Research On Wet Steam Condensation Flow Characteristics And Losses Control Methods In Steam Turbine

The later several stages of the low pressure cylinder of the condensing steam turbine and the full stages of the nuclear steam turbine all work in the wet steam zone.With the increase of the wetness,the wet steam brings two problems to the steam turbine.The first is that the steam condensing process deviates from the equilibrium state.The second is that the secondary droplets carried by the wet steam will produce erosion and impact on the turbine blade,which threaten the safe operation of the steam turbine.Therefore,the study of heat and mass transfer mechanism in the process of wet steam two-phase condensing flow is of great theoretical significance to improve the efficiency of steam turbine.It has very important application value to carry out the optimal design of the flow through in steam turbine.In view of the difficulties in theoretical and experimental research,numerical simulation has become an effective means to explore the wet steam two-phase condensing flow.Although the predecessors have studied the field and achieved some valuable achievements,there are still many problems to be further explored.The current condensation theory and numerical model still have defects,and the effectiveness of the condensing flow model in the three-dimensional complex flow needs to be examined.In addition,the mechanism of wetness losses caused by various complex factors in steam turbines is still not clear.The related research achievements of wetness losses are difficult to directly guide the design of turbine blading components.Based on the careful study of the related literature at home and abroad,based on the thermodynamics theory,the nucleation model and the water droplet growth model were studied,and a correction method was proposed combined with the existing experimental data.Two-fluid models of homogeneous and heterogeneous condensation are established from the point of volume averaging.The complex flow phenomena of the condensing flow and its effects on the wetness losses are analyzed.The quantitative calculation method of wetness losses and the control method of wet steam condensing flow were put forward.The main work includes:(1)A two-fluid numerical model is established,which takes into account the interphase heat and mass transfer,the effect of velocity slip and the turbulence.The Becker-Doring nucleation theory is deduced based on classical nucleation kinetics.The application range and calculation accuracy of the existing nucleation models are investigated by the experimental data.The Wolk-Strey nucleation model and non-isothermal modified nucleation model are reasonable for calculation of condensing flow in turbine.Based on the Langmuir-Maxwell model,a coupled model of heat and mass balance for droplet growth was established.The applicable range and calculation precision of the current droplet growth models were analyzed.The coupled model of heat and mass balance for droplet growth can be used to solve the droplet growth rate under any Kn,and has high calculation precision.(2)The distribution and influence factors of condensing flow in the final stage of steam turbine were analyzed.Combined with plane cascade pressure distribution test data obtained by White and Bakhtar,the correctness of the numerical model in this paper is verified.The influence of the coupling between the condensing flow and the high turbulent flow wake is studied.The aerodynamic shock wave and the condensation shock wave in the wet steam two-phase flow field are captured.The mechanism of wetness losses caused by the complex phenomena of shock wave and unsteady flow is revealed.(3)The quantitative evaluation method of wetness losses considering multiple factors is proposed.The wetness losses in the final stage of the steam turbine are calculated by wetness losses evaluation model.The influence of wet steam condensation on the final stage aerodynamic performance and work capacity is analyzed.When the inlet steam state deviates from the saturation state greater,the larger the wetness losses is,and the lower the stage efficiency is.(4)The methods of reducing the wetness losses by controlling the nucleation process and the water droplet growth process are studied.The influence of the surface roughness of the blade on the condensing flow characteristics of the stator is analyzed.The increase of the surface roughness of the blade leads to the increase of the dynamic loss in the cascade,but it can reduce the thermodynamic loss of the condensation to a certain extent.With the increase of surface roughness,the peak value of nucleation rate and the outlet wetness decrease.The positive/cosine function was proposed for endwall modification in the final stage of a steam turbine.The design method presented in this paper can be used to produce endwall shapes with different heights at different axial chords.The effect of asymmetric endwall modeling on the condensation characteristics of wet steam was studied.By comparison with the original cascade,the total pressure loss coefficient and outlet wetness were found to be ideal when the endwall was located at the middle of the stator cascade and the protruding parts were 3%of the blade height.The influence of blade surface heating on condensation characteristics in the final stage is analyzed.The volume heating of the stator blade surface can well restrain the condensation of the steam and raise the stage efficiency to a certain extent.