Conjugate Heat Transfer Analyses of a Turbine Vane With Different RANS Turbulance Models/2013
Thermal efficiency of a gas turbine engine is highly dependent on the turbine inlet gas temperature. Increasing the turbine inlet gas temperature increases its thermal efficiency whereas it creates some problems in relation to the endurance of the materials from which its blades are manufactured as well as the length of their operational lives. Different cooling techniques have been developed to overcome this problem. Internal, impingement and film cooling techniques are the most pronounced ones and widely utilized in modern gas turbine engines. In internal cooling, relatively cold and pressurized bleed air from compressor is fed into the internal channels of turbine rotor and NGV (Nozzle Guide Vane) to keep metal temperature of these components in the desired range. Through flow or serpentine-like internal cooling channels are usually equipped with pins and ribs to enhance the heat transfer rate between cooling air and blade material. Maximum cooling effectiveness has to be obtained with minimum cooling air to sustain desired engine performance.
Conjugate heat transfer analyses which use popular RANS equations do not provide the desired accuracy for turbine cooling applications. The main reason of this inaccuracy stems from the assumption of isotropic turbulence (excluding Reynolds Stress Model) causing insensitivity to curvature. Simulations possess some difficulties because of high curvatures of blade external surface and bends in the cooling channels which possibly cause flow separation. V2-f model is one of the new turbulence models which has been introduced to overcome these inaccuracies. In this research, the V2-f turbulence model together with different RANS models (k-ε Realizable, k- SST, RSM) are used to simulate the conjugate heat transfer for turbine vane which has cylindrical internal cooling passages. The experimental studies conducted by Hylton et. al.  are used as test cases to validate the analyses. Two cases (one subsonic and one supersonic) are simulated and compared with the experimental data. Findings are presented in the form of surface heat transfer coefficient and surface temperature.
Results of the V2-f model and RSM are good agreement with experimental data for two flow conditions which have different Reynolds Number, turbulent intensity and exit Mach number. V2-f model showed a better convergent behaviour as compared to RSM model. High coupling between momentum and turbulent stress equations result in stable divergence although under-relaxation values are moderate. Second-order predictions of Reynolds stresses cause fluctuations. From this point of view V2-f model is proven to be the most suitable way of implementing conjugate heat transfer analyses if one decides to use RANS equations.
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