PRESSURE AND TEMPERATURE SENSITIVE PAINT ABSTRACT
Pressure Sensitive Paints (PSP) and Temperature Sensitive Paints
(TSP) are optical sensorsfor measuring the temperature and pressure
of a remote surface. These sensors are based on the quenching of
luminescent molecules that are sensitive to the local temperature
or pressure. The following Report describes the basics of TSP and
PSP. More detailed reviews of PSP and TSP.Also this report include
working principles of PSP and TSP are based on the oxygen and
thermal quenching processes of luminescence, respectively. Also to
understand how to measure pressure & temperature from PSP &
TSP image an detailed section explains the basic measurement
apparatus and its setup. Also the PSP data were also compared
withCFD data to estimate the capability of PSP as a CFD validation
tool.
To clearly understand the concept this report also includes the
application of PSP/TSP in Aerodynamics, turbo machinery and
Aerospace.
Lastly, the advantage and dis advantage of using PSP/TSP is
explained.
INTRODUCTION
Pressure- and temperature-sensitive paints (PSP and TSP) are
thin luminescent polymer coatings for measuring surface pressure
and temperature fields by utilizing the quenching mechanisms of
luminescence. PSP and TSP are able to provide noncontact,
high-resolution, and quantitative mapping of surface pressure and
temperature on complex models at a lower cost. In both PSP and TSP,
luminescent molecules are used as sensing probes that are
incorporated into a polymer coating on a surface. In general,
luminophore and polymer binders are dissolved in a solvent, and the
resulting paint can be applied to a surface using a sprayer or a
brush. After the solvent evaporates, a solid polymer coating in
which the luminescent molecules are immobilized remains on the
surface. When a light of a proper wavelength illuminates the
coating, the luminescent molecules are excited and the luminescent
light of a longer wavelength is emitted from the excited
molecules.
The main photo-physical process in PSP is the oxygen quenching
that causes a decrease of the luminescent intensity as the partial
pressure of oxygen or air pressure increases. A polymer binder for
PSP should be oxygen permeable, which allows oxygen molecules to
interact with the luminescent molecules in the binder.
The relevant mechanism in TSP is the thermal quenching that
reduces the luminescent intensity as temperature increases. TSP is
not sensitive to air pressure since a polymer binder used for TSP
is oxygen impermeable. It is noted that PSP is also affected by the
thermal quenching and, therefore, it is intrinsically temperature
sensitive.
In principle, once PSP and TSP are calibrated, surface pressure
and temperature can be remotely measured by detecting the
luminescent emission radiance. Temperature Sensitive Paint(TSP)
A typical TSP consists of the luminescent molecule and an oxygen
impermeable binder. The basis of the temperature sensitive paint
method is the sensitivity of the luminescent molecules to their
thermal environment. The luminescent molecule is placed in an
excited state by absorption of a photon. The excited molecule
deactivates through the emission of a photon. A rise in temperature
of the luminescent molecule will increase the probability that the
molecule will return to the ground state by a radiation less
process; this is known as thermal quenching. The temperature of the
painted surface can be determined by monitoring the fluorescent
intensity of the painted surface.
Pressure Sensitive Paint(PSP)
The PSP method is based on the sensitivity of certain
luminescent molecules to the presence of oxygen. A typical PSP is
comprised of two main parts, an oxygen-sensitive fluorescent
molecule, and an oxygen permeable binder. When a luminescent
molecule absorbs a photon, it is excited to an upper singlet energy
state. The molecule then typically recovers to theground state by
the emission of a photon of a longer wavelength. Pressure
sensitivityof the luminescent molecules results when an excited
luminophore interacts with anoxygen molecule and transfers some of
the excited state energy to a vibrational mode of theoxygen
molecule. The resulting transition to the ground state is
radiationless, this process isknown as oxygen quenching. The rate
at which the quenching process competeswith the radiation process
is dependent on the partial pressure of oxygen present, witha
higher oxygen pressure quenching the molecule more, thus reducing
fluorescence.
Conceptually a PSP system is composed of a PSP, an
illuminationsource, a detector, and a long-pass filter. The PSP is
distributed over the modelsurface and the surface is then
illuminated by the excitation source causing the PSP toluminesce.
The luminescent intensity from the PSP is recorded by the detector
and converted topressure using a previously determined calibration.
Unfortunately, the luminescent intensityfrom a pressure-sensitive
coating can be a function of several parameters such as;
spatialvariations in excitation illumination, pressure-sensitive
luminophore concentration, paint layer thickness, and camera
sensitivity.
WORKING PRINCIPLE (PHOTOPHYSICAL FOUNDATION)
The working principles of PSP and TSP are based on the oxygen
and thermal quenching processes of luminescence, respectively. The
general principles of luminescence are described by Rabek (1987).
The different energy levels and photophysical processes of
luminescence for a simple luminophore can be clearly described by
the Jablonski energy-level diagram, as shown in Figure. The lowest
horizontal line represents the ground-state energy of the molecule,
which is normally a singlet state denoted by S0. The upper lines
are energy levels for the vibrational states of excited electronic
states. The successive excited singlet states are denoted by S1 and
S2, while the triplet state is denoted by T1. As is normally the
case, the energy of the first excited triplet state T1 is lower
than the energy of the corresponding singlet state S1.
Jablonski energy-level diagram.
A photon of radiation is absorbed to excite the luminophore from
the ground electronic state to excited electronic states (S0 S1 and
S0 S2). The excitation processis symbolically expressed as S0 + _
S1, where _ is the Planck constant and is the frequency of the
excitation light. Each electronic state has different vibrational
states, and each vibrational state has different rotational states.
The excited electron returns to the unexcited ground state by a
combination of radiative and radiationless processes. Emission
occurs through the radiative processes called luminescence. The
radiation transition from the lowest excited singlet state to the
ground state is called fluorescence, which is expressed as S1 S0 +
_f . Fluorescence is a spin-allowed radiative transition between
two states of the same multiplicity. The radiative transition from
the triplet state to the ground state is called phosphorescence (T1
S0 + _p), which is a spin-forbidden radiative transition between
two states of different multiplicity. The lowest excited triplet
state T1 is formed through a radiationless transition from S1 by
intersystem crossing (S1 T1). Since phosphorescence is a forbidden
transition, the phosphorescent lifetime is typically longer than
the fluorescent lifetime. Luminescence is a general term for both
fluorescence and phosphorescence.
Radiationless deactivation processes mainly include internal
conversion, intersystem crossing, and external conversion.
Deactivation of an excited electronic state may involve interaction
and energy transfer between the excited molecules and the
environment-like solutes, which are called external conversion. The
excited singlet and triplet states can be deactivated by
interaction of the excited molecules with the components of a
system. These bimolecular processes are quenching processes,
including collisional quenching (diffusion or nondiffusion
controlled), concentration quenching, oxygen quenching, and energy
transfer quenching. The oxygen quenching of luminescence is the
major photophysical mechanism for PSP. The quantum efficiency of
luminescence in most molecules decreases with increasing
temperature because the increased frequency of collisions at
elevated temperatures improves the possibility for deactivation by
the external conversion. This effect associated with temperature is
the thermal quenching, which is the major photophysical mechanism
for TSP.
Pressure-Sensitive Paint Measurement
PSP measurement is a molecular sensor based optical measurement
technique. Figure is the schematic of PSP measurement. Illumination
light is to supply the luminescent energy to PSP and CCD camera
measures the luminescence intensity. PSP consists of two layers;
white undercoat and active layer. Active layer is a mixture of
probe molecule and oxygen permeable polymer. White undercoat is
used to enhance the PSP luminescence by diffusive reflection.
Schematic of PSP measurement
The principle of PSP measurement stands on the oxygen quenching
of the luminescence ofpressure-sensitive probe molecule. Thus, PSP
can sense the oxygen partial pressure in actual.Air includes 21%
oxygen, therefore pressure value can be measured using
luminescentintensity variation of PSP. Theoretically, its relation
is represented by following Stern-Volmer relation; where, I and P
are the luminescent intensity and pressure of wind-on test
condition and Iref and Pref are those of wind-off reference
one.
Temperature Dependency of PSPPSP has not only the pressure
sensitivity but also the temperature dependency. PSPintensity and
coefficients of equation depend on temperature. Thus, when the
pressure iscalculated using PSP, it is very important to compensate
the temperature effect in some way.
The simplest method is to use a thermometer. It is convenient to
measure representative temperature. However, its drawback is the
lack of spatial resolution. Another solution is to use an infrared
(IR) camera. It can measure global temperature distribution.
However, the utilized wavelength of IR camera is so long that
conventional glass or transparent plastic cannot transmit it. And
the setting space to install IR camera in addition to CCD camera
also becomes a problem. The bestsolution are to produce
temperature-insensitive PSP, which has developed at NASA Langley or
PSP/TSP binary paint. However, there are chemical and
spectrographic difficulties on developing such ideal paints and few
examplesare reported.In this study, it is handled by temperature
measurement using Temperature-Sensitive Paint(TSP). One side of a
test model is painted by PSP and the other is painted by TSP. Then
thetemperature effect of PSP is compensated by TSP data assuming
symmetrical temperaturedistribution. This method has the limitation
of symmetrical assumption, however,one CCD camera can measure PSP
and TSP at same time and no limitation on wavelengthtransmittance
of window material.
The temperature dependency of PSP is compensated using both PSP
and TSP data. Tosolve it needs numerical iteration process. The
relation between pressure, temperature, PSPdata (Iref/I)PSP and TSP
data (I/Iref)TSP are represented below;
Measurement Methods/Systems
- Intensity based Methods (most common) Full-field using camera
Point systems using scanning laser
- Time based Methods (lifetime decay) Full-field using camera
Point systems using scanning laser
- Frequency based Methods (phase shift from excitation)
Full-field using camera Point based system using scanning laser
1. Intensity based Methods (most common)
Requires two readings, a reference at constant pressure
(windoff) and an unknown data point (wind-on)
Ratio of intensities IREF/I is inversely proportional to the air
pressure
The excitation and detection systems must be spectrally
separated, (>10-6 attenuation in stop band) -Excitation system:-
Continuous Sources: LEDs, Filtered lamps (Halogen, Xenon), Lasers
Pulsed Sources for instantaneous or periodic measurements: LEDs,
Xenon strobes/flash
- Detectors Cooled Scientific grade CCD cameras (slow scan, low
noise), PMT, PD
Simplest technique, most sensitive
Very sensitive to motion between wind-off and wind-on
Intensity Methods: Types of Testing Imaging Techniques
Most aero data is taken during steady state conditions with
constant illumination Steady state data extracted from a pulsed
synchronization illumination with a periodic experiment (rotating)
Dynamic data from a pulsed synchronized illumination with a
periodic experiment with time delay off of a trigger signal
Point Techniques
CW laser and PMT to get time history data at a single point both
steady and unsteady data Laser can be stationary or scanned
Advantages: Eliminate wind off images and image registration
problems. It works in theory but do to homogeneity problems of
dispersing two probes equally it actually requires a double set of
ratios,often called ratio of ratios method Measure temperature to
compensate for temperature sensitivity of PSP.
2. Time based Methods (lifetime decay) Easiest to do with a
point measurement, but can use time resolved cameras to measure
lifetime decays of the probe molecules. Point measurements require
a pulsed light source and detector(PMT, PD) Time resolved imaging
requires a double pulse type experiment to measure the decay times
(gated camera, interline transfer camera capable of multiple flash
integration). Determination of pressure and temperature from a
single probe. The time decay signal has embedded temperature and
pressure information Requires three gates to generate two equations
of gate ratios to solve for pressure and temperature at each point
(pixel) Significant processing for imaging applications
3. Frequency based Methods (phase shift from excitation) If
modulation frequency is fixed, then the phase angle is a function
of the lifetime =f(P,T) Phase angle can be measured directly with a
lock-in amplifier Phase delay can be measured using two images from
a camera locked in phase to the excitation, the second image is
acquired out of phase.
PSP MESUREMENT COMPONENT SETUP
Figure below is the PSP measurement setup. There are three sets
of CCD camera and illumination. They correspond to upper
measurement system and side one. Sidemeasurement system consists of
both of left and right system. Because if the test model is painted
both PSP and TSP symmetrically, upper system can measure both PSP
and TSP side at the same time. Components of PSP measurement
systemMajor components of PSP measurement apparatus are
illumination light source, imageacquisition device and optical
filters.
Illumination Light Source
The Xenon lamp with stabilizing circuit is used as the
illumination light source tosupply energy to PSP and TSP. The
output illumination light is transmitted to theillumination light
head through a light guide. The light guide is used to handle the
lighttransmission easily. The illumination light head is lens
system to illuminate a PSP painted model. Mostly there are two
types of the illumination light head which are the
standardillumination light head and the wide one. Appropriate one
is selected depending on the measurement area.
Image Acquisition Device
The CCD cameras are used to detect the luminescence intensity
from PSP and TSP. It isnecessary to have high signal-to-noise (S/N)
ratio and high quantum efficiency because thePSP and TSP
luminescence intensity are small. To increase S/N ratio, our CCD
cameras areslow-scan and cooled (up to -60C) type.
Optical Filters
The PSP and TSP luminescence is much smaller than illumination
light intensity. Thus, itis necessary to eliminate the illumination
light from CCD camera image. Optical filters areinstalled in front
of the illumination light head (illumination filter) and CCD
camera(luminescent filter). The illumination filter transmits only
the wavelength of violet and blue,which corresponds to the
absorption wavelength of probe molecule of PSP and TSP.
Theluminescent filter transmits only the wavelength of red, which
corresponds to the luminescence wavelength of PSP and TSP.
IMAGE PROCESSING
Image processing procedure is as follows:1. Averaging of PSP
images.2. Marker detection for image registration.3. Image
registration between wind-on & wind-off image.4. Convert TSP
image to temperature image.5. Temperature correction of PSP image
using temperature image,6. PSP & TSP image is calculated with
calibration curve by a-prior method & by pressure taps PSP
image is calibrated.
PreprocessingAlthough CCDs are excellent light detectors,
corrections need to be made.Preprocessing includes the image
averaging and dark image subtraction. Imageaveraging is one mean to
decrease the shot noise and increases the S/N ratio. And
darkcomponent of CCD camera image needs to subtract from all CCD
camera images using dark image. This is dark image subtraction. And
spatial filter to reduce shot noise,for example, wiener filter, is
also applied on these images.These preprocessing is common for
bothPSP and TSP data.
Image RegistrationTo calculate pressure value using it is
necessary to make the ratio image. It is the map of Iref/I for PSP
or I/Iref for TSP. The model location on CCD camera image between
windonand wind-off is different because a test model is deformed by
aerodynamic force duringwind-on condition. Thus, an image
processing to align wind-on deformed image to wind-offreference one
is necessary. It is the image registration. The markers on a model
areutilized as reference points for this alignment to fix the image
transformation function fromwind-on image to reference one. Then
whole of the wind-on image is transformed using thisfunction. Then
PSP and TSP ratio image are constructed.
Pressure CalculationAfter construction of the PSP and TSP ratio
image, pressure value is calculated by equation. Temperature
compensation of PSP is included implicitly. To solve equation is
numerical iteration process, thus Newton-Raphson method is used to
enhance the convergence.
Pressure Calculation Method:-Data CalibrationCalibration between
pressure and luminescence intensity is necessary to transfer camera
image to pressure and temperature map. There are two conventional
PSP calibrationmethods, a-priori and in-situ.
A-priori MethodA-priori method is a pressure calculation method
using an off-line PSP and TSPcalibration. This calibration uses the
PSP and TSP sample coupon which have samecharacteristic with a test
model.
Before the wind tunnel test, PSP characteristics are calibrated
using automatic calibration stand shown in Figure below which can
set the matrix of the discretionary pressure and temperature
calibration points. The calibration results were shown in Figure
Pressure value is calculated by equation using PSP and TSP data and
PSP and TSP characteristic surfaces.
Automatic Calibration Stand Schematic of a-priori method
A-priori method is attractive that it needs no pressure taps on
the test model and pressure test can be conducted using force test
model. However, a-priori method is easily affected various factor,
for example, intensity variation of illumination light, error on
compensated temperature, PSP photodegradation, etc. The data
accuracy of a-priori method is slightly less than that of in-situ
methods.
A-priori/In-situ Hybrid Method
In-situ method is an on-site calibration which makes the
relationship between pressure tap data and corresponding PSP
intensity data simultaneously. Conventional insitumethod is widely
used for practical PSP tests because it can produce high accuracy
datadue to introduction of pressure tap data. Schematic of
Conventional In-situ MethodHowever, conventional in-situ method
calibrates the PSP characteristics from givenpressure tap data.
Depending on the arrangement of pressure tap, there is region which
local pressure exceeds the range of pressure taps. In-situ method
has the possibility of such extrapolation.To improve such
extrapolation effect and keep high accuracy due to introduction of
pressure tap data, a-priori/in-situ hybrid method is employed as
primary pressure calculation.
A-priori/insitu hybrid method includes temperature distribution
compensation. The essences of this method are;- compensate PSPs
temperature dependency and TSPs pressure dependency using PSPand
TSP data in a mutually complementary manner.- introduction of the
correction coefficient to compensation global unknown error
source.
The formulation of a-priori/in-situ hybridmethod is following
where, CPSP is the correction coefficient of PSP data and CTSP is
the correction coefficient ofTSP data.
Post Processing
Post processing includes the display of the pressure map, the
comparison between PSPand pressure tap, and so on.
Transform image plane (2D) to model plane (3D) coordinates using
photogrammetry techniques for mapping PSP data on to CFD generated
grids Multiple views allow full 360 views of pressure data to
berepresented Techniques typically based on central projections
from the painted model through a optical point to the image plane
Reference marks on model are measured to give the needed inputs to
solve the transformations matrices Colinearity equations of
photogrammetry Direct linear transform
Uncertainty
Characterization of the paint and calibration errors (a-priori,
insitu calibration, photodegradation, paint contamination, paint
intrusiveness, time response) Measurement system errors (detector
noise, illumination spectral and temporal stability, spectral
leakage) Signal analysis errors (registration from model motion and
deformation, incomplete temperature compensation, self
illumination, resectioning on a non-deformed grid) The major
contributor is temperature uncertainty which can account for up to
90% of the total uncertainty.
APPLICATION OF PSP AND TSP
PSP measurements can be more effectively made in high subsonic,
transonic, and supersonic flows since PSP is more sensitive in a
range of Mach numbers from 0.3 to 3.0. Experiments on various
aerodynamic models with PSP in large production wind tunnels have
been made. More recently, advances have been made in PSP
measurements in hypersonic, unsteady, and rarified gas flows.
Besides applications of PSP in external aerodynamic flows, PSP has
also been used to study supersonic internal flows with complex
shock wave structures in turbomachinery.
TSP has been used to measure heat flux distributions on air
vehicle models in hypersonic tunnels. The global surface heat
transfer distributions at Mach 10 were measured. Transition from
laminar to turbulent flow was clearly identified as an abrupt
change from low to high heat transfer. Movement of the transition
line toward the leading edge was also observed as the laminar
region diminished when the surface temperature increased with time
TSP has been utilized for visualizing flow transition. Since
convection heat transfer is much higher in turbulent flow than in
laminar flow, TSP can visualize a surface temperature difference
between the laminar and turbulent flow regions. Typically, in
low-speed, subsonic, transonic, and supersonic flows, a model (or
the incoming flow) should be heated or cooled in order to generate
a sufficient temperature change across the transition line.
However, in hypersonic flows, aerothermodynamic heating is able to
produce a significant temperature difference between the laminar
and turbulent flow regions for measurements (visualization). In
addition, cryogenic TSPs were used to detect transition on airfoils
and wings in cryogenic transonic wind tunnels.
1. Investigations of large scale aircraft models at transonic
speeds.
PSP measurements technique are realized in two large transonic
wind tunnels. Binary (two-color) PSP is used. The binary paint
contains additional reference luminophor that is insensitive to
pressure and emits light with intensity directly proportional to
the excitation light intensity. Powder of europium doped crystal
phosphor is used as a reference luminophor in combination with
pyren derivative. Both luminophors (pressure sensitive and
reference) are excited by the light of the same wavelength but emit
in different spectral ranges, providing separate recording of
sensitive and reference images, e.g. using two cameras with
appropriate optical filters.
Luminescence of reference luminophore is used for pixel-by-pixel
correction of excitation light intensity variations. Four CCD
cameras and two flash lamps are used to measure pressure
distribution on two model sides.
Example of pressure fields on upper and lower surfaces of wing
of training aircraft with deflected slat (=20) at Mach number =0.85
and angle-of-attack AoA=8 is presented in below. Pressure
distribution allows understanding flow physics and verifying CFD
results. The integration of the pressure distribution over the
surface of model elements yields the total loads and moments
applied to these elements, for example allows determination of
hinge moments of flaps, ailerons and slats.
Pressure distribution (P) on upper and lower surfaces of wing of
training aircraft with deflected slat
2. Pressure field measurement on the propeller blade surface
Pressure field measurement on the propeller blade surface is an
extremely complicated problem of experimental aerodynamics.
Classical technique, i.e. creation of propeller model with vast
amount of pressure taps, is quite time-consuming, complex and
expensive. PSP provides alternative, quite rapid and economical
method to obtain pressure distribution on the propellers.
Application of PSP technology to propellers has some specific
features. The main problem is the image acquisition of moving
blade. Linear speed of blade tip can reach up to 200-300m/sec. To
eliminate blade displacement during image acquisition the
measurement time must not exceed 12 sec that corresponds to blade
tip displacement of 0.20.5 mm. Longer measurement time will lead to
unacceptable blur of blade image.The problem is solved by using the
pulsed nitrogen laser operating in stroboscopic mode (light pulse
duration is 6-8 nsec).
The other parameter affecting on the spatial resolution of
pressure measurement on blades is luminescence decay time of PSP
after excitation light pulse. Its effect on image blur is the same
as an effect of illumination time. PSP formulations based on pyren
derivative are optimal for the models moving with high speed since
the lifetime of pyren derivative molecules is less than 400 nsec
(lifetime in vacuum). Unfortunately, only single-component PSP can
be used for pressure measurements on the blades. Reference
component of our binary paint has too large lifetime (0.5 msec).
For correction of excitation light intensity variation the spot of
Luminescent Reference Paint (LRP) is applied on the model
surface.
Figure below shows pressure distribution on propeller blade in
comparison with CFD prediction. Pressure fields are similar to each
other, but PSP pressure level is lower by 4000 Pa that correspond
to 4% of maximum pressure level on the blade.
(a) (b) Pressure field on propeller blade PSP (a) and CFD
(b)
3. PSP application in hypersonic flowsPSP application in
hypersonic flows is problematic because of significant PSP
temperature sensitivity. Temperature problem is overcome by: a)
Executing the tests in short duration wind tunnel.b) Model
manufacturing from heat-conducting material (Aluminium alloy).c)
Application of PSP with fast response time.
Response time of PSP is determined by oxygen diffusion in the
polymer layer and is directly proportional to the squared polymer
layer thickness. Usage of permeable polymer applied as very thin
layer (about 2 micrometers) allows getting response time less than
5 msec. Binary PSP for transonic applications contains crystal
phosphor and is too thick for short duration facilities. To
compensate excitation light variations in these tests we use a
separate referencelayer applied on the model surface before
sensitive PSP layer.
PSP method is wildly used at Mach numbers =5, 6 and 8 in wind
tunnel with flow duration 40 m/sec. One of the investigated
problems is the interaction of the oblique shock waves, generated
by a single fin or a fin pair, with boundary and entropy layers of
blunted plate on which they are installed. Model with single fin
and flow scheme are shown in Figure below. Pressure distributions
on the plate and on the fin are presented. To exclude luminescent
light re-reflection problem the pressure distributions on the plate
and on the fin were measured in separate wind tunnel tests, tuning
the investigated surface perpendicularly to illumination and
observation direction. Investigated model and flow structure: 1
plate, 2 wedge, 3 - oblique shock wave, 4 bow shock wave, 5
separation line, 6 reattachment line.
Pressure distribution measured with PSP on the surface.
4. Oscillating airfoil
PSP has also been used for investigations of the unsteady
pressure distribution on oscillating airfoils It is demonstrated
that the measurement system including the PSP is adequately
described as a linear time-invariant system, allowing for
characterization of the paint by a transfer function. Fast Binary
PSP to determine the fluctuating Cp distribution on a NACA 0012
airfoil and an oscillation frequency up to 30 Hz at a Mach number
of 0.72 and an angle of attack of 1.120.6. Phase-locked pressure
field measurements were compared to conventional pressure
measurements for a chordal section. Correction of the pressure time
series was conducted, by computing the Fourier series, truncating
it at f=120 Hz, and correcting phase and amplitude by means of the
transfer function determined for the PSP.
5. Nozzle Flow
The application of the PSP technique to two different scramjet
nozzles in a Mach 7 flow in hypersonic wind tunnel has been
successfully demonstrated. The agreement with results from pressure
taps is very good. Surface pressure measurements enabled deeper
insights into the physical flow phenomena, and also helped to
explain the anomalous (wavy) behavior of the static pressure
distribution seen along the centerline ( also shown below the
positions of the pressure orifices on left, center and right lines)
which had been observed in previous pressure tap measurements; they
also showed the way for further optimizing the location of gas
injection into the nozzle. An error analysis revealed that a major
source of error in the PSP results was due to the non-uniform
temperature distribution over the nozzle surface. PSP measurement
results for scramjet nozzle
6. Turbomachinery Application -Transonic compressor: Also an
technique for using PSP in turbomachinery applications is
debeloped. New pressure-and temperature-sensitive paints have been
developed for application to a state-of-the-art transonic
compressor where pressures up to 2 atm and surface temperatures up
to 140C are expected for the first-stage rotor. PSP and TSP data
has been acquired from the suction surface of the first-stage rotor
of a transonic compressor operating at its peak-efficiency
condition. Visual comparison of the final PSP image presented in
Figure and the CFD prediction reveal similar pressure trends.
Steady blade pressure data, normalized with respect to inlet
reference pressure, P/Pref
TURBINE BLADE COOLING
A few studies have been carried out on film cooling of turbine
blades using PSP namely. The film-cooling effectiveness
distributions were measured on the blade tip using the
pressure-sensitive paint technique. PSP is a photoluminescent
material that emits light with intensity proportional to the
surrounding partial pressure of oxygen. Any pressure variation on
the PSP-coated surface causes emitting light intensity to change
because of an oxygen-quenching process. A CCD camera measures this
change of intensity. To measure the film-cooling effectiveness and
to obtain the intensity ratio from PSP, four kinds of images are
required. A reference image (with illumination, no mainstream flow,
surrounding pressure uniform at 1 atm), an air image (with
illumination and mainstream flow, air used as coolant), an
air/nitrogen image (with illumination and mainstream flow, nitrogen
gas used as coolant), and a black image (no illumination and no
mainstream and coolant flow) to remove noise effects due to the
camera.
Another set of experiments on a rotating blade platform using a
pressure sensitive paint for film cooling effectiveness
measurements. The PSP technique for film cooling effectiveness is
based on mass transfer analogy and is free from heat conduction
related errors frequently encountered with other heat transfer
measurement techniques measuring adiabatic effectiveness. A
schematic of the optical components setup for these measurements is
depicted and effectiveness results obtained from using PSP for the
reference rotating condition of 2550 rpm are plotted below.
Distributions of pressure ratio (Pt/P) for plane blade tip (top
row) and squealer blade tip (bottom row) for coolant injection
through tip holes Optical components setup for the model turbine
and PSPThis work provided detailed data on film cooling on a
rotating platform using PSP measurements for the first time in open
literature. Turbine researchers and designers will be better
equipped with knowledge for film cooling under rotating conditions,
by utilizing these results for film cooling.
7. Pressure and Temperature Measurements on the Aft-Body of a
Capsule Re-entry Vehicle
Pressure Sensitive Paint (PSP) and Temperature Sensitive Paint
(TSP) were used tovisualize and quantify the surface interactions
of reaction control system (RCS) jets on the aft body of capsule
reentry vehicle shapes. The first model tested was an Apollo-like
configuration and was used to focus primarily on the effects of the
forward facing roll and yaw jets. The second model tested was an
early Orion Crew Module configuration blowing only out of its
forward-most yaw jet, which was expected to have the most intense
aerodynamic heating augmentation on the model surface. This is
especially true in hypersonic flight conditions for vehicle
concepts such as reentry capsules, where complex flow phenomena
such as flow transition, shock layer interactions, impinging jets,
etc., often occur.
Aft body pieces for the Apollo-era (inset) and Orion-derived
model shapes including various RCS jet configurations.
ADVANTAGE AND DIS-ADVANTAGE OF PSP & TSPADVANTAGE:- Pressure
sensitive paint has numerous advantages over conventional pressure
taps and transducers. The most obvious is that PSP is a field
measurement, allowing for a surface pressure determination over the
entire model, not just at discrete points. Hence, PSP provides a
much greater spatial resolution than pressure taps, and
disturbances in the flow are immediately observable.
PSP also has the advantage of being a non-intrusive technique.
Use of PSP, for the most part, does not affect the flow around the
model, allowing its use over the entire model surface. The use of
PSP eliminates the need for a large number of pressure taps, which
leads to more than one benefit. Since pressure taps do not need to
be installed, models can be constructed in less time, and with less
money than before. Also, since holes do not need to be drilled in
the model for the installation of taps, the model strength is
increased, and higher Reynolds numbers can be obtained. Not only
does the PSP method reduce the cost of the model construction, but
it also reduces the cost of the instrumentation needed for data
collection. In addition, the equipment needed for PSP costs less
than pressure taps, but it can also be easily reused for numerous
models.
In aircraft design, PSP has the potential to save both time and
money. The continuous data distribution on the model provided by
PSP can easily be integrated over specific components, which can
provide detailed surface loads. Since a model for use with the PSP
technique is faster to construct, this allows for load data to be
known much earlier in the design process.
DIS-ADVANTAGE:- One of these characteristics is that the
response of the luminescent molecules in the PSP coating degrades
with time of exposure to the excitation illumination. This
degradation occurs because of a photochemical reaction that occurs
when themolecules are excited. Eventually, this degradation of the
molecules determines the useful life of the PSPcoating. This
characteristic becomes more important for larger models, as the
cost and time of PSP reapplication becomes a significant
factor.
A second undesirable characteristic of PSP is that the emission
intensity is affected by the local temperature. This behavior is
due to the effect temperature has on the energy state of
theluminescent molecules, and the oxygen permeability of the
binder. This temperature dependence becomes even more significant
in compressible flowtests, where the recovery temperature over the
model surface is not uniform.
Dept. of Aeronautical Engineering,DSCE25