Study On The Flow Characteristics Of Low Pressure Section In Steam Turbine Under Low Volume Flow Conditions

The improvement of the distribution and aerodynamic performance in the low pressure section is of great significance for efficiency of steam turbine.As the important part of the low pressure section,exhaust hood is used to recover leaving kinetic energy of the last stage and transport steam to condenser.The flow and aerodynamic loss in the last stage and exhaust hood of steam turbine would be deteriorated under low volume flow condition.In order to analyze the impact of low volume flow condition on flow,CFX is applied to process the coupling calculation of the flow distribution in last stage and exhaust hood field from design condition to low volume flow condition to get the characteristics of vortex system in flow field.On account of the flow characteristics at low volume flow condition,increasing the inclination angle of the inside guide ring to improve the flow field distribution and aerodynamic performance,and verify the rationality of the program.Result indicates that the steam from last stage has a significant influence on the inlet distribution of the exhaust hood,the pressure and speed distribution is closer to the real inlet condition.More concretely,the upward tendency of streamline in the exit of last stage leads to the flow separation.When the volume flow continues to decrease,the vortex occurs at the top of the gap between the stationary blade and rotating blade.This phenomenon shows that the last stage is in blast condition.There would be vortex near the pressure surface of rotating blade in the horizontal section of 50% blade height with the decrease of volume flow.The flow separation and vortex weaken the power of the rotating blade.Changes in flow characters of the last stage directly impact on the flow distribution of exhaust hood.Under the low volume flow condition,a vortex shows up in the domain near inside guide flow ring,it grows larger and squeezes the mainstream area.Under severe conditions,the vortex develops to the top wall of the volute casing,greatly reducing the capacity of diffuser.In addition,the deterioration of flow in the diffuser would directly lead to the deterioration of fluid mixing in the exhaust passage,increasing the energy loss and reducing the aerodynamic performance of the exhaust hood.The flow in exit of exhaust hood becomes more unstable.The comparison shows that the effect of diffuser is an important factor to affect the aerodynamic performance of the exhaust hood.The inclination angle of the inside guide ring in the diffuser of the exhaust hood can be optimized so as to improve its adaptability under the low volume flow conditions.After modification,the result shows that properly increasing the inclination angle of the inside guide ring can optimize the flow condition and aerodynamic performance of the exhaust hood under low volume flow conditions.