Study On The Condensation Characteristics Of CO₂ In Different Structure Of Supersonic Separator

In this paper,supersonic swirling separation technology is applied to remove CO₂from natural gas.Based on the theories of gas dynamics,thermodynamics and nucleation dynamics,using the method of combining theoretical research with numerical simulation,and taking CH₄-CO₂mixture as the research object,mathematical model and numerical method of supersonic condensation flow were established,the structure of nozzle and hydrocyclone were optimized,which is the key components of supersonic swirling separator,and the condensation flow of CH₄-CO₂mixture under supersonic flow and supersonic swirl was studied systematically.In this paper,the main studies and results are as follows:（1）The mathematical model and numerical method of supersonic gas condensation flow in nozzle are established.Based on the governing equations of gas-liquid flow,standard k-ωturbulence model,ICCT nucleation theory,the Gyarmathy’s droplet growth theory,and droplet surface tension model of CO₂,UDF was compiled,and operation steps and numerical methods were explained in detail.Finally,the accuracy of mathematical models and numerical methods is verified by experimental data in literature.（2）Based on the supersonic condensation flow characteristics of CH₄-CO₂mixture,the convergent section of Laval nozzle was designed by Translation of Witoszynski curve.The mathematical model and numerical method established in this paper were used to simulate CH₄-CO₂mixture gas flowing in the nozzle,which convergent section is designed by Bicubic parametric curve,Quintic polynomial curve,Witoszynski curve and Translation of Witoszynski curve,respectively.The distribution of key parameters of flow and condensation in the nozzle was obtained.The results show that the pressure and temperature change most smoothly in the contraction section designed by Translation of Witoszynski curve,the spontaneous condensation occurs first and the"condensation shock"phenomenon is the most severe,the maximum nucleation rate is the largest,secondary nucleation occurs in the rear of nozzle,the number of droplets is much higher than the other three with a trend of continuous increase.The liquid mass fraction is 22.58%at the nozzle outlet,so the Translation of Witoszynski curve is recommended to design convergent section.（3）Based on the supersonic condensation flow characteristics of CH₄-CO₂mixture,the nozzle expansion section was optimized.The condensation flow of CH₄-CO₂mixture was simulated numerically in the nozzle which convergent section of nozzle was designed by Translation of Witoszynski curve,and the length of expengding section was from 70 mm to 130 mm.The results show that when the expanding section length is in the range of 60 mm≤L₂<120 mm,the pressure recovery ability becomes worse,the effect of temperature drop becomes obvious,the phenomenon of secondary nucleation occurs gradually,and it becomes more intense,the change of droplet number and radius is more obvious,and the liquid mass fraction and liquefaction efficiency increase with the increase of length.The influence of expansion half-angle on condensation flow process was investigated by choosing the length of expengding section as 100 mm.The results show that the change of expansion half-angle mainly affects the condensation flow characteristics in the nozzle expengding section,and the outlet pressure and temperature of the nozzle decrease with the increase of expansion half-angle.Secondary nucleation occurs when the expansion half-angle is greater than 2 degrees.The nucleation phenomenon becomes more and more intense,nucleation rate and droplet number of secondary nucleation,liquefaction efficiency of nozzle increase gradually with the increase of expansion half-half-angle.Therefore,the optimized nozzle expansion section has a half-angle of 4 degrees and a length of 100 mm.（4）The effects of nozzle inlet design parameters and outlet pressure on condensation flow were studied.The nozzle inlet pressure,temperature and CO₂mole fraction were changed for numerical simulation,and the results show that when the inlet pressure is not higher than 8 MPa,the inlet pressure is increased and the liquefaction efficiency of the nozzle is improved.When the inlet pressure is 9 MPa,the temperature fluctuation causes the evaporation of CO₂droplets.The liquefaction efficiency of nozzle can be improved by reducing the inlet temperature of nozzle.The change of the molar fraction of CO₂at the inlet mainly affects the condensation process in the nozzle,and the liquefaction efficiency is maintained between 66%and 67%.There is no secondary condensation occuring in the nozzle when the molar fraction of CO₂in the inlet rises from 25%to 35%.With the increase of pressure ratio,shock waves gradually appear and move to the throat,affecting the secondary nucleation and the first nucleation condensation flow process,so the pressure ratio should be no more than 0.37.（5）The structural parameters of swirl blades were optimized and the spontaneous condensation process of CO₂under supersonic swirling was studied.The installation position,length and rotation angle of the blade were numerically simulated and the results show that the pressure fluctuation is greater,the tangential velocity in the throat is greater,and the overflow capacity is weaker,but it does not affect the shock location and the flow in the expanding section,when the swirl blade is close to the nozzle throat.The longer the blade length is,the greater the pressure fluctuation is,and the flow is suppressed,but it has little effect on the shock,however the shorter the swirl blade is,the smaller the tangential velocity is.Therefore,the blade is recommended to be installed at x=70 mm with a rotating angle of 180°and a length of 20 mm.The spontaneous condensation of CO₂has hysteresis at the central axis of the optimized nozzle.The spontaneous condensation process of CO₂is not over at outlet,the nucleation rate decreases from the extreme value,the number of droplets increases continuously,the radius of droplets decreases first and then increases,and the liquid mass fraction increases continuously,reaching the maximum at the exit of nozzle with an upward trend.