Using CFD to Enhance the Preliminary Design of High-Pressure Steam Turbines

This paper focuses on the use of the CFD for improving a steam turbine preliminary design tool. Three-dimensional RANS analyzes were carried out in order to investigate independently the effects of profile, secondary flow and tip clearance losses on the efficiency of two high-pressure steam turbine stages. The parametric study included geometrical features such as stagger angle, aspect ratio and radius ratio, and was conducted for a wide range of flow coefficients to cover the whole operating envelope. The results are reported in terms of stage performance curves, enthalpy loss coefficient and span-wise distribution of the blade-to-blade exit angle. A detailed discussion of these results is provided in order to highlight the different aerodynamic behavior of the two geometries. Once the analysis was concluded, the tuning of a preliminary steam turbine design tool, based on a correlative approach, was carried out. In lack of a large set of experimental data, the information get by the post-processing of the CFD computations were applied to update the current correlations, in order to improve the accuracy of the efficiency evaluation for both the stages. Finally, the prediction of the tuned preliminary design tool were compared, in terms of stage efficiency, with the results of the CFD computations, in a broad range of flow coefficients and in different real machine layouts.