The accurate prediction of the life cycle is one of the most challenging issues in turbomachinery design. Nowadays life cycle calculations for radial turbines focus on mechanical loads such as centrifugal and vibration forces. Because of enhanced turbine inlet temperatures with inevitably increasing thermal stress, the requirements in the design process of turbine wheels become higher. Therefore, it is desirable to know the temperature profile and thus the thermal stress in the turbine wheel as early as possible in the design process for steady state operating points and transient operation.
This paper reports the development of a fast empirical method to calculate the heat transfer coefficients on the surface of a radial turbine wheel for steady state operating points. In order to do this, steady state Conjugate Heat Transfer (CHT) investigations of a turbocharger turbine wheel for commercial application were performed to model the heat transfer between the fluid and the solid state. These investigations provide a basis for the analysis and characterization of the heat transfer distribution at the turbine wheel and the flow phenomena that cause these. The empirical method for determining heat transfer coefficients of a turbine wheel is developed based on the numerical results. To model the local different heat transfer coefficients the turbine wheel is divided in several surface segments which correspond to the geometry of a radial turbine wheel.
To validate the method the heat transfer coefficients from the empirical model are used as boundary conditions for a subsequent Finite Element Analysis (FEA). The calculated temperatures of the FEA results are compared to those of the CHT simulation and to the experimental data. For operating points with a circumferential velocity of u ≤ 0.75 u0 a good agreement are reached. The deviations increase for higher circumferential velocities. Furthermore the number of surface segments is varied to show the influence of the segmentation level to the temperature profile. It is also possible to reach a good agreement for operating points of u ≥ 0.75 u0 if the blade is segmented over its height. With the presented method a fast prediction of the heat transfer coefficients and the steady state temperature field of the turbine wheel are possible for steady state operating points.