Optimization Of Cyclone Structure And Study On CO₂ Supersonic Swirling Condensation Characteristics

Natural gas is getting more and more attention as a clean energy.In the process of pipeline transportation,excessive CO₂ will reduce the comprehensive calorific value of natural gas and pipeline transportation capacity,form dry ice to block the pipeline,and produce corrosive substances,etc.Therefore,it is very important to remove CO₂ from natural gas before pipeline transportation.Conventional CO₂ removal methods include condensation separation,absorption,adsorption and membrane separation,etc.,but all the above methods have problems such as high energy consumption,large investment,complex equipment and low CO₂ recovery rate.Therefore,it is very urgent to develop efficient,energy saving and environmental protection natural gas CO₂ removal new technology.It is found that supersonic separation technology can effectively separate CO₂ from natural gas,and overcome the defects of the above methods.There are few studies on the removal of CO₂ from natural gas by supersonic separation technology at home and abroad,and most of them are in the research stage of two-dimensional axial flow,so the supersonic swirling condensation characteristics of CO₂ are still not clear.Therefore,this paper adopted the theories of aerodynamics,phase change dynamics,heat transfer and engineering fluid mechanics to systematically study the key issues such as the optimization of internal flow field of supersonic separator,droplet motion law and supersonic swirling condensation characteristics of CO₂ gas from the aspects of theoretical analysis and numerical simulation.The main contents are as follows:Firstly,the structure design and optimization of supersonic separator were carried out.A mathematical model of supersonic swirl flow was established to simulate the gas flow process in the supersonic separator,and the distribution characteristics of gas dynamic parameters in the separator were illustrated.Based on the Discrete Particle Model,the trajectory of droplets in the separator was analyzed,and the influence of cyclone structure on the stability of flow field and the separation efficiency was studied by droplet capture situation and flow field distribution.The results showed that:The new type of wall-mounted cyclone can increase the swirl intensity and delay the generation of shock wave.With the increase of the blade number,rotation angle and discharge gap,the separation efficiency of droplets is improved,but at this time,the influence on the flow field in the separator increases,resulting in insufficient gas expansion,shock wave advance and other problems.Considering the flow field stability and the separation efficiency,the cyclone length was determined to be 0.02 m,the blade number was 3,the blade thickness was 0.25 mm,the blade angle was 60°,and the discharge gap was2.5 mm.Then,the supersonic swirling condensation characteristics of CO₂ were studied.Based on the gas-liquid flow equation and condensation model（condensation nucleation,droplet growth and surface tension model）,an Euler mathematical model of swirling condensation flow of CO₂-CH₄ gas was established.Computational fluid dynamics software Fluent was used to simulate the supersonic condensation process of CO₂ under axial flow and swirl flow conditions respectively,and the condensation parameter distribution under different conditions was obtained.The influence of swirl intensity on CO₂ condensation characteristics was studied.The results show that:The change trend of CO₂ spontaneous condensation parameters along the axial direction is basically consistent with that of the axial flow.The super-cooling degree shows an upward trend at the beginning,when it increases to a certain value,the phenomenon of spontaneous condensation occurs.The process of condensation nucleation is very intense,and a large number of condensation nuclei are generated in a very short distance.Then it enters the phase of droplet growth,in which CO₂ vapor molecules gather on the surface of condensation nuclei and release a large amount of latent heat,which destroys the nucleation process and makes the gas return to equilibrium.Radially,the strong swirl and the boundary layer near the wall lead to uneven distribution of condensation parameters,and the maximum nucleation rate,the maximum liquid mass fraction and the maximum liquid drop number all show an annular distribution.With the increase of swirl intensity,CO₂ condensation occurs in advance,the maximum nucleation rate and droplet number decreases,the liquid mass fraction and droplet radius increases,but the flow capacity of the nozzle is limited.Then,the effect of Laval nozzle structure on the swirling condensation characteristics of CO₂ was studied.The nozzle contraction section was designed using Bicubic parametric curve,Quintic Polynomial curve,Witoszynski curve and Translation of Witoszynski curve respectively,and the expansion section was designed using equal-slope method.The results show that:As the nozzle expansion angle increases,the temperature drop inside the nozzle increases,the spontaneous condensation position of CO₂ is advanced,the nozzle liquefaction efficiency increases,the droplet radius decreases,and the shock wave influence increases.When the length of expansion section is small,extending the expansion section length of the nozzle will increase the droplet growth time,droplet radius and nozzle liquefaction efficiency.The nozzle designed by Witoszynski curve and Translation of Witoszynski curve has good performance of CO₂ liquefaction,but the droplet radius in the nozzle is small,which is not conducive to the separation of droplets.The nozzle design using Bicubic parametric curve and Quintic polynomial curve has poor liquefaction performance,but the droplet size in the nozzle is large.Finally,the influence of aerodynamic parameter on the swirling condensation characteristics of CO₂ was studied.Based on the mathematical model of swirling condensation flow and numerical method established above,the effects of inlet temperature,pressure and CO₂ content on CO₂ swirl condensation characteristics were studied.The results show that:Reducing the inlet temperature or increasing the pressure will lead to the advance of spontaneous condensation position of CO₂ and the improvement of nozzle liquefaction efficiency,especially when the inlet temperature is lower than 285 K or the pressure is lower than 7.5 MPa,the improvement is more significant.The increase of CO₂ content will increase the partial pressure of CO₂ vapor and make the gas more easily reach the super-saturation state,but the nozzle liquefaction efficiency is basically unchanged and always maintains at about 60%.