Numerical Optimization of Stator Blade Sweep and Lean in an LP Turbine Stage

The paper describes results of direct constrained optimization of stator blade axial sweep and circumferential lean for the exit stage of a large power steam turbine, using Nelder-Mead’s method of deformed polyhedron. Values of the minimised objective function, that is stage losses with the exit energy are found from 3D viscous compressible computations. Turbulence effects are taken into account using Menter’s SST model. Among the optimized parameters are stator and rotor stagger angles, stator straight sweep and straight lean, stator compound sweep at the tip and compound lean at the root, giving the total number of 8 optimized parameters. The blade sections (profiles) are assumed not to change during the optimization. There are constraints imposed on the mass flow rate, exit swirl angle and reactions. A number of optimization runs is reported. First, optimization of the exit stage with stator blade circumferential lean, second, optimization of the stage with stator blade axial sweep, then, the stage is optimized with both sweep and lean of stator blades. In the above tasks, the process of optimization is carried out for a nominal load, however, due to the fact that exit stages of steam turbines operate over a wide range of flow rates away from the nominal conditions, the original and final geometries are also checked for low and high loads. Optimization gives designs with new 3D stacking lines of stator blades, and with significantly increased efficiencies, over large part of the assumed range of load, compared to the original design.