Research On Non-eqilibrium Phase Change Flow And Shock Wave In Wet Steam Zone Of Steam Turbine

As the core energy conversion equipment in the energy and power industry,the flow phenomena in the low-pressure cylinder of the steam turbine affect the efficiency most.Because the cascades of the low-pressure cylinder run in the wet steam area,the stage efficiency of the steam turbine is seriously affected.In the final stage cascade,the high-speed expansion process of wet steam deviates from the thermodynamic equilibrium state,releasing the latent heat of condensation and appearing non-equilibrium phase transition phenomena,which will show various unsteady phenomena,especially the appearance of condensation shock wave and droplet evaporation,which have a great influence on the thermodynamic parameters of wet steam.Therefore,further research on non-equilibrium phase transition flow of wet steam and unsteady phenomena such as condensation shock wave and droplet evaporation are of great significance for improving efficiency and safety of steam turbine.In this paper,the relationship between nonequilibrium phase transition flow of wet steam and condensation shock wave and droplet evaporation is studied by means of numerical simulation,and the following contents are studied:First,on the basis of previous research,based on Euler-Euler method,condensation theory and heat and mass transfer mechanism,a numerical model of condensation phase transition of wet steam is established,which considers the effect of Condensation Shock wave.The control equation is solved by differential solution in high-resolution second-order TVD format to achieve accurate shock capture effect,and the error between the numerical model and experimental data is compared.The reliability and accuracy of mathematical model and numerical method are checked.Secondly,the non-equilibrium phase transition of wet steam is simulated by numerical simulation,and condensation shock phenomena occurring during the two-phase flow of wet steam are captured.It is found that condensation of wet steam occurs at Wilson point after the throat of Laval nozzle and causes Condensation Shock phenomena.The shock wave shows the characteristics of sudden pressure jump,temperature increase and sudden decrease of Mach number.Then,the boundary conditions of Laval nozzle with different inlet and outlet steam temperatures and pressures are calculated.The results show that lower inlet temperature and higher inlet pressure will increase the intensity of Condensation Shock wave,while higher back pressure will reduce shock interference and decrease the flow field humidity.Then,wet vapor condensation and shock wave are numerically calculated in two-dimensional Dykas cascade.The effects of back pressure and relative saturation of inlet vapor on the condensation process of Dykas cascade are studied.It is concluded that both the decrease of back pressure and the increase of relative saturation result in the decrease of shock angle at the rear edge of cascade,the increase of condensation shock wave intensity,the increase of droplets by nearly four orders of magnitude and the increase of humidity by 1-2%.The relationship between the condensation shock of wet steam and boundary conditions studied in this paper provides a reference for the study of unsteady phenomena in non-equilibrium phase transition.Finally,the existence of droplet evaporation in non-equilibrium condensation flow of wet steam is calculated and verified.Based on the two-phase condensation control equation of wet steam,a calculation method for non-equilibrium phase transition flow of wet steam with droplet evaporation is proposed.Non-equilibrium phase transition flows in Laval nozzles at different inlet steam temperatures were numerically calculated.It is found that the higher the inlet temperature,the larger the influence range of droplet evaporation on flow field,but the less the total droplet evaporation amount,the slower the change of thermodynamic parameters of wet steam,and the wet steam flow with evaporation is more suitable for the complex flow field inside the actual turbine.The influence of relative saturation of wet steam on nonequilibrium phase transition flow of wet steam in Dykas cascade channel is further studied.With the increase of relative saturation,the intensity of condensation shock wave increases and droplet evaporation is enhanced.The flow in the inner flow passage of the cascade is more complex.The more droplets are,the more complex the change of humidity gradient is.Compared with non-evaporation,the flow in the inner flow passage of the cascade is more complicated when droplet evaporation is considered.