The subject of this paper is the optimization of a steam turbine impulse wheel control stage by flow path profiling of the shroud. The investigated control stage is derived from an existing industrial steam turbine design. The shroud contour is varied in radial direction within specified restrictions by an evolutionary algorithm. The algorithm is directly connected to a mesh generator and a CFD solver. The optimization goal is the reduction of the total pressure loss over the guide vanes. The geometry of the rotor blade has been retained unchanged within the presented investigations. The flow field of the varied stage is compared with the baseline geometry. The optimum candidates are further investigated with CFD simulations for different operating point scenarios. Numerical results show that the axisymetric flowpath profiling of the shroud has a considerable effect on the loss behavior of the whole stage over a wide range of pressure ratios. Due to flowpath profiling the boundary layer in the nozzle is significantly affected which results in a more uniformly shaped exit flow angle profile over the nozzle span and a significant reduction of the global secondary flow effects in the guide vanes. Both observations have a positive influence on the flow conditions to the subsequent rotor blade.

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