Numerical and experimental investigation of a new film cooling geometry with high P/D ratio

In order to improve the coolant surface coverage, in the past years new geometries have been proposed with higher lateral fan-shaped angle and/or greater inter-hole pitch distance (P/D). Unfortunately it is not possible to increase the fan angle or the pitch distance even further without inducing a coolant separation and a drop in the overall effectiveness. This study proposes an innovative design which improves the lateral coverage and reduces the jet lift off. The results have been validated by a combination of numerical and experimental analyses: the experimental work has been assessed on a flat plate using thermo chromic liquid crystals and the results have been confirmed numerically by the CFD with the same conditions. The CFD simulations have been carried out considering a stochastic distribution for the free stream Mach number and the coolant blowing ratio. The experimental and computational results show that the inducing lateral pressure gradients there is a minimum increase in lateral averaged adiabatic effectiveness of +30% than the baseline case until a distance downstream of 20 times the coolant diameter. © 2013 Elsevier Ltd. All rights reserved

Internal crossflow effects on turbine airfoil film cooling adiabatic effectiveness with compound angle round holes

textInternal crossflow is an important element to actual gas turbine blade cooling; however, there are very few studies in open literature that have documented its effects on turbine blade film cooling. Experiments measuring adiabatic effectiveness were conducted to investigate the effects of perpendicular crossflow on a row of 45 degree compound angle, cylindrical film cooling holes. Tests included a standard plenum condition, a baseline crossflow case consisting of a smooth-walled channel, and various crossflow configurations with ribs. The ribs were angled to the direction of prevailing internal crossflow at 45 and 135 degrees and were positioned at different locations. Experiments were conducted at a density ratio of DR=1.5 for a range of blowing ratios including M=0.5, 0.75, 1.0, 1.5, and 2.0. Results showed that internal crossflow can significantly influence adiabatic effectiveness when compared to the standard plenum condition. The implementation of ribs generally decreased the adiabatic effectiveness when compared to the smooth-walled crossflow case. The highest adiabatic effectiveness measurements were recorded for the smooth-walled case in which crossflow was directed against the spanwise hole orientation angle. Tests indicated that the direction of perpendicular crossflow in relation to the hole orientation can significantly influence the adiabatic effectiveness. Among the rib crossflow tests, rib configurations that directed the coolant forward in the direction of the mainstream resulted in higher adiabatic effectiveness measurements. However, no other parameters could consistently be identified correlating to increased film cooling performance. It is likely that a combination of factors are responsible for influencing performance, including internal local pressure caused by the ribs, the internal channel flow field, jet exit velocity profiles, and in-hole vortices.Mechanical Engineerin

Numerical investigation of film cooling fluid flow and heat transfer using large eddy simulations

Large eddy simulations of film cooling from discrete holes inclined at 35° with a feeding plenum chamber are performed at a density ratio of 2 and blowing ratios from 0.5 to 2.0 in order to gauge the suitability and performance of different hole shapes. Cylindrical holes at length to diameter ratios of 1.75 and 3.5 as well as shaped holes (laterally diffused and console holes) at a length to diameter ratio of 3.5 are simulated issuing into a laminar crossflow at a Reynolds number of approximately 16,000 based on freestream velocity and hole diameter. The domain extends 15 hole diameters downstream of a single coolant hole, and periodic boundary conditions on the lateral faces of the domain are used. The results are validated in terms of the flow field and surface adiabatic effectiveness to experiments for cylindrical hole cases. Horseshoe vortices, DSSN vortices, and hairpin vortices are resolved and isolated. Jetting is found to have significant effects on effectiveness in cylindrical hole cases (with less jetting at the exit plane and better cooling performance from the longer holes) and shaped hole cases (with a laterally split jetting action occurring around a central recirculation region). The performance of the shaped holes is dramatically better than the performance of the cylindrical holes in terms of surface adiabatic effectiveness, with the console holes performing slightly better than the laterally diffused holes. In terms of aerodynamic loss, the console and cylindrical hole far outperformed the laterally diffused hole

Numerical modeling and optimization study for the geometry of film cooling holes

Film cooling is one of the essential approaches developed to protect the gas turbine blades and vanes from the passing, very high temperature, gases. It does so through covering the surface with a film of coolant air. Experimental and numerical studies have identified the important parameters affecting the film cooling aerodynamic and thermal behavior. Nevertheless, researchers are still concerned with more enhancement of film cooling performance through deeper understanding of the parameters that control coolant jet behavior. Being an important controlling parameter, the coolant nozzle geometry has been optimized in the current study. The analysis was performed in terms of adiabatic film effectiveness and heat transfer coefficient. The remaining key parameters were neutralized through fixing the coolant pipe inlet area and the pitch-to-hole-width ratio. Realizable k-ε model with scalable wall function has been selected to perform the current numerical study, being the most suitable RANS-based tested model in predicting the experimentally reported film cooling indicators at the blowing ratio of one. After proving the model accuracy, it was utilized to verify the cooling superiority of the racetrack slot (rectangular slot with fully round ends) over the typical round hole. Afterwards, the cooling performance of the racetrack slot was investigated at different aspect ratios. The optimum recorded racetrack geometry, having an aspect ratio of seven, served as a starting point for further optimization of the coolant pipe shape utilizing ANSYS Fluent Adjoint solver. The numerical optimization tool allows for a powerful, less-constrained, irregular shape optimization. Starting from the optimum racetrack geometry, the optimum irregular pipe shape was designated in two optimization steps, through which the average adiabatic film effectiveness over the test surface has increased from 0.24 to 0.34

AEROTHERMODYNAMIC INVESTIGATION OF THE TURBINE BLADE FILM COOLING WITH LARGE EDDY SIMULATION METHOD

Gas turbines have been widely used in the aviation, marine, power plant, etc., which plays an indispensable role in modern industries. Nowadays, higher efficiency and larger output power is demanded, consequently, the turbine inlet temperature (TIT) has to be increased to up to 2260K in some turbofan engines. The turbine blades or vanes operate in a harsh environment and the operating temperature significantly surpasses the melting point of the turbine blade material. Therefore, the turbine cooling technology is of vital importance aiming to guarantee the lifespan of the turbine blades and the safe operation. Apart from the internal cooling methods, the external film cooling is a necessary and effective solution to protect the outer surface of the blades against the extreme high temperature mainstream from the combustion chamber. In this thesis, the thermal performance of the laidback fan-shaped film hole structure was numerically studied, which is known as 7-7-7 laidback fan-shaped film hole proposed by Thole [1]. Large eddy simulation (LES) method was implemented to investigate the thermal performance of the shaped film hole, and the LES result was compared with the RANS simulation with various turbulent models and verified by the experimental data from Thole. Besides, a comparative study was conducted between the conventional cylindrical film hole and the 7-7-7 shaped film hole. The results show the better cooling effectiveness with sufficient spread in spanwise direction as the blowing ratio increases, and proper orthogonal decomposition (POD) method was employed to present the coherent structure in flow field. Additionally, the effects of the blowing ratios M on the shaped film hole were simulated with LES in the range of M=0.5-3.0. Three different mainstream inlet turbulence intensities included between Tu=0.5% and Tu=20% were chosen to research the effects on the cooling effectiveness. Three mainstream inlet velocity profiles were applied for the LES calculation. The convex curved bottom surface was also investigated and compared with the flat bottom wall. The results show that M=1.5 can obtain a relative better performance for the same turbulence intensity of 0.5%. The cooling effectiveness deteriorates as the mainstream turbulence intensity increases from 0.5% to 20%. The mainstream inlet velocity profile causes less effects on the effectiveness relative to the blowing ratio and inlet turbulence intensity. The effectiveness of the convex curved bottom surface decays at higher blowing ratio condition. In addition, aerothermal performance of film cooled C3X vane was analyzed, and a comparison between the cylindrical and shaped film hole cases was presented. The effects of the two different film hole structures on the pressure and temperature distributions were studied. The present work evaluated the LES accuracy through a comparison with the experimental data and presented the reason of the different predictions between the LES and RANS. The numerical research on the film cooling can be considered as a baseline for further comparison and investigation of the film cooling in turbine blades or vanes

Surface Measurements And Predictions Of Full-coverage Film Cooling

Full-coverage film cooling is investigated both experimentally and numerically. First, surface measurements local of adiabatic film cooling effectiveness and heat transfer augmentation for four different arrays are described. Reported next is a comparison between two very common turbulence models, Realizable k-ε and SST k-ω, and their ability to predict local film cooling effectiveness throughout a full-coverage array. The objective of the experimental study is the quantification of local heat transfer augmentation and adiabatic film cooling effectiveness for four surfaces cooled by large, both in hole count and in non-dimensional spacing, arrays of film cooling holes. The four arrays are of two different hole-to-hole spacings (P/D = X/D = 14.5, 19.8) and two different hole inclination angles (α = 30◦ , 45◦ ), with cylindrical holes compounded relative to the flow (β = 45◦ ) and arranged in a staggered configuration. Arrays of up to 30 rows are tested so that the superposition effect of the coolant film can be studied. In addition, shortened arrays of up to 20 rows of coolant holes are also tested so that the decay of the coolant film following injection can be studied. Levels of laterally averaged effectiveness reach values as high as ¯η = 0.5, and are not yet at the asymptotic limit even after 20 − 30 rows of injection for all cases studied. Levels of heat transfer augmentation asymptotically approach values of h/h0 ≈ 1.35 rather quickly, iii only after 10 rows. It is conjectured that the heat transfer augmentation levels off very quickly due to the boundary layer reaching an equilibrium in which the perturbation from additional film rows has reached a balance with the damping effect resulting from viscosity. The levels of laterally averaged adiabatic film cooling effectiveness far exceeding ¯η = 0.5 are much higher than expected. The heat transfer augmentation levels off quickly as opposed to the film effectiveness which continues to rise (although asymptotically) at large row numbers. This ensures that an increased row count represents coolant well spent. The numerical predictions are carried out in order to test the ability of the two most common turbulence models to properly predict full coverage film cooling. The two models chosen, Realizable k − ε (RKE) and Shear Stress T ransport k − ω (SSTKW), are both two-equation models coupled with Reynolds Averaged governing equations which make several gross physical assumptions and require several empirical values. Hence, the models are not expected to provide perfect results. However, very good average values are seen to be obtained through these simple models. Using RKE in order to model full-coverage film cooling will yield results with 30% less error than selecting SSTKW

EFFECTS OF CROSSFLOW IN AN INTERNAL-COOLING CHANNEL ON FILM COOLING OF A FLAT PLATE THROUGH COMPOUND-ANGLE HOLES

The film-cooling holes in turbine blades are fed from an internal cooling channel. This channel imposes a crossflow at the entrance of the holes that can significantly affect the performance of the cooling jets that emanate from those holes. In this study, CFD simulations based on steady RANS with the shear-stress transport (SST) and the realizable k -ε turbulence models were performed to study film cooling of a flat plate with cooling jets issuing from eight round holes with a compound angle of 45 degrees, where the coolant channel that fed the cooling jets was oriented perpendicular to the direction of the hot-gas flow. One case was also performed by using large-eddy simulation (LES) to get a sense of the unsteady nature of the flow. Operating conditions were chosen to match the laboratory conditions, which maintained a density ratio of 1.5 between the coolant and the hot gas. Parameters studied include internal crossflow direction and blowing ratios of 0.5, 1.0, and 1.5. Results obtained showed an unsteady vortex forms inside the hole, causing a side-to-side shedding of the coolant jet. Values of adiabatic effectiveness predicted by the CFD simulations were compared with experimentally measured values. Steady RANS was found to be inconsistent in its ability to predict adiabatic effectiveness with relative error ranging from 10% to over 100%. LES was able to predict adiabatic effectiveness with reasonable accuracy

Discrete Film Cooling in a Rocket with Curved Walls

This study quantified the effects of discrete wall-based film cooling in a rocket with curved walls. Simulations and experiments showed decreasing with wall radius of curvature, holding jet diameter constant, improves net heat flux reduction (NHFR) and adiabatic effectiveness (η) for 90˚ compound injected cylindrical jets, though η is reduced at the highest curvature. NHFR and η improved further with a high favorable stream-wise pressure gradient (K=2.1x10-5) at all tested blowing ratios, but were affected little by a high density ratio (DR=1.76) using carbon dioxide as the coolant. Experiments were run at a Reynolds number of 31K and a free-stream turbulence intensity of 26% with varying wall and jet radii. Simulations showed the Rannie transpiration model may be used to predict the cooling performance of a wall with full coverage film cooling using a correction formula based on the hole coverage area. Three improvements were made to the method of simultaneous acquisition of adiabatic wall temperature and heat flux coefficient: solving for the needed variables via a multi- point non-linear least squares curve fit instead of a two-point direct solution; correctly applying the free-stream fluid temperature boundary condition to account for drifting temperature instead of assuming it to be constant; and showing a repeatable way to reduce uncertainty in the test start time

Benchmarking of Computational Models against Experimental Data for Velocity Profile Effects on CFD Analysis of Adiabatic Film-Cooling Effectiveness for Large Spacing Compound Angle Full Coverage Film Cooling Arrays

This study aims to benchmark experimental data that tested the effects of blowing ratio, surface angle, and hole spacing for full coverage geometries composed of cylindrical staggered holes at a compounded angle of 45 degrees. These holes had an inclination angle of 45 degrees, while maintaining a lateral and axial spacing of 14.5 hole diameters. Within this study, the local film cooling effectiveness was obtained from 30 rows for the 14.5 diameter spacing. The goal of this research was to test the effects of utilizing a realistic vs a uniform velocity profile at the crossflow inlet and find any significant differences in the results produced when compared to experimental data. The results displayed differences between the spanwise average adiabatic effectiveness for both the uniform velocity profile case and the velocity profile replication of the experimental data when using the Realizable k-ε turbulence model. These differences were found to be due to the differences in the thermal boundary layer predicted by the turbulence model for the two test cases