An Analysis of the Merits of CFD for the Performance Prediction of a Low Pressure Steam Turbine Radial Diffuser

Abstract

The performance of the last stage and the diffuser of a low pressure steam turbine module are inherently linked. Design alterations in one part must take into consideration the subsequent effects imposed upon the other. Through a linked design system, including both the last stage and the exhaust, the last stage performance can be predicted more accurately and hence further performance improvements are possible. The importance of a valid prediction for the effects of the diffuser upon last stage power and efficiency become even more important when considering retrofit applications. In a retrofit project, a new flow path including modernised blades are installed in an existing older steam turbine casing. In these cases, the exhaust geometry does not lend itself to allowing the last stage to perform to its originally predicted level, but an optimization has to be established for a diffuser design, that links the last stage blade to the exhaust. This paper presents the coupled CFD design system, used by Alstom Power, to highlight the importance of including the 3D exhaust geometry, coupled to the last stage, when conducting last stage performance calculations. The test case is of a typical retrofit application. A comparison with measured test data shows the significance of the fidelity of any CFD system claiming to predict last stage performance, especially when used during the optimisation process of the diffuser.