Influence of Blade Shape and Water Droplet Size on Fractional Deposition in the Last Stages of Steam Turbine

The improvement in Rankine steam cycle power generation efficiency is dependent upon lowering heat rejection during condensation for enhanced work ratio that is limited by allowable wetness of steam at turbine exhaust. In this paper the influence of the blade shape and droplet size on the deposition in the last stage blades of the condensing steam turbine is investigated. The blade profile of the steam turbine has been adopted from an experimental set up along a reference test case. While passing through the lowest pressure stages of turbine, steam crosses the saturation line with a variable wetness thus reducing the efficiency of the turbo- cylinder. These water droplets deposit on the stator guide blade surface, and coagulate in to films and rivulets, which are dragged towards the trailing edge by viscous drag. The large droplets then hit the rotor blade due to strong aerodynamic force which causes erosion on the rotor blade surface. The two prominent phenomenons responsible for deposition in blade surface are inertial impaction and turbulent-diffusion. With increasing stagger angle, the fractional deposition departs highly from single value from leading edge to trailing edge showing strong influence of blade shape on fractional deposition. From this background, the aim of this work is to analytically outline the unsaturated water vapour condensing mechanism in lowest pressure stages of steam turbine and investigate the influence of droplet size and rotor blade profile on deposition.