Controlling Mixing at the Interaction Zone Through Systematic Variation of the Inlet Rotor Cavity Volume and Length Scale

Abstract

Mixing losses due to cavity related flows in axial steam turbines contribute considerably to overall aerodynamic losses. The coherent study presented in this paper examines the influence of rotor inlet cavity geometry on stage efficiency. The experimental work is supported by computational analysis. Inlet cavity geometry has been varied by reducing the axial and radial cavity lengths along with the volume. Six different configurations have been examined, focusing mainly on the flow interactions occurring at the zone between the cavity and main flow and their impact on stage efficiency. An upper stator-casing platform prolonged by 17% and 34%, and a radial wall length shortened by 13% and 25% offered a cavity volume reduction of 14% and 28%, respectively, compared to the initial cavity volume. The axial cavity wall length reduction impacts drastically on the vortex formation inside the cavity. A 17% length reduction leads initially to the bifurcation and re-connection of the vortex during inflow, whereas the 34% length reduction completely eliminates the presence of any vortex. On the other hand, the radial cavity wall length reduction affects the vortex positioning. Generally, the cases with radial wall length reduction show higher efficiency relative to the axial cavity length reduction. For the 14% cavity volume reduction cases this difference rises to 1%, and for the 28% cavity volume reduction the difference is even higher, attaining a 1.7% efficiency increase.