A Combined Numerical Simulation And Optimization Of Low Pressure Exhaust Hood And The Last Stage In High Power Steam Turbine

The low-pressure exhaust hood in steam turbine connecting the last stage and condenser is userd to recover leaving kinetic energy of the last stage and convert it to pressure, while guiding the steam flow from the last stage to conderser. The aerodynamic characteristics of exhaust hood have a dirct bearing on the pressure of turbine outlet, the recovery of leaving kinetic energy of the last stage and the steam condition of conderser inlet. Therefore, the analysis of flow field structure and sources of aerodynamic loss in exhaust hood as well as the study of its modified method of optimizaition ,which aim to improve its aerodynamic performance, have great important significance to increasing the efficiency of steam turbine unit.This paper firstly simulated and analyzed the complex flow field inside an exhaust hood model with dry steam model under different inlet swirl conditions. Result indicates that there are many complex vortices which seem to be the major sources of kinetic consumption and presssuse loss in exhaust hood. It is shown clearly by comparing different conditions that a certain inlet swirl angle has a favorable effect on the pressure recovery and aerodynamic efficiency of exhast hood, while also has an unfavorable effect on the uniform of outflow.In order to obtain more accurate inlet conditons of exhaust hood, a combined numerical simulation which couples the exhaust hood with the whole annular cascades, was carried out with real wet steam. Result shows that there are strong flow interactions between the last stage and exhaust hood which have a strong impact on the aerodynamic performance of the last stage and exhaust hood. There is a strong flow separation in the diffuser which leads to deterioration of pressure recovery and increase of aerodynamic loss. For this reason, The optimization of exhast hood was carried out. The flow separation is weakened by reducing the diffusion coefficient of the diffuser and increasing the leaning angle of the front wall. The pressure recovery and aerodynamic efficiency of the exhaust hood has been inproved by the optimization.