The Format Criterion Of Doctoral Dissertation Of CUP

In the natural gas liquefaction process,expansion valves are often used to reduce pressure,which will cause flash steam.The application of low-temperature hydraulic turbine can significantly improve the liquefaction rate and recover high-pressure liquid pressure energy.The cryogenic hydraulic turbine uses cryogenic liquefied natural gas as the medium,and cavitation is easy to occur at the runner outlet and in the downstream draft tube.Due to the strong lateral velocity at the outlet of the runner,there is a "braided bundle" flow at the midline of the draft tube,resulting in a low pressure area,resulting in liquid cavitation and expanding downstream,which will affect the flow in the subsequent pipeline and produce additional gas.Reduce the liquefaction rate.Therefore,when carrying out the optimal design of the cryogenic hydraulic turbine runner,it is necessary not only to ensure the external characteristics of the hydraulic turbine,but also to restrain the cavitation in the draft tube.These characteristics make the optimization of cryogenic hydraulic turbine more complex.In order to solve the above problems,firstly,the cavitation flow simulation is carried out in the medium of cryogenic liquid,and the thermodynamic effect of cryogenic liquid is described by modifying the source phase of the cavitation model.The accuracy of the cavitation model is verified by comparing with the cryogenic experimental results.The numerical simulation of the designed initial model of cryogenic hydraulic turbine is carried out,and the accuracy of the simulation results is verified by comparing with the experimental results with water and liquid nitrogen as medium.Furthermore,the gas distribution in the hydraulic turbine under cavitation conditions is analyzed,and the pressure pulsation characteristics in the guide vane and runner of the hydraulic turbine under cavitation conditions are obtained by means of unsteady numerical simulation.In order to accurately obtain the source of hydraulic loss in hydraulic turbine under cavitation condition,the theory of entropy production based on the second law of thermodynamics is introduced to analyze the cavitation flow field in hydraulic turbine,and the result is compared with that of traditional pressure difference calculation.the rationality of the calculation method of entropy generation hydraulic loss is verified.The results show that,compared with the non-cavitation condition,the entropy production in the guide vane and runner channel changes little under the cavitation condition,but in the draft tube,the entropy production increases obviously,the local entropy production is the highest at the entrance,and the entropy production is larger near the wall of the cavitation area,this is due to the severe reflux and flow separation in this area.In the low flow condition,although the cavitation area in the draft tube is small,the hydraulic loss is more serious because the flow changes more violently.Secondly,the optimal design method of runner is established.In this study,the runner full three-dimensional inverse problem design method,CFD numerical simulation method,response surface agent model and multi-objective genetic algorithm are organically combined.Taking the runner shaft shape control parameters,blade load control parameters and high pressure edge inclination angle as design variables,the relationship between design variables and objective function is constructed by using CFD simulation method and response surface model,and the runner design is completed finally.The scheme has the advantages of small number of samples,accurate prediction results,and can take into account multiple optimization objectives.Finally,the numerical simulation method and optimization scheme of cryogenic cavitation are applied to the optimization study of cryogenic hydraulic turbine.In order to ensure the external characteristics of the turbine and improve the flow in the draft tube,the energy loss coefficient in the runner head and draft tube is optimized.The optimization results show that the wheel performance is significantly improved under the backward force on the hub and the forward force on the flange,and under the non-cavitation condition,the larger the inclination angle of the high pressure side is beneficial to the improvement of the hydraulic performance of the turbine,but under the cavitation condition,in order to reduce the vaporization rate in the draft tube,the absolute value of the inclination angle of the high pressure side should not exceed 5 °.