescence signal due to quenching and absorption line shape broadening with increasing pressure and the estimated signal reduction was of the order of 100 in the 10–100 bar range.
Interference filter centered at 315 nm with FWHM of 38 nm
(i) The simulated Voigt profile matched well with the measured one (ii) OH concentration from absorption measurements with 30% accuracy agreed well with numerical predictions (ii) Feasibility of applying numerical modeling to obtain effect of quenching and line broadening on fluorescence efficiency was mentioned
45
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
Comparison of A-X (1,0), (0,0) and (3,0) schemes for quantitative fluorescence imaging . The A-X(1,0) scheme is highlighted here
- (i) 10 mJ/cm2
(4 mJ for 80x0.5 mm sheet) is considered to ensure fluorescence in linear regime within + 5% down to zero energy (ii) Need to apply corrections for spatial laser profile variation
(i) Need to choose rotational transition with low temperature dependence (ii) Overlap integral variation in a non-isobaric flow(1–5 bar) is 30–40% for lasers with line width of 0.2–0.5cm-1 (iii) Overlap integral variation with temperature (1000–2500K) is less then + 5% for line widths less than 0.5 cm-1 (iv) quench rates vary only by <10% in regions of OH concentration
(i) Assumption: Fluorescence is emitted equally into 4πSr (ii) Random noise in the detector(ICCD) is contributed by shot noise, quantum efficiency, electron gain, dark, readout and digitization noise (iii) Pulse to pulse variation in laser bandwidth contributes to error in OH concentration measurement.
(i) Actual laser induced excitation and emission can deviate from the two state, two step quasi steady model leading to systematic errors (ii) Nonlinear responses to change in laser energy, population fraction and depletion are well within the range of A-X(1,0) excitation scheme.
46
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
- (i) The need for spectroscopic data for quenching correction was mentioned (ii) Colliding species and temperature field data was obtained from equilibrium calculations
- (i) To obtain OH concentration an initial calibration was carried out in a lean H2/air flame in a Hencken burner
47
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
Paul [53] Temperature dependent collisional model for OH in 250–2500 K range
- - - - (i) A function for predicting temperature dependent cross section for collisional quenching of OH by various molecules is provided (ii) A model for fluorescence yield in A-X(1,0) excitation scheme by incorporating effect of VET in the excited electronic state(A)
48
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
Imaging OH in 1–10 bar heptane, methanol and ethanol-air flame
Excitation(OH) 283 nm, A-X(1,0) Detection(OH) 316-371 nm ICCD camera Interference 100 ns gate time to avoid background luminosity and chemiluminescent gas emissions.
(i)3 mJ per pulse. (ii) laser attenuation was estimated as ~30% due to absorption by hydrocarbons
(i) Effect of pressure on fluorescence signal intensity in linear regime was analyzed based on steady state and multi level transient approach
A combination of filters transmitted fluorescence from 316–371 nm
(i) As long as fluorescence was in linear regime, quasi steady state model used in deriving fluorescence yield was valid for the experimental conditions investigated.
49
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
LIF measurements of OH and NO in 1–10 bar methane flames: Fluorescence modeling and experimental validation
Excitation(OH) P1(8),285.685 nm, A-X(1,0) Detection(OH) A-X(0,0) Photomultiplier tube(PMT) Interference No significant interference near 285 nm
(i)100 μJ per pulse to ensure fluorescence in linear regime (ii) Judicious selection of absorption transition to avoid significant laser attenuation
(i) Fluorescence signal was modeled as two state two step steady process in linear regime (ii) Use of laser with large bandwidths to minimize effect of pressure on overlap integral. (iii) Laser with large bandwidths provided more flexibility in tuning the centre line of the absorption profile
- (i) The single point OH equilibrium concentration from LIF measurements agreed well with the calculated equilibrium values of OH. (ii) This implied that the effect due to overlap integral, absorption line strength variation due to temperature and fluorescence yield were well accounted by the model used to predict them
50
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
- (i) Rate coefficients from fluorescence life time was converted into quenching cross sections by dividing with average collisional velocity of the species pair (ii) Quenching model formulated by Paul53 could predict the temperature dependent behavior observed from experiments
Bandpass filterer, 310+5 nm
At 2300 K, the ratio of the measured quenching cross section to quenching model [53] predicted values for H2O and O2 are 1.12 and 0.537 respectively
51
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
- (i) The species concentration and temperature obtained from STANJAN was used to calibrate the measurement from a H2-air and CH4flame. (ii) The fluorescence signal was corrected for the variation in Boltzmann fraction and collisional quench rate.
- (i) Uncertainties in measurement were identified as 10.5% due to shot noise and 5% due to wavelength drift. (ii) Location and growth of shear layer were determined from the OH distribution. (iii) The super equilibrium OH concentration were nearly four times higher than the equilibrium counter parts
52
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
(i)40 μJ per pulse to ensure fluorescence in linear regime (ii) O12(8) transition was chosen to avoid significant laser attenuation
(i) For electronic quenching corrections, the temperature and colliding species concentration data were obtained from Raman/Rayleigh measurements. (ii) The OH number density was calibrated against the equilibrium OH composition corresponding to the measured Rayleigh temperature in a lean CH4-air flame
Combination of color glass filters (WG-295 & Hoya U-340)
(i) In the study, it was observed that the temperature and OH concentrations at the inner flame zones could be well predicted using a one dimensional premixed laminar flame model incorporating finite rate chemistry.
53
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
(i)1 mJ per pulse (ii) Back-ground subtraction to avoid light reflections (iii) laser spatial variation was corrected
(i) Boltzmann fraction variation was 10 % in the range of 1300–3200K (ii) Absorption line shape was measured by careful scanning of P1(8) line. (iii) Temperature data was obtained from CARS and numerical simulation
(i) WG295 filter was used to suppress the elastic scattering (ii) Spatial variation (pixel to pixel) of camera sensitivity was corrected
(i) Absolute concentration was obtained from 1D absorption measurements
Atkan et al. [62]
OH LIF, 2D and spectroscopic measurements at 5–36 bar in a laminar premixed methane/air flames
Excitation(OH) 280–287nm, A-X(1,0) Detection(OH) 310 nm, A-X(0,0) ,(1,1) CCD camera Interference No interference from other molecule
(i) 14 mJ per pulse (ii) Estimated laser absorption was less than 10 %
(i) Scanned excitation spectra and simulated excitation spectra matched very well and signified that there are no interferences from other molecules in the A-X(1,0) 280–287 nm range (O2 interference for A-X(3-0) scheme)
Bandpass filter(WG305 and UG11) centered at 310 nm, FWHM-16 nm and peak transmission efficiency of 5.5%
(i) The advantages of the A-X(1,0) LIF detection scheme were identified as devoid of fluorescence trapping (A-X(0,0))
54
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
Fluorescence imaging of combustion species in gas turbines up to 20 bar and associated complexities
Excitation(OH) 283nm, A-X(1,0) Detection(OH) 310-320 nm, A-X(0,0) ,(1,1) ICCD camera Interference Interferences from PAH as they are broadband absorbers and emitters in the emission spectrum of OH, flame emissions, laser light scattering & wall luminescence
(i) 16 mJ per pulse (ii) laser sheet non-uniformity was corrected by obtaining the quartz reflected images of the laser sheet (iii) Back-ground subtraction of the non-resonant images
(i) Pressure induced line broadening and quenching effects which tend to decrease the fluorescence signal. (ii) Selection of line transitions with weak absorption coefficients to avoid considerable laser absorption and attenuation
(i) Combination of WG-305 & UG 11 filters (transmission efficiency ~56% in the 310–320 nm range). (ii) Weak signals require pixel binning in the camera (iii) Determination of camera magnification and its accurate alignment.
Complexities in OH imaging (i) Test rig and optical system vibration and displacement (ii) Optical window cleanliness(soot formation on the windows), cooling and structural integrity (iii) laser wavelength drift (iv) Optical thickness of the medium (v) Noisy spikes in the collected signal due to abrupt rise in laser intensity
55
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
(i) Flame temperatures were measured from the excitation scans. (ii) For quenching rate determination, equilibrium compositions of the colliding species concentration were calculated. (iii) Quenching contributions from individual colliding species was calculated based on temperature dependent rate expression
- (i) The measured quench rate and the calculated quench rate based on temperature and species dependent quench rate model agreed very well. (ii) The excellent agreement between the calculations and the experiments showed that collisional quench rate could be well predicted from knowledge of gas temperature and colliding species concentration
56
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
OH-PLIF to investigate shear coaxial cryogenic jet flames
Excitation(OH) Q1(6), 283.92 nm A-X(1,0) Detection(OH) A-X(0,0) ICCD Interference Raman signal at 296 nm from the liquid phase
30 mJ per pulse
(i) The Q1(6) transition was selected to minimize the temperature dependence
UG-5 and WG-305 fliter
(i) LOx jets scattered and dispersed the laser sheet thereby affecting OH fluorescence
Meier et al. [65]
Species and temperature measurements from piston engine(10 bar) and aero engine test rig(6 bar)
Excitation(OH) 282-286 nm A-X(1,0) Detection(OH) 315 nm, A-X(0,0) ICCD camera Interference Interference from fuel fluorescence
(i) 5–10 mJ per pulse. (ii) Laser sheet spatial variation was corrected by normalized acetone fluorescence images on an average basis (iii) laser shot to shot energy fluctuation was monitored using a fast photodiode
(i) Transition was selected to minimize the variation of state population in 1000–3000 K temperature range
(i) Interference filters centered around 315 nm with FWHM 30 nm
(i) Areas of OH concentration were used to identify zones of homogenous combustion and high heat realse.
57
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
OH-PLIF in heptane and Jet-A spray flames at 5, 7 and 11 bar
Excitation(OH) 283nm A-X(1,0) Detection(OH) 315 nm, A-X (0,0),(1,1) ICCD camera Interference Scattering from fuel droplets
(i)3 mJ per pulse (ii) Laser attenuation across the flame at higher pressures was attributed to increased OH number density and hydrocarbons
(i) At high pressures there was considerable decrease in fluorescence signal due to quenching and line broadening.
Interference filters
(i)The OH distribution was used to analyze the turbulent spray structure
Hicks et al. [67]
OH-PLIF applied to combustors burning Jet-A fuel at pressures of 9 and 18 bar
Excitation(OH) 282nm A-X(1,0) Detection(OH) 316 nm, A-X (0,0),(1,1) ICCD camera Interference Scattering from fuel droplets
25–30 mJ per pulse
- Interference filters centered at 316 nm with FWHM 2.6 nm and peak transmission of 16%
(i) OH-PLIF images were used to mark flame and recirculation zones. (ii) The use of OH-PLIF images in fuel injector design and kinetic modeling was highlighted.
58
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
Identification of rotational lines of OH in H2/O2 and methane/air flame
Excitation lines at A-X(0, 0), (1,0), (2,0) and (3,0) were recorded using spectrograph
5 mJ per pulse - - (i) The entire rovibrational transitions in the 240–325 nm range were excited, detected, identified and tabulated
Thiele et al. [69]
OH-PLIF in spark ignited combustion of H2/air mixtures
Excitation(OH) 283nm A-X(1,0) Detection(OH) 310 nm, ICCD camera
0.2 mJ per pulse
- - (i) The raw gray scale images were filtered using a 2-D Gaussian filter to reduce noise. (ii) The flame front position was identified as the region of steepest gradient in the flame/OH image. (iii) Temporal evolution of the flame kernel was identified from the OH images.
59
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
Excitation(OH) Q1(5) A-X(1,0) Detection(OH) A-X (0,0),(1,1) ICCD camera
(i)<10 μJ per pulse (ii) Laser shot to shot power fluctuation was monitored
(i) For calculation of collisional quench rate, the colliding species concentration and temperature were obtained from adiabatic equilibrium conditions
UG5 and WG 295 filters
(i) OH calibration was carried out in a H2/air flame of equivalence ratio 0.9. (ii) Fluorescence intensity of OH along the height was compared with the simulated OH profiles. (iii) The estimated uncertainty in absolute OH concentration was 20%
61
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
Excitation(OH) Q1(9) A-X(1,0) Detection(OH) A-X (0,0),(1,1) ICCD camera
(i) 24 mJ per pulse (ii) shot to shot power fluctuations was estimated as + 5%
(i) Boltzmann fraction variation was + 12.5 % in the range of 1100–2400K (ii) The collisional quenching rate variation with species concentration and temperatures corresponding to equivalence ratios of 0.5 to 3 was estimated to be + 30 % in that range
WG 295 and UG 11
(i) Laser energy absorption due to OH and droplet scattering accounted to + 10 % uncertainty.
Kalitan et al. [18]
OH-PLIF in LOx/methane flames at 41 bar
Excitation(OH) Q1(9) A-X(1,0) Detection(OH) A-X (0,0),(1,1) ICCD camera
(i) Signal attenuation due to laser absorption by OH. (ii) Light scattering as it traversed through spray
- UG11 and WG305 filter
(i) OH images were used as indicators of combustion zones of high temperatures
62
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
(i) 42 mJ per pulse (ii) laser beam absorption by OH was estimated to be 10–30 % (iii) Laser beam is absorbed and scattered by LOX core in the centre
(i) The variation of Boltzmann fraction accounted to 10% in the range of 2000–2500 K and was considered insignificant (ii) The collider species concentration and temperature field for quench rate at 63 bar was calculated based on the collider species mole fraction and temperature field of a counter flow LOx/GH2 flame at 1 bar
Bandpass filter (i) A detailed description of fluorescence modeling was provided. (ii)Quenching did not strongly perturb the spatial fluorescence (iii) The collected fluorescence spectra matched well with the simulated spectra from LIFBASE (iv) The mean position of the flame, flame stabilization, corrugation and unsteadiness of the jet were observed from OH images
63
Table 2-1. Continued. Authors Target Species (I)
Fluorescence strategy and interference signals
(II) Laser energy, spatial profile and attenuation
(III) Absorption &excitation line shape and fluorescence efficiency
- (i) Filter scheme 1: 56% transmission at 310–320 nm (ii) Filter scheme 2: 25% at 308 nm with FWHM of 15 nm
(i) The limiting factor for OH-PLIF in oxygen/methane flame above 25 bar was PAH fluorescence interference when compared to laser absorption in hydrogen/oxygen flames (ii) The LOx/methane flame was less stabilized compared to LOx/H2 flame
64
Based on these studies certain observations are useful as follows
Fluorescence Strategy and Interference Signals
The choice of excitation at A-X(1,0) and detection at A-X(0,0), (1,1) in the 306–320 nm
range has the advantage that the elastic and laser internal scattering can be effectively blocked.
Moreover there are no interferences from molecules like H2O and O2 in the combustion
environment. The radiative trapping which is predominant in the A-X(0,0) scheme is negligible
[62]. To suppress the flame emissions and to collect all the fluorescence, gate width of the order
of ~150 ns for the detection system, in this case ICCD camera can be employed. The background
emissions need to be corrected depending on the signal strength.
Laser
The laser pulse energy / area of 10 mJ/cm2 could ensure fluorescence in the linear regime
within + 5% [50]. Hence laser energy typically of 2–3 mJ with a sheet cross section of 40 mm x
0.5 mm could be effectively used to ensure fluorescence in the linear regime. The laser
attenuation across flame/ combustion environment depends on the strength of the absorption
transition excited, the number density of the molecule, unwanted absorption by molecules like
hydrocarbons and the path length traversed by the laser. The laser absorption and attenuation can
vary from less than 5 % to 100 % across the flame depending on the flame conditions. In a
GH2/LOx flame the absorption of laser by other combustion species like H2O was found
negligible at 283 nm [20, 70].
The use of a strong transition with a laser of large line widths and low energy to ensure
fluorescence in the linear regime can be viewed as a potential method to obtain fluorescence of
good signal strength. The use of lasers with relatively larger line widths can lead to excitation of
rovibrational lines neighboring the strong transitions. Hence the excitation of these lines needs to
be taken into account in the fluorescence modeling. The laser shot to shot power fluctuation
65
could be monitored and corrected or the mean value of the laser energy can be used provided the
uncertainty from energy fluctuation is accounted in quantitative measurements. To correct for the
laser sheet profile variation in space, acetone fluorescence images [65] from the same excitation
wavelength, 283 nm in this case could be used.
Absorption & Excitation, Line Shape and Fluorescence Efficiency
In quantitative measurement, one of the major uncertainties is due to the initial state
population variation with temperature and is normally 10–15% in the 1500–2500 K range. This
problem could be approached in the following ways. The transition could be selected such that
the variation of the state population in the temperature range of interest is negligible so that it
does not contribute to the uncertainty in the measurements. The second approach is to obtain the
temperature field information in the region of interest either from calculations based on
equilibrium conditions, detailed numerical simulation of the combustion field /reference flame or
by calibration measurements via thermocouple measurements, Raman / Rayleigh measurements
in actual combustion environment/ reference flame. However, each of these approaches has
themselves a degree of uncertainty. The third approach is to determine the uncertainties in the
measurements due to state population variation in the temperature region of interest and account
for the uncertainties in the quantitative measurements [73].
The line broadening and shifting at higher pressures reduce the overlap integral and hence
the fluorescence signal. The variation in the overlap integral needs to be, therefore taken into
account in quantitative measurements. The reduction in fluorescence signal due to decrease in
the overlap integral can be overcome by employing lasers with larger line widths [55].
The effect of collisional quenching in reducing the fluorescence efficiency can lead to
signal reduction of the order of a factor of 100 in the 10–100 bar range [49]. Most of the works
which utilize OH-PLIF for applied spectroscopy uses collisional quench model given by
66
Equation 2-5 for calculation of collisional quench rate. This requires the knowledge of the
pressure, temperature, mole fraction and temperature dependent cross section of the colliding
species. The expression for temperature dependent cross section from a complex collision model
and the measured temperature dependent cross sections of the colliding species such as H2, H2O,
and O2 are available in the literatures [53, 58, 64]. The knowledge of temperature and mole
fraction of the colliding species can be obtained in the same way as the temperature field data is
obtained to account for initial state population variation. The quenching rate variation across the
combustion field is not significant [20] and could normally account for less than 10% variation
[50].
Experimental Constants
The fluorescence process is considered such that it is emitted equally in all directions and
that the photons are Poisson distributed [35]. There is, thus an uncertainty in the exact number of
photons detected and this uncertainty is called shot noise. The uncertainties in detection system
also arise due to the quantum efficiency of the photocathode, the thermal current in the CCD
chip known as dark current, readout noise in the A/D conversion and the digitization noise [35,
50]. The spatial variation of camera sensitivity across the chip also adds to the uncertainties. For
weak fluorescence signal detection pixel binning at the cost of resolution is also recommended.
Thus the challenges involved in applying OH-PLIF at high pressures and temperatures in
the linear regime could be recognized as 1) rotational level population dependence on
temperature, 2) reduced fluorescence efficiency due to absorption line shape broadening and
The GH2/GO2 combustion experiments lasted for 9.75 s following ignition for 10 bar case
where as for all the other test cases the combustion run time was limited to 7.75 seconds
following ignition.
OH-PLIF Diagnostics
For the PLIF experiments third harmonic output at 355 nm from a Nd-YAG (Continuum
Surelite II) pulsed laser was used to pump the OPO (Continuum Panther). The FWHM spectral
width of output beam measured using a Burliegh WA-4500 wavemeter was ~5 cm-1 and the
centerline of the laser before doubling corresponded to 563.03 as shown in the Figure 3-4. The
measured spectral width was in agreement with the manufacturer’s specification. The output
from the OPO was frequency-doubled to obtain a UV beam at 283 nm.
73
Figure 3-4. Laser spectral profile measured using Burleigh Wavemeter before doubling to 283 nm
The UV beam had a measured pulse energy of 0.89 mJ and was used to excite the OH A-X
(1,0) rotational transitions. The laser beam at 283 nm was formed into a sheet of 4 cm x 0.05 cm
cross section using a series of fused silica lenses. The sheet was made 4 cm in height; however
the central portion of 2 cm of the light sheet passed through the chamber to ensure that the wings
of the Gaussian beam are not used for PLIF diagnostics.
The schematic of the OH-PLIF diagnostic setup is shown in Figure 3-5. Fluorescence
images were collected perpendicular to the direction of laser beam propagation using an ICCD
camera (DiCam-Pro Cooke Corp.) equipped with 105mm/4 telephoto UV lens. The laser and the
camera were synchronized using a pulse generator (DG 535 Stanford Research Systems) and
were operating at 10 Hz. The camera was used in double shutter mode such that it collected
fluorescence for 100ns in synchronization with the laser in the first image. The second image
was collected 500ns after the first image for the same duration of 100ns to capture flame
74
emissions. The effective resolution of the camera was 66 micrometer/pixel in 4 x 4 binning
mode. A combination of 3mm WG 305 Schott and 3 mm UG 11 filters were used to collect
fluorescence from 306–320 nm while effectively blocking elastic scattering. The combined
transmission efficiency of the filters was about 55% between 306 and 320 nm.
Figure 3-5. OH-PLIF Experimental Set-up
OH-PLIF images were acquired for the entire run time at the rate of 10 Hz with 100 ns
exposure time. Out of the instantaneous OH-PLIF images recorded, thirteen of the instantaneous
images recorded at the near steady state at the end of the experiments were averaged and
represented as averaged OH-PLIF image. Correspondingly, thirteen background emission images
recorded by operating the camera in the double shutter mode were averaged. The instantaneous
and averaged background emission images were then subtracted from the instantaneous and
averaged OH-PLIF image respectively.
75
Wall Boundary Conditions
The wall boundary conditions consist of wall heat fluxes determined from temperature
measurements in the combustion chamber. Conley et al. [76] calculated the heat fluxes from the
temperature measurements. In this and in the previous study [75, 76] the heat fluxes were
calculated by solving the steady state one dimensional heat conduction equation and adding a
correction term to compensate the heat absorption by the chamber as shown below
( ) ,2 ,1A ,2 ,2q
2o o
i o
T TC xk T Tx t
−⎛ ⎞Δ= − + ⎜ ⎟Δ Δ⎝ ⎠
r (3-1)
where Aq , is the heat flux, Δx is the distance between the thermocouple pairs, the subscripts ‘i’
is assigned for thermocouple close to the inner chamber wall, ‘o’ represents the one farthest and
1 and 2 represents the initial and final times respectively.
The very nature of heat transfer in the combustion chamber is three dimensional. Thus
calculation of heat fluxes based on 1D assumption can introduce errors. Thus in the current work
the heat flux was calculated by numerically solving the unsteady 3D heat conduction equation.
The method will be discussed in detail in the following section.
The chamber extensions shown in Figure 3-3 incorporate thermocouple pairs placed next
to each other and separated by 7 mm in the transverse direction. For each thermocouple pair, the
temperatures are measured at 3.2 and 9.5 mm from the inner chamber walls respectively. The
temperature at location 3.2 mm from the inner wall is denoted as Tinner and that measured at 9.5
mm is denoted as Tmiddle.
A 3D model of the central portion of the chamber from 37 to 102 mm from the injector
face was chosen as the computational domain. The central portion of the chamber was selected
since the temperature measured outside of this domain did not indicate an axial temperature
76
gradient. The outer wall was assumed to be insulated such that the heat released during the
experiment was assumed to be accumulated in the chamber. The validity of the insulated wall
assumption was checked by imposing forced convection at the outer walls, assuming outer wall
temperature to be at 100oC and ambient air temperature set at 27oC. Forced convection was
calculated by assuming an air velocity of 10 m/s. These conditions are considerably more
dissipative than experienced during the experiments. The heat transfer for the case of a laminar
forced convection past a flat plate with the prescribed values was calculated. The heat transfer
thus determined was 0.1% of heat flux values in the chamber walls due to combustion and thus
all the heat released during the transient process was assumed to accumulate in the walls.
The computational temperatures which evolved over the period of 7.75 seconds were
matched with the actual temperatures obtained from the experimental run at the inner and middle
locations which are at 3.2 and 9.5 mm from the inner chamber walls, respectively. The imposed
heat flux at the inner chamber wall was changed for different sets of computation so that the
temperatures Tinner and Tmiddle obtained from the computations, matched the experimental results
within 4 to5 oC. The discretized 3D heat conduction equation [77] is
( )( )
( )( )
( )( )
( )( )
1, , , , , , 1, , , , 1, , , , , , 1, ,2 2
, , 1, , , , , , 1, , ,, ,2 2
2 2
2 =
i j l t i j l t i j l t i j l t i j l t i j l t
i j l t i j l t i j l t i j ti j t
k kT T T T T Tx y
Ck T T T T Tz
+ − + −
+ − +
− + + − +
+ − + −dt
d dr
d dt
(3-2)
Here, the density, r , and heat capacity, C , for Copper 110 are 8700 kg/m3 and 385 J/(kg K),
respectively The computational domain consisted of a 51 x 51 x 51 grid and the time step was
0.0001 seconds. The heat flux obtained through this procedure is used to accompany the in-flow
species concentration measurement in the process of code validation. The Matlab scripts used for
data processing are detailed in Appendix A.Equation Section (Next)
77
CHAPTER 4 OH-PLIF IMAGE PROCESSING AND QUANTITATIVE ANALYSIS
The OH-PLIF image processing, the methodology for determining the OH concentration
and the uncertainties in the measurement analyses are discussed below.
Fluorescence and Interference Signals
The background emission was subtracted from the fluorescence + background images. The
images for the four different test cases are shown in Figure 4-1 to 4-4. Figure 4-1(a) to 4-4(a)
show the raw image which has been corrected by subtracting the background shown in Figure 4-
1(b) to 4-4(b). The intensity levels of the background subtracted OH-PLIF images are shown in
Figure 4-1(c) to 4-4(c).
78
Height (mm)
Wid
th (m
m)
O2→H2→
H2→
0 5 10 15
-2
0
2
10 20 30 40 50 (a)
Height (mm)
Wid
th (m
m)
O2→H2→
H2→
0 5 10 15
-2
0
2
10 20 30 40 50 (b)
Height (mm)
Wid
th (m
m)
O2→H2→
H2→
0 5 10 15
-2
0
2
10 20 30 40 (c)
Figure 4-1. Average of 13 instantaneous images obtained at near steady state for chamber
pressure of 10 bar; (a) OH-PLIF + background emission image, (b) background emission image and (c) OH-PLIF image
79
Height (mm)
Wid
th (m
m)
O2→H2→
H2→
0 5 10 15
-2
0
2
10 20 30 40 50 60 70 (a)
Height (mm)
Wid
th (m
m)
O2→H2→
H2→
0 5 10 15
-2
0
2
10 20 30 40 50 60 70 (b)
Height (mm)
Wid
th (m
m)
O2→H2→
H2→
0 5 10 15
-2
0
2
5 10 15 20 25 30 (c)
Figure 4-2. Average of 13 instantaneous images obtained at near steady state for chamber
pressure of 27 bar; (a) OH-PLIF + background emission image, (b) background emission image and (c) OH-PLIF image
80
Height (mm)
Wid
th (m
m)
O2→H2→
H2→
0 5 10 15
-2
0
2
20 40 60 80 (a)
Height (mm)
Wid
th (m
m)
O2→H2→
H2→
0 5 10 15
-2
0
2
20 40 60 80 (b)
Height (mm)
Wid
th (m
m)
O2→H2→
H2→
0 5 10 15
-2
0
2
5 10 15 20 25 (c)
Figure 4-3. Average of 13 instantaneous images obtained at near steady state for chamber
pressure of 37 bar; (a) OH-PLIF + background emission image, (b) background emission image and (c) OH-PLIF image.
81
Height (mm)
Wid
th (m
m)
O2→H2→
H2→
0 5 10 15
-2
0
2
20 40 60 80 100 120 140 (a)
Height (mm)
Wid
th (m
m)
O2→H2→
H2→
0 5 10 15
-2
0
2
20 40 60 80 100 120 140 (b)
Height (mm)
Wid
th (m
m)
O2→H2→
H2→
0 5 10 15
-2
0
2
5 10 15 20 25 (c)
Figure 4-4. Average of 13 instantaneous images obtained at near steady state for chamber
pressure of 53 bar; (a) OH-PLIF + background emission image, (b) background emission image and (c) OH-PLIF image
82
From Figure 4-1 to 4-4, it is evident that at higher pressures of 37 and 53 bar, the
background emissions are stronger than at 10 bar. This shows that collection of fluorescence
with a gate-width narrowed to 100 ns was not sufficient to suppress the flame emissions. The
sources of the background emissions are recognized as typical flame emissions from OH and
water molecules in a H2/O2 flame [8]. The spectra of background emissions from a LOx/GH2
flame in the range of 300 to1100 nm at 60 bar was measured by Mayer et al. [15] and found that
the contributions from the OH and O2 flame emissions lied in the 300 to 400 nm range, the
predominant spectra being OH A-X (0,0) branch at 310 nm. The contributions from H2O could
be found spanning the 400 to1000 nm range. It is noteworthy to note the transmission of UV
filters used in this study to block the elastic scattering and transmitted light in the range of 300 to
400 nm and above 650 nm. The UV filters WG 305 & UG 11 served as the best combination
considering the low laser energy of 0.89 mJ /pulse and the large laser line width of 5cm-1 used
here, which both tend to decrease the fluorescence signal strength. Thus considering the
emissions from H2/O2 flame and the transmission range of UV filters, the background emissions
observed in the current study were identified as due to OH, O2 and H2O.
Laser
The laser shot to shot power fluctuation was monitored for 290 pulses. The average of the
290 laser pulse energies accounted to 0.89 mJ/ pulse with a standard deviation of 0.10 mJ/ pulse.
The fluctuation in the laser energy accounted for an uncertainty of 11 %.
The laser sheet profile variation in space was corrected from calibration using acetone
fluorescence. The laser sheet at 283 nm was passed through the chamber filled with acetone
vapor and the 2D fluorescence was collected by effectively blocking the elastic scattering using
the UV filters and ICCD camera. Ninety acetone fluorescence images were averaged and
normalized with the maximum intensity/counts along the width to obtain the spatial variation of
83
the laser sheet in the region of interest. The normalized laser sheet profile variation in a
percentage intensity scale is shown in Figure 4-5. The laser sheet had maximum intensity of
above 90% at heights of 11 to 15 mm while it gradually decreased to 25% at heights of 1 and 19
mm on either side.
Figure 4-5. Normalized laser sheet intensity profile variation obtained from acetone fluorescence images. The intensity is provided in percentage scales. The intensity is above 90 % at heights of 11 to 15 mm and gradually decreases to 25% at heights of 1 and 19 mm
Based on the normalized acetone fluorescence images shown in Figure 4-5, the laser sheet
intensity variation in space was corrected for all the OH-PLIF images acquired in the current
study and the resultant uncertainty calculated as the ratio of the standard deviation to the average
values of the 90 normalized fluorescence images was 5.9 %.
The absorption of the laser sheet by OH and other molecules that interact with the laser
beam as it passed through the combustion chamber need to be further discussed. In GH2/GO2
combustion one of the main combustion products is water vapor. The absorption cross section of
H2O between 190 and 320 nm at 900-3000 K temperature ranges and pressures up to 70 bar
increases as a function of temperature [70]. However the effect of absorption is small enough for
wavelengths above 280 nm such that the absorption by H2O can be neglected. The laser beam
absorption, as it traversed through the region of interest was insignificant as seen in Figure 4-1 to
84
4-4. The laser beam absorption by OH will be estimated based on the Beer-Lambert’s law, once
the OH number density is determined.
Absorption and Excitation, Line Shape, and Fluorescence Efficiency
The fluorescence signal in Equation 2-19 is rearranged and the number density of OH,
dependent due to Bf . The line shape absΦ in ( )laser abs dνΦ Φ∫ is both temperature and pressure
dependent and the quench rate 21Q determined from Equation 2-5 also requires the knowledge of
temperature field and colliding species mole fraction as discussed in Chapter 2.
The different approaches to circumvent this problem were identified from the review of
OH-PLIF diagnostic studies as calibration measurements via Rayleigh/Raman measurements of
temperature & species and calculations / numerical simulation based on equilibrium conditions.
The other approach is to obtain the variation in12
9'
1
BBf∑ , laser abs dνΦ Φ∫ and 21Q with
temperature and colliding species mole fraction corresponding to a broad range of equivalence
ratio. The approach used in this study is to use the average values of 12
9'
1
BBf∑ , laser abs dνΦ Φ∫
and 21Q for the calculations, and determining the uncertainty in the OH concentration due to the
variation over a broad range of equivalence ratio. The resultant variation and corresponding
uncertainties will be presented and discussed in chapter 5.
The term 21
21 21
AA Q
⎛ ⎞⎜ ⎟+⎝ ⎠
in Equation 4-2 known as the fluorescence yield needs to be further
analyzed. The effect of quenching becomes predominant at high pressures when A21<< Q21. In
86
GO2/GH2 combustion the colliding species are mainly H2O, O2 and H2 molecules. The
corresponding colliding cross sections [53] are given in Table 4-1.
Table 4-1. Colliding Species Cross Section for Collisional Quenching Species H2O O2 H2
Colliding species cross section (Å2) 25 7 5
The fluorescence yield based on the Equation 2-5 is well represented for A-X (0-0)
transitions. For transitions also involving A-X (1-1) the expression for the fluorescence yield
needs to be modified [20, 53] to
1
21(0,0) 21(1,1) 0 10 10
21 21(0,0) 1 1 1
A A1
Q A
−⎛ ⎞⎛ ⎞= + +⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠
s s sFs s s
(4-3)
where 21(0,0)A and 21(0,0)A represent the spontaneous emission rates from A-X(0,0) and A-X(1,1),
respectively. 0s and 1s represent the total effective cross sections for quenching from vibration
levels 'u = 0 and 'u = 1 respectively. The vibrational energy transfer from 'u =1 to 0 is
represented by 10s . The approximate value [53] for 21(1,1)
21(0,0)
AA
, 0
1
ss
, 10
1
ss
are 0.575, 1and 0.58
respectively.
Experimental Constants
The OH-PLIF diagnostic in this study is associated with 2D imaging of the fluorescence on
a CCD chip. Thus the volume V (cm3) in Equation 4-2 corresponding to the collected
fluorescence signal intensity in each pixel in the camera is equal to the product of the pixel
projection area Pixel ProjectionA (cm2) and the laser sheet thickness l (cm).
The uncertainty associated with the volume probed is due to the accurate determination of
the pixel resolution. The pixel resolution was obtained by calibrating it against the accurately
87
known dimensions of a wire with constant diameter and length. The resultant uncertainty due to
the variation in pixel resolution accounted to 2.8 % uncertainty in the probe volume.
To obtain absolute OH concentration in number density, the arbitrary selected unit, counts
of the camera, are to be converted to photometric units. This was done by calibration of the
camera against a light source of known irradiance. The light source used in this study was a
thousand watt, quartz halogen, tungsten filament lamp with designation of 7-1121 from Oriel
Instruments. The uncertainty in the irradiance levels near the 310 nm wavelength was 2.3 % as
mentioned in the lamp specifications. The camera calibration corresponding to the detection
strategy employed in the OH-PLIF measurements and region of interest is shown in Figure 4-6.
0 100 200 300 400 500 6000
100
200
300
400
500
600
700
800
900
Counts
Num
ber o
f Pho
tons
(Np) Np = 1.59 *Counts
Data PhotonsLinear fit
Figure 4-6. Camera calibration corresponding to the detection strategy employed in the OH-PLIF measurements and region of interest. The uncertainty in the calibration due to non linearity associated with the fit of 1.8 % together with the uncertainty in the lamp irradiance of 2.3 % amounted to a net uncertainty of 2.9 % in photon calibration.
The uncertainty in the photon calibration due to the non-linearity associated with the fit is
calculated and is 1.8 %. The net uncertainty in the photon calibration due to lamp irradiance
uncertainty of 2.3 % and uncertainty of 1.8% due to non linearity in calibration fit accounted to
88
2.9%. A proposed new methodology to calibrate the camera as part of the thesis study is
explained in Appendix B. The calibration obtained from the new method is compared with the
conventional calibration shown in Figure 4-6.
As the photons are Poisson distributed [35], there is uncertainty in the exact number of
photons detected; this is called shot noise. The uncertainty contribution from shot noise due to
the Poisson distributed photon number was calculated as the ratio of the standard deviation to the
average photon arrival from the OH-PLIF images at 10, 27, 37 and 53 bar. The uncertainty
contribution due to shot noise at 10, 27, 37 and 53 bar accounted to 6.9, 7.05, 6.8 and 6.7%
respectively.
The camera has spatial variation of pixel intensities. The systematic and random spatial
variation is eliminated by linear filtering in which the value of an output pixel in the image is
computed as a weighted average of neighboring pixels [11, 78]. In the current study, each pixel
value was computed as a weighted average of the neighboring 5 x 5 matrix of pixels with equal
weights.
The uncertainty due to systematic and random spatial variation of pixel intensities,
minimized by linear filtering [11, 78] in which the value of an output pixel in the image is
computed as a weighted average of neighboring pixels, was calculated as the ratio of the
difference in pixel intensities before and after filtering to their corresponding averaged values.
The uncertainty contribution due to pixel smoothening of the OH-PLIF images at 10, 27, 37 and
53 bar accounted to 7, 7, 6.3 and 6 % respectively.
The Matlab scripts used for data processing are detailed in Appendix A.
89
CHAPTER 5 RESULTS AND UNCERTAINTY ANALYSIS
Experiments at high pressure GH2/GO2 combustion covered Oxygen to Fuel (O/F) mass
flow ratio of 3.77 corresponding toF =2.15 and pressures of 10, 27, 37 and 53 bar. The results
presented here include (i) Experimental Conditions and Chamber pressure measurements and (ii)
Image processed OH-PLIF measurements. The uncertainty analysis includes determination of
uncertainties for the OH-PLIF measurements at 37 bar.
Chamber Pressure Measurements
In the GH2/GO2 experiments the chamber pressure was increased by increasing the
propellant mass flow rates while keeping the O/F mass flow and velocity ratios constant for a
constant exhaust nozzle area. The chamber pressure rise in time for the four experimental
conditions of GH2/GO2 combustion is shown in Figure 5-1 to 5-4. The sequence included
nitrogen pressurization followed by fuel injection and ignition. It should be noted that ignition is
identified in the figures by the high oscillation induced in the sensor recording. The pressure
increases in time and for higher pressures, which are of interest here, attains a near steady state at
7–8 sec following ignition. To attain steady-state at lower pressures longer experimental time
would have been required.
90
0 2 4 6 8 10 12 14 160
2
4
6
8
10
12
14
16
time(s)
Pres
sure
(bar
)
Chamber Pressure
Nitrogen pre-pressurization
Ignition
Combustionshut-off
OH-PLIF images range
Figure 5-1. Chamber pressure versus time for GH2/GO2 combustion for 10 bar and O/F mass flow of 3.7. The experiment continued for 10 sec following ignition.
0 2 4 6 8 10 12 140
5
10
15
20
25
30
35
time(s)
Pres
sure
(bar
)
Chamber Pressure
Nitrogen pre-pressurization
Ignition
Combustionshut-off
OH-PLIF images range
Figure 5-2. Chamber pressure versus time for GH2/GO2 combustion for 27 bar and O/F mass flow of 3.7 The experiment continued for 8 s following ignition.
91
0 2 4 6 8 10 12 140
5
10
15
20
25
30
35
40
45
50
time(s)
Pres
sure
(bar
)
Chamber Pressure
Nitrogenpre-pressurization
Ignition
Combustionshut-off
OH-PLIF imagesat near steady state
Figure 5-3. Chamber pressure versus time for GH2/GO2 combustion for 37 bar and O/F mass flow of 3.7. The pressure attains a near steady state at the end of 8 s following ignition.
0 2 4 6 8 10 12 140
10
20
30
40
50
60
70
time(s)
Pres
sure
(bar
)
Chamber Pressure
Nitrogen pre-pressurization
Ignition
Combustionshut-off
OH-PLIF images at near steady state
Figure 5-4. Chamber pressure versus time for GH2/GO2 combustion for 53 bar and O/F mass flow of 3.7. The pressure attains a near steady state at the end of 8 s following ignition.
92
OH-PLIF Measurements
The OH-PLIF images acquired for the experiments include thirteen instantaneous images.
At each pressure the instantaneous images were averaged. The instantaneous images, acquired
over a period of 100 ns each are needed for validation of LES codes while the average is used for
validation of RANS codes. The OH-PLIF images shown in Figure 5-5 to 5-6 were image
processed to
• eliminate background emissions; • correct for spatial variation in laser intensity; • smoothen the images and minimize the spatial variation of pixel intensities;
Figure 5-5 to 5-6 show an example of an instantaneous image and average image for each
pressure case.
93
(a)
(b)
(c)
(d)
Figure 5-5. Instantaneous image-processed OH-PLIF images at near steady state chamber
pressure of (a) 10, (b) 27, (c) 37 and (d) 53 bar
94
(a)
(b)
(c)
(d)
Figure 5-6. Average of thirteen instantaneous image-processed OH-PLIF images at near steady
state chamber pressure of (a) 10, (b) 27, (c) 37 and (d) 53 bar.
95
To ascertain the repeatability of the OH-PLIF measurements the average of image-
processed OH-PLIF images acquired at 35, 36 and 37 bar pressure cases for similar experimental
conditions were compared and are shown in Figure 5-7 (a–c). The average OH-PLIF images in
Figure 5-7 (a–c) shows that the OH-PLIF measurements were repeatable and can be used for
determination of OH concentration with confidence.
(a)
(b)
(c)
Figure 5-7. Average of thirteen instantaneous image-processed OH-PLIF images at near steady
state chamber pressure of (a) 35, (b) 36, and (c) 37 bar indicating the repeatability and reliability of OH-PLIF measurements for determination of OH concentration.
96
The intensity levels of the image processed OH-PLIF images shown in Figure 5-5 to 5-6
are related to the number density of OH by Equation 4-2. Theses image need to be processed as
described in Chapter 4 and thus the OH number density is determined. The OH-PLIF images in
Figure 5-5 to 5-6 show certain interesting features that are noteworthy. From the OH-PLIF image
at 10 bar, it can be observed that the flame is smooth and less corrugated as seen from OH
images at higher pressures. For the four experimental conditions, O/F velocity and density ratios
governing the development of shear layer remain the same. The difference in the four
experiments is the turbulence and the heat release. In addition to these there can be also Soret
and Dufour cross diffusion effects arising from concentration and temperature gradients. These
secondary effects need to be evaluated in complimentary CFD efforts.
As noted, the OH radical in a non premixed flame is considered to be a good marker of the
reaction zone. Similar to the study described in Reference 32, the stoichiometric contour was
traced from the axial evolution of the location of maximum OH intensity in the flame, as
indicative of the mean position of the reaction zone [32]. Thus from all four average OH PLIF
images, shown in Figure 5-6(a–d), the mean position of the reaction zone was quantitatively
determined and is shown in Figure 5-8(a–d).
97
(a)
(b)
(c)
(d)
Figure 5-8. Mean position of reaction zone determined from the average OH-PLIF images at (a)
10, (b) 27, (c) 37 and (d) 53 bar. The OH-PLIF signal at 27 and 37 bar would indicate a lifted flame, however this is an effect of strong background correction.
98
The mean position of the reaction zone at 10 bar shows that the flame is anchored at the lip
of the oxidizer post and is typical of the coaxial shear flames [20]. For test cases at higher
pressures of 27 and 37 bar the OH-PLIF signal would indicate a lifted flame, however this is an
effect of strong background correction. In all cases the flame is anchored at the lip. The shear
layers merge at 16–17 mm from the injector face at pressures of 27, 37 and 53 bar. The location
of the maximum OH concentration is similar for all cases, explaining the similar effect of same
density and velocity ratio on shear layer development regardless of the difference in turbulence
and heat release rates.
To analyze the effect of turbulence the Reynolds number of GO2 and GH2 were calculated
as 1UDρμ
and ( )3 2U D D−ρμ
whereρ is the density, U, the velocity andμ the dynamic
viscosity of the gas, and D1, D2 and D3 are the dimensions of injector as shown in Figure 3-2
respectively. The Reynolds number, ReD for GO2 was 38100, 75380, 112767 and 148637 and for
GH2, ReD was 5752, 11534, 16936 and 21900 at 10, 27, 37 and 53 bar respectively. The ReD of
GH2 and GO2 clearly indicates that flow regime is turbulent. The momentum flux ratio was
defined as J =( )( )
2
2
2
2
U
UGO
GH
ρ
ρ. For all the pressure cases the momentum flux ratio that governs the
growth of the shear layer remained the same and was 2.7.
In the study by Seitzman et al. [25] the OH structures in turbulent non-premixed hydrogen
flame were characterized at ReD of 2300, 8600, 25000 and 49500. It was found that, as the flow
transits from laminar to turbulent regime, there is significant change in the OH structures from
low strain rate, thick filament zones to high strain rate, thin filament, more diffuse regions.
Another notable observation was that at higher Reynolds number the OH structures became
increasingly convoluted and similar behavior was observed in the current study also.
99
The studies which focused on shear coaxial cryogenic flames [11, 12, 13, 19, 20, 21]
identified the wrinkling, corrugation and flapping of the flame to be caused by the combined
effects of turbulence and instabilities in the flow field. Singla et al. [20, 21] proposed stability
criteria based on the ratio of oxidizer lip thickness to the flame thickness and for the flame to be
stable the ratio needed to be greater than one. As the flame anchors on the oxidizer lip, the size
and dynamics of the recirculation region in the lip wake influences the flame stability.
Thus, in the current study the wrinkling and corrugation of the flame, at higher pressures
with higher Reynolds number, is attributed to the increased turbulence where as the flapping of
the flame which is evident from the instantaneous OH distribution in Appendix D is attributed to
the instability dictated by two factors: (i) the size of the recirculation zone in the wake of
oxidizer lip and, (ii) the large scale flow fluctuation in the recirculation region formed on the
injection face around the jet injectors .
100
Quantification of OH Concentration and Uncertainty at 10, 27, 37 and 53 bar
To determine the number density from the image-processed OH-PLIF images in Figure 5-5
to 5-6, the values of 12
9'
1
BBf∑ , laser abs dνΦ Φ∫ and 21Q in Equation 4-2 are calculated as follows.
A broad range of equivalence ratio for GH2/GO2 combustion was considered and covered
0.5–3 range [73]. The OH radical probed in the shear reaction zone of the GH2/GO2 combustion
is known to exist mostly around the region of stoichiometry. Hence, the equivalence ratio of 0.5–
3 could be considered a very broad range of conditions in the flame. Therefore this assumption is
quite conservative and is expected to yield a larger uncertainty than actually encountered in the
experiment. However given the lack of data it was adopted here to bracket with confidence the
possible experimental uncertainty.
In a first approximation equilibrium conditions are assumed. The equilibrium conditions
for the chemical reactions pertaining to the GH2/GO2 experiments carried out at 10, 27, 37, and
53 bar was calculated using STANJAN [79]. The variation of temperature and mole fraction of
species, H2O, H2, and O2 with equivalence ratio of 0.5–3.0 is shown in Figure 5-9(a–d). It was
found that, the temperature varied between 2500 and 3500 K, with the maximum at
stoichiometry.
101
0.5 1 1.5 2 2.5 30
0.5
1
Mol
e fra
ctio
n
Equivalence ratio (φ)2000
2500
3000
3500
4000
Tem
pera
ture
(o C)
Mole fraction H2O 10bar Mole fraction H2 10 bar Mole fraction O2 10 barTemperature 10 bar
(a)
0.5 1 1.5 2 2.5 30
0.5
1
Mol
e fra
ctio
n
Equivalence ratio (φ)2000
2500
3000
3500
4000
Tem
pera
ture
(o C)
Mole fraction H2O 27 bar Mole fraction H2 27 bar Mole fraction O2 27 barTemperature 27 bar
(b)
102
0.5 1 1.5 2 2.5 30
0.5
1
Mol
e fra
ctio
n
Equivalence ratio (φ)0.5 1 1.5 2 2.5 3
2000
2500
3000
3500
4000
Tem
pera
ture
(o C)
Mole fraction H2O 37 bar Mole fraction H2 37 bar Mole fraction O2 37 barTemperature 37 bar
(c)
0.5 1 1.5 2 2.5 30
0.5
1
Mol
e fra
ctio
n
Equivalence ratio (φ)0.5 1 1.5 2 2.5 32000
2500
3000
3500
4000
Tem
pera
ture
(o C)
Mole fraction H2O 53 bar Mole fraction H2 53 bar Mole fraction O2 53 barTemperature 53 bar
(d)
Figure 5-9. Temperature and specie mole fraction variation based on equilibrium calculations
with equivalence ratios of 0.5–3 at (a) 10, (b) 27, (c) 37 and (d) 53 bar. The temperature has a maximum value of 3500 K at stoichiometry and decreases to a minimum of 2500 K at equivalence ratio of 3.
103
The variation of 12
9'
1
BBf∑ (cmJ-1) with temperature which in turn varies with the
equivalence ratio is shown in Figure 5-10(a–d).
0 1 2 3 4 524
25
26
27
28
29
30
31
32
33
34
Equivalence ratio (φ)
Abs
orpt
ion
Coe
ffici
ent (
cmJ-1
)
Absorption Coefficient 10 barMean Absorption Coefficient 10 bar
(a)
0 1 2 3 4 522
24
26
28
30
32
34
Equivalence ratio (φ)
Abso
rptio
n C
oeffi
cien
t (cm
J-1)
Absorption Coefficient 27 barMean Absorption Coefficient 27 bar
(b)
104
0 1 2 3 4 522
24
26
28
30
32
34
Equivalence ratio (φ)
Abso
rptio
n C
oeffi
cien
t (cm
J-1)
Absorption Coefficient 37 barMean Absorption Coefficient 37 bar
(c)
0 1 2 3 4 522
24
26
28
30
32
34
Equivalence ratio (φ)
Abs
orpt
ion
Coe
ffici
ent (
cmJ-1
)
Absorption Coefficient 53 barMean Absorption Coefficient 53 bar
(d)
Figure 5-10. Absorption coefficient (12
9'
1
BBf∑ ) variation with equivalence ratio and temperature
(2500–3500 K) at (a) 10, (b) 27, (c) 37 and (d) 53 bar showing that the variation with respect to mean is 12.4, 14.6, 14.5 and 15.1% respectively.
The mean value of 12
9'
1
BBf∑ is used for the calculation. The uncertainty due to the
variation with temperature/equivalence ratio with respect to mean at 10, 27, 37 and 53 bar is
12.4, 14.6, 14.5 and 15.1 % respectively.
105
The absorption profiles of OH at 10, 27, 37 and 53 bar were simulated using LIFBASE
[37]. To simulate the absorption profile absΦ , the collisional and Doppler widths were obtained
from Equation 2-13 and Equation 2-15 respectively. The absorption profiles at 3017 K and 10
bar, 3085 K and 27 bar, 3100 K and 37 bar, and 3125 K and 53 bar for an equivalence ratio of 2
along with the laser spectral profile are shown in Figure 5-11(a–d).
Laser ProfileOH Absorption Profile at T =3125 and 53 bar
(d)
Figure 5-11. Absorption profile of OH at (a) 3017 K and 10 bar, (b) 3085 K and 27 bar, (c) 3103
K and 37 bar, and (d) 3125 K and 53 bar simulated using LIFBASE showing a complete overlap with the laser spectral profile at all pressures.
The OH absorption profiles at 10, 27, 37 and 53 bar were simulated for a broad
temperature range of 2500–3500 K corresponding to the equivalence ratio of 0.5–3 and are
107
provided in the Appendix C. The OH absorption profile broadens and the centre line of the OH
absorption profile shifts with pressure and temperature.
The overlap integral of the absorption profile of OH at 10, 27, 37 and 53 bar, and the
Gaussian spectral laser profile, laserΦ is calculated by laser abs dνΦ Φ∫ . As a result of the variation
of the absorption profile with temperature, the determined overlap integral also varies for each
pressure case over the broad range of temperature in the flame. To find out the variation the
overlap integral is calculated for a temperature range of 2500–3500 K corresponding to an
equivalence ratio of 0.5–3 at 10, 27, 37 and 53 bar. The results are shown in Figure 5-12(a–d).
0 1 2 3 4 50.133
0.134
0.135
0.136
0.137
0.138
0.139
0.14
φ
Ove
rlap(
cm)
Overlap Integral 10 barMean Overlap Integral 10 bar
(a)
108
0 1 2 3 4 50.128
0.1285
0.129
0.1295
0.13
0.1305
0.131
0.1315
0.132
0.1325
0.133
φ
Ove
rlap(
cm)
Overlap Integral 27 barMean Overlap Integral 27 bar
(b)
0 1 2 3 4 50.126
0.1265
0.127
0.1275
0.128
0.1285
0.129
0.1295
0.13
φ
Ove
rlap(
cm)
Overlap Integral 37 barMean Overlap Integral 37 bar
(c)
109
0 1 2 3 4 50.122
0.1222
0.1224
0.1226
0.1228
0.123
0.1232
0.1234
0.1236
0.1238
0.124
φ
Ove
rlap(
cm)
Overlap Integral 53 barMean Overlap Integral 53 bar
(d)
Figure 5-12. Overlap integral laser abs dνΦ Φ∫ variation at (a) 10, (b) 27, (c) 37 and (d) 53 bar with
temperature corresponding to equivalence ratio of 0.5–3, indicating that the variation with respect to mean is 1.3, 1, 0.8 and 0.5% respectively and can be assumed negligible.
The uncertainty due to the variation in the overlap integral at 10, 27, 37 and 53 bar is
determined as 1.3, 1, 0.8 and 0.5 % respectively over the broad temperature range of 2500–3500
K and is therefore assumed negligible. The overlap integral could also vary due to the centre line
shift of the laser profile. The center line of the laser profile was measured as 283.015 from the
Burleigh Wavemeter. The uncertainty in the overlap integral variation at 10, 27, 37 and 53 bar
due to the laser center line shift accounts to 2.8, 1.6, 1 and 0.2 % respectively.
The absorption profile broadens and gets shifted as pressure increases. Hence in most of
the studies carried out using lasers with small spectral widths of less than 1 cm-1, the centre line
wavelength of the laser needs to be shifted in order to overlap with the center line wavelength of
the OH absorption profile. The most important area of concern is the pressure broadening. The
overlap between the laser spectral profile and the pressure broadened absorption profile
110
decreases leading to a decrease in the strength of the collected fluorescence signal as the pressure
increases. The spectral width of the laser employed in this study was larger than the spectral
width of the broadened absorption profile even at elevated pressures such as 37 and 53 bars. This
can be considered as an advantage since it was ensured that the laser profile overlapped with the
absorption profile at all pressures thereby ensuring fluorescence with good signal strengths.
The variation of quench rate, 21Q at 10, 27, 37 and 53 bar with temperature and the species
mole fraction corresponding to equivalence ratio of 0.5–3 is shown in Figure 5-13(a–d).
0 1 2 3 4 51.08
1.1
1.12
1.14
1.16
1.18
1.2
1.22x 1010
Equivalence ratio (φ)
Col
lisio
nal Q
uenc
h R
ate(
s-1)
Collisional Quench rate 10 barMean Collisional Quench rate 10 bar
(a)
111
0 1 2 3 4 52.95
3
3.05
3.1
3.15
3.2
3.25
3.3x 1010
Equivalence ratio (φ)
Col
lisio
nal Q
uenc
h R
ate(
s-1)
Collisional Quench rate 27 barMean Collisional Quench rate 27 bar
(b)
0 1 2 3 4 53.9
3.95
4
4.05
4.1
4.15
4.2
4.25
4.3
4.35
4.4x 1010
Equivalence ratio (φ)
Col
lisio
nal Q
uenc
h R
ate(
s-1)
Collisional Quench rate 37 barMean Collisional Quench rate 37 bar
(c)
112
0 1 2 3 4 55.8
5.9
6
6.1
6.2
6.3
6.4
6.5x 1010
Equivalence ratio (φ)
Col
lisio
nal Q
uenc
h R
ate(
s-1)
Collisional Quench rate 53 barMean Collisional Quench rate 53 bar
(d)
Figure 5-13. Collisional quench rate Q21 variation at (a) 10, (b) 27, (c) 37 and (d) 53 bar with
temperature and colliding species mole fraction corresponding to equivalence ratio of 0.5–3 indicating that the variation with respect to mean is 4.1, 3.9, 3.8 and 3.7 % respectively.
The mean value of 21Q is used to calculate F in Equation 4-3. The uncertainty due to the
variation of 21Q at 10, 27, 37 and 53 bar with temperature and colliding species mole fraction
corresponding to equivalence ratio of 0.5-3 with respect to mean is 4.1, 3.9, 3.8 and 3.7 %,
respectively.
All the parameters in Equation 4-2 required for determination of OH number density was
calculated and the image-processed OH-PLIF images in Figure 5-6 and 5-7 were converted into
absolute concentration. Figure 5-14(a–d) and 5-15(a–d) represents instantaneous and averaged
OH concentration at 10, 27, 37 and 53 bar respectively. Appendix D includes complete set of
instantaneous OH number density contours for all the pressure cases.
113
(a)
(b)
(c)
(d)
Figure 5-14. Instantaneous OH number density contours at near steady state chamber pressure of
(a) 10, (b) 27, (c) 37 and (d) 53 bar
114
(a)
(b)
(c)
(d)
Figure 5-15. Average of thirteen instantaneous OH number density contours at near steady state
chamber pressure of (a) 10, (b) 27, (c) 37 and (d) 53 bar.
115
The uncertainty due to laser absorption by OH creating a horizontal incident photon flux
gradient, estimated from average OH number density using Beer-Lambert’s law [35],
( )9
121
OH2o
BI exp -h
I co
laser abs
Bfd znν ν
⎛ ⎞⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟= Φ Φ⎜ ⎟⎜ ⎟
⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠
∑∫ , for z= 1mm path length of the laser at 10, 27,
37 and 53 bar was 2, 3.3, 3.8 and 4.7 % respectively.
In summary, the uncertainties associated with OH quantitative measurement based on the
conservative assumptions made here were quantified for 10, 27, 37 and 53 bar cases as shown in
Figure 5-16. OH-PLIF measurement uncertainties at (a) 10, (b) 27, (c) 37 and (d) 53 bar. The rms error include the contributions from (i) camera calibration (ii) shot noise, (iii) pixel smoothening, (iv) laser power variation, (v) laser spatial variation, (vi) laser absorption by OH, (vii) absorption coefficient, (viii) overlap integral, (ix) quench rate variation and (x) pixel area accuracy and accounted to total rms error of 21.4, 22.8, 22.5 and 22.9 % at 10, 27, 37 and 53 bar respectively.
The rms error includes the contributions from:
i. Camera calibration ii. Shot noise
iii. Pixel smoothening iv. Laser power variation v. Laser spatial variation
vi. Laser absorption by OH vii. Absorption coefficient
viii. Overlap integral ix. Quench rate variation x. Pixel area accuracy
118
The uncertainty due to camera calibration of 2.9%, laser shot to shot power fluctuation of
11%, laser sheet spatial variation of 5.9 % and pixel area of 2.8 % remain the same for all the
pressure cases. The uncertainties due to the laser shot-to-shot power fluctuation could be
eliminated by monitoring the laser energy variation during the experiments. The uncertainty in
laser sheet spatial variation in could be eliminated by monitoring the spatial profile during
experiments from a separate test cell, uniformly filled with a fluorescing substance like acetone.
The shot noise accounted to 6–7 % in all the pressure cases. The average number of
photons collected in all the pressure cases was in the 200–225 range. As the pressure increases
the decrease in the OH signal strength is expected due to collisional quenching. But in the
current study the increase in the pressure was achieved by increasing the propellant mass flow
rate resulting in increased OH production at higher pressures. Thus as the pressure increased the
strength of the OH signal collected primarily depended on the collisonal quenching and
increased OH production. The uncertainty due to pixel smoothening used to minimize the
contribution of camera sensor randomness was also 6–7% for all the pressure cases.
The uncertainty due to the laser absorption by OH was estimated to increase from 2 to 4.7
% in the 10–53 bar range. From the OH-PLIF images in Fig. 5 it could be recognized that the
effect of laser absorption is negligible for all the pressure cases and as indicated by the
uncertainty estimation too.
Of all the uncertainties the variation of the absorption coefficient with temperature was the
highest and was 12 to 15 % in the 10 to 53 bar range. The uncertainty due to this can be reduced,
provided the 2D temperature field is available through measurements or calculations. For flames
with wrinkling, corrugation and large fluctuations the use of temperature field data from
119
numerical simulation or a reference flame could lead to additional errors as the instantaneous
temperature field of the actual flame and simulated/reference flame cannot be precisely matched.
The uncertainty due to the variation of overlap integral due to line shape broadening
decreased from 1.3 to 0.5 at 10–53 bar pressure range. Similarly the uncertainty in the overlap
integral due to the shift in the center line of the laser decreased from 2.8 to 0.2 % in the 10 to 53
bar range. The relatively low variations in the overlap integral is attributed to the use of large
laser line width of 5cm-1 thereby obtaining a complete overlap between laser spectral and OH
absorption profile at all pressures. Moreover the mean value of the overlap integral was reduced
by only 10 % from 10 to 53 bar in the current study compared to the 30–40% reduction of
overlap integral in other studies [50] due to the use of lasers with small line widths of the order
of 0.5–1cm-1. The uncertainty due to variation in collisional quenching was nearly 4 % in all the
pressure cases and is less significant compared to the absorption coefficient variation of 12–15%
with temperature.
The uncertainty contributions from spectroscopic constants and uncertainty in the
mathematical model describing the fluorescence process, collisional/Doppler widths and shifts,
and quench rate are identified as negligible in this study. Thus the total rms uncertainty in the
OH number density measurements for a GH2/GO2 flame determined from a broad range of
uncertainty sources accounted to of 21.4, 22.8, 22.5 and 22.9 % at 10, 27, 37 and 53 bar
respectively.
The improvements identified in the study includes elimination of uncertainties from laser
shot to shot power fluctuation, laser spatial sheet variation, and minimizing the uncertainty due
to temperature variation from simultaneous temperature measurements. The incorporation of the
improvements suggested in the study could potentially reduce the uncertainty from the present
120
uncertainty of 21–23 % to nearly 11 % for all the pressure cases thereby improving the quality of
the data for CFD code validation.
The boundary conditions included temperature measurements and evaluation of wall heat
fluxes. These data are collected simultaneously with the OH concentration and are used in the
computational studies that parallel this work. These results are included in Appendix E.
121
CHAPTER 6 CONCLUSIONS
The purpose of this work was to provide a database of in-flow planar species concentration
and to quantify the uncertainty associated with these measurements. The experimental conditions
investigated used O/F mass flow ratio fixed at 3.77 and chamber pressures of 10, 27, 37 and 53
bar. Nine rovibrational lines at A-X(1,0) transition of OH excited at 283 nm was employed to
obtain OH distribution in the shear reaction zone near the coaxial injector. The following are the
conclusions:
• Benchmark inflow OH concentration data was generated in the same experimental facility with the same propellant system and instrumentation method for a range of pressures from 10–53 bar. The instantaneous OH concentration and the averaged concentration in number densities which were inexistent in previous single injector studies over the 10–53 bar range can be used to validate of LES and RANS CFD codes respectively. This is the first contribution of the current study.
• The wrinkling, corrugation and flapping of the flame at higher pressures of 27–53 bar is
due to the combined effects of turbulence due to increased Reynolds number and jet instability caused by size and dynamics of the recirculation region in the wake of oxidizer post lip.
• The quality of the benchmark inflow data was improved by a thorough and
comprehensive uncertainty analysis and assessment, and this is the second contribution of the study.
• The systematic uncertainties, which remained the same irrespective of the experimental
conditions, were evaluated at all pressures; uncertainty due to camera calibration, laser shot to shot power fluctuation, laser sheet spatial variation and pixel area accuracy was 2.9, 11, 5.9 and 2.3 % respectively. The uncertainties due to laser shot to shot power fluctuation and laser sheet spatial variation could be potentially minimized in future studies.
• The uncertainty due to shot noise and pixel smoothening were, each 6–7% for all the
pressures cases. • The laser elastic scattering was effectively blocked and contributions from the
background flame emissions were eliminated for accurate quantitative measurements. • The uncertainty due to absorption of laser across the flame by H2O was negligible and by
OH was 2–5 % in 10–50 bar range.
122
• The uncertainty in absorption coefficient variation with temperature was 12–15 % in 10–50 bar range and was the maximum among all the uncertainties. The uncertainty could be potentially minimized provided there is availability of temperature field data from experiments/ computations.
• The uncertainty in overlap integral with temperature variation was 1.3–0.5 % and 2.8–
0.2% with laser centerline shift and the mean value of overlap integral was reduced by 10% in the 10–50 bar range. The use of lasers with larger line widths is recommended for OH-PLIF measurements at high pressures for minimizing the uncertainty due to overlap integral.
• The uncertainty in collisional quench rate variation with temperature and colliding
species mole fraction was nearly 4% at all pressures and is insignificant compared to the 12–15% variation of absorption coefficient with temperature.
• The uncertainty in the spectroscopic constants, mathematical model used to describe
fluorescence process, collisional and Doppler widths, and collisional quenching are negligible.
• The total rms uncertainty contributions in OH number density analyzed and determined
from 18 sources at 10, 27, 37 and 53 bar was 21.9, 22.8, 22.5 and 22.9 % respectively. The quality of the inflow data was improved from uncertainty assessment of two sources in previous studies to 18 sources; by quantifying 12, eliminating 2 and identifying as negligible the rest 4.The information is valuable for CFD validation as it brackets the reliability of the experimental data base. To reduce the uncertainty to nearly 11 % from the current 23%, potential areas for future improvements include elimination of the uncertainties due to laser power and spatial variation, and absorption coefficient variation with temperature.
123
CHAPTER 7 FUTURE WORK
The future work should be directed towards improving the accuracy of the OH
concentration in areas highlighted in the current study. The uncertainty in OH concentration
measured today reaches nearly 23% for all the pressure cases. Some of the major uncertainties
that were significant and that can be minimized include:
i. 12–15 % from temperature dependence of absorption coefficient ii. 11% from laser shot to shot power fluctuations
iii. 6 % from laser sheet spatial variation in intensity
The uncertainty in the absorption coefficient of 12–15 % can be minimized, if there is
availability of the temperature field data either from simultaneous temperature measurements or
from CFD simulations. This remains problematic for a number of reasons. Planar temperature
measurements in a high pressure reacting flow has not been attempted today, given the difficulty
to adapt the point wise absorption technique to the current flow field. CFD simulation either in
time averaged or time accurate formulations have uncertainties that far exceed the 12–15%
evaluated in this study. Hence while several CFD-experimental combined studies may improve
this item the future work will require considerable effort. An experimental technique that may be
attempted is a two-line OH thermometry. The advantage of using simultaneous temperature
measurements is that both the OH and temperature field data can be spatially matched. The
disadvantages results from the considerable complexity of the experimental setup and additional
contributions to overall uncertainty from the temperature measurement errors.
Thus the following procedure need to be adopted to revise both the experimental and CFD
data;
124
Step 1: the OH number density data simulated from RANS simulations should be validated
against the experimental data and the CFD code should be improved till it predicts the measured
OH concentration within the current uncertainty limits.
Step 2: the temperature field data from the improved CFD code can be used to refine the
calculation of OH concentration from experimental measurements.
The procedures in Step 1 and Step 2 should be followed iteratively till the uncertainty
contribution due to unknown temperature field attains a minimum value.
The second major uncertainty source is the laser shot to shot power fluctuation. To
eliminate it, a fixed percentage of the total laser power could be monitored through out the
experiments. Additional equipment would require a laser power meter than can be synchronized
with OH-PLIF shot to shot images.
Similarly the uncertainty from the averaged laser sheet spatial intensity could be reduced
by monitoring the spatial profile throughout the experiments. This could be done by tracking the
spatial intensity profile of the laser from a separate test cell, uniformly filled with a fluorescing
substance like acetone and synchronized with the shot to shot OH-PLIF images. Additional
experimental challenges include separate optical setup and detection electronics, and extraction
of a part of total laser power at 283 nm for acetone fluorescence.
It is estimated that incorporation of the improvements suggested here in future works
would minimize the uncertainty in the OH concentration measurements from 23 % to nearly 11
% and this is the third contribution of the study.
125
APPENDIX A MATLAB® SCRIPTS USED FOR DATA PROCESSING
The Matlab scripts used for data processing are provided here:
i. 3D heat flux processing -37 bar ii. Elimination of background emissions- 37 bar
iii. Laser sheet spatial profile uncertainty iv. Conventional photon calibration v. Poisson photon calibration - photon count 300 ns
vi. Poisson photon calibration - camera calibration vii. OH number density contours-37 bar
Laser ProfileOH Absorption Profile at T =2480 and 10 bar
(f)
Figure C-1. Absorption profile of OH simulated using LIFBASE at equivalence ratio of (a) 0.5, (b) 1, (c) 1.5, (d) 2, (e) 2.5 and (f) 3 corresponding to temperatures of 2500–3500 K for gaseous H2-O2 flame at 10 bar.
163
OH Absorption Profiles at 27 bar and 2500–3500 K Temperature Range
Laser ProfileOH Absorption Profile at T =2492 and 27 bar
(f)
Figure C-2. Absorption profile of OH simulated using LIFBASE at equivalence ratio of (a) 0.5, (b) 1, (c) 1.5, (d) 2, (e) 2.5 and (f) 3 corresponding to temperatures of 2500–3500 K for gaseous H2-O2 flame at 27 bar.
166
OH Absorption Profiles at 37 bar and 2500–3500 K Temperature Range
Laser ProfileOH Absorption Profile at T =2500 and 53 bar
(f)
Figure C-4. Absorption profile of OH simulated using LIFBASE at equivalence ratio of (a) 0.5, (b) 1, (c) 1.5, (d) 2, (e) 2.5 and (f) 3 corresponding to temperatures of 2500–3500 K for gaseous H2-O2 flame at 53 bar.
172
APPENDIX D OH NUMBER DENSITY CONTOURS
Thirteen Instantaneous OH Number Density Contours at 10 bar
(a)
(b)
(c)
173
(d)
(e)
(f)
174
(g)
(h)
(i)
175
(j)
(k)
(l)
176
(m)
Figure D-1. Thirteen instantaneous OH number density contours at near steady state chamber
pressure of 10 bar; (a)-(m).
Thirteen Instantaneous OH Number Density Contours at 27 bar
(a)
(b)
177
(c)
(d)
(e)
178
(f)
(g)
(h)
179
(i)
(j)
(k)
180
(l)
(m)
Figure D-2. Thirteen instantaneous OH number density contours at near steady state chamber
pressure of 27 bar; (a)-(m).
Thirteen Instantaneous OH number Density Contours at 37 bar
(a)
181
(b)
(c)
(d)
182
(e)
(f)
(g)
183
(h)
(i)
(j)
184
(k)
(l)
(m)
Figure D-3. Thirteen instantaneous OH number density contours at near steady state chamber
pressure of 37 bar; (a)-(m).
185
Thirteen Instantaneous OH Number Density Contours at 53 bar
(a)
(b)
(c)
186
(d)
(e)
(f)
187
(g)
(h)
(i)
188
(j)
(k)
(l)
189
(m)
Figure D-4. Thirteen instantaneous OH number density contours at near steady state chamber
pressure of 53 bar; (a)-(m).
190
APPENDIX E TEMPERTURE MEASUREMENTS AND BOUNDARY CONDITIONS
The wall heat flux boundary conditions for 37 bar were calculated from temperature
measurements along the chamber wall. The chamber wall temperatures at inner and middle
locations defined in Figure 3-3 are shown in Figure E-1 and E-2.
Figure E-10. Experimental and computational temperatures at 102 mm axial location
The heat fluxes thus determined in the axial direction are shown in Figure C-11 along
with the heat flux calculated from the linear+ unsteady term assumption in Equation 3-1. The
recent heat transfer studies conducted by Marshall et al [75] and Conley et al [76] used the linear
+ unsteady calculation for determining heat fluxes. It can be seen from Figure E-11 that both
calculations based on 3D computations and linear assumption showed the same trend, the heat
fluxes determined from the latter being relatively low compared to that from the former.
195
0 20 40 60 80 100 1200
0.5
1
1.5
2
2.5
3
3.5
4
Distance from Injector Face (mm)
Hea
t Flu
x (M
W/m
2 )
Heatflux Computational 3DHeatflux Linear
Figure E-11. Chamber wall heat fluxes calculated based on 3D computations and linear + unsteady assumption at 37 bar
The heat flux has a peak value at 70 mm indicating the location of shear layer re-
attachment. The matching of experimental and computational temperatures at the end of 8 s for
37 bar are also shown in Figure E-12.
20 40 60 80 100 120 14080
100
120
140
160
180
200
220
Distance from Injector Face (mm)
Tem
pera
ture
(o C
)
Tinner ExpTinner CompTmiddle ExpTmiddle Comp
Figure E-12. Computational and Experimental Temperatures for 37 bar at the end of 8s.
196
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