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