Effect of water spray in exhaust passage of steam turbine on flow field of the last stage during windage

Abstract

Under the windage condition, the steam in the last stage of steam turbine absorbs energy from rotor blade, which leads to overtemperature of the exhaust steam. Water spray is an effective way to lower the temperature in exhaust passage. The Euler-Lagrange particle tracking method is used to study the heat and mass transfer of the water spray in exhaust passage. The quantity of water spray under different conditions is predicted and the effect of water spray on the last stage flow field is analyzed. The results show that there is a small temperature difference between steam and liquid in the exhaust passage and a minimum evaporation of droplets for high steam velocity. The water spray quantity has a negative correlation with the inlet steam mass flow rate under different conditions and the ratio of water spray quantity to the inlet mass flow rate (water/steam ratio) has a quadratic relationship with the operating conditions. From 20%THA (Turbine Heat Acceptance) to 7%THA, the ratio of steam evaporated by water to mixture-vapor (H2O/mixture-vapor ratio) increases from 0.05% to 0.95%, and the water/steam ratio increases from 22% to 300%. The water spray has little effect on the rotor-stator clearance vortex and the separation vortex, but the height and velocity of the back flow vortex are increased. Meanwhile, the pressure of the last stage and the pressure ratio of the rotor blade increase, which result in the dissipating shaft power of the rotor blade reduces. Water spray has an effect on the flow field of the last stage, which is of great significance to correctly predict the windage conditions and study the dynamic characteristics of the rotor blade.