Analysis on optimal angle for hot steam injection dehumidification in a twisted blade cascade using entropy-weighted TOPSIS method

Abstract

The last stage(LS) of steam turbine operates in a wet steam environment, where the presence of the water droplets not only reduces the stage efficiency but also threatens the blade safety. Using the hot steam injection(HSI) into the hollow blades to reduce the wetness in the 3D twisted blades of the LS was proposed in this paper. Five steam injection cases with the angles ranging from -30° to 60° on the suction surface(SS) were designed. Based on the defined “KQJEI evaluation criteria”, the entropy-weighted TOPSIS multi-objective optimization method was introduced to determine the optimal injection angle. The results indicate that the efficiency ratio (E_R) holds the highest weight, while the condensation loss ratio (Q_R) has the lowest weight. An injection angle of 0° is identified as the optimal case, achieving the maximum relative closeness coefficient of 0.667. At 57% chord on the suction surface at 10% blade height under the optimal case, the peak nucleation rate and average droplet radius are 55.47% and 52.63% of the no hot steam injection(NHSI) case, respectively, with condensation loss reduced by 58.91%. At 50% blade height, the average droplet radius is only 0.005 μm. No condensation occurs above 75% blade height, and all liquid-phase parameters are zero. Steam injection at the 0° angle, which perpendicular to the blade surface, facilitates the manufacturing and processing. This study provides an important theoretical basis for dehumidification in the hollow twisted blades operating in the wet steam zone of steam turbines.