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

The geometry of the exit shroud cavity where the rotor shroud leakage flow reenters the main passage flow is very important due to the dominant influence of the leakage flow on the aerodynamics of low aspect ratio turbines. The work presented in this paper investigates, both experimentally and numerically, possibilities for the control of shroud leakage flow by modifications to the exit shroud cavity. The processes through which the leakage flow affects the mainstream aerodynamics identified in the first part of this study were used to develop promising strategies for reducing the influence of shroud leakage flow. The experimental program of this study was conducted on a three-stage model air turbine, which was extensively supported by CFD analysis. Three different concepts for shroud leakage flow control in the exit cavity were analyzed and tested: (a) profiled exit cavity downstream end wall, (b) axial deflector, and (c) radial deflector concepts. Reductions in aerodynamic losses associated with shroud leakage were achieved by controlling the position and direction at which the leakage jet reenters the mainstream when it leaves the exit shroud cavity. Suggestions are made for an optimum shroud and cavity geometry.