Numerical Simulation of Some Heat Transfer and Fluid Flow Phenomena for Gas Turbine Blades and a Transonic Turbine Stage
Abstract: With the attempts to achieve higher thermal efficiency, turbine blades are exposed to very high temperature gases and may undergo severe thermal stress and fatigue. Thus, in order to develop optimal cooling strategies and reduce the heat transfer it is important to obtain a good understanding of both the complex flow field and the heat transfer characteristics in turbine rotor/stator hot-gas passages. The flow field in a high pressure gas turbine is very complex. It is strongly three-dimensional, unsteady, viscous, with several types of secondary flows and vortices (passage vortex, leakage flow, horseshoe vortex, etc.). Transitional flow and high turbulence intensity result in additional complexities. The most significant contribution to the unsteadiness of the flow field is the relative motion of the blade rows. The understanding of such complex flow fields and heat transfer characteristics is necessary to improve the blade design and prediction in terms of efficiency as well as the evaluation of mechanical and thermal fatigue. This thesis aims to investigate the convective flow and heat transfer processes in turbine rotor/stator hot-gas passages. The focus is on turbine aerodynamics and heat transfer behaviour at the mid-span location, and at the rotor tip and casing region. The heat transfer and fluid flow has been numerically simulated by CFD (Computational Fluid Dynamics) methods for turbulent and compressible flow conditions. A commercial finite volume based Navier-Stokes solver FLUENT was extended to multi-block and parallel computations, with implementation of some turbulence models. Grid and scheme independence has been verified, and a few general guidelines about the numerics are summarized. In this study numerical simulations of heat transfer and fluid flow have been performed using different turbulent models (the Spalart-Allmaras model, the standard high Re k-? model, the low Re k-? model, the low Re k-? (SST) model and model). Firstly, the study was focused on the different turbulence models in order to assess the capability of the models to correctly predict the blade heat transfer. Secondly, the effect of different tip clearances on the blade tip and casing, the effect of free-stream turbulence, length scale and variations in rotational speed of the rotor on heat transfer and fluid flow was studied. Also, in this work, a numerical study has been performed to simulate the unsteady fluid flow and heat transfer in a transonic high-pressure turbine stage. The predicted heat transfer and static pressure distributions show reasonable agreement with the experimental data. In general, it is shown that the model yield the best agreement with measurements. It was also observed that the tip clearance has a significant influence on the local tip heat transfer coefficient distribution. Comparison of the different length scales at the same turbulence intensity showed that the stagnation heat transfer was significantly increased as the length scale increased. However, the inlet length scale showed no significant effect on the blade tip or rotor casing heat transfer. Also, the results presented in this thesis show that the rotational speed in addition to the turbulence intensity and length scale have an important contribution to the turbine blade aerodynamics and heat transfer.
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