The numerically and experimentally investigated industrial steam turbine control stage is derived from a real design. Due to the production process and costs of the guide vanes for control stages of steam turbines the flowpath profiling is rotationally symmetric. However the combination of the two-dimensional shroud contour and the flow deflection in the guide vane results in a fully three-dimensional end wall contour having a strong influence on the secondary flow features in the turbine control stage. To obtain an improved profile for the nozzle shroud the reduction of the total pressure loss over the guide vanes is taken as an optimization criterion. The three-dimensional contour generates a diffuser flowpath between the suction and the pressure side of two guide vanes perpendicular to the main flow direction. This diffuser geometry affects the pressure distribution over the guide vane and therefore the formation mechanisms of secondary flows. For the experimental and numerical investigations a baseline shroud design and two additional profiled contours are analyzed in detail. The control stage test rig is operated with air and is capable to represent a wide range of operating conditions. The measurements show a considerable increase of the stage efficiency and power output. The effect of the flowpath profiling on the pressure distribution over the guide vane is clearly proved.

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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