Experimental and Numerical Study on the Effects of the Relative Position of Film Hole and Orientation Ribs on the Film Cooling With Ribbed Cross-Flow Coolant Channel

To investigate the application of ribbed cross-flow coolant channels with film hole effusion and the effects of the internal cooling configuration on film cooling, experimental and numerical studies are conducted on the effect of the relative position of the film holes and different orientation ribs on the film cooling performance. Three cases of the relative position of the film holes and different orientation ribs (post-rib, centered, and pre-rib) in two ribbed cross-flow channels (135° and 45° orientation ribs) are investigated. The film cooling performances are measured under three blowing ratios by the transient liquid crystal measurement technique. A RANS simulation with the realizable k-ε turbulence model and enhanced wall treatment is performed. The results show that the cooling effectiveness and the downstream heat transfer coefficient for the 135° rib are basically the same in the three position cases, and the differences between the local effectiveness average values for the three are no more than 0.04. The differences between the heat transfer coefficients are no more than 0.1. The “pre-rib” and “centered” cases are studied for the 45° rib, and the position of the structures has little effect on the film cooling performance. In the different position cases, the outlet velocity distribution of the film holes, the jet pattern and the discharge coefficient are consistent with the variation in the cross flow. The related research previously published by the authors showed that the inclination of the ribs with respect to the holes affects the film cooling performance. This study reveals that the relative positions of the ribs and holes have little effect on the film cooling performance. This paper expands and improves the study of the effect of the internal cooling configuration on film cooling and makes a significant contribution to the design and industrial application of the internal cooling channel of a turbine blade.

Effects of Film Hole Shape and Turbulence Intensity on the Thermal Field Downstream of Single Row Film Holes

A nylon mesh coated with broadband thermochromic liquid crystal was set in different planes perpendicular to the mainstream direction at various locations downstream of the film hole. By the temperature visualization technique, the colorful non-dimensional temperature images on the nylon mesh of cylindrical hole, water-drop hole and dustpan shaped hole at different blowing ratios and turbulence at angle of 30° and 60° were visualized. The visualization experiment visually studied the effects of hole shape, hole inclination angle, blowing ratio and mainstream turbulence on the distribution of the film. The results show that stream-wise diffusion of water-drop hole reduces kidney vortex intensity, making higher attachment of the film of water-drop than that of cylindrical hole, consequently the lateral coverage range of water-drop hole film is wider than that of cylindrical hole film. The lateral diffusion of dustpan shaped hole further reduces the kidney vortex intensity. This obviously increases the film coverage and strengthens the adhesion of film of dustpan shaped hole. Increasing the inclination angle of the hole and the blowing ratio will increase the normal velocity of the jet and increase the thickness of the film. however, increasing inclination angle and blowing ratio will enhance kidney vortex intensity and decrease the film cooling effectiveness. The high turbulent intensity of mainstream will enhance the lateral diffusion of the film and enhance the mixing of the secondary flow and mainstream, so the continuity and uniformity of film are better. However, the intense mix of secondary flow and mainstream results in the non-dimensional temperature of the film drops sharply and the film coverage reduced accordingly.

Scalable Heat Transfer Characterization on Film Cooled Geometries Based on Discrete Green’s Functions

The aerothermal characterization of film cooled geometries is traditionally performed at reduced temperature conditions, which then requires a debatable procedure to scale the convective heat transfer performance to engine conditions. This paper describes an alternative engine-scalable approach, based on Discrete Green’s Functions (DGF) to evaluate the convective heat flux along film cooled geometries. The DGF method relies on the determination of a sensitivity matrix that accounts for the convective heat transfer propagation across the different elements in the domain. To characterize a given test article, the surface is discretized in multiple elements that are independently exposed to perturbations in heat flux to retrieve the sensitivity of adjacent elements, exploiting the linearized superposition. The local heat transfer augmentation on each segment of the domain is normalized by the exposed thermal conditions and the given heat input. The resulting DGF matrix becomes independent from the thermal boundary conditions, and the heat flux measurements can be scaled to any conditions given that Reynolds number, Mach number, and temperature ratios are maintained. The procedure is applied to two different geometries, a cantilever flat plate and a film cooled flat plate with a 30 degree 0.125” cylindrical injection orifice with length-to-diameter ratio of 6. First, a numerical procedure is applied based on conjugate 3D Unsteady Reynolds Averaged Navier Stokes simulations to assess the applicability and accuracy of this approach. Finally, experiments performed on a flat plate geometry are described to validate the method and its applicability. Wall-mounted thermocouples are used to monitor the surface temperature evolution, while a 10 kHz burst-mode laser is used to generate heat flux addition on each of the discretized elements of the DGF sensitivity matrix.

Upstream Film Cooling on the Contoured Endwall of a Transonic Turbine Vane in an Annular Cascade

The effects of mainstream flow velocity, density ratio (DR), and coolant-to-mainstream mass flow ratio (MFR) were investigated on a vane endwall in a transonic, annular cascade. A blow down facility consisting of five vanes was used. The film cooling effectiveness was measured using binary pressure sensitive paint (BPSP). The mainstream flow was set using isentropic exit Mach numbers of 0.7 and 0.9. The coolant-to-mainstream density ratio varied from 1.0 to 2.0. The coolant to mainstream MFR varied from 0.75% to 1.25%. The endwall was cooled by eighteen discrete holes located upstream of the vane passage to provide cooling to the upstream half of the endwall. Due to the curvature of the vane endwall, the upstream holes provided uniform coverage entering the endwall passage. The coverage was effective leading to the throat of the passage, where the downstream holes could provide additional protection. Increasing the coolant flowrate increased the effectiveness provided by the film cooling holes. Increasing the density of the coolant increases the effectiveness on the endwall while enhancing the lateral spread of the coolant. Finally, increasing the velocity of the mainstream while holding the MFR constant also yields increased protection on the endwall. Over the range of flow conditions considered in this study, the binary pressure sensitive paint proved to be a valuable tool for obtaining detailed pressure and film effectiveness distributions.

Aerothermal Effect of Cavity Welding Beads on a Transonic Squealer Tip

High pressure turbine blade tips are critical for gas turbine performance and are sensitive to small geometric variations. For this reason, it is increasingly important for experiments and simulations to consider real geometry features. One commonly absent detail is the presence of welding beads on the cavity of the blade tip, which are an inherent by-product of the blade manufacturing process. This paper therefore investigates how such welds affect the Nusselt number, film cooling effectiveness and aerodynamic performance. Measurements are performed on a linear cascade of high pressure turbine blades at engine realistic Mach and Reynolds numbers. Two cooled blade tip geometries were tested: a baseline squealer geometry without welding beads, and a case with representative welding beads added to the tip cavity. Combinations of two tip gaps and several coolant mass flow rates were analysed. Pressure sensitive paint was used to measure the adiabatic film cooling effectiveness on the tip, which is supplemented by heat transfer coefficient measurements obtained via infrared thermography. Drawing from all of this data, it is shown that the weld beads have a generally detrimental impact on thermal performance, but with local variations. Aerodynamic loss measured downstream of the cascade is shown to be largely insensitive to the weld beads.

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.