Jonathon E. Slepski

Aerodynamic Performance Assessment of Part-Span Connector of Last Stage Bucket of Low Pressure Steam Turbine

Modern steam turbines often utilize very long last stage buckets (LSB’s) in their low-pressure sections to improve efficiency. Some of these LSB’s can range in the order of 5 feet long. These long buckets (aka “blades”) are typically supported at their tip by a tip-shroud and near the mid span by a part span shroud or part span connector (PSC). The PSC is a structural element that connects all the rotor blades, generally at the mid span. It is primarily designed to address various structural issues, often with little attention to its aerodynamic effects. The objective of the current work is to quantify the impact of PSC on aerodynamic performance of the last stage of a LP steam turbine by using detailed CFD analyses. A commercial CFD solver, ANSYS CFX™, is used to solve the last stage domain by setting steam as the working fluid with linear variation of specific heat ratio with temperature. A tetrahedral grid with prismatic layers near the solid walls is generated using ANSYS WORKBENCH™. The results show a cylindrical PSC reduces the efficiency of the last stage by 0.32 pts, of which 0.20 pts is due to the fillet attaching the PSC to the blade. The results also show insignificant interaction of the PSC with the bucket tip aerodynamics. The work presents a detailed flow field analysis and shows the impact of PSC geometry on the aerodynamic performance of last stage of steam turbine. Present work is useful to turbine designer for trade-off studies of performance and reliability of LSB design with or without PSC.Copyright