Inlet angle distribution of last stage moving blades for large steam turbines

Abstract

A simplified analytical method is presented for use in preliminary design studies on the final stages of large steam turbines to derive the required moving blade twist distribution to achieve near-zero incidence. The method incorporates the influence of nozzle twist and of casing flare which is shown to enter the analysis mainly through its effect on the distribution of streamline curvature. Provided flare is properly incorporated into the design, there appears to be no inherent reason why higher flare should limit performance unduly. Optimisation of moving blade twist for variations in load and back pressure is also investigated. Blade skeletons derived analytically are subjected to full throughflow calculations and refined where necessary to improve their likely performance. The results are compared with results of calculations with published commercial blade angle distributions, designed for both low- and high-flare turbines. The results of the throughflow calculations on the blades derived here, and the comparisons with apparently successful commercial blades, suggest that this method can be used as a preliminary design tool, freeing the designer from repetitive trial runs with a full throughflow calculation.