Aerodynamic Concept for Very Large Steam Turbine Last Stages

Abstract

A move to ever larger low pressure steam turbines inevitably leads to an increase in the blade height to hub diameter ratio and the extent and magnitude of supersonic flow. This paper looks at the options available to the designer to mitigate the aerodynamic challenges associated with an increase in turbine size. Combinations of the effects of lean and sweep are explained using simple arguments based on flow field streamlines, then demonstrated in a stationary blade design by numerical and experimental methods. Developments in the principal regions of transonic and supersonic flow are shown by stationary blade hub and rotating blade tip examples. A stationary blade design which optimises the flow field effects shown is presented. The paper concludes by evaluating the unsteady flow in a very large final stage design.