Hongde Jiang

Effect of Turning Vane Configurations on Heat Transfer and Pressure Drop in a Ribbed Internal Cooling System

The ribbed serpentine blade cooling system is a typical configuration in the modern gas turbine airfoil. In this study, experimental and the numerical efforts were carried out to investigate the local heat transfer and pressure drop distribution of a ribbed blade cooling system with different configurations in the turn region. A test rig containing a ribbed rectangular U-duct with a 180 deg round turn was built in Tsinghua University for this study. The transient liquid crystal method was applied to get the heat transfer distribution. Nine test cases with three turn configurations under three Reynolds numbers were carried out in the experiment. Pressure was measured along the duct in order to determine the influence of turning vane configurations on pressure drop. The test cases were also analyzed numerically based on Reynolds-averaged Navier-Stokes (RANS) with three different turbulence models: the k-e model, the SST reattachment model, and the Omega Reynolds stress (ORS) turbulence model. Both the experimental and numerical results showed a significant influence of the turning vane configuration on the heat transfer and pressure drop in the convective cooling channel. Among the three configurations, the loss coefficient of turn in configuration 2 was lowest due to the introduction of turning vane. Even the ribs were added in the turn region of configuration 3, the loss coefficient and friction factor are reduced by 23% and 17.5%, respectively. Meanwhile, the heat transfer in baseline configuration is still the highest. As the introduction of turning vane, the heat transfer in the region after turn was reduced by 35%. In configuration 3, the heat transfer in the turn region was enhanced by 15% as the ribs installed in the turn region. In the before turn region, the pressure drop and heat transfer was not influenced by the turn configuration. All the turbulence models captured the trend of heat transfer and pressure drop distribution of three test sections correctly, but all provide overpredicted heat transfer results. Among the models, the ORS turbulence model provided the best prediction. While aiming at high heat transfer level and low pressure drop, it is suggested that a suitable turn configuration, especially with the turning vane and/or the ribs, is a promising way to meet the conflicted requirements of the heat transfer and pressure drop in the convective cooling system.

Unsteady Structure and Development of a Row of Impingement Jets, Including Kelvin–Helmholtz Vortex Development

Considered is a cylinder channel with a single row of ten aligned impinging jets, with exit flow in the axial direction at one end of the channel. For the present predictions, an unsteady Reynolds-Averaged Navier–Stokes (RANS) solver is employed for predictions of flow characteristics within and nearby the ten impingement jets, where the jet Reynolds number is 15,000. Spectrum analysis of different flow quantities is also utilized to provide data on associated frequency content. Visualizations of three-dimensional, unsteady flow structural characteristics are also included, including instantaneous distributions of Y-component vorticity, three-dimensional streamlines, shear layer parameter ψ, and local static pressure. Kelvin–Helmholtz vortex development is then related to local, instantaneous variations of these quantities. Of particular importance are the cumulative influences of cross flows, which result in locally increased shear stress magnitudes, enhanced Kelvin–Helmholtz vortex generation instabilities, and increased magnitudes and frequencies of local flow unsteadiness, as subsequent jets are encountered with streamwise development.

Optimization of the Hole Exit Shaping of Film Holes Without and With Compound Angles for Maximal Film Cooling Effectiveness

Shaped film holes can achieve higher film cooling effectiveness compared with the simple cylindrical film holes. According to former studies, the geometry of the shaped film holes has significant influence on the cooling performance. In order to maximize the film cooling effectiveness of the shaped holes, a two-level design optimization methodology of the hole exit shaping is developed in the present study.The optimization methodology consists of a parametric design and CFD mesh generation tool called Coolmesh, a RANS CFD solver, a database of film cooling effectiveness distributions, a metamodel, and a genetic algorithm (GA) for evolutionary optimization. A binary parametric representation of the 2D hole exit shaping is initiated based on the B-spline methods. The metamodel can efficiently predict the detailed distribution of film cooling effectiveness using the CFD results in the database, which is continuously updated for higher accuracy. In each first-level iteration, a second-level GA optimization search is carried out coupled with the metamodel, and then the optimal geometry is evaluated using CFD methods and added to the database. An anisotropic turbulence model is applied to the CFD solver for higher accuracy according to a detailed experimental validation using PSP measurements.In the present study, three design optimizations of the shaped holes without and with compound angles are carried out on a flat plate. The optimization methodology can efficiently find the optimal geometries of shaped holes using only hundreds of CFD runs. For the shaped holes with compound angle, the optimized geometry can generate a back flow vortex which interacts with the shear vortex and weakens the mixing of coolant and hot gas, resulting in a higher film cooling effectiveness on the plate.Copyright

Film Cooling Performance of the Embedded Holes in Trenches With Compound Angles

Film cooling performance for a row of cylindrical holes can be enhanced by embedding the row in a suitable transverse slot. The compound angle of the holes can even more affects the cooling performance at downstream of the injections. In this study the cooling performance of the embedded holes in transverse trenches with different compound angles are explored both by pressure sensitive paint (PSP) experiment technology and RANS algorithm. A film cooling test rig was built up in Tsinghua University, which contains an accelerating free stream section to model the surface of a turbine airfoil. The PSP technology is applied in the tests to obtain the film cooling effectiveness. The experiments are performed for a single mainstream Reynolds number based on free-stream velocity and film hole diameter of 4000. Considering three compound angles, 0°, 45° and 90°, and with or without transverse trenches. All six cases are tested at three different coolant-to-mainstream blowing ratios of 0.5, 1.0, and 1.5. Meanwhile, the test cases are numerically simulated based on RANS with k-e turbulence model to show the detail of the flow patterns. Both the experimental and numerical results show that the adiabatic film effectiveness is relative insensitive to the blowing ratio in the case of holes with trenches. Moreover, it could be improved with a more uniform spanwise distribution. It is mainly due to the blockage of the ejected coolant at the downstream edge of the trench, which forces a portion of the cooling air to spread laterally within the trench prior to issuing onto the upper surface. Both 45° and 90° compound angles can further enhance the film cooling effectiveness over the axial ejection, this is mainly due to the lateral momentum component of the ejection. A lateral passage vortex is formed inside the trench which strengthens the lateral spreading of the jets. The 45° compound angle gives a higher film cooling effectiveness overall.Copyright

Geometrical Optimization and Experimental Validation of a Tripod Film Cooling Hole With Asymmetric Side Holes

A tripod cylindrical film hole with asymmetric side holes is studied numerically and experimentally on a flat plate for higher film cooling effectiveness. First, the influences of geometrical parameters are studied and the optimum configurations of the asymmetric tripod hole are found in a design of experiments (DoE) optimization study based on an improved numerical model for film cooling prediction, in which more than 100 3D computational fluid dynamics (CFD) simulations are carried out. Then, one optimum configuration of the asymmetric tripod hole is examined experimentally using pressure-sensitive paint (PSP) measurements and compared to the experimental results of the simple cylindrical film hole and a well-designed shaped film hole. The flow and heat transferring characteristics of the asymmetric tripod holes were explored from the DoE results. The side holes can form a shear vortex system or an antikidney vortex system when proper spanwise distances between them are adopted, which laterally transports the coolant and form a favorable coolant coverage. According to the experimental results on flat plate, the optimal configuration of the asymmetric tripod hole is significantly better than cylindrical hole, especially at high blowing ratios. Furthermore, it can provide equivalent or even higher film cooling effectiveness than a well-designed shaped hole.

Experimental Investigation of Local and Average Heat Transfer Coefficients Under an Incline Impinging Jet Array, Including Jets With Low Impingement Distance and Inclined Angle

Comprehensive impingement heat transfer coefficients data are presented with varied Reynolds number, hole spacing, jet-to-target distance and hole inclination utilizing transient liquid crystal. The impingement configurations include: streamwise and spanwise jet-to-jet spacing (X/D, Y/D) are 4~8 and jet-to-target plate distance (Z/D) is 0.75~3, which composed a test matrix of 36 different geometries. The Reynolds numbers vary between 5,000 and 25,000. Additionally, hole inclination pointing to the upstream direction (?: 0°~40°) is also investigated to compare with normal impingement jets. Local and averged heat transfer coefficients data are presented to illustrate that 1) surface Nusselt numbers increase with streamwise development for low impingement distance, while decrease for large impingement distance. The increase or decrease variations are also influenced by Reynolds number, streamwise and spanwise spacings. 2) Nusselt numbers of impingement jets with inclined angle are similar to those of normal impingement jets. Due to the increase or decrease variations corresponding to small or large impingement distance, a two-regime based correlation, based on that of Florschuetz et al., is developed to predict row-averaged Nusselt number. The new correlation is capable to cover low Z/D~0.75 and presents better prediction of row-averaged Nusselt number, which proves to be an effective impingement design tool.

Optimal Arrangement of Combined-Hole for Improving Film Cooling Effectiveness

Film cooling technique is widely used to protect the components from being destroyed by hot mainstream in a modern gas turbine. Combining round-holes is a promising way of improving film cooling effectiveness. A DoE (design of experiment) simulation of 396 cases focusing on the arrangement of the combined-hole with double holes for improving film cooling performance is carried out in this work, and the influence of an aerodynamic parameter, blowing ratio is considered as well. The dimensionless lateral distance (PoD) and compound angle (CA) of the double holes have relative influence on the film cooling performance of the combined-hole unit. At the low blowing ratio, increasing symmetrical compound angle (SCA) has positive influence on the area-average effectiveness (EFF) of the combined-hole. But at the intermediate and large blowing ratio, the influence of SCA on the area-average EFF depends on the range of PoD. At the small PoD, the area-average EFF ascends basically along SCA axis. However, the area-average EFF first ascends and subsequently descends along SCA axis at the large PoD. Asymmetrical compound angle (ACA) is also considered to fit the antikidney vortexes produced in the combined-hole film cooling compared to their ideal schematic. However, the film cooling effect of the cases with ACA is not as good as expected. The area-average EFF of ACA cases locates in the level between that of the adjacent SCA cases. The optimal arrangement of combined-hole unit for improving film cooling effectiveness is relative to the local flow field. The optimal arrangement of PoD and CA for improving the combined-hole film cooling performance is different at different blowing ratios.

Effects of Streamwise Pressure Gradient and Convex Curvature on Film Cooling Effectiveness

The effects of streamwise pressure gradient and convex wall curvature on film cooling effectiveness are investigated using PSP technology. Film cooling under five different main stream pressure gradients on both flat and convex wall is examined. The cooling hole has an inclined angle of 30° and no compound angle with a hole length of L/D = 4. The convex wall has a constant radius of r/D = 30. Numerical simulations are also conducted to gain more flow field information. For the flat wall case, film cooling effectiveness is higher with greater favorable pressure gradient for low blowing ratios. While for high blowing ratio cases, cooling effectiveness doesn’t vary much with streamwise pressure gradient. Film cooling effectiveness is increased significantly on convex wall compared with flat wall for M<0.5. The effect of streamwise pressure gradient is greater on convex wall and becomes unneglectable. The influence of streamwise pressure gradient and convex wall curvature is conjugated and should be discussed together. For all blowing ratios, film cooling effectiveness is apparently higher for larger mainstream favorable pressure gradient on convex wall.Copyright

Multi-Dimensional Platform for Cooling Design of Air-Cooled Turbine Blades

Progresses in conjugate heat transfer simulation method provide a new approach for turbine blade cooling design. The procedure of air-cooled blade design introduced in this paper consists of two levels, a schematic design using 1D flow network calculation method, and a detailed design based on CFX conjugate heat transfer simulation.The program platform for the design method was developed, including an air-cooled blade design program, a 1D cooling structure flow network solver, and conjugate mesh generation tools for air-cooled blade.The design platform contains parametric methods for blade profile, cooling channel, and various cooling structures. The key parametric algorithm named Element Design Method was invented and introduced, which brings about parametric design for complex cooling channel, and accelerates the calculation model generation during the schematic design and detailed design.The flow network solver for schematic design consists of a pressure solving program, a temperature solving program, and a film cooling solving program. The pressure solving program uses linear method to solve the momentum equation, so higher stability of the flow network solver can be achieved. In detailed design, CFD pre-treatment using the commercial software is time-consuming. The mesh generation tools, combined with parametric design programs, can automatically create hexahedron/mixed mesh for turbine cascade and cooling structures with considerable speed and quality, which significantly reduces the difficulty of pre-treatment during detailed design.Copyright

Effect of Film Cooling Arrangement on Impingement Heat Transfer on Turbine Blade Leading Edge

Impingement cooling plays an important role in gas turbine blade leading edge where proper heat transfer distribution is needed for extremely high and nonuniform thermal load.A 2/3 cylinder leading edge model with 3 arrays of film cooling holes was investigated with 8 film cooling arrangements. The impingement parameters and the jet Reynolds number were kept the same for the 8 configurations. The transient liquid crystal (TLC) measurement was applied on heat transfer coefficient on the leading edge. A 3D numerical method with the SST k-ω model was verified by experimental data, which shows a heat transfer error less than 15%.The film suction creates both local heat transfer enhancement and limit effect to wall jets. The hole position of film cooling holes significantly affects the shape of high heat transfer area and cooling of the intermediate area. The array angle of film cooling holes affects the spread of heat transfer laterally. The Nu in stagnation zone decreases with the increase of array angle of film cooling holes. Smaller pitch of film cooling holes helps decrease the size of fountaining flow and heat transfer valley. The Nu in stagnation zone increases with the decrease of pitch of film cooling holes.The hole position of x0/P = 0.125 is recommended for the best cooling performance in the intermediate area. The configurations with θ = 13 or P/pf = 3 work best in this study.Copyright