Wall Temperature Effects on Heat Transfer Coefficient for High-Pressure Turbines

For external convective heat transfer, a common assumption based on the conventional wisdom is that aerodynamics determines the heat transfer coefficient without being affected by wall temperature. In this paper, a systematic computational study has been carried out on the effects of the wall thermal boundary condition on the external heat transfer coefficient for a two-dimensional nozzle guide vane section. Heat transfer coefficients are worked out first in a typical uncoupled manner with specified wall temperatures. As an alternative verification of the findings, further computations are also made using the coupled solid-fluid conjugate heat transfer mode of the solver. The present results show a strong dependence of heat transfer coefficient on wall temperature, in a locally varying manner, which cannot be corrected by using the existing correlations based on a global correction. A detailed examination of the boundary layer highlights the impact of upstream cooling history on the local heat transfer coefficient distribution. Following on these findings, a new three-point nonlinear method to obtain the heat transfer coefficient is proposed. In this method, the heat transfer coefficient has been considered to vary with wall temperature rather than being independent of it as normally assumed. Heat transfer coefficient predictions for different wall temperatures have been compared between the conventional two-point method and the newly proposed one. The effectiveness of the new method has been clearly demonstrated.