Pressure-sensitive Paint Measurements Research Articles

Experimental study on heat transfer and throughflow losses characteristics of single/triple-row holes with an engine-representative density ratio

In present study, the heat transfer and the throughflow losses characteristics of single/triple-row of fan-shaped and cylindrical film cooling holes with a fixed outlet width are studied experimentally with an engine-representative density ratio. The pressure-sensitive paint (PSP) measurement technology is utilized to test the film cooling performance. The film cooling holes with the inclination angle α which ranged from 20° to 30° and the diffusion angle β which ranged from 0° to 15° are considered. The heat transfer and flow resistance characteristics of film cooling holes under the engine-representative density ratio, DR, of 1.52 as well as the blowing ratio, BR, which ranged from 0.3 to 2.0 are studied. The critical blowing ratios of the secondary flow detaches from wall surface of each hole shape and the hole shape parameters which acquire the highest span-wise averaged adiabatic film cooling efficiency are obtained. Moreover, the experimental results indicate that, within the scope of the investigative parameters pondered in present study, the discharge coefficient and the film cooling performance of the cylindrical hole are significantly lower than that of the fan-shaped film cooling holes. The coolant jet which ejects from the fan-shaped holes does not blow off from the wall surface when BR ≤ 1.5. However, when BR = 2.0, a part of coolant jet begins to detach from the wall surface. In addition, when BR ≤ 1.5, the fan-shaped film cooling hole with the hole shape parameters of α = 20°, β = 15° generates the highest film cooling effectiveness. However, when BR > 1.5, the highest film cooling effectiveness is obtained by the fan-shaped hole with hole shape parameters of α = 25°, β = 10°. Importantly, the discharge coefficient does not change monotonously with the variation of the hole shape parameters. In addition, the fan-shaped film cooling holes with the hole shape parameters of α = 25°, β = 13° as well as those with α = 30°, β = 10° offer the highest discharge coefficient. Finally, the correlations for the discharge coefficients of different hole shape parameters are developed.

Influence of Coolant Density on Turbine Blade Tip Film Cooling at Transonic Cascade Flow Conditions Using the Pressure-Sensitive Paint Technique

Abstract This work is an experimental study of film cooling effectiveness on a blade tip in a stationary, linear cascade. The cascade is mounted in a blowdown facility with controlled inlet and exit Mach numbers of 0.29 and 0.75, respectively. The freestream turbulence intensity is measured to be 13.5% upstream of the blade’s leading edge. A flat tip design is studied, having a tip gap of 1.6%. The blade tip is designed to have 15 shaped film cooling holes along the near-tip pressure side (PS) surface. Fifteen vertical film cooling holes are placed on the tip near the pressure side. The cooling holes are divided into a two-zone plenum to locally maintain the desired blowing ratios based on the external pressure field. Two coolant injection scenarios are considered by injecting coolant through the tip holes only and both tip and PS surface holes together. The blowing ratio (M) and density ratio (DR) effects are studied by testing at blowing ratios of 0.5, 1.0, and 1.5 and three density ratios of 1.0, 1.5, and 2.0. Three different foreign gases are used to create density ratio effect. Over-tip flow leakage is also studied by measuring the static pressure distributions on the blade tip using the pressure-sensitive paint (PSP) measurement technique. In addition, detailed film cooling effectiveness is acquired to quantify the parametric effect of blowing ratio and density ratio on a plane tip design. Increasing the blowing ratio and density ratio resulted in increased film cooling effectiveness at all injection scenarios. Injecting coolant on the PS and the tip surface also resulted in reduced leakage over the tip. The conclusions from this study will provide the gas turbine designer with additional insight on controlling different parameters and strategically placing the holes during the design process.

Differential luminescent imaging method for pressure field measurement

A differential luminescent imaging method is applied to a pressure-sensitive paint (PSP) measurement to capture a quasi-steady acoustic pressure field. Conventionally, PSP measurements capture an absolute luminescent intensity and convert it to pressure. When the mean component of the absolute luminescent intensity occupies a substantial portion of the dynamic range of the image acquisition device, it leads to a coarse signal resolution. Differential luminescent imaging, utilizing two-color luminescent intensities and a differential amplifier, extracts the fluctuation component from the absolute luminescent intensity. The developed differential luminescent imaging system gives a point-wise pressure measurement. These measurements are combined to produce a spatially resolved image. It is capable of resolving the periodic pressure wave over the acoustic field where the resolution of a conventional PSP measurement falls into one digital count. This comparison to a conventional PSP measurement is given as a reference by which to evaluate the spatiotemporal resolution of the differential luminescent imaging method.

Mechanism of supersonic mixing enhancement by a wall-mounted three-dimensional cavity

The detailed mechanism of supersonic mixing enhancement by a novel device is investigated computationally based on the Navier-Stokes equations. The device is a wall-mounted three-dimensional cavity, several parts of the rear face of which are cut out, and the lower wall of the cut-out part has a slope directed toward the injector from which the injectant is discharged. The flow structures and behaviors observed in the computational results are confirmed in the experimental results. Experimental information about the high-frequency (several tens of kHz) pressure oscillation in the cut-out part is acquired in this study using a pressure-sensitive paint (PSP) responding to rapid change in pressure. The computational results reveal that a hairpin-like vortex is formed inside the cavity. The vortex works on the shear layer across the cavity to deflect it upward. The deflected shear layer shields the injectant from the primary flow, which causes the injectant to penetrate deep into the primary flow. The computational results also clarify that the expansion and compression waves produced in the rectangular part of the cavity are delivered effectively to the injector along the slope of the cut-out part, which is confirmed in the PSP measurement results. The delivered waves actuate the jet issuing from the injector to fluctuate up and down periodically with a large amplitude.

Pressure-sensitive paint measurements with temperature correction on the wing of AGARD-B under transonic flow conditions

This study experimentally examined the flow phenomena and pressure distribution on the wing of Advisory Group for Aerospace Research and Development Model B (AGARD-B) at a Mach number of 0.83. For simultaneous pressure and temperature measurement, a mesoporous pressure-sensitive paint (PSP) sensor was applied to one of the wings of AGARD-B at an angle of attack (AOA) of 0°–8° and a temperature-sensitive paint sensor was applied to the other wing. The collected temperature data were used for pixel-by-pixel temperature correction in a PSP experiment. The pressure distribution obtained on the AGARD-B wing through the PSP experiment with temperature correction agreed well with that obtained through computational flow dynamics (CFD) simulation and accurately indicated the low-pressure regions introduced by the vortex generated from the leading edge at an AOA larger than 4°. The lift coefficient calculated using the PSP data agreed well with that calculated using CFD.

Global Visualization of Compressible Swept Convex-Corner Flow Using Pressure-Sensitive Paint

This study used pressure-sensitive paint (PSP) and determined the surface pressure distributions for a compressible swept convex-corner flow. The freestream Mach numbers were 0.64 and 0.83. The convex-corner angle and swept angle were, respectively, 10–17° and 5–15°. Expansion and compression near the corner apex were clearly visualized. For the test case of shock-induced boundary layer separation, there were greater spanwise pressure gradient and curved shocks. The acquired PSP data agree with the experimental data measured using the Kulite pressure transducers for a subsonic expansion flow. For a transonic expansion flow, the discrepancy was significant. The assumption of a constant recovery factor is not valid in the separation region, and temperature correction for PSP measurements is required.

Luminescent intensity enhancement of pressure-sensitive paint by optimization of mole fraction of oxygen

Pressure-sensitive paint (PSP) relates luminescent output to the partial pressure of oxygen in a gas-dynamic medium. Luminescent intensity enhancement of a PSP is directly related to the improvement of the pressure resolution of a PSP measurement. An analytical model is proposed that gives the optimum mole fraction of oxygen of a gas-dynamic medium for a given PSP. This model also gives the desired PSP sensitivity under a given mole fraction of oxygen. Experimental validation is given and compared to the proposed model. A gas-dynamic medium can be any oxygen-containing gas, and is not restricted to measurements in air. The proposed analytical model is versatile and can be applied to various gas-dynamic mediums for PSP measurements, for instance, cryogenic, microfluidic, and Mars wind tunnel applications.

Global flow visualization of transonic cavity flow with various yaw angles

This study experimentally investigated transonic cavity flows with different length-to-depth ( L/ h) ratios and yaw angles. Two rectangular models with L/ h = 6.14 and 21.5 were examined with yaw angles of 10°, 30°, and 45° under a flow of Mach 0.83. The flow was visualized using pressure-sensitive paint (PSP) to obtain the detailed pressure distribution inside the cavity models. The acquired PSP data were compared with experimental data measured using Kulite transducers, and these data showed favorable agreement. Gradual pressure increases inside the cavity model with L/ h = 6.14 were observed from the PSP measurements as open cavity flow. The flow impingement at the bottom of the cavity and the significant pressure rise inside the cavity model with L/ h = 21.5 were observed as closed cavity flow. The present study quantitatively visualized the evolution of the pressure distribution from symmetric to asymmetric for different yaw angles using porous PSP sensors.

Scaling Considerations for Thermal and Pressure-Sensitive Paint Methods Used to Determine Adiabatic Effectiveness

Abstract To be representative of engine conditions, a measurement of film cooling behavior with an experiment must involve matching certain nondimensional parameters, such as freestream Reynolds number. However, the coolant flowrate must also be scaled between the experiments and engine conditions to accurately predict film cooling effectiveness. This process is complicated by gas property variation with temperature. Additionally, selection of the appropriate coolant flowrate parameter to scale from low to high temperatures is a topic of continued uncertainty. Furthermore, experiments are commonly conducted using thermal measurement techniques with infrared thermography (IR), but the use of pressure-sensitive paints (PSPs) implementing the heat-mass transfer analogy is also common. Thus, the question arises of how the adiabatic effectiveness distributions compare between mass transfer experimental methods and thermal experimental methods and whether these two methods are sensitive to coolant flowrate parameters in different ways. In this study, a thermal technique with IR was compared with a heat-mass transfer method with a PSP on a flat plate model with a 7-7-7 film cooling hole. While adiabatic effectiveness is best scaled by accounting for specific heats with the advective capacity ratio (ACR) using thermal techniques, results revealed that PSP measurements are scaled best with the mass flux ratio (M). The difference in these methods has significant implications for engine designers that rely on PSP experimental data to predict engine thermal behavior as PSP is fundamentally not sensitive to the same relevant physical mechanisms to which thermal methods are sensitive.

Sub-millimeter resolution pressure measurement on free flight model at Mach 1.5 using novel non-intrusive optical technique

A motion-capturing pressure sensitive paint (PSP) system is developed and demonstrated by collecting transient pressure data on the surface of a 30 mm diameter sphere model in free flight at Mach 1.5, where a conventional pressure tap is unable to be mounted. The developed non-intrusive measurement technique is able to cancel out changes in signal intensity due to variations in illumination during free flight. Images of the sphere model are collected using a high-speed camera at a rate of 100 kHz with an exposure time of 3.75 μs. Motion blur is estimated as 6.5% of the sphere diameter. The sphere is fully in view for 7 frames, or 70 μs. Transient data from each frame is collected to produce a free flight pressure map on the surface. In order to validate the measurement, a time-averaged CFD simulation of a sphere model at Mach 1.5 is run using far field pressure boundary conditions and a mesh of 6.96 million cells. An uncertainty analysis is performed using the signal to noise ratio of the PSP measurement. The PSP measurement is in agreement with the CFD simulation quantitatively across a centerline pressure trace. The accuracy of the PSP measurement compared to the CFD simulation was within 7.4 kPa.

Uncertainty analysis of motion-capturing pressure-sensitive paint method based on unsteady surface-pressure measurement on fluttering airfoil

Motion-capturing pressure-sensitive paint (PSP) methods give a pressure map over an unsteady aerodynamic surface. To collect a quantitative pressure measurement using this method, unsteady analysis is important. Due to the unsteady motion of an aerodynamic surface, the luminescent output acquired by a stationary imaging device is a time-dependent value. This creates a time-dependent change in signal-to-noise ratio. An uncertainty analysis is given based on the signal-to-noise ratio of the luminescent output. Unsteady surface pressure over a high aspect-ratio (YXX) wing with 11.5% thickness ratio at 30% chord is considered for analysis. Under Mach 0.8 flow, the YXX wing flutters at 13.7 Hz with a shock oscillation over the surface. With the proposed uncertainty analysis, the overall pressure uncertainty, based off of traditional unsteady pressure taps, of the PSP measurement is given as 3.5 [%]. The measurement region that gives the high signal-to-noise ratio had uncertainty as low as 2.9 [%].

A two-dimensional temperature correction method for pressure-sensitive paint measurement on helicopter rotor blades

Pressure-sensitive paint (PSP) has become an important technique for global pressure measurement on rotating surfaces such as helicopter rotor blades. Due to the temperature sensitivity of PSP, it is essential to apply accurate temperature correction method in PSP measurements. In previous studies, one-dimensional (1D) correction methods were generally adopted which only considered the radial temperature gradient on the blade. However, in the current study, it was found that the systematic error of this 1D assumption was non-negligible in large-scale experiments due to the growing chordwise temperature gradient. Accordingly, a two-dimensional (2D) temperature correction method considering both radial and chordwise temperature gradients was developed to improve the accuracy of PSP. In this method, a 2D temperature map was firstly generated based on the TSP results. Calibrations were then conducted under a series of temperature and pressure values to generate a calibration surface. According to the calibration results, the pressure and temperature fields on the rotor blade were solved directly. The proposed 2D method was validated through PSP measurements on a 2 m diameter BO-105 helicopter rotor in hover. Compared with 1D temperature correction, the results from the current method showed clearly improved agreement with the CFD results and reduced overall measurement error.

Design of Real-time Image Processing System for Measurement and Calibration of PSP

The measurement of surface pressure in wind tunnel or in flight tests gives very important data for aerodynamics analysis. In fact, the pressure measurement gives valuable information about air flow phenomena such as shock, flow separation and boundary layer transition. In this paper a real-time image processing system for pressure sensitive paint (PSP) measurement and calibration has been designed using a calibrated pressure sensor. In the implementation of the system, TCL paper is used to luminaphore inside a windowed enclosure and a factory-calibrated pressure sensor for validation of measurement. The pressure measurement of PSP is based on the images acquired from a CCD camera sent to the MyRIO processor module in which the real-time image processing has been applied. The results are compared with the calibrated sensor from which the A and B coefficients of the Estern Volmer Equation have been calculated. During the several tests done for the pressure measurement, the accuracy of the PSP measurement has been calculated in this paper. Both normal and real-time image processing analysis have been compared as well.

Quantification of Airfoil Aerodynamic Uncertainty due to Pressure-Sensitive Paint Thickness

In this paper, uncertainty quantification is used to investigate the propagation of the uncertainty of the pressure-sensitive paint (PSP) thickness distribution to the uncertainty of aerodynamic force measured. Specifically, airfoil surface pressure coefficient () and aerodynamic lift coefficient () of a natural laminar flow wing are analyzed with uncertain PSP thickness distribution. The airfoil with PSP applied is parameterized by using a novel parameterization method based on radial basis function interpolation. As a characteristic of wing aerodynamic performance, is determined by surface pressure distribution, which can be affected by uncertainties of PSP binder thickness. A kriging response surface method has been used to develop a surrogate model to represent the Reynolds-averaged Navier–Stokes solution of the flow around the airfoil with PSP. This enables the use of Monte Carlo simulations to obtain stochastic output, including the probability of the occurrence of significantly inaccurate output values affected by the inputs subjecting to Gaussian distributions. An error margin and associated probability for its occurrence are recommended to account for uncertainties for the aerodynamic force occurring in the PSP measurement.

Spectral signal quality of fast pressure sensitive paint measurements in turbulent shock-wave/boundary layer interactions

This study presents a critical investigation of the signal quality obtained using candidate fast pressure sensitive paint measurements in realistic, spectrally complex flows. The evaluations focus on characterizing the importance of the paint layer frequency response in conjunction with the fundamental signal-to-noise ratio limitations of the imaging system. Fast fluoro-isopropyl-butyl polymer-based paint is used primarily because of an identified characteristic attenuation pattern. Complementary high-speed pressure sensitive paint and wall static pressure measurements are performed in a turbulent, separated, scooped compression ramp shock-wave/boundary layer interaction at Mach 2.5 using a sampling rate of 4 kHz. The results reveal a systematic underprediction of the RMS pressure due to the limited frequency response of the paint. A simple frequency domain correction methodology utilizing the experimental data and a diffusion-based model of the paint response are employed to compensate for these effects. A method for determining the spectral signal-to-noise ratio of the imaging system is then presented, which is found to impose a major constraint on the upper limit of the resolvable pressure frequency. The correction is observed to provide a significant improvement in the agreement between the pressure sensitive paint and transducers, but only below the noise-dominated cutoff frequency. The relative significance of these factors is then investigated for polymer–ceramic-based, 8 kHz measurements of a planar compression ramp shock-wave/boundary layer interaction in a rectangular channel.

Film Cooling on the Geometrically Modified Trailing Edge Model of Gas Turbine Blade

The objective of this study is to examine the effect of the geometrical modification of the land on the overall film cooling effectiveness on the cutback region of a turbine blade model. A room-temperature experiment was conducted, in which nitrogen serves as the cooling stream, and the mainstream flow is air. The adiabatic film cooling effectiveness was mapped employing the pressure-sensitive paint (PSP) technique. Data was acquired at five different blowing ratios (from 0.45 to 1.65) for both the baseline and the modified model. Detailed film cooling effectiveness from PSP measurements in correlation with the flow map in streamwise and spanwise planes from particle image velocimetry (PIV) measurements was performed, characterizing the effect of rounding the edges of the lands. The results show that the rounded edges enable the coolant flow to reach the top surface of the land area more readily, especially at low blowing ratios. Superior coolant coverage on the land surface observed in the PSP measurements are well correlated with the PIV measurements. At the high blowing ratio of 1.65, the round edge of the lands helps regulate the mixing between the coolant and mainstream flows, therefore the film cooling effectiveness in the slot region is also improved.

Pressure-sensitive paint with imprinted pattern for full-field endoscopic measurement using a color camera

An endoscopic system for pressure-sensitive paint (PSP) measurement was presented in the current study, which was composed of a fiberscope, a diode laser and a color camera. The fiberscope was integrated by optical fibers (for transmitting the laser excitation) and a set of lenses (for delivering the PSP images to the camera). Here, a unique PSP with imprinted pattern was developed to address the key issue of limited viewing angle/area encountered during endoscopic measurement. This coating was based on a pressure-sensing luminophore PtTFPP and a recently developed mesoporous-particle binder, but with the addition of a pressure-insensitive reference dye (coumarin 6) forming a random dot pattern. The emission of PtTFPP and reference dye matched with the red and green channels of the color camera, respectively. The randomly distributed dots provided highly distinguishable features required for image stitching operation. This allowed the establishment of image processing algorithms that could combine a series of PSP images (scanned over the measurement field) into a full-field image. Meanwhile, the issue of spectral leakage from the emission of reference dye into the red channel was identified, and a correction method was proposed to remove the related measurement error. This PSP along with the endoscopic system was demonstrated in film cooling measurement on a blade in low-speed flow. The current method provides a practical solution for PSP applications in test environments with limited or even no optical access.

Turbine Vane Endwall Film Cooling Comparison From Five Film-Hole Design Patterns and Three Upstream Injection Angles

Abstract The effects of upstream injection angle on film cooling effectiveness of a turbine vane end wall with various endwall film-hole designs were examined by applying pressure-sensitive paint (PSP) measurement technique. As the leakage flow from the slot between the combustor and the turbine vane is not considered an active source to protect the vane endwall in certain engine designs, discrete cylindrical holes are implemented near the slot to create an additional controllable upstream film to cool the vane end wall. Three potential injection angles were studied: 30 deg, 40 deg, and 50 deg. To explore the optimum endwall cooling design, five different film-hole patterns were tested: axial row, cross row, cluster, midchord row, and downstream row. Experiments were conducted in a four-passage linear cascade facility in a blowdown wind tunnel at the exit isentropic Mach number of 0.5 corresponding to inlet Reynolds number of 380,000 based on turbine vane axial chord length. A freestream turbulence intensity of 19% with an integral length scale of 1.7 cm was generated at the cascade inlet plane. Detailed film cooling effectiveness for each design was analyzed and compared at the design operation conditions (coolant mass flow ratio (MFR) 1% and density ratio 1.5). The results are presented in terms of high-fidelity film effectiveness contours and laterally (spanwise) averaged effectiveness. This paper will provide the gas turbine designers valuable information on how to select the best endwall cooling pattern with minimum cooling air consumption over a range of upstream injection angle.

Development and Testing of Data Reduction Software for Measurements Using Pressure Sensitive Paints

Abstract The paper concentrates on post-processing of data necessary for pressure measurements using Pressure Sensitive Paints (PSP). The purpose of the study was to develop and test procedures for extraction of the surface pressure distribution from the images captured during PSP tests. The core issues addressed were reduction of the influence of model movement and deformation during wind tunnel run and synchronization between conventional pressure tap measurements and PSP data, necessary for in-situ calibration. In the course of the studies, two approaches on image registration were proposed: the first based on geometric transformation of control points pairs with cross-correlation tuning and the second based on similarity finding and estimation of geometric transformation of the images. Performance of the developed algorithm was tested with use of experimental set-up allowing for controlled movement of the imagined target with micrometer resolution. Both of the proposed approaches to PSP image resection proved to perform well. After testing of the software, the PSP system was used for determination of the pressure field on flat plate exposed to impinging jet. The presented procedures and results can be useful for research groups developing in-house PSP measurements systems for wind tunnel tests and internal flow investigations.

Detection of small-amplitude periodic surface pressure fluctuation by pressure-sensitive paint measurements using frequency-domain methods

Image measurement using pressure-sensitive paint (PSP) is an effective tool for analyzing the unsteady pressure field on the surface of a body in a low-speed air flow, which is associated with wind noise. In this study, the surface pressure fluctuation due to the tonal trailing edge (TE) noise for a two-dimensional NACA 0012 airfoil was quantitatively detected using a porous anodized aluminum PSP (AA-PSP). The emission from the PSP upon illumination by a blue laser diode was captured using a 12-bit high-speed complementary metal-oxide-semiconductor (CMOS) camera. The intensities of the captured images were converted to pressures using a standard intensity-based method. Three image-processing methods based on the fast Fourier transform (FFT) were tested to determine their efficiency in improving the signal-to-noise ratio (SNR) of the unsteady PSP data. In addition to two fundamental FFT techniques (the full data and ensemble averaging FFTs), a technique using the coherent output power (COP), which involves the cross correlation between the PSP data and the signal measured using a pointwise sound-level meter, was tested. Preliminary tests indicated that random photon shot noise dominates the intensity fluctuations in the captured PSP emissions above 200 Hz. Pressure fluctuations associated with the TE noise, whose dominant frequency is approximately 940 Hz, were successfully measured by analyzing 40,960 sequential PSP images recorded at 10 kfps. Quantitative validation using the power spectrum indicates that the COP technique is the most effective method of identification of the pressure fluctuation directly related to TE noise. It is possible to distinguish power differences with a resolution of 10 Pa $$^2$$ (4 Pa in amplitude) when the COP was employed without use of another wind-off data. This resolution cannot be achieved by the ensemble averaging FFT because of an insufficient elimination of the background noise.