Abstract The paper presents an experimental data base on transitional boundary layers developing on a flat plate installed within a variable area opening endwall channel. Measurements have been carried out by means of time-resolved PIV. The overall test matrix spans 3 Reynolds numbers, 4 free-stream turbulence intensity levels and 4 different flow adverse pressure gradients. For each condition, 16000 instantaneous flow fields have been acquired in order to obtain high statistical accuracy. The flow parameters have been varied in order to provide a gradual shift of the mode of transition from a bypass process occurring with mild adverse pressure gradients at high free-stream turbulence, to separated flow transition, occurring with low Reynolds number, low free-stream turbulence intensity and elevated adverse pressure gradient. In order to quantify the influence of the flow parameter variation on the boundary layer transition process, the transition onset and end positions, and the turbulent spot production rate have been evaluated with a wavelet based intermittency detection technique. This post-processing technique is in fact able to identify the vortical structures developing within the boundary layer, the intermittency function is then automatically evaluated for each tested condition counting the number of such structures and defining the cumulative probability function. The by-pass transition mode has the longest transition length that decreases with increasing the Reynolds number. The transition length of the separated flow case is smaller than the by-pass one, and the variation of the flow parameters has a similar impact. Similarly, the dimensionless turbulent spot production rate reduces when the Reynolds number is increasing. The variation of the inlet turbulence intensity has a small influence on this parameter except for the condition at the highest turbulence intensity, that always shows the lowest turbulent spot production rate because a by-pass type transition occurs. This large amount of data has been used to develop new correlations used to predict the spot production rate and the transition length in attached and separated flows.

Abstract In the present work, a wetting and drying model is coupled with Eulerian Thin-Film model (ETFM) to analyze the wetting and drying behavior inside the bearing chamber. In the enhanced model, an additional source term is included to account for the contact angle effect. These models were coupled with volume-of-fluid (VOF) such that the core region is resolved by VOF and region close to the chamber walls, where a thin film is expected is resolved by either ETFM or enhanced ETFM model. Numerical studies are conducted for a shaft speed of 5,000 rpm, lubricant and air flow rates of 100 1/hr and 10 g/s respectively, at a scavenging ratio of 4. In the case of enhanced ETFM model lubricant to surface contact angle was varied from 10° to 45°. The performance of enhanced ETFM model is evaluated to capture drying and wetting behavior on a flat plate and found to be satisfactory. Film thickness prediction of enhanced ETFM model is found to be comparable with the VOF predictions reported in the literature. The effect of contact angle on the spreading of oil and film thickness is found to be small for the investigated conditions on an aero-engine bearing chamber.

Abstract Thermal barrier coatings (TBC) found on turbine blades are a key element in the performance and reliability of modern gas turbines. TBC reduces the heat transfer into turbine blades by introducing an additional surface thermal resistance; consequently allowing for higher gas temperatures. During the service life of the blades, the TBC surface may be damaged due to manufacturing imperfections, handling damage, service spalling, or service impact damage, producing chips in the coating. While an increase in aerofoil temperature is expected, it is unknown to what degree the blade will be affected and what parameters of the chip shape affect this result. During routine inspections, the severity of the chipping will often fall to the discretion of the inspecting engineer. Without a quantitative understanding of the flow and heat transfer around these chips, there is potential for premature removal or possible blade failure if left to operate. The goal of this preliminary study is to identify the major driving parameters that lead to the increase in metal temperature when TBC is damaged, such that more quantitative estimates of blade life and refurbishing needs can be made. A two-dimensional computational Conjugate Heat Transfer model was developed; fully resolving the hot gas path and TBC, bond-coat, and super alloy solids. Representative convective conditions were applied to the cold side to emulate the characteristics of a cooled turbine blade. The hot gas path properties included an inlet temperature of 1600 K with varying Mach numbers of 0.30, 0.59, and 0.80 and Reynolds number of 5.1×10 5 , 7.0×10 5 , and 9.0×10 5 as referenced from the leading edge of the model. The cold side was given a coolant temperature of 750 K and a heat transfer coefficient of 1500 W/m 2 *K. The assigned thermal conductivities of the TBC, bond-coat, and metal alloys were 0.7 W/m*K, 7.0 W/m*K, and 11.0 W/m*K, respectively, and layer thicknesses of 0.50 mm, 0.25 mm, and 1.50 mm, respectively. A flat plate model without the presence of the chip was first evaluated to provide a basis of validation by comparison to existing correlations. Comparing heat transfer coefficients, the flat plate model matched within uncertainty to the Chilton-Colburn analogy. In addition, flat plate results captured the boundary layer thickness when compared with Prandtl’s 1/7th power-law. A chip was then introduced into the model, varying the chip width and the edge geometry. The most sensitive driving parameters were identified to be the chip width and Mach number. In cases where the chip width reached 16 times the TBC thickness, temperatures increased by almost 30% when compared to the undamaged equivalents. Additionally, increasing the Mach number of the incoming flow also increased metal temperatures. While the Reynolds number based on the leading edge of the model was deemed negligible, the Reynolds number based on the chip width was found to have a noticeable impact on the blade temperature. In conclusion, this study found that chip edge geometry was a negligible factor, while the Mach number, chip width, and Reynolds number based on the chip width had a significant effect on the total metal temperature.

Abstract In this study the effects of dust composition on particle deposition in an effusion cooling geometry were investigated through a series of experiments. Single mineral dusts made from five different minerals, Quartz, Dolomite, Albite, Salt, and Gypsum, were milled to similar size distributions (approx. 0–10μm diameter). These dusts were then used in particle deposition tests on a flat plate effusion hole test article which was heated in a kiln to 1116K and supplied with coolant flow heated to 950K. Percent mass flow reduction per gram and deposit morphology were recorded for each test. Results for the different minerals varied greatly ranging from 7.8% to 160% reduction in mass flow per gram injected, with the albite dust producing the greatest blockage. The different dusts also produced varying shapes of deposits. These five dusts were then combined to form a dust blend with the same mass fractions found in AFRL02, a commercially available test dust, and additional tests were conducted using this dust. Results from the tests using the OSU mixed AFRL02 were compared with an estimated blockage per gram found by taking a weighted average of the blockage per gram for each single mineral dust on a percent volume basis. When tested, the mixed AFRL02 produced a lower blockage per gram than the estimate, indicating that an estimate based on volume fraction alone is not sufficient to predict the deposition of dusts composed of a mixture of minerals.

Abstract For film-cooling of a flat plate with cooling jets issuing from round holes, turbulent mixing has been shown to scale with the velocity ratio (VR). In this paper, large-eddy simulations (LES) were performed to investigate the effect of varying blowing ratio (BR = 0.5 – 1.3), density ratio (DR = 1.1 – 2.1), and momentum-flux ratio (MR = 0.2–0.8) on adiabatic effectiveness and turbulence, while keeping the VR fixed at 0.46 and 0.63. Simulations based on Reynolds-Averaged Navier-Stokes (RANS) equations with the realizable k-ϵ and shear-stress transport k-ω models were also performed. The LES results show that separation and spreading of the film-cooling jet increase as BR, DR, and MR increase while VR remains constant. For a given VR, the LES predicts an absolute difference between the minimum adiabatic effectiveness of the lowest and highest MRs to be 2 to 5 times greater than those predicted by RANS. This is because RANS with either model cannot respond appropriately to changes in MR. However, RANS can correctly predict that adiabatic effectiveness decreases as VR increases. The LES results show the turbulent kinetic energy and Reynolds stresses near the film-cooling hole to change considerably with MRs at a constant VR, while turbulent heat flux changes negligibly. This suggests that while improved turbulence models for heat flux can improve RANS prediction of spreading, capturing trends, however, requires improved modeling of the Reynolds stresses.

Abstract The present study focuses on evaluating fully-coupled conjugate heat transfer simulation in a ribbed cooling passage with a fully developed flow assumption using LES with the immersed boundary method (IBM-LES-CHT). The IBM-LES and the IBM-CHT frameworks are validated prior to the main simulations by simulating purely convective heat transfer in the ribbed duct, and a laminar boundary layer flow over a 2D flat plate with heat conduction, respectively. For the main conjugate simulations, a ribbed duct geometry with a blockage ratio of 0.3 is simulated at a bulk Reynolds number of 10,000 with a conjugate boundary condition applied to the rib surface. The nominal Biot number is kept at 1, which is similar to the comparative experiment. As a means to overcome a large time scale disparity between the fluid and the solid regions, the use of a high artificial solid thermal diffusivity is compared to the physical diffusivity. It is shown that while the diffusivity impacts the instantaneous fluctuations in temperature, heat transfer and Nusselt numbers, it has an insignificantly small effect on the mean Nusselt number. Comparison between IBM-LES-CHT and iso-flux heat transfer simulations shows that the iso-flux case predicts higher local Nusselt numbers at the back face of the rib. Furthermore, the local Nusselt number augmentation ratio (EF) predicted by IBM-LES-CHT is compared to experiment and another LES conjugate simulation. Even though there is a mismatch between IBM-LES-CHT predictions and other two studies at the front face of the rib, the area-averaged EF compares reasonably well in other regions between IBM-LES-CHT prediction and the comparative studies.

Abstract Effective design of film cooled engine components requires the ability to predict behavior at engine conditions. This is commonly accomplished through low temperature testing on scaled up geometries. The adiabatic effectiveness, η, is one indicator of the performance of a film cooling scheme. Performing an experiment to measure η in a low temperature wind tunnel requires appropriate selection of the coolant flow rate. Perhaps the most common flow rate parameter that is used to characterize the coolant flow relative to the freestream is the mass flux ratio, or blowing ratio, M. This is usually used in lieu of the velocity ratio to account for the fact that the density of the coolant is typically much larger than that of the hot freestream gas. Numerous studies have taken place evaluating the ability of M to properly scale the effects of density ratio and its performance has produced mixed results. The momentum flux ratio, I, is an alternative that is also found to have mixed success, leading some to recommend matching the density ratio to allow simultaneous matching of M and I. Nevertheless, widely varying results in the literature regarding the efficacy of these coolant flow rate parameters to scale the density ratio suggests there may be other largely ignored effects playing a role in the thermal physics. In the present work, thermal experiments were performed to measure adiabatic effectiveness on a flat plate with a single 7-7-7 shaped hole. Various coolant gases were used to give a large range of thermodynamic property variations. It is shown that a relatively new coolant flow rate parameter that accounts for not only density variations but also specific heat variations, the advective capacity ratio (ACR), far exceeds the ability of either M or I to provide matched adiabatic effectiveness between the various coolant gases that exhibit extreme property differences. Particularly considering that the specific heat of the coolant in an engine is significantly lower than the specific heat of the freestream gas, ACR is shown to be appropriate for characterizing non-separating coolant flow situations.

Abstract The mechanisms of blade row interaction affecting rotor film cooling are identified in order to make recommendations for the design of film cooling in the real, unsteady turbine environment. Present design practice makes the simplifying assumption of steady boundary conditions, despite intrinsic unsteadiness due to blade row interaction; we argue that if film cooling responds non-linearly to unsteadiness, the time-averaged performance will then be in error. Non-linear behaviour is confirmed using experimental measurements of flat-plate cylindrical film cooling holes, main-stream unsteadiness causing a reduction in film effectiveness of up to 31% at constant time-averaged boundary condition. Unsteady computations are used to identify the blade row interaction mechanisms in a high-pressure turbine rotor: a ‘negative jet’ associated with the upstream vane wake, and frozen and propagating vane potential field interactions. A quasi-steady model is used to predict unsteady excursions in momentum flux ratio of rotor cooling holes, with fluctuations of at least ±30% observed for all hole locations. Computations with modified upstream vanes are used to vary the relative strength of wake and potential field interactions. In general, both mechanisms contribute to rotor film cooling unsteadiness. It is recommended that the designer should choose a cooling configuration which behaves linearly over the expected unsteady excursions in momentum flux ratio as predicted by a quasi-steady hole model.

Abstract A new experimental dataset focusing on the influence of high free-stream turbulence and large pressure gradients on boundary layer transition is presented. The experiments are conducted in a new wind tunnel equipped with a flat plate test section and a new kind of turbulence generator which allows for a continuous variation of turbulence intensity. The flat plate features an elliptic nose and is mounted midway between contoured top and bottom walls. Two different wall contours can be implemented to create pressure distributions on the flat plate that are typical for the pressure and suction side of high pressure turbine cascades. A large variation of Reynolds number from 3.0 · 10 5 to 7.5 · 10 5 and inlet turbulence intensity between 1.1 % and 8 % is realized, resulting in more than 100 test cases. Measurements comprise highly resolved heat transfer, near-wall intermittency and free-stream Reynolds stress distributions. Near-wall intermittency is measured using a traversable hotfilm sensor embedded in a steel-belt that is running around the flat plate while free-stream Reynolds stresses are measured simultaneously at the same position with a revolvable X-wire probe. Additionally, turbulent length scales are analyzed using the X-wire signal along the flat plate. Results show that heat transfer and near wall intermittency distributions are in good agreement and that heat transfer at high turbulence levels increases prior to the formation of first turbulence spots. Transition onset is found to be influenced by the turbulence Reynolds number, i.e. turbulent length scales. At constant inlet turbulence intensity, transition onset moves upstream, when the turbulent Reynolds number is decreased.

Abstract Gas turbine hot gas path components are protected via coolant that travels through internal passageways before being ejected as external film cooling. Modern computational approaches facilitate simulation of the conjugate heat transfer that takes place within turbine components, allowing prediction of the actual metal temperature, usually nondimensionalized in the form of the overall effectiveness. Efforts aimed at improving cooling are often focused on either only the internal cooling or the film cooling; however, the common coolant flow means the internal and external cooling schemes are inextricably linked and the coolant holes themselves provide another convective path for heat transfer to the coolant. The relative influence of internal cooling, external cooling, and convection through the film cooling holes on overall effectiveness is not well understood. Computational fluid dynamics (CFD) simulations were performed in order to isolate each cooling mechanism, and thereby determine their relative contributions to overall effectiveness. The conjugate CFD model was a flat plate with five staggered rows of shaped film cooling holes. Unique boundary conditions were used to isolate the cooling mechanisms. The internal cooling was modeled with and without heat transfer on the internal surface in order to isolate the effects of plenum cooling. Convection through the coolant holes was isolated by making the inside of the film cooling holes adiabatic. This was done in order to evaluate the influence of the internal cooling provided by the cooling holes themselves. The effect of film cooling was removed through the novel use of an outlet boundary condition at the exit of each hole that allowed unaltered internal coolant flow without external coolant ejection.

Abstract This study introduces a novel and simple strategy; compound angle upstream sister holes (CAUSH) to increase film cooling performance of the cylindrical hole by combining two techniques: Sister holes; (two small round holes placed upstream the primary hole) and compound angle hole. Whereas the upstream sister holes were injected at several compound angles β = 0°, 45°, 75°, and 90°, while the main hole was injected to the streamwise direction at 35° on a flat plate. FLUENT-ANSYS code was used to perform the simulation by solving the 3D Reynolds Averaged Navier-Stokes Equations. The capability of three types of k-ε turbulence modeling combined with the enhanced wall treatment is investigated to predict the film cooling performance of sister holes. A detailed computational analysis of the cooling performance of the (CAUSH) and the flow field was done at a density ratio equal to two (D.R = 2) and four blowing ratios M = 0.25, 0.5, 1.0 and 1.5 to predict the centerline and laterally averaged film cooling performance. The centerline effectiveness results showed that the highest cooling performance from the examined (CAUSH) was obtained at β = 0°, 45°, and 90° for low and high blowing ratio, the highest laterally averaged film cooling performance was captured at β = 0° and 90° for all tested blowing ratios. Also, the results indicated that the upstream sister hole with 90° compound angle holes has the best overall film cooling effectiveness while the worst performance is attained at β = 75°.

Abstract The interaction of coolant jets is significant in effusion settings as a result of the short streamwise and spanwise distance between films. This complicates the design of effusion cooling devices because computing the interaction between numerous, closely spaced rows of films is a challenging task. A flat plate effusion cooling model is investigated at both low and high blowing ratios. Pressure sensitive paint (PSP) is used to measure film effectiveness. Computational fluid dynamics (CFD) calculations are performed to examine the cooling flow features in detail. The mesh sensitivity is studied to demonstrate the effect of mesh size on film effectiveness. The solution obtained by coarse mesh may not capture the correct trend with blowing ratio variation. Results of the computational work by fine mesh demonstrate good agreement with the measured effectiveness. Coolant jets interaction is also investigated. The profile of quantities such as velocity, temperature, kinetic energy and Reynolds stress at several locations in the flow field is compared. The boundary layer profiles are scaled by the thermal boundary layer thickness to study the feature of heat transfer. It is observed that profiles of the flow quantities are self-similar. Two distinct scalings are found: an outer scaling based on boundary layer thickness which collapses the upper part of the profiles; an inner scaling which collapses the profiles at distances from the wall comparable to the penetration depth of a single jet. The latter scaling is based on the distance from the wall to the minimum temperature profile. This distance identifies the location of the coolant leaving the effusion cooling device.

Abstract Further improvements in gas turbine efficiency can be sought through more advanced cooling systems — such as the double-wall, effusion system — which provide high cooling effectiveness with low coolant utilisation. The double-wall system, as described here, comprises two walls, one with a regular array of impingement holes, the other with a closely-packed, regular array of film holes (characteristic of effusion systems). These walls are mechanically and thermally connected to one another via a bank of pedestals which increase coolant wetted area and turbulent flow features. However, a lack of data exists in the open literature on these systems. This study presents a novel experimental heat transfer facility designed with the intent of investigating flat plate versions of such double-wall geometries. Key features of the facility are presented including the use of recirculation to increase mainstream-to-coolant temperature ratio and the use of infrared thermography to obtain thermal measurements. Some rig commissioning characteristics are also provided which demonstrate well-conditioned, uniform flow. Both coolant and mainstream Reynolds numbers are matched to engine conditions, with Biot number within around 15% of engine conditions. The facility is used to assess the cooling performance of four double-wall effusion geometries which incorporate various geometrical features. Both overall effectiveness and film effectiveness measurements are presented at a range of coolant mass flows with conclusions drawn as to preferable features from a cooling perspective. The results from a fully conjugate CFD model of the facility are presented which utilised boundary conditions obtained during experimental runs. Additionally, a computationally efficient decoupled conjugate method developed previously by the authors was adapted to assess the experimental geometries with the results comparing favourably.

Abstract Performance of film cooling holes is generally evaluated near ambient conditions, so scaling the film cooling performance from ambient to engine conditions is important. These performance metrics can be reported in terms of net heat flux reduction, heat transfer enhancement, and effectiveness. There are several challenges and penalties in conducting experiments on a real engine or even on a stationary flat plate test article at near engine conditions. In this study, an experimental methodology capable of delivering film cooling performance for the experiments conducted in the high-temperature high-pressure test facility at NETL will be explored. More specifically, this study focuses on various challenges that are present and how these concerns are addressed to evaluate the heat transfer coefficient on the test article. Steady state experiments were conducted on a flat plate test article whose surface temperature distribution was obtained using an IR camera calibrated with a combination of bench top experiments and in-situ calibrations based on embedded thermocouple measurements. A detailed radiation analysis is conducted to enable differentiation of heat transfer by convection from heat transfer by radiation. The estimated surface heat flux and heat transfer coefficient were found to be higher than the one obtained from flat plate correlation.

Abstract Discrete hole film cooling has been commonly used as an effective cooling technique to protect gas turbine blades from hot gas. There have been numerous investigations on the cylindrical hole and shaped hole, but few experimental investigations on the cooling mechanism of the novel film holes with side holes (anti-vortex hole and sister hole) are available. This paper presents an experimental and numerical investigation to study the film cooling performance and flow structure of four kinds of film holes (cylindrical hole, fan-shaped hole, anti-vortex hole and sister hole) on the flat plate. The film holes have the same main hole diameter of 4mm and the same inclination angle of 45°. The adiabatic film cooling effectiveness is obtained by the steady-state Thermochromic Liquid Crystal (TLC). The flow visualization experiment and numerical investigation are performed to investigate the flow structure and counter-rotating vortex pair (CRVP) intensity. The smoke is selected as the tracer particle in the flow visualization experiment. The mainstream Reynolds number is 2900, the blowing ratio ranges from 0.3 to 2.0, and the density ratio of coolant to mainstream is 1.065. Experimental results show that compared with the cylindrical hole, the film cooling performance of the anti-vortex hole and sister hole shows significant improvement at all blowing ratios. The sister hole can achieve the best cooling performance at blowing ratios of 0.3 to 1.5. The fan-shaped hole only performs well at high blowing ratios and it performs best at the blowing ratio of 2.0. Flow visualization experiment and numerical investigation reveal that the anti-vortex hole and sister hole can decrease the CRVP intensity of the main hole and suppress the coolant lift-off because of side holes, which increases the film coverage and cooling effectiveness. For the sister hole, the side holes are parallel to the main hole, but for the anti-vortex hole, there are lateral angles between them. The coolant interaction between the side holes and main hole of the sister hole is stronger than that of the anti-vortex hole. Therefore, the sister hole provides better film cooling performance than the anti-vortex hole.

Abstract The effect of film cooling hole inlet geometry is experimentally investigated in this study. Detailed film cooling effectiveness distributions are obtained on a flat plate using Pressure Sensitive Paint (PSP). The inlet of a traditional 12°-12°-12°, laidback, fanshaped hole varies from a traditional “round” opening to an oblong, racetrack shaped opening. In this study, a single racetrack inlet with an aspect ratio of 2:1 is compared to the round inlet. For both designs, the holes are inclined at θ = 30° relative to the mainstream. Blowing ratios of 0.5, 1.0, and 1.5 are considered as the coolant–to–mainstream density ratio varies between 1.0 and 4.0. For all cases, the freestream turbulence intensity is maintained at 7.5%. With the introduction of the racetrack shaped inlet, the coolant spreads laterally across the diffuse, laidback fanshaped outlet. The centerline film cooling effectiveness is reduced with the enhanced lateral spread of the coolant. However, the benefit of the shaped inlet is also observed with an increase in the area averaged film cooling effectiveness, compared to the traditional round inlet. Not only does the shaped inlet promote spreading of the coolant, it is also believed the racetrack shape suppresses turbulence within the hole allowing for enhanced film cooling protection near the film cooling holes.

Abstract Gas turbine film cooling creates complicated and highly unsteady flow structures. This study seeks to examine the unsteady characteristics created by different film hole inlet geometries using a fast-response pressure sensitive paint (PSP) technique able to capture time-accurate measurements at 2000 frames per second, resolving frequencies up to 1000 Hz. Time accurate and time-averaged measurements are used to evaluate the performance of a plenum-style inlet and a crossflow-style inlet in varying turbulence environments over a flat plate. The results of this study are intended to begin the process of breaking down widely accepted time-averaged film effectiveness contours into the cumulative effects of smaller oscillating cooling jets. Jet behaviors observed in this study include a sweeping oscillation, unsteady attachment and separation from the plate, and time accurate and time average flow bias. The behavior and performance of higher blowing ratio, separated film cooling jets depend heavily on the momentum flux ratio. Crossflow fed cooling holes show bias to the upstream side of the cooling hole with respect to the internal crossflow direction. Plenum fed cooling holes outperform crossflow fed cooling holes, and the difference increases with increasing momentum flux ratio. Cooling hole inlet geometry and momentum flux ratio affect the core of the jet, and freestream turbulence affects the periphery of the jet. Fluctuating frequencies of plenum fed and crossflow fed cooling holes were seen to be influenced by the turbulent velocity fluctuation frequency. The resulting mode shapes showed dominant side-to-side fluctuations for higher turbulence environments and a separation and reattachment motion for lower turbulence environments.

Abstract An investigation of the experimental heat transfer and cooling effectiveness for a modern fully-cooled high-pressure turbine (HPT) inlet vane is presented. Conjugate Heat Transfer (CHT) Computational Fluid Dynamics (CFD) is conducted to simulate experiments using thin-film heat-flux gauges on full-scale 3D vanes at engine-representative conditions from Part 1 of this paper. Pressure side (PS) film cooling performance is compared for a baseline and optimized configuration, in which the latter was previously developed using genetic algorithm (GA) optimization. The optimized vane was iterated using hundreds of computationally efficient 3D Reynolds Averaged Navier Stokes (RANS) CFD simulations with a transpiration boundary condition to simulate film cooling. This combination of CFD and GAs determined surface-optimized cooling hole orientations and placement. Steady-state flat plate infrared thermography experiments that followed also determined the best cooling hole shapes to use on different sections of the vane pressure side surface. This ultimately generated the cooling design to be fabricated using realistic materials and experimentally tested in Part 1 and simulated using CHT CFD in the current work (Part 2). Here, spanwise and streamwise heat transfer distributions for the baseline and optimized cooling design are validated against experimental data. 3D CHT CFD results are then assessed at the same conditions, providing relevance and credence to the overall cooling design methods. Ultimately, surface-optimized film cooling designs can be used to reduce the adverse effects of sub-optimal heat distribution on critical high temperature engine parts, increasing the life of the part. Alternatively, such a design could lead to increases in engine efficiency since less cooling air is required from the mainstream per part.

Abstract Surface curvature has been shown to have significant effects on the film cooling performance of round holes, but the literature includes few studies of its effects on shaped holes despite their prevalence in gas turbines. Experiments were performed using two rows of holes placed on the suction side of a scaled-up turbine blade in a low-Mach-number linear cascade wind tunnel with low freestream turbulence. The rows were placed in regions of high and low convex surface curvature, respectively. Geometries and flow conditions for the rows were matched to those from previous flat plate studies. Comparison of the adiabatic effectiveness results from the high curvature and flat plate rows revealed the same trends as those in the literature using round holes — with increased performance for the high curvature row at lower blowing ratios and the opposite at higher ones. The low curvature row had similar performance to the flat plate row at lower blowing ratios, suggesting the mild convex curvature had little beneficial effect. At higher blowing ratios, the low curvature row had inferior performance, which was attributed to the local freestream adverse pressure gradient that generated additional turbulence, promoting jet-to-mainstream mixing and decreasing performance.

Abstract Heat transfer experiments were done on a flat plate to study the effect of internal counter-flow backside cooling on adiabatic film cooling effectiveness and heat transfer coefficient. In addition, the effects of density ratio (DR), blowing ratio (BR), diagonal length over diameter (L/D) ratio, and Reynolds number were studied using this new configuration. The results are compared to a conventional plenum fed case. Data were collected up to X/D = 23 where X = 0 at the holes, an S/D = 1.65 and L/D = 1 and 2. Testing was done at low L/D ratios since short holes are normally found in double wall cooling applications in turbine components. A DR of 2 was used in order to simulate engine-like conditions and this was compared to a DR of 0.92 since relevant research is done at similar low DR. The BR range of 0.5 to 1.5 was chosen to simulate turbine conditions as well. In addition, previous research shows that peak effectiveness is found within this range. Infrared (IR) thermography was used to capture temperature contours on the surface of interest and the images were calibrated using a thermocouple and data analyzed through MATLAB software. A heated secondary fluid was used as ‘coolant’ in the present study. A steady state heat transfer model was used to perform the data reduction procedure. Results show that backside cooling configuration has a higher adiabatic film cooling effectiveness when compared to plenum fed configurations at the same conditions. In addition, the trend for effectiveness with varying BR is reversed when compared with traditional plenum fed cases. Yarn flow visualization tests show that flow exiting the holes in the backside cooling configuration is significantly different when compared to flow exiting the plenum fed holes. We hypothesize that backside cooling configuration has flow exiting the holes in various directions, including laterally, and behaving similar to slot film cooling, explaining the differences in trends. Increasing DR at constant BR shows an increase in adiabatic effectiveness and HTC in both backside cooling and plenum fed configurations due to the decreased momentum of the coolant, making film attachment to the surface more probable. The effects of L/D ratio in this study were negligible since both ratios used were small. This shows that the coolant flow is still underdeveloped at both L/D ratios. The study also showed that increasing turbulence through increasing Reynolds number decreased adiabatic effectiveness.