Wall Temperature Effects on Heat Transfer Coefficient

Abstract

Developments of High Pressure Turbine (HPT) blades and vanes are particularly affected by the accuracy and consistency in heat transfer prediction techniques. The conventional wisdom in heat transfer analysis considers that aerodynamics fully determines Heat Transfer Coefficient (HTC) distribution along the blade surface, so that wall temperature should have no influence on HTC. The effect of the wall temperature on the heat transfer coefficient has been rarely studied, although there have been some rather scattered correlations regarding the influence of the wall-inflow temperature ratio on HTC, largely based on simple geometry configurations. There seems to be a need to answer two related questions: a) to what extent the conventional wisdom is valid, in particular for a transonic HPT blading? b) what is a consistent way by which the HTC should be worked out in CFD (as well as in experiment)? In this paper, computational analyses are carried out on the effects of wall thermal boundary condition on external HTC for a 2D Nozzle Guide Vane (NGV) profile. The study is performed by using Fluent® commercial Reynolds-Averaged Navier Stokes Equations solver. Further computations are also made using a coupled solid-fluid Conjugate Heat Transfer (CHT) method with an internally cooled blade. The present results show a strong dependence of HTC on wall temperature, far above that predicted by using the existing correlations, highlighting the importance of upstream boundary layer history on the HTC distribution. Influence of the wall temperature on the trailing edge shock position has been also observed. Based on the results found, a new method is proposed, allowing to model the HTC dependence on wall temperature with much improved accuracy as demonstrated.