The cooling air in the secondary air system of gas turbines is routed through the inside of the rotor shaft. The air enters the rotor through an internal extraction in the compressor section and flows through different components to the turbine blades. Constant improvements of the secondary air system is a basic element to increase efficiency and power of heavy duty gas turbines. It is becoming more and more important to have a precise calculation of the heat transfer and air temperature in the internal cooling air system. This influences the cooling behavior, the material temperature and consequently the cooling efficiency. The material temperature influences the stresses and the creep behavior which is important for the life time prediction and the reliability of the components of the engine. Furthermore, the material temperature influences the clearances and again the cooling flow, e.g. the amount of mass flow rate, hot gas ingestion etc. This paper deals with an investigation of the influence of heat transfer on the internal cooling air system and on the material temperature. It shows a comparison between numerical calculations with and without heat transfer. Firstly, the Navier-Stokes CFD calculation shows the cooling flow physics of different parts of the secondary air system passages with solid heat transfer. In the second approach, the study is expanded to consider the cooling flow physics under conditions without heat transfer. On the basis of these investigations, the paper shows a comparison between the flow with and without heat transfer. The results of the simulation with heat transfer show a negligible influence on the cooling flow temperature and a stronger influence on the material temperature. The results of the calculations are compared with measured data. The influence on the material temperature is verified with measured material temperatures from a Siemens Model V84.3A gas turbine prototype.

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