Pressure Sensitive Paint Research Articles

Film cooling performance on pressure side of turbine blade with different number of hole rows under rotating state

Film cooling is a major cooling method for turbine blades. Due to short film trajectories on the pressure side, the film coverage could be improved by increasing the number of hole rows. To study the effect of different hole rows on the film cooling performance of pressure side, this paper experimentally measures the film cooling effectiveness (η) of the pressure side with different number of hole rows (3, 4, and 5 rows) by employing pressure-sensitive paint (PSP) technology at a speed of 600 rpm. The effects of mass flow ratio (MFR), blowing ratio (BR), and hole arrangement (in-line and staggered) are analyzed. Numerical simulations are also performed for further analysis of the flow field. Under the mainstream parameters and blade structure in this study, the results show that: the three-row film hole cases have better film cooling performance at lower MFRs because of a larger film coverage; for four-row and five-row film hole cases, a belt region with poor film coverage appears at higher BRs, and the film cooling is influenced by the hole arrangement as well as behaves differently at lower and higher BRs.

Resolving dynamic features of kilohertz pressure fluctuations using fast-responding pressure-sensitive paint: measurement of inclined jet impingement

Resolving dynamic features of kilohertz pressure fluctuations using fast-responding pressure-sensitive paint: measurement of inclined jet impingement

Effects of crossflow-fed-shaped holes on the adiabatic film cooling effectiveness

The effects of internal crossflow-fed-shaped holes on the unsteady behavior of the adiabatic film cooling effectiveness were extensively investigated using high-resolution fast-response pressure-sensitive paint (fast-PSP). During the experiment, coolant (DR ≈ 1.0) was fed into either plenum chamber or crossflow channel at coolant to mainstream velocity ratio of VRc = 0.4-1.6, channel to mainstream velocity ratio of VRch = 0.4-0.7, and channel to coolant (hole-inlet) velocity ratio of VRin = 0.25-1.75. The mean and unsteady effectiveness were presented in the near-hole region (9D), where the significance of the internal crossflow was captured. The results affirmed the asymmetrical coolant deterioration behind the crossflow-fed-shaped holes compared with the plenum feed. The unsteadiness was originated from the hole entrance (i.e., in-hole counter-rotating vortex-pair (CRVP) and the in-hole vortex-tube structures for plenum and crossflow, respectively). The vortex-tube formed a swirl-vortex, which interacted with the CRVP at the windward side, leading to asymmetric CRVP and declined effectiveness. Meanwhile, the coolant spreading behind the crossflow-fed-shaped holes was sensitive to the hole-inlet velocity (VRin), showing symmetric spreading at VRin ≈ 1.0, biased to the windward at VRin < 1.0, and biased to the leeward when VRin > 1.0. The combined results insight our understanding into the coolant unsteadiness of the shaped-holes in-flow conditions, recommends modifying the hole-inlet shape in order to enhance the effectiveness and damp the corresponding unsteadiness.

Influences of groove configuration and density ratio on grooved leading-edge showerhead film cooling using the pressure sensitive paint measurement technique

In response to the cooling requirements of high adiabatic cooling effectiveness and low convective heat transfer coefficient on the outer wall of the showerhead leading-edge of the turbine vane, a groove configuration is set near-stagnation region, and a grooved leading-edge showerhead cooling structure is designed. Using the steady-state pressure-sensitive paint technology as the measurement method, the interference effect of the groove configuration on the leading-edge showerhead film cooling was experimentally investigated. The influences of the depth and width of the groove on the adiabatic cooling effectiveness of the showerhead leading-edge were compared and analyzed in detail. The present study investigated the adiabatic cooling effectiveness of the showerhead leading-edge with counter-inclined film-holes under varied momentum flux ratio MR varies in the wide range of 0.09–6.04. The influence of coolant mass flow rate on the showerhead cooling effectiveness for the original leading-edge with counter-inclined holes was studied thoroughly; that is, the area-averaged adiabatic cooling effectiveness increases first, then decreases, and then increases again as MR increases. The arrangement of the grooves also enables the grooved structure to ensure the trend mentioned above. Compared with the original case, the effects of the groove on the showerhead cooling effectiveness are mainly reflected in the coolant retention within the groove, the jet-out and superimposition of the downstream of the outer rows. As the MR continues to increase, the grooved case is more likely to have a counter-inclined film distribution in advance, and the turning point of the adiabatic cooling effectiveness drops first and then rises earlier than the original case at a higher MR. The film-coverage of the shallow-grooved case is slightly better than that of the deep-grooved case. The widening of the groove width is not conducive to improving the film-coverage of the showerhead leading-edge. On the contrary, the interference of flow within the groove on the outflow causes a negative effect under lower coolant injection rates. The influence of the density ratio on the showerhead cooling effectiveness of the grooved leading-edge is similar to the original case. At the constant MR, the cooling effectiveness increases as the density ratio increases, and the core region of film-coverage downstream is broader and more extended.

Pressure-sensitivity stability of poly[1-(trimethylsilyl)-1-propyne]-based pressure-sensitive paint for low-pressure conditions

Pressure-sensitive paintsusing poly[1-(trimethylsilyl)-1-propyne] (PTMSP) as a binder polymerare capable of capturing subtle pressure fluctuationsunder low-pressure conditions, butthe pressure sensitivity isunstable. The dependence of the pressure sensitivitystability onenvironmental factors and coating parameterswas evaluated under low-pressure conditions. The pressure sensitivity decreased from 56.3to 42.5%/kPa at an extremely low pressureafter60 min, and the stability decreased significantly as thebackground pressure was reduced.An apparent change inthe pressure sensitivities and stability was observed from 278 to 318 K, withoptimal performancebeingobtained at 298 K. A comparison of the pressure sensitivities and emission intensities ofshaded and irradiated samples stored at low pressuresindicated that the excitation light hardly agedthe binders. The pressure sensitivity stability decreased as the luminophore concentration increased, while an increased in the paint thickness improved both the pressure sensitivity and stability.

Effect of Coolant Crossflow on Film Cooling Effectiveness of Diffusion Slot Hole With and Without Ribs

Abstract The coolant flow condition at the film hole entrance has a notable effect on film cooling effectiveness. The previous studies about internal ribbed crossflow effects are the hole with a circular inlet cross section. In this study, a diffusion slot hole with a race-track inlet cross section is examined in a low-speed wind tunnel by the pressure-sensitive paint (PSP) technique. The film cooling effectiveness of three different hole configurations are investigated: a normal diffusion slot hole, a compound angle diffusion slot hole, and a lateral inclination angle diffusion slot hole. Two internal perpendicular crossflow conditions with and without ribs for the 45 deg inline crossflow and the 135 deg counter crossflow are discussed. The blowing ratios are varied from 0.5 to 2.5 at a constant crossflow-to-mainstream velocity ratio of 0.36. A detailed method for the uncertainty analysis of the PSP is also presented. It is found that the normal diffusion slot hole always shows the best cooling performance than other holes. The ribs under the crossflow condition have a positive effect on the film cooling effectiveness for the normal diffusion slot hole even though the compound angle changes the hole’s orientation. However, the ribs have a negative effect on the film cooling effectiveness for the geometrically asymmetric diffusion slot hole.

An experimental and numerical investigation of film cooling effectiveness on a gas turbine blade tip region

In this paper, the effects of coolant flow rate, location of film holes, tip clearance height, and density ratio on blade tip film cooling effectiveness were experimentally investigated using pressure-sensitive paint. And numerical simulation was used to analyze the flow characteristic. Three different turbine blade tips were studied under a mainstream Reynolds number of 6.7 × 105. The pressure ratio was 1.7 and the blowing ratio ranged from 0.4 to 2.2. The results indicated that the cooling effectiveness gradually increased as the blowing ratio increased. The film tends to lift off under a larger blowing ratio, resulting in a decrease in the cooling effectiveness. The squealer tip with an inclined pressure side rim exhibited better film cooling effectiveness under the design conditions. The film cooling effectiveness on the pressure-side rim was lower than that on the suction-side rim under the same operating conditions. The analyses also indicated that an increase in the density ratio effectively enhances film cooling. Compared with the most published investigations conducted in a low-speed wind tunnel, these studies can be better applied to turbine blade cooling design.

Film Cooling Effectiveness Measurement of Fan-Shaped Holes Manufactured Using Electrical Discharge Machining Technique

Abstract In this study, the influence of geometric factors such as hole diameter (D), length-to-diameter ratio (L/D), injection angle (α), and lateral expansion angle (β) on film cooling effectiveness of holes made using electrical discharge machining (EDM) is experimentally investigated. Nine different cooling configurations were tested on a flat plate wind tunnel at various coolant Reynolds number (Rec) and coolant-to-mainstream blowing ratio (M). The considered flat plate model incorporates engine-sized V-shaped holes. EDM reliability is assessed through a hole qualification process, while effectiveness was measured by the pressure-sensitive paint (PSP) technique. Results confirm the suitability of EDM for V-shaped hole manufacturing as long as a correct tolerance on β is prescribed. An accurate qualification of hole morphology is also recommended.

Transonic vane film cooling with crescent-shaped craters using an endoscopic pressure-sensitive paint technique

This study examined the cooling performance of a crescent-shaped crater (CSC) hole design along the pressure side of a transonic vane. The adiabatic effectiveness of the CSC holes and circular holes was measured via an endoscopic pressure-sensitive paint (PSP) technique in a transonic wind tunnel at Ma = 0.84. Coolant flow (CO2) was discharged into the mainstream flow through two rows of holes, with the blowing ratio (M) varying from 0.6 to 1.2. The measured cooling effectiveness of the CSC hole was significantly higher than that of the circular design, demonstrating ∼ 57% and ∼ 38% enhancements in area-averaged effectiveness at M = 0.8 and 1.0, respectively. The physics of the CSC hole design was further explored by numerical simulations, which reveal the generated vortex structures and aerodynamic losses under various conditions. Specifically, the simulations demonstrated the features of anti-counter-rotating vortex pairs for CSC holes, which partially counteracted the detrimental effect of counter-rotating vortex pairs, thereby greatly improving the cooling effectiveness along the vane’s pressure side.

Numerical and Experimental Studies of Transpiration Cooling Film Effectiveness over Porous Materials

Comprehensive experimental and numerical studies were performed to determine cooling film effectiveness of transpiration cooling over porous materials. The cooling film effectiveness was evaluated experimentally using pressure-sensitive paint by invoking heat/mass transfer analogy over the surface of the porous samples. It was found that transpiration cooling can reduce total surface heat flux by two to three times and provide solid surface cooling film effectiveness on average 40% higher than the state-of-the-art multihole effusion cooling. This improvement increases to 200% when high coolant flow rates are used due to effusion cooling film liftoff. Numerical simulations allowed detailed investigations of the flow evolution in the porous media and its ability to create a thermal protection film. Modeling results revealed slight lateral motion of the flow inside the porous media in the same direction as the main flow. This was attributed to the viscous effect of the channel flow and low flow resistance of the porous samples. As a result, a larger portion of coolant exits the porous transpiration cooling surface toward the trailing edge of the test coupon, creating a nonuniform cooling film over the test samples surface. The model also suggested that cooling film effectiveness is a function of the porous media physical properties (permeability and inertia coefficient). In contrast to the reduction in cooling film effectiveness in multihole effusion cooling resulting from cooling film liftoff at high coolant flow rates, this study indicates that high coolant flow rates enhance cooling film protection in transpiration cooling.

Experimental Study of Dynamical Airfoil and Aerodynamic Prediction

Dynamic stall is a critical limiting factor for airfoil aerodynamics and a challenging problem for active flow control. In this experimental study, dynamic stall was measured by high-frequency surface pressure tapes and pressure-sensitive paint (PSP). The influence of the oscillation frequency was examined. Dynamic mode decomposition (DMD) with time-delay embedding was proposed to predict the pressure field on the oscillating airfoil based on scattered pressure measurements. DMD with time-delay embedding was able to reconstruct and predict the dynamic stall based on scattered measurements with much higher accuracy than standard DMD. The reconstruction accuracy of this method increased with the number of delay steps, but this also prolonged the computation time. In summary, using the Koopman operator obtained by DMD with time-delay embedding, the future dynamic pressure on an oscillating airfoil can be accurately predicted. This method provides powerful support for active flow control of dynamic stall.

Time-resolved particle image velocimetry and pressure sensitive paint measurements of afterbody flow dynamics

A pair of counter-rotating vortices is produced in the wake of typical cargo aircraft fuselages due to their ramp shape. With a fast polymer-ceramic pressure sensitive paint (PC-PSP), we measure the global pressure fluctuations at very low velocities (15 to 75 m/s) in a surrogate cylinder with a slanted edge model. Through spectral proper orthogonal decomposition, we observe a family of convective pressure waves with peak energy at frequencies matching the vortex wandering frequencies in this flow.

Spatiotemporal distributions of sweeping jet film cooling with a compact geometry

The spatiotemporal distributions of the coolant coverage behind the 777-shaped hole, sweeping jet (SJ), and compact SJ were quantified comprehensively. Nitrogen gas was selected as the coolant, and its blowing ratio (M) was set from M = 1.0 to 3.0. The fast pressure-sensitive paint technique was applied to measure the instantaneous, mean, and unsteady film cooling effectiveness of the three configurations, and these data are compared side-by-side with the 777-shaped hole. The measured velocity spectra demonstrated a close level of Strouhal number for the SJ (St = 1.6–1.7) and compact SJ (St = 1.5–1.6). Due to the dynamic nature, both the SJ and compact SJ exhibited a highly unstable cooling effectiveness over the surface. Their effectiveness values were found to be lower than the 777 hole at relatively low M, but the compact SJ surpassed it and showed the highest effectiveness (i.e., the best cooling performance) as M ≥ 2.0 due to the widest coolant spreading. Compared with the SJ, the effectiveness of the compact SJ was consistently higher, but its coherence of flow structure was reduced, as revealed by a proper orthogonal decomposition analysis. Further simulations vividly describe the flow structures and oscillating processes inside the sweeping actuators. The SJ with compact geometry exhibited a lower exit momentum and more uniform coolant coverage than the SJ, leading to augmented adiabatic effectiveness.

Experimental comparisons of film cooling performance for the multi-cavity tips at two different tip gaps

The unshrouded blade tip in a gas turbine needs to be cooled down due to the severe heat load. In this paper, multi-cavity tip structures were applied in the tip region to help enhance the cooling performance. Distributions of film cooling effectiveness (η) on multi-cavity tips were measured by pressure sensitive paint (PSP) approach under five different coolant blowing ratios. A turbulence-validated computational prediction was performed to help indicate the flow field of multi-cavity tips gap. Then, impact of tip gap on the multi-cavity tips film cooling performance was studied. Results show that multi-cavity tips mainly enhance the cooling performance at tip mid-portion compared with the classic one-cavity tip, but weaken it near the trailing edge (TE). Increasing the tip gap reduces cooling effectiveness values for most portion of the tip, but increases it near the TE. Lower aerodynamic losses were obtained in multi-cavity tips, especially for the leakage loss region.

Pressure field measurements on large-scale propeller blades using pressure-sensitive paint

Pressure field measurements on large-scale propeller blades using pressure-sensitive paint

Fast PSP measurement of three-dimensional low-frequency unsteadiness in sidewall-confined shock wave/turbulent boundary layer interaction

The presence of sidewalls exerts three-dimensional (3D) effects on shock wave/turbulent boundary layer interaction (SBLI), which results in experimental challenges due to the highly unsteady flow with complex 3D patterns. The low-frequency large-scale flow patterns of a 24° sidewall-confined compression ramp interaction were investigated using fast pressure-sensitive paint. The experiment was conducted in a Mach 2.5 wind tunnel, and the unit Reynolds number was Re = 1.12 × 107/m. The sampling rate of the fast pressure-sensitive paint was 4 kHz, and the spatial resolution was 0.175 mm/pixel. In-situ calibration via several pressure transducers was used to determine the time-resolved global pressure distributions. Correlation analysis and proper orthogonal decomposition analysis were used to extract the large-scale unsteady flow patterns. The time-averaged results provide a global view of the typical flow structure, which consists of primary interaction zone and corner interaction zones. The time-resolved results show the asymmetry of the instantaneous pressure distribution induced by the sidewalls, and spanwise differences of the pressure fluctuation amplitudes in the interaction zones are observed. In addition to the common streamwise breathing mode, an interesting spanwise antisymmetric mode is revealed that provides clear evidence of the strong 3D effects in sidewall-confined SBLI.

Polymerizable Oxygen Probe Derived from Platinum-Based Porphyrins for Oxygen Sensing and Pressure-Sensitive Paints

In this work, a polymerizable oxygen probe (PtTFPP-1MA) derived from platinum(II)-5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)-porphyrin (PtTFPP) was synthesized and characterized. The probe PtTFPP-1MA with a methacrylate unit can be copolymerized with other monomers; herein, we used t-butyl styrene (tBs) as a representative monomer for the oxygen sensor and pressure-sensitive paint (PSP). Three copolymers of poly(tBs-co-(PtTFPP-1MA)) (P1a–c) with different PtTFPP-1MA contents were synthesized. Results showed that the oxygen-sensing ability of these polymers exhibited similar oxygen sensitivity, indicating that the compositions of the probe concentrations in our chosen ranges did not have significant influences on sensing. To determine the properties’ difference of probe between the physical doping approach with the chemical copolymerization method in the polymer matrix, poly(tBs) (P2) without PtTFPP-1MA were prepared. Results showed that the polymers (P1a–c) prepared by the chemical copolymerization method exhibited better performance in oxygen- and pressure-sensing tests. In order to investigate whether steric hindrance of the bulky t-butyl group has a significant effect on sensing, poly(St-co-(PtTFPP-1MA)) (P3) using polystyrene with the t-butyl moiety as the matrix was synthesized for comparison. Results confirmed that P1a with the bulky t-butyl group possesses much better sensing performance than P3. P1a’s pressure sensitivity (0.75%/kPa) at 20 °C is among the highest values in reported PSPs, and its temperature sensitivity (−0.63%/°C) at 100 kPa is among the lowest in reported PSPs. It is expected that the polymerizable oxygen probe PtTFPP-1MA could be widely used in oxygen sensors and PSPs.

Markerless Image Alignment Method for Pressure-Sensitive Paint Image.

We propose a markerless image alignment method for pressure-sensitive paint measurement data replacing the time-consuming conventional alignment method in which the black markers are placed on the model and are detected manually. In the proposed method, feature points are detected by a boundary detection method, in which the PSP boundary is detected using the Moore-Neighbor tracing algorithm. The performance of the proposed method is compared with the conventional method based on black markers, the difference of Gaussian (DoG) detector, and the Hessian corner detector. The results by the proposed method and the DoG detector are equivalent to each other. On the other hand, the performances of the image alignment using the black marker and the Hessian corner detector are slightly worse compared with the DoG and the proposed method. The computational cost of the proposed method is half of that of the DoG method. The proposed method is a promising for the image alignment in the PSP application in the viewpoint of the alignment precision and computational cost.

Correction of self-illumination for pressure and temperature sensitive paints

The present work reports self-illumination correction procedure and results of its experimental testing on a simple model. In the procedure reflectivity value is assessed directly from the set of model images captured at wind-off conditions with different reflecting surfaces covered or removed. The paper covers the theoretical part of the problem and describes exact implementation of a processing sequence, which included image mapping on a 3-D surface, building a mesh of surface elements, evaluation of radiative exchange between elements and calculation of the corrected brightness map.

Pressure-Sensitive Paint Measurement at Low-speeds and in Special Environments

Pressure-Sensitive Paint Measurement at Low-speeds and in Special Environments