Pressure Sensitive Paint Technique Research Articles

Experimental investigation of pressure distribution characteristics of a flying-wing model using PSP

To investigate the static pressure distribution characteristics of a flying-wing model, an advanced binary pressure sensitive paint (PSP) technique is introduced. It has low-temperature sensitivity and can compensate the errors induced by temperature. The pressure measurement test was performed in 0.6 m trisonic wind tunnel at angles of attack ranging from 0[Formula: see text] to 12[Formula: see text] in supersonic condition, adopting a low-aspect-ratio flying wing model. The binary PSP is sprayed on the upper surface of the model while pressure taps are installed on the upper surface of the right wing. Luminescent images of two probes are acquired with a color charge-coupled-device camera system and processed with calibration results. During the test, the surface pressure is measured by PSP and transducer, respectively. The results obtained show that the binary paint is of advantage to the surface pressure measurement and flow characteristic analysis. The high-resolution pressure spectra at different angle of attack clearly reveal the impact of leading edge vortex on the upper surface pressure distributions. The pressure measured by PSP also agrees well with the pressure tap results. The root mean square error of pressure coefficient is 0.01 at [Formula: see text], [Formula: see text].

A Parametric Investigation of Vane Showerhead Film Cooling by Pressure-Sensitive Paint Technique

Abstract In this study, a parametric analysis of the thermal performance of a nozzle vane cascade with a showerhead cooling system made of four rows of cylindrical holes was carried out by using the pressure-sensitive paint (PSP) technique. Coolant-to-mainstream blowing ratio (BR), density ratio (DR), main flow isentropic exit Mach number (Ma2is), and turbulence intensity level (Tu1) were the considered parameters. The cascade was tested in an atmospheric wind tunnel at Ma2is values ranging from 0.2 to 0.6, with an inlet turbulence intensity level of 1.6% and 9%, at variable injection conditions of BR = 2.0, 3.0, 4.0. Moreover, the influence of the DR on the leading-edge film-cooling performance was investigated: the testing was carried out at DR = 1.0, using nitrogen as foreign gas, and DR = 1.5, with carbon dioxide serving as a coolant. In the near-hole region, higher BR and Ma2is resulted in higher effectiveness, while higher mainstream turbulence intensity reduced the thermal coverage in between the rows of holes, whatever the BR is. Further downstream along the vane pressure side, the effectiveness was negatively affected by rising the BR but positively influenced by lowering the mainstream turbulence intensity. Moreover, a decrease in the DR caused a reduction in the film-cooling performance, whose extent depends on the injection condition.

Turbine Vane Endwall Film Cooling Effectiveness of Different Purge Slot Configurations in a Linear Cascade

Abstract This study is concerned with the film cooling effectiveness of the flow issuing from the gap between the nozzle guide vane (NGV) and the transition duct on the NGV endwall, i.e., the purge slot. Different slot widths, positions, and injection angles were examined in order to represent changes due to thermal expansion as well as design modifications. Apart from these geometric variations, different blowing ratios (BRs) and density ratios (DRs) were realized to investigate the effects of the interaction between secondary flow and film cooling effectiveness. The experimental tests were performed in a linear scale-1 cascade equipped with four highly loaded turbine vanes at the Institute of Fluid Mechanics and Fluid Machinery of the University of Kaiserslautern. The mainstream flow parameters were, with a Reynolds number of 300,000 and a Mach number (outlet) of 0.6, set to meet real engine conditions. By using various flow conditioners, periodic flow was obtained in the region of interest (ROI). The adiabatic film cooling effectiveness was determined using the pressure sensitive paint (PSP) technique. In this context, nitrogen and carbon dioxide were used as tracer gases realizing two different density ratios DR = 1.0 and 1.6. The investigation was conducted for a broad range of blowing ratios with 0.25 ≤ BR ≤ 1.50. In combination with 10 geometry variations and the aforementioned blowing and density ratio variations, 100 single operating points were investigated. For a better understanding of the coolant distribution, the secondary flows on the endwall were visualized by oil dye. The measurement results will be discussed based on the areal distribution of film cooling effectiveness, its lateral spanwise, as well as its area average. The results will provide a better insight into various parametric effects of gap variations on turbine vane endwall film cooling performance—notably under realistic engine conditions.

Experimental investigation of film cooling effectiveness on a gas turbine blade pressure surface with diffusion slot holes

The film cooling effectiveness of diffusion slot hole was experimentally examined using pressure sensitive paint technique at a low-speed linear cascade. As a comparison, the film cooling effectiveness of fan-shaped hole was also measured. The mainstream Mach number at the cascade inlet and outlet was 0.1 and 0.19, respectively. The effects of blowing ratio, density ratio and hole location on the film cooling effectiveness were investigated. The experimental results demonstrated that the film cooling effectiveness of diffusion slot hole is obviously higher than that of fan-shaped hole. The difference of area-averaged effectiveness from surface arc length-to-hole diameter ratio of S/D = 1 to S/D = 30 between the two film hole schemes is maximum at first row, and the absolute increment is over 0.2 when the blowing ratio is larger than 1.0. The increase of blowing ratio elevates the film cooling effectiveness of the diffusion slot hole. Generally, the farther the film hole away from the blade leading edge, the higher the film cooling effectiveness. Meanwhile, the diffusion slot hole shows less sensitivity to film hole location compared to fan-shaped hole. In addition, the elevating density ratio leads to a positive influence on the film cooling effectiveness of diffusion slot hole.

Uncertainty Quantification of Film Cooling Performance of an Industrial Gas Turbine Vane.

Computational Fluid Dynamics (CFD) results are often presented in a deterministic way despite the uncertainties related to boundary conditions, numerical modelling, and discretization error. Uncertainty quantification is the field studying how these phenomena affect the numerical result. With these methods, the results obtained are directly comparable with the experimental ones, for which the uncertainty related to the measurement is always shown. This work presents an uncertainty quantification approach applied to CFD: the test case consists of an industrial prismatic gas turbine vane with standard film cooling shaped holes system on the suction side only. The vane was subject of a previous experimental test campaign which had the objective to evaluate the film cooling effectiveness through pressure-sensitive paint technique. CFD analyses are conducted coherently with the experiments: the analogy between heat and mass transfer is adopted to draw out the adiabatic film effectiveness, solving an additional transport equation to track the concentration of CO2 used as a coolant fluid. Both steady and unsteady simulations are carried out: the first one using a RANS approach with k-ω SST turbulence model the latter using a hybrid LES-RANS approach. Regarding uncertainty quantification, three geometrical input parameters are chosen: the hole dimension, the streamwise inclination angle of the holes, and the inlet fillet radius of the holes. Polynomial-chaos approach in conjunction with the probabilistic collocation method is used for the analysis: a first-order polynomial approximation was adopted which required eight evaluations only. RANS approach is used for the uncertainty quantification analysis in order to reduce the computational cost. Results show the confidence interval for the analysis as well as the probabilistic output. Moreover, a sensitivity analysis through Sobol’s indices was carried out which prove how these input parameters contribute to the film cooling effectiveness, in particular, when dealing with the additive manufacturing process.

Experimental optimization of a fan-shaped film cooling hole with 30 degrees-injection angle and 6-hole length-to-diameter ratio

In this study, the effect of the shape parameters of fan-shaped holes on film cooling effectiveness was experimentally investigated and the optimum values of each shape parameter to maximize the overall averaged film cooling effectiveness were presented. Among the shape parameters of a fan-shape hole, the injection angle and the hole length to diameter ratio (L/D) were fixed as 30 degrees and 6, respectively, and the three shape parameters, the forward expansion angle, the lateral expansion angle, and the metering length ratio, were selected as variables. The design points of each parameter were selected using the Box-Behnken design method, and the optimized values of each parameter were obtained using RSM (Response Surface Methodology). The film cooling effectiveness was measured using the PSP (pressure sensitive paint) technique. The experiment was conducted under the condition of two density ratios (1.0 and 2.0) and four blowing ratios (1.0, 1.5, 2.0, and 2.5). Results showed that the film cooling effectiveness was generally higher for the higher density ratio condition, but the effect of the blowing ratio was not consistent. The main effect analysis showed that film cooling effectiveness increased as the forward and the lateral expansion angles increased, and the metering length became shorter. However, optimization results showed that the optimized hole had a smaller forward expansion angle due to the interaction effect between each shape parameter. The film cooling effectiveness of the optimized hole was higher than that of the reference hole by 52.4% near the gas turbine operating condition.

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.

Flat Plate and Turbine Vane Film-Cooling Performance with Laid-Back Fan-Shaped Holes

Shaped holes are considered as an effective solution to enhance gas turbine film-cooling performance, as they allow to increase the coolant mass-flux, while limiting the detrimental lift-off phenomena. A great amount of work has been carried out in past years on basic flat plate configurations while a reduced number of experimental works deals with a quantitative assessment of the influence of curvature and vane pressure gradient. In the present work PSP (Pressure Sensitive Paint) technique is used to detail the adiabatic effectiveness generated by axial shaped holes with high value of Area Ratio close to 7, in three different configurations with the same 1:1 scale: first of all, a flat plate configuration is examined; after that, the film-cooled pressure and suction sides of a turbine vane model are investigated. Tests were performed varying the blowing ratio and imposing a density ratio of 2.5 . The experimental results are finally compared to the predictions of two different correlations, developed for flat plate configurations.

An Experimental Investigation of Adiabatic Film Cooling Effectiveness and Heat Transfer Coefficient on a Transonic Squealer Tip

The effect of film cooling on a transonic squealer tip has been examined in a high speed linear cascade, which operates at engine-realistic Mach and Reynolds numbers. Tests have been performed on two uncooled tip geometries with differing pressure side rim edge radii, and a cooled tip matching one of the uncooled cases. The pressure sensitive paint technique has been used to measure adiabatic film cooling effectiveness on the blade tip at a range of tip gaps and coolant mass flow rates. Complementary tip heat transfer coefficients have been measured using transient infrared thermography, and the effects of the coolant film on the tip heat transfer and engine heat flux were examined. The uncooled data show that the tip heat transfer coefficient distribution is governed by the nature of flow reattachments and impingements. The squealer tip can be broken down into three regions, each exhibiting a distinct response to a change in the tip gap, depending on the local behavior of the overtip leakage flow. Complementary computational fluid dynamics (CFD) shows that the addition of casing motion causes no change in the flow over the pressure side rim. Injected coolant interacts with the overtip leakage flow, which can locally enhance the tip heat transfer coefficient. The film effectiveness is dependent on both the coolant mass flow rate and tip clearance. At increased coolant mass flow, areas of high film effectiveness on the pressure side rim coincide strongly with a net heat flux reduction and in the subsonic tip region with low heat transfer coefficient.

Overall effectiveness of film-cooled leading edge model with normal and tangential impinging jets

This paper investigates the overall effectiveness of film-cooled leading edge model with normal and tangential impinging jets. The leading edge has three rows of cylindrical film cooling holes, located at the stagnation line (0°) and ± 40° measured from the stagnation line. Film cooling holes are at an inclined angle of 25° relatives to the surface. The normal jet has one row of normal jet impinging holes and the tangential jet has two rows of tangential jet impinging holes. Mainstream Reynolds number based on the leading edge cylinder external diameter is about 100,000, and the mainstream turbulence intensity is about 7%. Three blowing ratios of 0.77, 1.54, and 2.31 are considered. Pressure sensitive paint technique is applied to measure the adiabatic effectiveness with the density ratio of 0.97. Liquid crystal technique is used to obtain the overall effectiveness with the density ratio of 0.94. Numerical simulations using realizable k-ɛ (RKE) turbulence model with enhanced wall treatment were applied to calculate the adiabatic effectiveness and overall effectiveness. Compared with experiments, numerical simulations overpredict overall effectiveness about 20% to 30% between the 0° row and the 40° row, and the difference between simulations and experiments is about 10% to 20% after the 40° row. The results provide baseline information for turbine blade leading edge film cooling design and heat transfer analysis.

Numerical evaluation of showerhead film cooling aerothermal performance on a first-stage vane

A wide set of experimental data has been used to validate numerical simulations of leading edge film cooling in a high-pressure turbine vane, according with steady-state RANS and delayed detached-eddy simulation (DDES) modelling. A typical showerhead configuration was adopted, namely 4 staggered rows of cylindrical holes. Boundary conditions were fully based on the wind tunnel tests: low speed flow (exit isentropic Mach number of Ma2is = 0.2), low inlet turbulence intensity of Tu1 = 1.6% and low density ratio of DR ∼ 1 were dictated by constraints relating to Particle Image Velocimetry (PIV) setup. One only blowing ratio of BR = 3.0 was taken into account since it is representative of effective thermal protection, while offering a challenging scenario of jet separation and reattachment on a curved surface. Predicted off-the-wall aerodynamic features of the flow in the leading edge region were compared against PIV data and flow visualizations. Moreover, contours of adiabatic effectiveness, measured with the pressure sensitive paint (PSP) technique by injecting nitrogen as coolant (DR ∼ 1), were computed by means of the species transport model (i.e. mass transfer (MT)), in addition to the conventional thermal method (i.e. heat transfer (HT)). The MT approach yielded more accurate predictions of adiabatic effectiveness (η) than the alternative HT method. Therefore it can be considered particularly suited to the validation of the PSP experimental results. DDES was indispensable to capture the highly unsteady, anisotropic mixing of the cooling jets with the mainstream, despite overestimation of the jet vertical penetration, and hence slight underprediction of η outside the showerhead region.

Optimization of the configuration of the laidback fan-shaped film cooling hole with a lateral expansion angle of 10 degrees

Film cooling techniques have been widely applied to gas turbine engines to protect the components from hot combustion gas. In this study, the shape of the laidback fan-shaped hole with a lateral expansion angle of 10 degrees was optimized through the Reynolds Averaged Navier-Stokes (RANS) analysis. Results by numerical analysis were validated with the corresponding experimental results obtained by the pressure sensitive paint technique (PSP). For optimal hole shape derivation, the design points were selected through the Latin Hypercube Sampling (LHS) method for three design variables: injection angle, metering length, and forward expansion angle. The overall averaged effectiveness was used as the objective function. Results showed that the overall film cooling effectiveness of the optimal holes derived by the response surface method and the Kriging method were higher than that of the reference hole by 4.5% and 7.5%, respectively. Generally, holes with a higher area ratio (AR) showed higher overall film cooling effectiveness. However, the interaction between each shape parameter should be considered to derive the optimal hole shape.

Film Cooling Effectiveness From Two Rows of Compound Angled Cylindrical Holes Using Pressure-Sensitive Paint Technique

This study investigates the effects of blowing ratio, density ratio, and spanwise pitch on the flat plate film cooling from two rows of compound angled cylindrical holes. Two arrangements of two-row compound angled cylindrical holes are tested: (a) the first row and the second row are oriented in staggered and same compound angled direction (β = +45 deg for the first row and +45 deg for the second row); (b) the first row and the second row are oriented in inline and opposite direction (β = +45 deg for the first row and −45 deg for the second row). The cooling hole is 4 mm in diameter with an inclined angle of 30 deg. The streamwise row-to-row spacing is fixed at 3d, and the spanwise hole-to-hole (p) is varying from 4d, 6d to 8d for both designs. The film cooling effectiveness measurements were performed in a low-speed wind tunnel in which the turbulence intensity is kept at 6%. There are 36 cases for each design including four blowing ratios (M = 0.5, 1.0, 1.5, and 2.0), three density ratios (DR = 1.0, 1.5, and 2.0), and three hole-to-hole spacing (p/d = 4, 6, and 8). The detailed film cooling effectiveness distributions were obtained by using the steady-state pressure-sensitive paint (PSP) technique. The spanwise-averaged cooling effectiveness are compared over the range of flow parameters. Some interesting observations are discovered including blowing ratio effect strongly depending on geometric design; staggered arrangement of the hole with same orientation does not yield better effectiveness at higher blowing ratio. Currently, film cooling effectiveness correlation of two-row compound angled cylindrical holes is not available, so this study developed the correlations for the inline arrangement of holes with opposing angles and the staggered arrangement of holes with same angles. The results and correlations are expected to provide useful information for the two-row flat plate film cooling analysis.

Heat Transfer Coefficient and Film Cooling Effectiveness on the Partial Cavity Tip of a Gas Turbine Blade

Leakage flow between the rotating turbine blade tip and the fixed casing causes high heat loads and thermal stress on the tip and near the tip region. For this study, new squealer tips called partial cavity tips, which combine the advantages of plane and squealer tips, were suggested, and the effects of the cavity shape on the tip heat transfer coefficient and film cooling effectiveness were investigated experimentally in a low-speed linear cascade. The suggested blade tips had a flat surface near the leading edge and a squealer cavity from the mid-chord to trailing edge region to achieve the advantages of both blade tip types. The heat transfer coefficient was measured via the 1-D transient heat transfer technique using an IR camera, and the film cooling effectiveness was obtained via the pressure-sensitive paint (PSP) technique. Results showed that the heat transfer coefficient and film cooling effectiveness on the partial cavity tips strongly depended on the cavity shape. Near the leading edge, the heat transfer coefficients for the partial cavity tip cases were lower than that for the squealer tip case. However, the heat transfer coefficient on the cavity surface was higher for the partial cavity tip cases. The D10 tip showed a similar distribution of film cooling effectiveness to that of the plane (PLN) tip near the leading edge and the double side squealer (DSS) tip near the mid-chord region. However, the overall average film cooling effectiveness of the DSS tip was higher than that of the D10 tip.

Combined effects of unsteady wake and free-stream turbulence on turbine blade film cooling with laid-back fan-shaped holes using PSP technique

Detailed film cooling effectiveness distribution for a gas turbine blade under the effects of unsteady wakes and oncoming free-stream turbulence intensities was obtained using pressure sensitive paint (PSP) technique. Tests were performed on a linear cascade at Reynolds number of 3.85 × 105 based on the blade chord at cascade exit. Upstream unsteady wakes were simulated using a spoke-wheel type wake generator. The test blade has three rows of compound angled cylindrical film holes at the leading edge, five rows of laid-back fan-shaped holes on the pressure surface and three rows of laid-back fan-shaped holes on the suction surface. The wake Strouhal number was varied from 0 to 0.36 and three mass flux ratios were determined. The oncoming free-stream turbulence intensities are 2.7% and 26.9%, respectively. Results show that the effect of the mass flux ratio on the film cooling effectiveness decreases under the high turbulence intensity and unsteady wake conditions. In most regions of the blade surface, the film cooling effectiveness decreases with the increase of wake Strouhal number, and the free-stream turbulence superimposed on the unsteady wake reduces the film cooling effectiveness further. The effect of the unsteady wake decreases under the high free-stream turbulence conditions.

Unsteady analysis of adiabatic film cooling effectiveness for discrete hole with oscillating mainstream flow

The unsteady behaviors of adiabatic effectiveness under oscillating flow conditions were studied extensively using the fast-response pressure-sensitive paint technique. Nitrogen was used as the coolant and was discharged from a 35°-inclined round hole. The oscillation frequencies of the mainstream were set to 0, 5, 10, and 20 Hz. The unsteady features of the measured effectiveness were quantified in terms of standard deviations, cross correlations, proper orthogonal decomposition (POD), and dynamic mode decomposition (DMD). Compared to the steady state, the measured effectiveness for the f = 10 and 20 Hz cases varied greatly during each cycle and led to significant enhancement in fluctuations. Cross correlations were used to determine the spatial signatures of large-scale structures (i.e., counterrotating vortex pairs, the horseshoe vortex, and structures near the hole) at various oscillation frequencies. Further POD and DMD analyses successfully decomposed the instantaneous fluctuations into a series of energetic modes, which vividly illustrated the strong modulations in adiabatic effectiveness imposed by flow oscillations and unsteady events.

Adiabatic Effectiveness on High-Pressure Turbine Nozzle Guide Vanes Under Realistic Swirling Conditions

In modern lean-burn aero-engine combustors, highly swirling flow structures are adopted to control the fuel-air mixing and to provide the correct flame stabilization mechanisms. Aggressive swirl fields and high turbulence intensities are hence expected in the combustor-turbine interface. Moreover, to maximize the engine cycle efficiency, an accurate design of the high-pressure nozzle cooling system must be pursued: in a film-cooled nozzle, the air taken from last compressor stages is ejected through discrete holes drilled on vane surfaces to provide a cold layer between hot gases and turbine components. In this context, the interactions between the swirling combustor outflow and the vane film cooling flows play a major role in the definition of a well-performing cooling scheme, demanding for experimental campaigns at representative flow conditions. An annular three-sector combustor simulator with fully cooled high-pressure vanes has been designed and installed at THT Lab of University of Florence. The test rig is equipped with three axial swirlers, effusion-cooled liners, and six film-cooled high-pressure vanes passages, for a vortex-to-vane count ratio of 1:2. The relative clocking position between swirlers and vanes has been chosen in order to have the leading edge of the central airfoil aligned with the central swirler. In this experimental work, adiabatic film effectiveness measurements have been carried out in the central sector vanes, in order to characterize the film-cooling performance under swirling inflow conditions. The pressure-sensitive paint (PSP) technique, based on heat and mass transfer analogy, has been exploited to catch highly detailed 2D distributions. Carbon dioxide has been used as coolant in order to reach a coolant-to-mainstream density ratio of 1.5. Turbulence and five-hole probe measurements at inlet/outlet of the cascade have been carried out as well, in order to highlight the characteristics of the flow field passing through the cascade and to provide precise boundary conditions. Results have shown a relevant effect of the swirling mainflow on the film cooling behavior. Differences have been found between the central airfoil and the adjacent ones, both in terms of leading edge stagnation point position and of pressure and suction side film coverage characteristics.

Numerical evaluation of heat/mass transfer analogy for leading edge showerhead film cooling on a first-stage vane

A numerical study was conducted to evaluate the applicability of the heat/mass transfer analogy to leading edge film cooling on a first-stage vane. A typical showerhead configuration was taken into account, thus including four staggered rows of evenly spaced cylindrical holes, angled at 45° towards the tip. The investigation was carried out at low speed (isentropic exit Mach number of Ma2is = 0.2), inlet turbulence intensity of Tu1 = 1.6% and 13%, and three different blowing ratios (BR = 2.0, 3.0 and 4.0). Steady-state simulations were conceived to mimic the pressure sensitive paint (PSP) technique aiming to measure the adiabatic film cooling effectiveness (η) in the leading edge region, under a density ratio of DR = 1 and 1.5. With the mainstream flow assumed to be air at ambient temperature, the desired DR was obtained by injecting coolant as air at different temperature from the mainstream (namely, heat transfer approach) or as isothermal foreign gas (namely, mass transfer approach). Despite the dominant effect of the DR on η, results indicated that the heat/mass transfer properties of the coolant are to be considered when comparing film cooling performance, at matched BR conditions. At low inlet turbulence intensity, the heat/mass transfer analogy was found to be applicable only to the lowest BR of 2.0. Its validity was extended to the medium BR of 3.0, in case of DR = 1.5, provided that the leading edge is approached by high turbulence intensity flow.

Influence of mainstream flow oscillations on spatio-temporal variation of adiabatic film cooling effectiveness

The effect of mainstream oscillations on spatio-temporal variation of adiabatic effectiveness was quantified extensively by the fast-response pressure-sensitive paint (fast-PSP) technique. During the experiment, nitrogen (N2) was used as the coolant and was discharged from a 35°-inclined round hole. While the steady state (i.e., f = 0 Hz) was selected as the baseline, the oscillating cases were assessed in great detail to explore the frequency effect (i.e., f = 5, 10, and 20 Hz with a fixed amplitude) on film cooling performance. Measured instantaneous effectiveness results (i.e., η) were analyzed in terms of the ensemble-averaged (i.e., η¯) and phase-averaged results (i.e., η∼), where the latter one was further analyzed through triple decomposition. The ensemble-averaged results showed a strong effect of mainstream oscillations on adiabatic effectiveness: compared to the steady state, a reduction of 30–35% was observed in effectiveness for the M = 0.4 at f = 20 Hz cases. As for the phase-averaged results, a relatively weak dependence was observed at 5 Hz with respect to the phase angles. However, as the frequency increased to 20 Hz, obtained effectiveness exhibited a strong correlation with the velocity phase, where its contour was found to become longer and shorter during periodic cycles. In general, mainstream oscillations have a significant impact on the film cooling behavior. They not only degrade the cooling effectiveness, but also increase the unsteadiness in film cooling.

Assessment of film cooling’s surface quantities using pressure- and temperature-sensitive paint: Comparisons between shaped and sand-dune inspired holes

Following the previous work (Zhou and Hu, 2017), a comprehensive assessment was performed to further evaluate the film cooling’s surface quantities behind shaped and sand dune-inspired holes. Adiabatic film cooling effectiveness, heat transfer coefficient, and net heat flux reduction (NHFR) were measured at four blowing ratios (M = 0.40, 0.90, 1.40, and 2.00). The measured quantities were compared side-by-side between the shaped and sand dune-inspired holes. The pressure-sensitive paint (PSP) technique was used to acquire high-resolution adiabatic effectiveness and the temperature-sensitive paint (TSP) technique was used to map the corresponding heat transfer coefficient over the surface. Nitrogen and air, with a density ratio of about one, were used as the coolant for the PSP and TSP tests respectively. The measured results showed that the adiabatic effectiveness of the Barchan dune-shaped injection compound (BDSIC) was significantly higher than that of the shaped hole. Improvements of 20–150% in the centerline and 30–400% in the laterally averaged effectiveness were observed behind the BDSIC compared to the shaped hole. As for the heat transfer performance, although the BDSIC showed 10–20% higher heat transfer coefficient, hf/h0, the measured spatially averaged NHFR still demonstrated an augmentation of 50–150% in heat flux reduction in comparison to the shaped hole. This paper represents the first effort to comprehensively evaluate the surface quantities behind BDSIC film cooling concept using both PSP and TSP techniques.