Experimental and Computational Investigation of Shaped Film Cooling Holes Designed to Minimize Inlet Separation

Film cooling is used to protect turbine components from the extreme temperatures by ejecting coolant through arrays of holes to create an air buffer from the hot combustion gases. Limitations in traditional machining meant film cooling holes universally have sharp inlets which create separation regions at the hole entrance. The present study uses experimental and computational data to show that these inlet separation are a major cause of performance variation in crossflow fed film cooling holes. Three hole designs were experimentally tested by independently varying the coolant velocity ratio (VR) and the coolant channel velocitty ratio (VRc) to isolate the effects of crossflow on hole performance. Leveraging additive manufacturing technologies, the addition of a 0.25D radius fillet to the inlet of a 7-7-7 shaped hole is shown to significantly improve diffuser usage and significantly reduce variation in performance with VRc. A second AM design used a very large radius of curvature inlet to reduce biasing caused by the inlet crossflow. Experiments showed that this “swept” hole design did minimize biasing of coolant flow to one side of the shaped hole and it significantly reduced variations due to varying VRc. RANS simulations at six VR and three VRc conditions were made for each geometry to better understand how the new geometries changed the velocity field within the hole. The sharp and rounded inlets were seen to have very similar tangential velocity fields and jet biasing. Both AM inlets created more uniform, slower velocity fields entering the diffuser. The results of this paper indicate large improvements in film cooling performance can be found by leveraging AM technology.

Experimental Study of the Endwall Heat Transfer of a Transonic Nozzle Guide Vane With Upstream Jet Purge Cooling: Part 1 — Effect of Density Ratio

Nozzle guide vane platforms often employ complex cooling schemes to mitigate ever-increasing thermal loads on endwall. Understanding the impact of advanced cooling schemes amid the highly complex three-dimensional secondary flow is vital to engine efficiency and durability. This study analyzes and describes the effect of coolant to mainstream blowing ratio, momentum ratio and density ratio for a typical axisymmetric converging nozzle guide vane platform with an upstream doublet staggered, steep-injection, cylindrical hole jet purge cooling scheme. Nominal flow conditions were engine representative and as follows: Maexit = 0.85, Reexit/Cax = 1.5 × 106 and an inlet large-scale freestream turbulence intensity of 16%. Two blowing ratios were investigated, each corresponding to upper and lower engine extrema at M = 3.5 and 2.5, respectively. For each blowing ratio, the coolant to mainstream density ratio was varied between DR = 1.2, representing typical experimental neglect of coolant density, and DR = 1.95, representative of typical engine conditions. An optimal coolant momentum ratio between = 6.3 and 10.2 is identified for in-passage film effectiveness and net heat flux reduction, at which the coolant suppresses and overcomes secondary flows but imparts minimal turbulence and remains attached to endwall. Progression beyond this point leads to cooling effectiveness degradation and increased endwall heat flux. Endwall heat transfer does not scale well with one single parameter; increasing with increasing mass flux for the low density case but decreasing with increasing mass flux of high density coolant. From the results gathered, both coolant to mainstream density ratio and blowing ratio should be considered for accurate testing, analysis and prediction of purge jet cooling scheme performance.

The Influence of Turbulence and Reynolds Number on Multiple Slot Film Cooling Over the Suction Surface

Developing robust film cooling protection on the suction surface of a vane is critical to managing the high heat loads which exist there. Suction surface film cooling often produces high levels of film cooling but can be influenced by secondary flows and some dissipation due to free-stream turbulence. Directly downstream from suction surface film cooling, heat loads are often significantly mitigated and internal cooling levels can be modest. One thermodynamically efficient way to cool the suction surface of a vane is with a counter cooling scheme. This combined internal/external cooling method moves cooling air in a direction opposite to the external flow through an internal convection array. The coolant is then discharged upstream where the high level of film cooling can offset the reduced cooling potential of the spent cooling air. The present suction surface film cooling arrangement combines a slot film cooling discharge on the near suction surface from an incremental impingement cooling method with a second from a counter cooling section. A second counter cooling section is added further downstream on the suction surface. The internal cooling plenums replicate the geometry of the cooling methods to ensure the fluid dynamics of the flow discharging from the slots are representative of the actual internal cooling geometry. These film cooling flows have been tested at blowing ratios of 0.5 and 1.0 for the initial slot and blowing ratios of 0.15 and 0.3 for the two downstream slots. The measurements have been taken at exit chord Reynolds numbers of 500,000, 1,000,000, and 2,000,000 with inlet turbulence levels ranging from 0.7% to 12.6%. Film cooling effectiveness measurements were acquired using both thermocouples and infrared thermography. The infrared thermography shows the influence of secondary flows on film cooling coverage near the suction surface endwall junction. The film cooling effectiveness results at varied blowing ratios, turbulence levels and Reynolds numbers document the impact of these major variables on suction surface slot film cooling. The results provide a consistent picture of the slot film cooling for the present three slot arrangement on the suction surface and they support the development of an advanced double wall cooling method.

Conjugate Heat Transfer Characteristics of Double Wall Cooling on a Film Plate With Gradient Thickness

Double wall cooling, consisting of internal impingement cooling and external film cooling, is an advanced cooling method of gas turbines. In this paper, the flow and conjugate heat transfer characteristics of double wall cooling which has a film plate with gradient thickness are analyzed numerically. The detailed overall cooling effectiveness distributions are obtained by solving steady three dimensional Reynolds-averaged Navier-Stokes equations. In the double wall cooling scheme, seven vertical film holes and six impingement holes are staggered with same diameter (D), and the hole pitch of them are both set to 6D in flow direction and lateral direction. The gradient thickness along the flow direction is realized by setting the angle (α) between the lower surface of the film plate and the horizontal plane at −1.5 deg and 1.5 deg respectively. By comparing the results of four broadly used turbulence models with experimental data, SST k-ω is selected as the optimal turbulence model for double wall cooling analysis in this paper. In addition, the number of grids are finally determined to be 5.2 million by grid sensitivity calculation. The influence of the thickness gradient on the overall cooling effectiveness is revealed by comparing with the constant thickness film plate (Baseline 1 and 2), and all the cases are performed under four various coolant mass flow rates, which correspond to blowing ratios ranging from 0.25 to 1.5. The calculated results show that the thickening of the film plate downstream is beneficial to improve overall cooling effectiveness at low blowing ratio, which is benefit from two aspects. One is the thicken film plate weakens the flow separation in film hole and velocity of film hole outlet, another is the thicken film plate makes the impingement channels convergence, and impingement cooling is strengthened to some extent. However, with the increase of blowing ratio, the increasing trend gradually weakens due to the jet-off and limited impinge ability. For thickening film plate, the variations of the double wall cooling configurations are considered at initial film plate thickness tf of 2D and 3D, it is found that the ability to improve the overall cooling effectiveness by thickening the film plate downstream decrease as the initial film plate thickness increases, which is due to the increase of heat transfer resistance, and another finding is the cooling effectiveness of downstream thickening film plate with initial thickness of 2D is higher than that of 3D, which will provide a theoretical foundation both for improving cooling performance and reducing turbine blade weight at the same time. The influence of initial impingement gap H is also observed, and the study come to the fact that the best cooling performance occurred in H = 2D.

Analysis of the Unsteady Flow Field Inside a Fan-Shaped Cooling Hole Predicted by Large-Eddy Simulation

Film cooling is a common technique to manage turbine vane and blade thermal environment. Optimizing its cooling efficiency is furthermore an active research topic which goes in hand with a strong knowledge of the flow associated with a cooling hole. The following paper aims at developing deeper understanding of the flow physics associated with a standard cooling hole and helping guide future cooling optimization strategies. For this purpose, Large Eddy Simulations (LES) of the 7-7-7 fan-shaped cooling hole [1] is performed and the flow inside the cooling hole is studied and discussed. Use of mathematical techniques such as the Fast Fourier Transforms (FFT) and Dynamic Mode Decomposition (DMD) is done to quantitatively access the flow modal structure inside the hole based on the LES unsteady predictions. Using these techniques, distinct vortex features inside the cooling hole are captured. These features mainly coincide with the roll-up of the internal shear layer formed at the interface of the separation region at the hole inlet. The topology of these vortex features is discussed in detail and it is also shown how the expansion of the cross-section in case of shaped holes aids in breaking down these vortices. Indeed upon escaping, these large scale features are known to not be always beneficial to film cooling effectiveness.

Investigation of Film Cooling Using Time-Resolved Stereo Particle Image Velocimetry

In the present study an existing test rig at the Institute of Thermal Turbomachinery (ITS), Karlsruhe Institute of Technology (KIT) designed for generic film cooling studies is adopted to accommodate time resolved stereoscopic particle image velocimetry measurements. Through a similarity analysis the test rig geometry is scaled by a factor of about 20. Operating conditions of hot gas and cooling air inlet and exit can be imposed that are compliant with realistic engine conditions including density ratio. The cooling air is supplied by a parallel-to-hot gas coolant flow-configuration with a coolant Reynolds number of 30,000. Time-resolved and time-averaged stereo particle image velocimetry data for a film cooling flow at high density ratio and a range of blowing ratios is presented in this study. The investigated film cooling hole constitutes a 10°-10°-10° laidback fan-shaped hole with a wide spacing of P/D = 8 to insure the absence of jet interaction. The inclination angle amounts to 35°. The time-resolved data indicates transient behaviour of the film cooling jet.

Implementation of Hole-Pair in Ramp to Improve Film Cooling Effectiveness on a Plain Surface

Film cooling is one of the most critical technologies in modern gas turbine engine to protect the high temperature components from erosion. It allows gas turbines to operate above the thermal limits of blade materials by providing the protective cooling film layer on outer surfaces of blade against hot gases. To get a higher film cooling effect on plain surface, current study proposes a novel strategy with the implementation of hole-pair into ramp. To gain the film cooling effectiveness on the plain surface, RANS equations combined with k-ω turbulence model were solved with the commercial CFD solver ANSYS CFX11.0. In the numerical simulations, the density ratio (DR) is fixed at 1.6, and the film cooling effect on plain surface with different configurations (i.e. with only cooling hole, with only ramp, and with hole-pair in ramp) were numerically investigated at three blowing ratios M = 0.25, 0.5, and 0.75. The results show that the configuration with Hole-Pair in Ramp (HPR) upstream the cooling hole has a positive effect on film cooling enhancement on plain surface, especially along the spanwise direction. Compared with the baseline configuration, i.e. plain surface with cylindrical hole, the laterally-averaged film cooling effectiveness on plain surface with HPR is increased by 18%, while the laterally-averaged film cooling effectiveness on plain surface with only ramp is increased by 8% at M = 0.5. As the blowing ratio M increases from 0.25 to 0.75, the laterally-averaged film cooling effectiveness on plain surface with HPR is kept on increasing. At higher blowing ratio M = 0.75, film cooling effectiveness on plain surface with HPR is about 19% higher than the configuration with only ramp.

Numerical Study on Film Effectiveness and Heat Transfer Characteristics of Aero-Engine Nose Cone With Hot Air Film

When the aircraft works in an environment containing supercooled water droplets, it is easy to cause the engine inlet nose cone on the windward side to freeze, which not only affects the performance of the engine, but also leads to flight accidents. Therefore, it is necessary to research the anti-icing technology of aero-engine nose cone components. At present, the air intake cone of the aircraft mainly forms a thermal anti-icing system by means of hot film anti-icing and heating impingement anti-icing. In this paper, the effects of blowing ratio, film hole pitch, hole shape and film hole arrangement method on film heating effectiveness and heat transfer characteristics are studied by numerical simulation methods for the hot film anti-icing system of the nose cone. The results show that with the increase of the blowing ratio, the film heating effectiveness in the downstream area quickly decreases first and then increases, and the range of change aggrandizes. Under the given condition, as the pitch between the film holes decreases, the area covered by the film extends, and the heating effectiveness improves significantly. In the case of high blowing ratio, the advantage of film heating effectiveness of waist-shaped film hole is more apparent. The laterally-averaged film heating effectiveness of the staggered film holes is much higher than that of the aligned film holes. For the characteristics of film heat transfer, the hole pitch and hole shape has little effect on the heat transfer characteristics at low blowing ratio. In the case of high blowing ratio, the heat transfer effect will be greatly weakened when the hole pitch is increased, moreover the heat transfer efficiency of the waist-shaped hole is better. In the region near the exit of the secondary row film hole, the heat transfer characteristics of the staggered structure is stronger than that in the aligned structure. In addition, it is found that the high heat transfer region for staggered arrangement shows W–shaped.

Implementation of Wide Diffusion Angle V-Shaped Holes for Gas Turbine Cooling: Design Phase and Numerical Simulation

This paper describes the design process carried out to develop a new hole geometry. This geometry is able to increase the cooling coverage effect on a turbine blade, in order to have a higher efficiency compared to the standard holes. The first step of the activity described is a CFD analysis of the performances of different hole geometries on a flat plate. Starting from the cylindrical holes the performances of several geometries have been compared. This study allowed the determination of the geometrical parameters mostly responsible of the film effectiveness increase. In this way a criterion able to optimize the hole geometry has been found. Keeping as constraint the same inlet section for all the geometries, the shape of the outlet section was modified in order to maximize the film coverage performances. An optimized hole geometry had been determined. This solution, called V-Shaped hole is characterized by a wide lateral expansion angle and a negligible laidback angle and it is able to increase the cooling effectiveness compared to cylindrical and shaped holes with typical expansion angles (lateral and laidback about 10°). Finally, a comparison with an experimental campaign has been performed to confirm the main results of the CFD analysis.

Large Eddy Simulation of the 7-7-7 Shaped Film Cooling Hole at Axial and Compound Angle Orientations

Shaped film cooling holes are used extensively for film cooling in gas turbines due to their superior performance in keeping coolant attached to the surface, relative to cylindrical holes. However, fewer studies have examined the impact of the orientation of the shaped hole axis relative to the main flow direction, known as a compound angle. A compound angle can occur intentionally due to manufacturing, or unintentionally due to changes in the main flow direction at off-design conditions. In either case, the compound angle causes the film cooling jet to roll up into a strong streamwise vortex that changes the lateral distribution of coolant, relative to the pair of vortices that develop from an axially oriented film cooling hole. In this study, Large Eddy Simulation (LES) using the Wall-Adapting Local Eddy Viscosity (WALE) model was performed on the publicly available 7-7-7 shaped film cooling hole, at two orientations (0°, 30°) and two blowing ratios (M = 1, 3). Laterally-averaged film effectiveness was largely unchanged by a compound angle at a blowing ratio of 1, but improved at a blowing ratio of 3. For both blowing ratios, the lateral distribution of film was more uniform with the addition of a 30° compound angle. Both wall normal and lateral turbulent convective heat transfer was increased by the addition of a compound angle at both blowing ratios.