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Advances in Numerical Prediction of Turbine Blade Heat Transfer

DOI: 10.1615/ICHMT.1994.IntSympHetatTransTurb.340
pages 501-510

Suhas V. Patankar
Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, and Innovative Research, Inc., Plymouth, MN 55447, USA

Abstract

Modern gas turbine engines are designed to give high specific power and thrust and low specific fuel consumption. These design goals can be reached by increasing the turbine inlet temperature. Over the past few decades, the inlet temperatures in actual engines have risen substantially as shown in Fig. 1 (taken from Hennecke, 1984). Of course, the blade materials cannot withstand these high temperatures without serious loss of durability. Therefore, some form of cooling of the turbine blades is required to maintain the blades at a lower, acceptable temperature as shown in Fig. 1. Efficient cooling thus holds the key to the design of efficient gas turbine engines.
The blades are normally cooled by a combination of internal cooling and film cooling. For cooling purposes, some air is diverted from the main stream at the exit of the compressor. The resulting reduced flow through the turbine leads to lower power and efficiency. Even when the coolant air is reinjected into the main stream, the irreversibilities associated with the mixing process cause the efficiency to deteriorate. Further, the blades should be cooled such that the metal temperature remains reasonably uniform to avoid high thermal stresses. Few tasks are so critical as the design of turbine blade cooling with the smallest possible degradation of efficiency and durability.
In the design of the cooling system, the local heat transfer coefficient between the hot gas and the blade surface must be predicted for the actual operating conditions in a turbine. It is believed that, even after decades of intensive research, the uncertainty in predicting the heat transfer coefficients is about ± 30%. This leads to increased expenditure of time and money in designing new engines.
The reason for such a major difficulty in predicting the heat transfer coefficients on a turbine blade lies in the simultaneous presence of many complicating factors. These include: pressure gradient, free-stream turbulence, transition, relaminarization, curvature, rotational body forces, film cooling, local separation bubbles, three-dimensionality, generation of vortices, unsteadiness and periodicity, boundary layer/shock interaction, compressibility, fluid property variations, and surface roughness. No simple empirical correlation for the heat transfer coefficient is expected to cover a wide range of conditions with good accuracy. However, as a result of the research in the last twenty years, the numerical solution of the boundary layer equations with an appropriate turbulence model shows the promise that a satisfactory prediction of turbine blade heat transfer may indeed be achieved. Already some remarkable progress has been made and the outlook for the future seems bright. The purpose of the present paper is to describe some of these advances and to provide suggestions for future research.

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