-
Combustion and Flame 144 (2006)
205224www.elsevier.com/locate/combustflame
Investigations of swirl flames in a gas turbinemodel
combustor
I. Flow field, structures, temperature, andspecies
distributions
P. Weigand, W. Meier , X.R. Duan 1, W. Stricker, M.
AignerInstitut fr Verbrennungstechnik, Deutsches Zentrum fr Luft-
und Raumfahrt (DLR), Pfaffenwaldring 38,
D-70569 Stuttgart, Germany
Received 22 November 2004; received in revised form 2 June 2005;
accepted 8 July 2005
Available online 21 September 2005
Abstract
A gas turbine model combustor for swirling CH4/air diffusion
flames at atmospheric pressure with good opticalaccess for detailed
laser measurements is discussed. Three flames with thermal powers
between 7.6 and 34.9 kWand overall equivalence ratios between 0.55
and 0.75 were investigated. These behave differently with respect
tocombustion instabilities: Flame A burned stably, flame B
exhibited pronounced thermoacoustic oscillations, andflame C,
operated near the lean extinction limit, was subject to sudden
liftoff with partial extinction and reanchor-ing. One aim of the
studies was a detailed experimental characterization of flame
behavior to better understand theunderlying physical and chemical
processes leading to instabilities. The second goal of the work was
the estab-lishment of a comprehensive database that can be used for
validation and improvement of numerical combustionmodels. The flow
field was measured by laser Doppler velocimetry, the flame
structures were visualized by planarlaser-induced fluorescence
(PLIF) of OH and CH radicals, and the major species concentrations,
temperature, andmixture fraction were determined by laser Raman
scattering. The flow fields of the three flames were quite
sim-ilar, with high velocities in the region of the injected gases,
a pronounced inner recirculation zone, and an outerrecirculation
zone with low velocities. The flames were not attached to the fuel
nozzle and thus were partially pre-mixed before ignition. The near
field of the flames was characterized by fast mixing and
considerable finite-ratechemistry effects. CH PLIF images revealed
that the reaction zones were thin (0.5 mm) and strongly
corrugatedand that the flame zones were short (h 50 mm). Despite
the similar flow fields of the three flames, the oscillatingflame B
was flatter and opened more widely than the others. In the current
article, the flow field, structures, andmean and rms values of the
temperature, mixture fraction, and species concentrations are
discussed. Turbulenceintensities, mixing, heat release, and
reaction progress are addressed. In a second article, the
turbulencechemistryinteractions in the three flames are treated.
2005 The Combustion Institute. Published by Elsevier Inc. All
rights reserved.
* Corresponding author. Fax: +49 711 6862 578.E-mail address:
[email protected] (W. Meier).
1 Present address: Southwestern Institute of Physics, P.O. Box
432, 610041 Chengdu Sichuan, Peoples Republic of China.0010-2180/$
see front matter 2005 The Combustion Institute. Published by
Elsevier Inc. All rights
reserved.doi:10.1016/j.combustflame.2005.07.010
-
n and
lence
Rayleigh scattering, laser Raman scattering, coher- In the study
discussed here, a nozzle with two
ent anti-Stokes Raman scattering (CARS), or laser-induced
fluorescence (LIF); species concentrations byLIF or Raman
scattering; flame structures by planarLIF (PLIF) or PIV; and the
mixture fraction by Ra-
concentric air swirlers and an annular fuel supply be-tween them
was used for CH4/air diffusion flames,with thermal powers up to 35
kW at atmosphericpressure. The combustion chamber enabled almost206
P. Weigand et al. / Combustio
Keywords: Gas turbine; Model combustor; Swirl flame;
Turbutechniques
1. Introduction
Swirl flames are used extensively in practical com-bustion
systems because they enable high energy con-version in a small
volume and exhibit good ignitionand stabilization behavior over a
wide operating range[14]. In stationary gas turbine (GT)
combustors, theyare used mostly as premixed or partially
premixedflames, and in aero engines, as diffusion flames. Toreduce
pollutant emissions, especially NOx , today theflames are operated
generally very lean [57]. Un-der these conditions, the flames tend
to exhibit unde-sired instabilities, e.g., in the form of unsteady
flamestabilization or thermoacoustic oscillations. The un-derlying
mechanisms of the instabilities are basedon the complex interaction
between flow field, pres-sure, mixing, and chemical reactions, and
are notwell enough understood to date. Detailed measure-ments in
full-scale combustors are hardly possible,and very expensive and
numerical tools have not yetreached a sufficient level of
confidence to solve theproblems. A promising strategy lies
therefore in theestablishment of a laboratory-scale standard
com-bustor with practical relevance and detailed, com-prehensive
measurements using nonintrusive tech-niques with high accuracy. The
gained data set willbe used for validation and optimization of
numeri-cal combustion simulation codes which then can beapplied to
simulate the behavior of technical combus-tors. Intrusive probe
measurements are less suited forthese applications as they disturb
the local flow fieldand change the conditions for stabilization and
forreactionlocally or even in general [8,9]. In turbu-lent reacting
flows, the use of optical measurementtechniques is therefore
essential for reliable infor-mation. Laser-based tools are the
method of choiceoffering the potential to measure most of the
impor-tant quantities with high temporal and spatial reso-lution,
often as one- or two-dimensional images, andthe ability to perform
the simultaneous detection ofseveral quantities [1013]. The flow
field can be mea-sured by laser Doppler velocimetry (LDV) or
particleimaging velocimetry (PIV); the temperature by laserman
scattering.In recent years a variety of laser-based investiga-
tions in GT model combustors have been reportedFlame 144 (2006)
205224
chemistry interaction; Validation measurements; Laser
that, besides feasibility studies, concentrated on cer-tain
aspects of the combustion process or modelvalidation. For example,
Kaaling et al. [14] per-formed temperature measurements with CARS
ina RQL (rich-quench-lean) combustor, and Kamp-mann et al. [15]
used CARS simultaneously with 2-DRayleigh scattering to
characterize the temperaturedistribution in a double-cone burner.
In the same com-bustor, Dinkelacker et al. [16] studied the flame
frontstructures with PLIF of OH and 2-D Rayleigh scat-tering. Fink
et al. [17] investigated the influence ofpressure on the combustion
process by applying PLIFof OH and NO in a LPP (lean prevaporized
premixed)model combustor. With respect to NOx reductionstrategies,
Cooper and Laurendeau [18,19] performedquantitative NO LIF
measurements in a lean direct-injection spray flame at elevated
pressures. Shih etal. [20] applied PLIF of OH and seeded acetone in
alean premixed GT model combustor, and Deguchi etal. [21] used PLIF
of OH and NO in a large practicalGT combustor. Hedman and Warren
[22] used PLIFof OH, CARS, and LDV for the characterization ofa
GT-like combustor fired with propane in order toachieve a better
understanding of the fundamentalsof GT combustion. PLIF of OH was
also applied byLee et al. [23] to study flame structures and
instabil-ities in a lean premixed GT combustor, by Arnold etal.
[24] to visualize flame fronts in a GT combustorflame of 400 kW,
and by Fritz et al. [25] for revealingdetails of flashback. Lfstrm
et al. [26] performeda feasibility study of two-photon LIF of CO
and 2-Dtemperature mapping by LIF of seeded indium in alow-emission
GT combustor. A comparison of twodifferent laser excitation schemes
for major speciesconcentration measurements with laser Raman
scat-tering was performed by Gittins et al. [27] in a GTcombustion
simulator. At a high-pressure test rig ofthe DLR, various laser
techniques (LDV, CARS, PLIFof OH and kerosene, and 2-D temperature
imagingvia OH PLIF) were applied to GT combustors un-der technical
operating conditions to achieve a betterunderstanding of combustor
behavior and to validateCFD codes [2831].unrestricted optical
access to the flames and was,thus, ideally suited for the
application of laser mea-surement techniques. The velocity fields
were mea-
-
n andP. Weigand et al. / Combustio
sured by 3-D LDV, the flame structures by PLIF ofOH and CH, and
the joint probability density func-tions (PDFs) of temperature,
major species concentra-tions, and mixture fraction by laser Raman
scattering.Three flames with different characteristics were
in-vestigated: flame A was operated at a specific powerrate of 42.4
MW/(m3 bar) that is comparable to thevalues of aeronautical or
modern aero-derivative in-dustrial gas turbines, which are operated
around 25to 70 MW/(m3 bar); flame B was chosen at a powerrate of
12.5 MW/(m3 bar) that is comparable to mostindustrial gas turbines,
which are operated at 5 to20 MW/(m3 bar); and flame C was operated
at thesame airflow as flame B but with reduced fuel sup-ply close
to the lean extinction limit (with a powerrate of 9.2 MW/(m3 bar)).
This is of interest becausemodern gas turbines in power plants are
operated un-der extremely lean conditions to meet the
emissionlimits. In addition, the flames were operated at
threedifferent equivalence ratios to investigate the stabi-lization
of the flames. Flame A with an equivalenceratio of = 0.65 burned
stably, whereas flame B( = 0.75) emitted strong thermoacoustic
noise, andflame C with = 0.55 operated close to the blowofflimit
and randomly experienced sudden liftoff andreestablishment of
stable operation.
The advantage of the combustor setup used wasthe excellent
optical access to the flame zone, en-abling the collection of
information from the wholearea around the flame zone in a burner
that is closeto technical application. In particular, the
detailedvelocity measurements at the nozzle exit result
inwell-defined boundary conditions, which are impor-tant for
numerical methods. One major goal of thework was the detailed
experimental analysis of theflames to gain deeper insight into,
e.g., the mixingand stabilization processes, the shape of the
reactionzones and the regions of heat release, and effects
ofturbulencechemistry interactions. The second goalwas the
establishment of a comprehensive databasewhich can be used for the
verification and improve-ment of combustion simulation codes. The
presentarticle focuses on flow fields, on the distribution of
thetemperature, the major species concentrations and themixture
fractions, and on the instantaneous and meanflame structures. The
turbulencechemistry interac-tions, which play an important role in
these flames,are discussed in a second article [32]. The
thermoa-coustic oscillations of flame B were analyzed previ-ously
by phase-resolved measurements [3335]. Theresults from those
investigations represent a supple-ment of the measurements without
phase resolution
presented in the current article, and some of the find-ings are
used here to support the discussion of thecharacteristics of flame
B.Flame 144 (2006) 205224 207
2. Experimental
2.1. Combustor and flames
The gas turbine model combustor is schematicallyshown in Fig. 1.
The burner was a modified versionof a practical gas turbine
combustor with an air blastnozzle for liquid fuels [36].
Co-swirling dry air atroom temperature was supplied to the flame
through acentral nozzle (diameter 15 mm) and an annular noz-zle
(i.d. 17 mm, o.d. 25 mm contoured to an outerdiameter of 40 mm).
Both air flows were fed froma common plenum with an inner diameter
of 79 mmand a height of 65 mm. The radial swirlers consistedof 8
channels for the central nozzle and 12 chan-nels for the annular
nozzle. The ratio of the air massflows through the annular and
central nozzle was ap-proximately 1.5. Nonswirling CH4 was fed
through72 channels (0.5 0.5 mm), forming a ring betweenthe air
nozzles. Compared with an annular nozzlefor CH4 with a slit width
of
-
n and
min.8.3.0
l mixrature
All three flamesestablished under globally lean 290 Hz. Flame C
showed nearly the same frequency
conditionsshowed no soot production and burnedwith a light blue
color. The flames appeared as type 2swirl flames [1,37] with a
conically shaped toroidalflame zone at different opening angles
and, as wasconfirmed by the velocity measurements, showed
pro-nounced recirculation zones on the axis (inner recir-culation
zone, irz) and near the walls of the com-bustion chamber (outer
recirculation zone, orz). Thevisual appearance of the flames,
despite the squarecombustion chamber, revealed good rotational
sym-
spectrum as flame B but with a much reduced ampli-tude. Flame A
emitted a rather broadband noise withsmall peak amplitudes at about
380 Hz.
2.2. Measuring techniques
All three velocity components were measured si-multaneously
using commercial LDV systems (DISA/DANTEC) and a cw-Ar+ laser
(Coherent, INNOVA90, operated at 1 W). The optical arrangement
con-208 P. Weigand et al. / Combustio
Table 1Parameters of the three flames investigated
Air CH4sl/min g/min sl/min g/
A 850 1095 58.2 41B 218 281 17.2 12C 218 281 12.6 9
a Pth, thermal power; glob, equivalence ratio for the
overaladiabatic temperature for the overall mixture with inlet
tempe
to the flame. A conical top plate made of steel witha central
exhaust tube (diam 40 mm, length 50 mm)formed the exhaust gas exit.
The high velocity in theexhaust tube avoided any backflow from
outside thecombustion chamber.
The three flames investigated were: flame A, withPth = 34.9 kW
and an overall equivalence ratio ofglob = 0.65 that ran very
stably; flame B, withPth = 10.3 kW and glob = 0.75, which
exhibitedpronounced self-excited thermoacoustic oscillationsat a
very high noise level; and flame C, operated closeto the lean
extinction limit, with Pth = 7.6 kW andglob = 0.55, which randomly
lifted off and rean-chored to the normal stabilization height.
Table 1 listscharacteristic parameters of the investigated
flames,i.e., the volume and mass flow rates of air and fuelas well
as the resulting values for power and overallglobal values for
equivalence ratio, mixture frac-tion, and adiabatic temperature
(given the subscriptglob). The mass flows of the gases were
controlledwith Brooks flow controllers (Type 5853S for air andType
5851S for CH4) with an accuracy of typically0.5%. As can be seen
from Table 1, flames B andC had almost identical total flow rates
but differentflame parameters. To achieve this, the air mass
flowwas kept constant for both flames and only the fuelmass flow,
which represents 7.3% of the total flow forflame B and 5.5% for
flame C, was changed. This waschosen to achieve a high similarity
of the velocity dis-tributions to exclude flow field effects as a
source forthe different behavior of the two flames.metry.The swirl
number S was calculated from the ve-
locity profile just above the nozzle exit neglecting theFlame
144 (2006) 205224
Ptha
(kW)glob f glob T glob ad
(K)34.9 0.65 0.037 175010.3 0.75 0.042 1915
7.6 0.55 0.031 1570
ture; f glob, mixture fraction for the overall mixture; T glob
ad,T0 = 295 K.
pressure term according to
S = R
0 2uwr drR R
0 2u2r dr,
where u = axial velocity (m/s), w = circumferentialvelocity
(m/s), = density (kg/m3), r = radius (m),and R = maximum radius of
the nozzle exit (m). Theswirl numbers are S 0.9 for flame A and S
0.55for flames B and C. Given the fact that the nozzlewas contoured
and a combustion chamber was usedwith an expansion factor D/d = 3.4
(D = diameterof combustion chamber, d = diameter of nozzle),
vor-tex breakdown with establishment of an irz was to beexpected
[38]. Due to the confinement, an orz wasalso found. The nozzle
Reynolds number based on thecold inflow and the minimum outer
nozzle diameter(25 mm) was about 15,000 for flames B and C andabout
58,000 for flame A.
All three flames did not burn directly at the fuelnozzle exit,
but rather with a liftoff height of somemillimeters. In flame C,
sudden liftoff (partial ex-tinction) and flashback randomly
occurred (approx-imately 10 times per minute), as it was chosen
tooperate close to the lean extinction limit that wasfound to be at
= 0.53. The random liftoff whichreached up to a height of 3040 mm
lasted typically100150 ms. Thus, flame C was, for about 2% of
thetime, in the partially extinguished mode. Of the threeflames
discussed here, flame B exhibited the highestnoise level with a
quite discrete frequency of aboutsisted of a two-component system
(DISA 55X,laser = 488 and 514.5 nm) and a single-componentsystem
(DISA Flow Direction Adapter, laser =
-
n andP. Weigand et al. / Combustio
514.5 nm), which were arranged orthogonally andboth used in the
forward scatter mode. The laserbeams were transmitted via mono mode
fibers intothe optical modules. A frequency shift of 40 MHz
wasapplied for all three directions; the center frequencyof the
detection was adapted to each measuring point.Focusing lenses with
f = 300 mm were used; the re-sulting probe volumes were about 60 m
in diameterand 1.0 mm in length for x and y directions, and120 m in
diameter and 1.5 mm in length for the zdirection, corresponding to
the axial (u), radial (v),and tangential (w) directions of the
velocity, respec-tively. Because the spatial intensity distribution
of theMie scattering with its maximum in forward directionis more
than 100 times higher than at 90, the twosystems could be operated
at the same wavelength(here 514.5 nm) because the forward-scattered
sig-nals can be clearly discriminated by their intensities inthis
orthogonal geometry. The detection optics werearranged in the yz
plane at about 10 off axis andconsisted of commercial camera lenses
(f = 85 mm,f/1.8 for x and y directions and f = 105 mm, f/4for the
z direction) focusing the signals on photo-multiplier tubes. For
the simultaneous detection andanalysis of the photomultiplier
signals, three Dantecburst spectrum analyzers (BSA enhanced, 57N20
and57N35) were used with a record length of 64 points.ZrO2
particles with a diameter of approximately 2 mwere seeded as
scatterers into the airflow. Becauseof the small size, the
particles can follow flow fieldfluctuations up to a frequency of
1.2 kHz within anaccuracy of 99%. At each measuring location,
typi-cally 10,000 to 15,000 validated velocity data wererecorded,
except in the area of the flame zone, wheresometimes only 2000
samples were validated duringthe record time. For flames B and C,
measurementsat lower heights were carried out coincidentally witha
time filter of 2 s, thus providing also the Reynoldsstress tensors
and cross moments. Using only the co-incidental values, the
effective probe volume and,consequently, the data rate are
drastically reduced,leading to much longer acquisition time.
Therefore,this was done only at selected heights. For calcu-lating
mean values, the noncoincidental values wereused for an improved
statistic. The lowest height forLDV measurements was h = 1.5 mm for
flame A. Inflames B and C an improvement of the setup
enabledmeasurements as low as h = 1.0 mm. For simplicityin this
study, these levels are all labeled h = 1 mm.
Planar laser-induced fluorescence (PLIF) of OHand CH radicals
was applied to visualize the flamestructures. A Nd:YAG laser pumped
optical paramet-ric oscillator (Spectra Physics GCR 290 and
MOPO
730) was used to supply pulsed laser radiation for theexcitation
of OH and CH radicals. The laser beamwas formed to a light sheet (h
45 mm) and irradi-Flame 144 (2006) 205224 209
ated vertically into the flame intersecting the flameaxis. The
pulse energies were typically 3 mJ/pulsefor OH and 4 mJ/pulse for
CH with a bandwidthof about 0.45 cm1. The sheet thickness was
ap-proximately 0.25 mm in the imaged area. The re-sulting spectral
laser intensities are on the order of16 MW/cm2 cm1 for CH and 12
MW/cm2 cm1for OH. Compared with the saturation intensities,which
are around 1 MW/cm2 cm1 for the chosentransitions [39,40], the
applied laser intensities arerelatively high and a significant
degree of satura-tion is expected. The excited fluorescence was
col-lected at 90 by a lens (for OH: achromatic UV lens,f = 100 mm,
f/2, Halle Nachf.; for CH: camera lens,f = 50 mm, f/0.95, Canon)
and, after spectral filter-ing, detected with an intensified CCD
camera (forOH: LaVision Flamestar II; for CH: Roper Scientific).The
laser pulse duration was 5 ns; the temporal detec-tion gate of the
image intensifier was 50 ns for OHand 200 ns for CH (limited by
irising effects of theimage intensifier). OH radicals were excited
on theR1(8) line of the A2+X2 ( = 1, = 0) tran-sition at = 281.3 nm
[41] and the fluorescence wasdetected through an interference
filter in the wave-length region 312 10 nm. For CH, the Q1(7)line
of the B2X2 ( = 0, = 0) band wasexcited at 390.3 nm [42,43]. For
suppression of laser-scattered light and background radiation, a
filter com-bination of a KV418 (Schott) and a short-pass filterwith
a cutoff wavelength of 450 nm (Oriel) was usedin front of the
camera, and only the fluorescence inthe BX (0,1) and AX bands
around 430 nmwas detected. The A state is efficiently populated
bycollision-induced electronic energy transfer from theB to the A
electronic level [44,45].
For pointwise quantitative measurement of theconcentrations of
major species (O2, N2, CH4, H2,CO, CO2, H2O) and temperature, laser
Raman scat-tering was used [46]. The radiation of a
flashlamp-pumped dye laser (Candela LFDL 20, wavelength = 489 nm,
pulse energy Ep 3 J, pulse durationp 3 s) was focused into the
combustion cham-ber, and the Raman scattering emitted from the
mea-suring volume (length 0.6 mm, diam 0.6 mm)was collected by an
achromatic lens (D = 80 mm,f = 160 mm) and relayed to the entrance
slit of aspectrograph (SPEX 1802, f = 1 m, slit width 2
mm,dispersion 0.5 nm/mm). The dispersed and spatiallyseparated
signals from the different species were de-tected by
photomultiplier tubes in the exit plane of thespectrograph and
sampled by boxcar integrators. Thespecies number densities were
calculated from thesesignals using calibration measurements, and
the tem-
perature was deduced from the total number densityvia the ideal
gas law [46,47]. The simultaneous de-tection of all major species
with each laser pulse also
-
n and
and C
max3. Results and discussion
3.1. LDV measurements
As expected for this type of confined swirl flame,the mean flow
field of each of the three flames showsa strong inner recirculation
zone (irz) along the axialcenterline as well as an outer
recirculation zone (orz)near the walls of the combustion chamber,
as can beseen in the vectorplots of the combined uv
velocitiesdisplayed in Fig. 2. The inlet velocities at the
lowestmeasuring height h = 1 mm correspond to the massflows, with
the maximum values for the axial velocityof umax = 39.0 m/s in
flame A, umax = 13.3 m/s inflame B, and umax = 13.0 m/s in flame C.
The angleof the maximum mean velocity in the uv plane withrespect
to the axial centerline is about 26 for all threeflames for h =
1020 mm. The measurements also
C (S 0.55). Nevertheless, the resulting flow fieldsare very
similar, despite the different visible appear-ance of the three
flames. The normalized mean axialvelocities (umean/umax) at h = 1
mm, illustrated inFig. 3, are almost identical and show that the
inflowat this height extends radially from r = 5 to 15 mm.The
isolines of umean = 0, plotted in Fig. 4, indi-cate the boundaries
of the irz. It can be seen that forflames A and C, the irz reaches
up to h 73 mm,whereas in flame B it ends at h 62 mm. In the
nearfield of the nozzle, for h < 10 mm, the contours ofthe irz
are nearly identical for all three flames, and inall three flames
the irz extends below the lowest mea-suring level. It can therefore
be assumed that the irzreaches even into the central air nozzle,
which endsat h = 4.5 mm. In comparing the mean values ofthe
different flames, it must, however, be consideredthat flame B is
subject to periodic oscillations and that210 P. Weigand et al. /
Combustio
Fig. 2. Vectorplots of combined uv velocities for flames A,
B,the size of the combustion chamber.
enabled the determination of the instantaneous mix-ture fraction
[48]. At each measuring location 500single-pulse measurements were
performed within ascanning pattern of roughly 100 points, from
whichthe joint probability density functions (PDFs) werecomputed.
The choice of 500 samples turned out to bea good trade-off between
measuring time and conver-gence of the mean and rms values. Studies
in highlyturbulent regions of these flames revealed that the fi-nal
values are reached to within 2% after 300 to 400samples. The
measurement uncertainty for the meanvalues of temperature, mixture
fraction, and molefraction of O2, H2O, and CO2 is typically
34%.revealed (not shown) that the ratio of the tangentialvelocity w
and the axial velocity u is nearly doublefor flame A (S 0.9)
compared with flames B andFlame 144 (2006) 205224
; negative u velocities are displayed in red. The lines
indicate
Fig. 3. Radial profiles of the normalized mean axial
velocity(u/u ) at h = 1 mm for flames A, B, and C.the time-averaged
velocities represent an average notonly over turbulent fluctuations
but also over periodicvariations (see discussion below).
-
n andP. Weigand et al. / Combustio
Fig. 4. Isolines of umean = 0 representing the extension ofirz
for flames A, B, and C.
Fig. 5 illustrates the mean values and rms fluctua-tions of the
three velocity components of flame A forh = 1.5, 5, 15, and 45 mm.
At h = 1.5 mm, the profileof the axial velocity u reflects the
inflow of the freshgas at r 516 mm with maximum values around39 m/s
and the irz with velocities of u 20 m/s.The radial velocity
component v is negative for r >16 mm, reflecting the size of the
orz. In the inflow,v is roughly half as large as u. The tangential
ve-locity w is rather constant in the orz (w 10 m/s),and its radial
profile displays two maxima in the re-gion of the inflow, which
likely reflect the flows fromthe two air nozzles with the minimum
between themoriginating from the fuel nozzle and its wake. Forr =
05 mm, w increases linearly with r , reflectingthe solid body
rotation part of the vortex. The rmsvalues of u and v have a
pronounced maximum inthe shear layer between the inflow and the
irz, and vexhibits another maximum in the shear layer betweenthe
inflow and the orz. The high level of the rms val-ues close to the
flame axis demonstrates that the flowfield is subject to strong
turbulent fluctuations in thisregion of the flame. At h = 5 mm, the
gradients of theradial profiles of the mean and rms values have
be-come smaller in comparison to h = 1.5 mm, but thebasic features
of the flow are unchanged. In the orz,umean is close to zero but
urms is 69 m/s. This showsthat u changes frequently its direction
in the orz. At
h = 15 mm, the profiles have broadened and the re-verse flow on
the axis reaches its highest negative ve-locity of umean 26 m/s.
The orz has shrunk but isFlame 144 (2006) 205224 211
still discernible from the negative v component. Themean
tangential velocity component indicates a solidbody vortex up to r
10 mm. For r > 10 mm, wmeandeclines but the shape of the radial
profile does notresemble that of a potential vortex. wrms is quite
con-stant over the radius, whereas urms and vrms exhibitbroad
maxima. With increasing downstream position,the profiles smooth out
and the orz vanishes, whereasthe irz reaches a radial expansion of
r 13 mm ath = 45 mm.
In the shear layer between the inflow and the irz,large velocity
fluctuations and the low mean veloc-ity generally cause a very high
turbulence intensity(urms/umean 100%). Therefore, intense mixing
ofthe cold fresh gas with hot burned gases coming fromthe irz can
be expected in this region. Fig. 6 shows,for example, radial
profiles of normalized urms ath = 10 mm, normalized by the maximum
velocitiesat h = 1 mm; i.e., umax = 39.0, 13.3, and 13.0 m/sfor
flames A, B, and C, respectively. The broad peaksof the urms values
around r 6 mm indicate that theinstantaneous flow fields are
subject to strong turbu-lent fluctuations and that the shear layer
is not lo-cally stable. This finding is also supported by thesingle
shot 2-D LIF images that are discussed in thefollowing paragraphs.
In Fig. 6 it can also be seenthat for the three flames, the
relative velocity fluctu-ations urms/umax are very similar and the
values ofurms reach more than 50% of umax at r 310 mm.Therefore,
the irz should not be regarded as a sta-ble structure with the
fluid following streamlines thathardly vary their positions.
However, from the mea-surements performed in this study, there was
no indi-cation of coherent structures such as rotating
vortexpairs.
To demonstrate that the average flow conditions atthe nozzle
exit for flames B and C were similar, theradial velocity profiles
of u, v, and w at the exit of thenozzle are plotted in Fig. 7. As
clearly can be seen, themean profiles of all three velocity
components matchvery well as intended, so that the mean flow field
canbe excluded as the primary reason for the differentbehavior of
these two flames. However, periodic vari-ations would not be
revealed by the mean profiles.One also recognizes the good symmetry
of the time-averaged velocity profiles, as is expected for a
rota-tional symmetric flow. The dips in the profiles that canbe
seen at r 1012 mm result from the wake of thefuel nozzle and have
already disappeared at h = 5 mm(not shown) due to the high
turbulence. The orz be-comes apparent by the inward-directed radial
veloc-ity v in the region r > 15 mm. Further downstream,the
different thermal powers and combustion temper-
atures of flames B and C lead to a different thermalexpansion
which has an influence on the flow veloci-ties. To demonstrate this
effect, Fig. 8 shows the radial
-
n andFig. 5. Radial profiles of the mean values (left side) and
rms fluctuations (right side) of the three velocity components in
flame Aat different heights.
profiles of u, v, and w at h = 30 mm. Here, the ax-ial velocity
in flame B is significantly larger than inflame C; e.g., umax is
10.6 m/s in flame B and 8.3 m/sin flame C. The profiles of the
radial velocity com-ponent are almost identical and those of the
tangen-tial velocity show only slightly higher velocities forflame
B. Thus, the different thermal expansion of theflames influences
predominantly the axial velocity,
In flame B, LDV measurements have also beenperformed with phase
resolution at heights h = 1and 5 mm. In those measurements, the
phase ofthe acoustic oscillation was measured by a micro-phone
simultaneously with two velocity components[34,35]. An important
finding was that the irz andorz varied in size during an
oscillation cycle, resem-bling a pumping motion: The irz varied
mainly in212 P. Weigand et al. / Combustioas expected for confined
flames. It is also typical ofconfined flames that the swirl number
decreases withcombustion progress due to the axial
acceleration.Flame 144 (2006) 205224the axial direction, and the
orz in the radial direc-tion. Thus, although the mean flow fields
of flames Band C look very similar, they are inherently differ-
-
n andP. Weigand et al. / Combustio
Fig. 6. Radial profiles of urms at h = 10 mm for flames A,B, and
C.
ent, with flame B exhibiting periodic variations super-posed
onto turbulent fluctuations and flames A and Cexhibiting only
turbulent fluctuations.
3.2. Flame structures from OH LIF and CH LIFmeasurements
In flames, OH can be found in detectable concen-trations at
temperatures above approximately 1400 K,especially in fuel-lean
mixtures [49]. The equilib-rium OH concentration increases
exponentially withtemperature but differently for fuel-lean and
fuel-richmixtures. Furthermore, OH is formed in superequi-librium
concentrations in the reaction zones, and itsrelaxation to
equilibrium by three-body collisions isquite slow at atmospheric
pressure ( > 3 ms) [50].Thus, high OH LIF intensities can be
regarded as anindicator of hot gas and/or reacting fuel/air
mixtures.CH radicals are formed at high temperatures on
thefuel-rich side of the reaction zone and have a muchshorter
lifetime (10 ns) than OH radicals [51]. Thus,high CH concentrations
can be interpreted as a markerfor the fuel consumption layer of the
reaction zoneand, with some restrictions, as a qualitative
measureof the heat release rate [52]. For illustration, Fig. 9shows
the calculated profiles of the temperature andOH and CH mole
fractions as a function of the mix-ture fraction f for a strained
laminar CH4/air counter-flow diffusion flame with a strain rate of
a = 400 s1[53,54]. Significant CH concentrations are presentonly in
mixtures with f 0.050.08. In contrast, OHis also found in lean
mixtures and covers a range off 0.0150.08. It can also be seen that
CH is a fac-tor of about 500 lower in concentration than OH
and,thus, much harder to detect. Although the turbulentflames
investigated cannot be directly compared with
a counterflow diffusion flame, this example shows atleast
qualitatively the characteristic behavior of OHand CH.Flame 144
(2006) 205224 213
With respect to the interpretation of the LIF im-ages presented
here, one has to keep in mind thatthe LIF intensities are not
necessarily proportionalto the species number density. The relative
popula-tion of the initial state excited by the laser changeswith
temperature (Boltzmann fraction f B). For OH,the Boltzmann fraction
of the initial rotational state,N = 8, varies by less than 7% over
the temperaturerange of interest (T 14002200 K). For CH, f B ofN =
7 decreases by roughly 20% over the tempera-ture range 1700 to 2200
K. Because the LIF signalsare quenching dominated, variations in
quenching en-vironment can significantly influence the
fluorescenceyield. An estimation was performed for OH using
theLASKIN program [55] and the temperature and gascomposition from
the strained laminar flame calcula-tion with a = 400 s1 already
used in Fig. 9. It turnedout that the OH fluorescence yield varied
by about15% over the range of interest. Taking into accountthat the
composition of the flame under investigationmay deviate from the
strained laminar flame compo-sition, the measured LIF signal
intensities reflect theOH density roughly within 25%. For CH, the
situationis more complex because quenching in two electronicstates,
A and B, and predissociation in the B state playa role. However, in
the thin layer of the reaction zonewhere CH is present, the gas
composition and tem-perature do not change drastically and
variations inquenching are expected to be small. Rensberger et
al.[56] reported that changes in the fluorescence quan-tum yield
after excitation of the B( = 0) state indifferent flames were small
and that quenching var-ied by less than 50%. In the flames
investigated here,variations should not be larger.
Fig. 10 shows typical OH single-shot LIF distri-butions for the
three flames. The images display theregion r = 4141 mm and h 047
mm. For tem-peratures below 13001400 K, OH concentrationswere below
the detection limit (dark areas). For allthree flames, the OH
distributions cover broad areaswith strongly wrinkled contours and
sometimes iso-lated regions (at least in a 2-D cut). These
structuresyield a good impression of the turbulent transport
andmixing processes within the flames. These imagesshow that the
instantaneous flame structures (and verylikely also the
instantaneous flow fields) are much lessuniform as might be assumed
from the mean flowfield. The steep gradients of OH LIF intensities
thatfrequently occurred may represent either a reactionzone or the
boundary between cold and hot fluid. TheOH-free regions near the
nozzle reflect the inlet flowof mostly unreacted fuel and air,
which is directed di-agonally upward. The mean contours of these
regions
can be better seen in the averaged images of Fig. 11(left). The
highest mean OH LIF intensities and thus,within 25% uncertainty,
the highest mean OH con-
-
n and214 P. Weigand et al. / Combustio
Fig. 7. Radial profiles of the simultaneously measured
threevelocity components u, v, and w for flames B and C ath = 1
mm.
centrations are found in each flame in the irz shortlyabove the
nozzle, with a maximum at h 10 mm.Here, flame C has the lowest mean
OH concentration,followed by flame B with approximately 50% moreand
flame A with approximately 75% more. Aroundh = 30 mm on the axis,
the concentrations are ap-proximately three times less than at h =
10 mm foreach flame. These significant OH concentrations inthe irz
in the averaged images (and also in most of
the single shot images) indicate a high temperaturein this
region. The mixing of this hot gas from theirz with fresh gas from
the nozzles presumably playsFlame 144 (2006) 205224
Fig. 8. Radial profiles of the simultaneously measured
threevelocity components u, v, and w for flames B and C ath = 30
mm.
the key role in the ignition and stabilization of theflames.
Investigations in flame B using planar two-line OH LIF thermometry
showed that temperatureand OH concentrations were not generally
well cor-related in that flame and that superequilibrium
con-centrations contributed significantly to the high OHlevels
within and some millimeters downstream ofthe reaction zones [57].
Comparison of the OH LIFdistributions from the three flames further
shows that
flame C exhibits a smaller area containing OH, espe-cially in
the orz. This is explained by the lower overalltemperature level of
this flame (see Table 1) accord-
-
n andP. Weigand et al. / Combustio
Fig. 9. Calculated profiles for temperature and concentra-tions
of OH and CH radicals in a counterflow diffusion flamewith a strain
rate of a = 400 s1.
ing to the global equivalence ratio. It is also seen thatfor
flame A, the inlet flow of cold gas penetrates fur-ther downstream
in comparison to the other flames.
The averaged CH LIF distributions displayed inFig. 11 (right)
reflect the regions where flame reac-tions and heat release take
place. The shapes of thethree flames are quite different: while
flames A and Care conically shaped with opening angles /2 30and 45,
respectively, flame B has a significantlylarger opening angle and
is rather flat with /2 75.The difference between flames B and C is
surprisingbecause their mean flow fields are quite similar andthe
mean velocities are almost identical at h = 1 mm(see Fig. 7).
Comparing the velocity fields and theCH LIF images it becomes
apparent that the heat re-lease for flame C and especially for
flame B does nottake place predominantly in the shear layer
betweenthe irz and the inflow, as might be expected. The CHLIF
distribution and the region of the shear layer arein good
accordance only for flame A, whereas forflames B and C, the opening
angles of the flame zonesare larger than that of the shear layer.
The unusual be-havior of flame B is related to the periodic
pulsationsand is addressed in Section 3.3. Further, it is
impor-tant to note that for all three flames the regions ofheat
release do not begin at the fuel nozzle, but ath 5 mm, h 4 mm, and
h 6 mm for flames A,B, and C, respectively. Due to this liftoff,
fuel, air,and exhaust gas are already partially premixed
beforeignition. According to the CH LIF images, the heatrelease is
complete at h 20 mm and h 40 mm forflames B and C, respectively. In
flame A, there is stilla small amount of CH at the upper end of the
mea-sured area at h = 47 mm.
For the interpretation of the averaged distributionsone has to
keep in mind that flames B and C exhibitunsteady combustion
behavior. Flame A is highly tur-
bulent but steady, and mean and rms values are re-lated to
turbulent fluctuations. Flame C is subject tosudden partial
extinction and can be regarded as bi-Flame 144 (2006) 205224
215
modal, i.e., either burning stably or not burning up toh = 3040
mm. However, the partially extinguishedstate is present only about
2% of the time and its con-tribution to the time-averaged values is
small. Thus,flame C should be classified as nearly steady and
itsfluctuations are caused predominantly by turbulence.For flame B,
the situation is different, because thethermoacoustic pulsations
are permanent and tempo-ral changes of the flame include turbulent
fluctuationsas well as periodic variations. The mean species
andtemperature distributions discussed in this article areaveraged
over both turbulent and periodic changes. Todistinguish between
them, phase-resolved measure-ments have to be performed that yield
averaged valuesat distinct phase angles of the periodic pulsation.
Suchmeasurements have also been performed for flame B,and the
results are discussed in detail in separate pub-lications
[3335].
The single-shot LIF distributions of CH, displayedin Fig. 12,
show thin (0.30.5 mm) and strongly cor-rugated reaction zones which
are, at least in the 2-Dcut, sometimes interrupted. Their shapes
are domi-nated by the turbulent flow field and vary stronglyfrom
shot to shot. Some samples exhibit only weakflame reactions, while
others possess strongly dis-torted and intense reaction zones.
Analysis of a largernumber of single shots revealed that in flame
A, theCH layers are more intensely contorted and the flamesurface
area is larger than in flames B and C [58]. Fur-thermore, CH LIF
peak intensities differ in the threeflames. They are highest for
flame B (glob = 0.75)followed by flame A (glob = 0.65) and flame
C(glob = 0.55); i.e., the order is the same as for theglobal
equivalence ratios. This indicates that the reac-tions occur, on
average, at different local equivalenceratios and not generally
around local = 1, as wouldbe assumed for diffusion flames. This
observation isconfirmed by the Raman results concerning the
ther-mochemical states of the flames.
3.3. Mixture fraction, temperature, and species
molefractions
To yield an overview of the main features ofthe distributions of
mixture fraction f , temperatureT , and mole fraction X, the
results from the point-wise Raman measurements are displayed as
two-dimensional charts which were obtained by interpo-lating
between the measuring locations. With the op-tical setup of the
Raman system, measurements wererestricted to the region h 5 mm and
r 30 mm.
The distributions of the mean mixture fraction, asdisplayed in
Fig. 13, show that the highest f values
are found directly above the fuel nozzle exit, as ex-pected. It
is, however, remarkable that these valuesare already quite small at
h = 5 mm; e.g., fmax =
-
n andFig. 10. Single-shot OH LIF images of flames A, B, and
C.
0.072, 0.059, and 0.056 for flames A, B, and C, re-spectively.
This demonstrates the fast mixing result-ing from this nozzle
configuration. In comparison,the stoichiometric mixture fraction is
fstoich = 0.055and the overall mixture fractions of the flames
werefglob = 0.037 (A), 0.042 (B), and 0.031 (C) (see Ta-
the CH distributions (see Fig. 11 right), indicatingthat mixing
and the main flame reactions are closelylinked at these heights. It
is further seen that in thenear field of the nozzle, f is
considerably higher inthe irz (f > fglob) than in the orz where
f fglob.The relatively high f values in the irz enhance the216 P.
Weigand et al. / Combustioble 1). For flames A and C, mixing is
complete ath 40 mm, and for flame B, already at h 20 mm.These
values are in agreement with the heights ofFlame 144 (2006)
205224stabilizing effect of the irz on the flame, because
theyenable a temperature level that is higher than T glob ad.The
rms values of f (displayed in Fig. 13 left) exhibit
-
n andFig. 11. Averaged OH LIF images (left) and CH LIF images
(right).
a maximum of frms 0.04 at h = 5 mm and decreaserapidly with
height. For a closer look at the mixingbehavior of the three
flames, Fig. 14 shows the radialprofiles of fmean at h = 5 and h =
10 mm. Flame Areaches the largest fmean (r = 6 mm at h = 5 mm,r = 8
mm at h = 10 mm) of all three flames, which
f mean of flame B shows a significantly smaller vari-ation than
that of flame C, although the mean flowfields are quite similar.
The reason lies in the ther-moacoustic oscillations of flame B: In
addition tothe turbulent fluctuations, the periodic variations
alsocontribute to a homogenization of the time-averagedP. Weigand
et al. / Combustiois explained by the high exit velocities of this
flameand, thus, the shorter time for mixing before reachingh = 10
mm. It is surprising that the radial profile ofFlame 144 (2006)
205224 217mixture fraction distribution.The distributions of the
mean temperatures, dis-
played in Fig. 15, reflect the different shapes of the
-
n and218 P. Weigand et al. / Combustio
Fig. 12. Single-shot images of CH LIF.
flames, and it once more becomes evident that flame Bis very
different from the others. It is also seen thatthe final
temperatures of the flames (at large heights)are increasing with
their global equivalence ratios,as expected. To identify the
differences more clearly,Fig. 16 displays the axial profiles of T
mean andT rms. At h = 5 mm, all three flames exhibit a
similartemperature of T 1300 K. With increasing height,T mean
values increase strongly, reach a maximum ath = 1020 mm, and
decrease slowly afterward. Forflames A and B, the maximum mean
temperatures areeven higher than T glob ad due to the relatively
highf values in this region. The temperature fluctuationsreach a
level of 500600 K close to the nozzle anddecrease to 4070 K at
larger heights, which corre-
sponds to the inherent rms of typically 3% due tomeasurement
precision. Considering the temperaturefluctuations within the irz,
especially in the lower part,Flame 144 (2006) 205224
Fig. 13. Two-dimensional mixture fraction distribution
(rightside: mean values, left side: rms values).
it becomes obvious that the irz is not a stationary vor-tex
stable in time and space but, rather, is subject tosignificant
turbulent fluctuations, as was already indi-cated by the
single-shot images of OH and CH in thisregion and by the velocity
fluctuations. A similar re-sult was obtained by Ji and Gore [59] in
a differentswirl flame, where they showed by particle image
ve-locimetry that the instantaneous structure of the irz isoften
composed of a number of smaller vortices. Thismust be kept in mind
for the phenomenological un-
derstanding of flame behavior.
More details of the comparison between the threeflames are seen
in the radial profiles of T mean at
-
n andP. Weigand et al. / Combustio
Fig. 14. Radial profiles of mean mixture fraction ath = 5 mm and
h = 10 mm.
h = 5 and 10 mm in Fig. 17. The low-temperatureregions at r 617
mm reflect the inlet streams offresh gas. Here it is seen that
flame B reaches signifi-cantly higher temperatures than the other
two flames;e.g., at h = 5 mm the lowest mean temperatures areTmin =
408, 627, and 476 K for flames A, B, andC, respectively. The higher
temperatures of flame Bin this region are partly explained by the
more fre-quent occurrence of reactions (see CH distribution inFig.
11). However, at h = 5 mm and especially forr > 10 mm, the main
source of the elevated tempera-tures is mixing of hot exhaust gas
from the recircula-tion zones with fresh gas. The increased
temperaturelevel at h = 5 mm enhances, of course, the reactiv-ity
of the gas mixtures and, thus, the heat release andburnout [60].
This can clearly be seen in the temper-ature profiles at h = 10 mm,
where flame B alreadyreaches a minimum temperature of 1007 K,
whereasfor flames A and C the minimum temperatures are 554and 639
K, respectively. The transition between theinlet stream and the orz
at r 20 mm is clearly vis-ible in the profile of flame A at h = 5
mm. It is alsoobvious that for h 10 mm, the temperature level inthe
orz is in general lower than in the irz, which is dueto the leaner
mixtures (lower f values) and heat loss
to the wall in the orz.
Phase-correlated measurements in flame B re-vealed that the
phase-resolved mean temperature ofFlame 144 (2006) 205224 219
Fig. 15. Two-dimensional temperature distribution (rightside:
mean values, left side: rms values).
the inflowing gas at h = 5 mm varied by about 300 Kduring an
oscillation cycle [34]. This variation wascorrelated with a
periodic expansion of the recircula-tion zones: When the irz
penetrated into the centralair nozzle and the orz reached its
maximum expan-sion, large amounts of recirculating exhaust gas
weremixed into the fresh gas, increasing the temperaturewithin the
inflow. The measurements further indicatedthat variations of the
temperature level of the inflow
and the heat release rate were correlated, leading tothe
conclusion that the temperature of the inflow hada significant
influence on the heat release rate.
-
n and220 P. Weigand et al. / Combustio
Fig. 16. Axial profiles of temperature (mean + rms).
Fig. 17. Radial profiles of mean temperature at h = 5 mmand h =
10 mm.
The right part of Fig. 18 displays the distributionsof the mean
values of the differences between thequasi-adiabatic flame
temperature T a and the mea-sured temperature T . Quasi-adiabatic
flame tem-perature is defined here as the temperature for
theparticular mixture fraction taken from a calculationfor a
strained laminar counterflow diffusion flamewith a strain rate of a
= 1 s1 [53,54]. T a has beencalculated for the measured mixture
fraction at eachlocation for each single shot and from these
resultsTa T has been averaged. The use of the real adi-abatic flame
temperature (and composition) is not
meaningful for fuel-rich regions of turbulent flames,because the
thermal decomposition of CH4 (whichis complete for adiabatic
equilibrium) takes a longerFlame 144 (2006) 205224
Fig. 18. Right: two-dimensional distribution of the differ-ence
between locally possible adiabatic temperature T a andthe measured
mean temperature T . Left: two-dimensionaldistribution of the mean
H2O mole fraction. Both values canbe taken as a measure of the
reaction progress.
time than is typically available in these flames. Devi-ations
between T and T a can stem either from heatloss of the flame gases,
e.g., due to thermal radia-tion or wall contact, or from
finite-rate chemistry ef-fects. As long as heat loss is of minor
importance,the mean value of Ta T can be taken as a mea-sure of the
mean reaction progress in the flame, and
is an indirect way to display the effects of
finite-ratechemistry, which is discussed in more detail in
theaccompanying article [32]. The results displayed in
-
n andP. Weigand et al. / Combustio
Fig. 18 show that Ta T reaches significant valuesin all three
flames; e.g., the maximum mean valuesare >1400, >900, and
>1000 K for flames A, B, andC just above the nozzle exit. From
the large valuesof Ta T and its distributions, it becomes
obviousthat finite-rate chemistry effects play a very impor-tant
role in the flames investigated. The burned gasesreach a final
state with Ta T < 200 K that is closeto equilibrium. Comparison
of the results reveals thatnonequilibrium effects are quite
differently distrib-uted in the three flames: Flame A reaches a
constantlevel of Ta T at h 55 mm, and in flame C signifi-cant
effects of finite-rate chemistry are observed up toh 45 mm, whereas
in flame B a uniform Ta T isattained by h 20 mm. These heights are
in good ac-cordance with the CH LIF images, where for flames Band
C, the same heights are found for detectable CH,and for flame A, CH
is still present at the upper endof the image at h = 47 mm. The
observed differencein height between flames A and C is in
accordancewith the different flow velocities and Reynolds num-bers
of the flames. The much faster burnout of flame Bis again related
to the thermoacoustic pulsations andis probably caused mainly by
the relatively high tem-perature level of the gas in the inflow as
discussedbefore.
Finally, the mean distributions of the mole frac-tions X of H2O,
CH4, and O2 are presented inFigs. 18 and 19. The shapes of the
distributionsof X(H2O), displayed in Fig. 18 (left),
resemblestrongly those of temperature for each flame (seeFig. 15).
Inspection of the single-shot results (not dis-played) reveals that
the correlation between X(H2O)and T is in quite good agreement with
the correla-tions calculated for strained laminar flames. Fromthe
single-shot correlations it is, however, seen thatthe flames
experience a temperature loss in theorz, probably due to heat
conduction to the burnerplate [32].
As shown in Fig. 19 (left), the distributions ofX(CH4) exhibit
the highest values close to the fuelnozzle; however, for flames A
and C the maximaare not exactly above the CH4 injection but
shiftedslightly inward. Close to the nozzle (h < 15 mm), theCH4
distribution of flame B is significantly broaderthan those of
flames A and C. A similar trend wasalready seen in the mixture
fraction distributions(Figs. 12 and 13) and can also be observed
for theO2 distribution (Fig. 19). This result is a further
indi-cation that the periodic oscillations of the flow
fieldgenerate additional mixing of fuel, air, and exhaustgas, which
promotes reaction progress. The consump-tion of CH4 with increasing
distance from the nozzle
is in general accordance with the decrease in Ta Texcept for a
small discrepancy in the orz, whereTa T increases, due to the
above-mentioned tem-Flame 144 (2006) 205224 221
Fig. 19. Left: two-dimensional distribution of the mean CH4mole
fraction. Right: two-dimensional distribution of themean O2 mole
fraction.
perature loss. This can be seen comparing Fig. 18(right) and
Fig. 19 (left) at positions r > 25 mm andh = 010 mm. Here, the
CH4 concentration is aroundor smaller than 1% but the temperature
differenceTa T is larger than 200 K for flame A or even400 K for
flames B and C. The temperature differ-ence is not explainable by
the remaining fuel and is
probably due to heat transfer to the casing. In the irz,above h
10 mm, and in the exhaust gas region, noCH4 is found.
-
n and222 P. Weigand et al. / Combustio
The distributions of X(O2) (Fig. 19, right) re-flect again the
shapes of the flames and are in goodagreement with those of T and
Ta T ; i.e., X(O2)decreases as T increases. The fact that the
lowestmean concentrations of O2 are found near the flameaxis within
the irz confirms that the mixing charac-teristics of the burner
configuration generate a rel-atively fuel-rich (but still overall
lean) recirculatinggas flow which is of importance for flame
stabiliza-tion. The distributions of the remaining major
species(N2, CO2, CO, H2) are in good agreement with theresults
presented and are not displayed. The meanmole fractions of the
intermediate species CO andH2 are generally below 0.018 and 0.012,
respectively.The highest concentrations of these species are
foundin the shear layer between the inlet flow and the irzat h =
1020 mm with near-stoichiometric mixturefractions and temperatures
between 1000 and 1500 K.
4. Summary and conclusions
A laboratory-scale GT model combustor has beendescribed and
three CH4/air diffusion flames with dif-ferent flame
characteristics have been investigated us-ing LDV, PLIF of OH and
CH, and laser Raman scat-tering. The main goals of the studies were
(1) to carryout a detailed experimental analysis to improve
under-standing of the physical and chemical processes lead-ing to
the different behaviors of swirling flames, and(2) to provide a
useful database for the verificationand improvement of numerical
combustion models.The results presented in this article concern the
flowfields and the flame structures, as well as the meanvalues and
fluctuations of the major species concen-trations, mixture
fraction, and temperature. A sec-ond article addresses
turbulencechemistry interac-tions and their effect on mixing and
stabilization [32].
The shapes of the mean flow fields of all threeflames are quite
similar: The injected flows of CH4and air formed a cone with an
opening angle /2of about 26 with respect to the flame axis. Due
tothe swirl, a pronounced irz was established whichreached from h
70 mm down into the central airnozzle. This reverse flow of hot
combustion productsformed the major source for the ignition and
stabiliza-tion of the flames. The instantaneous flame structuresand
velocity fluctuations gave evidence that the irzwas, at least in
the lower part, subject to strong tur-bulent fluctuations of its
shape and composition. Anorz was established in the lower part of
the combus-tion chamber.
The flame structures were visualized by planar
laser-induced fluorescence of OH and CH. The re-sults showed
that the instantaneous flame shapes weredominated by turbulence and
that the three flamesFlame 144 (2006) 205224
could hardly be distinguished based on a single in-stantaneous
OH or CH image. The reaction zonesof all three flames were
generally thin (0.5 mm)and strongly wrinkled but more contorted in
flame A,which had a higher Reynolds number. The averagedimages of
CH PLIF showed the regions of heat re-lease. It could be seen that
none of the flames wasattached to the nozzle; i.e., reactions did
not startbelow h 5, 4, and 6 in flames A, B, and C, re-spectively.
The flames were thus partially premixedbefore ignition. Flame B had
a remarkably short flamelength of h 20 mm, whereas the other two
flamesreached up to h 50 mm (flame A) and h 40 mm(flame C), which
also represented a fast burnout. Theaveraged OH and CH
distributions also revealed thesignificantly different shapes of
the three flames. Theopening angles for the flame zones deviated
from theopening angle of the flow fields for flame C and
espe-cially for flame B, while for flame A the two anglesmatched
roughly.
While a certain similarity was seen for flames Aand C, with
opening angles /2 of 30 and 45, re-spectively, flame B had a
significantly larger openingangle (/2 = 75) and was much shorter.
The differ-ent shapes of flames B and C were surprising
becausetheir mean flow fields were very similar, especiallyat the
nozzle exit. Except for the velocity fields, allother measured
quantities confirmed that flame B ex-hibited a different behavior
than flames A and C. Mi-crophone measurements revealed that the
sound emis-sions of flame B were concentrated at a frequency of290
Hz, proving the occurrence of thermoacousticoscillations, and
previously reported phase-resolvedLDV and PLIF measurements showed
significant pe-riodic variations of the flame structure during an
os-cillation cycle. Periodic changes in the expansion ofthe inner
and outer recirculation zones enhanced themixing of fuel, air, and
exhaust gas, which in turn con-tributed to increased reaction
progress.
Measurements of the mixture fraction demon-strated the fast
mixing of fuel and air of this nozzleconfiguration. At h = 5 mm,
variations of the meanmixture fraction along the radial profile
remained be-tween fmean = 0.02 and 0.08 for flame A and wereeven
smaller for the other flames. Mixing was com-pleted at about the
same height as the heat release, i.e.,at h 40 mm for flames A and C
and at h 20 mmfor flame B. Within the irz, f and T were higher
andX(O2) was smaller than the global values. The flowand the mixing
characteristics of the burner enhancedthe effect of the irz with
respect to ignition and stabi-lization of the flame. Although
measurements could
not be performed below h = 0 mm, the results ob-tained indicated
that hot combustion products weretransported via the irz into the
central nozzle, where
-
n andP. Weigand et al. / Combustio
they mixed with air before reentering the combustionchamber.
The distributions of Ta T showed that in allthree flames,
pronounced finite-rate chemistry effectsoccurred which led to a
significant deviation fromequilibrium composition and temperature,
especiallywithin the high-velocity regions of the inlet flow andthe
neighboring shear layers. Finally, it should benoted that the
experimental data are available fromone of the authors (W.M.) on
request for validation ofnumerical codes for swirling turbulent
flames.
Acknowledgments
The work presented here was performed mainly inthe frame of the
project Combustion Control and Sim-ulation, funded by the State of
Baden-Wrttemberg,and as part of the DLR project NACOS. The
finan-cial support within these projects is gratefully
ac-knowledged by the authors. We furthermore thankB. Lehmann for
execution of the LDV measurementsand B. Noll for fruitful
discussions.
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Investigations of swirl flames in a gas turbine model
combustorIntroductionExperimentalCombustor and flamesMeasuring
techniques
Results and discussionLDV measurementsFlame structures from OH
LIF and CH LIF measurementsMixture fraction, temperature, and
species mole fractions
Summary and conclusionsAcknowledgmentsReferences