The Influence of Shroud and Cavity Geometry on Turbine Performance — An Experimental and Computational Study: Part I — Shroud Geometry

Imperfections in the turbine annulus geometry, caused by the presence of the shroud and associated cavity have a significant influence on the aerodynamics of the main passage flow path. In this paper the datum shroud geometry, representative of steam turbine industrial practice, was systematically varied and numerically tested. The study was carried out using a three-dimensional multi-block solver, which modelled the flow in a 1.5 stage turbine. The following geometry parameters were varied: - Inlet and exit cavity length, - Shroud overhang upstream of the rotor leading edge and downstream of the trailing edge, - Shroud thickness for fixed casing geometry and shroud cavity depth, and - Shroud cavity depth for the fixed shroud thickness. The aim of this study was to investigate the influence of the above geometric modifications on mainstream aerodynamics, and to obtain a map of the possible turbine efficiency changes caused by different shroud geometries. The paper then focuses on the influence of different leakage flow fractions on the mainstream aerodynamics. This work highlighted the main mechanisms through which leakage flow affects the mainstream flow and how the two interact for different geometrical variations and leakage flow mass fractions.