Study On The Collision Process Of Two Droplets In Gas-liquid Dispersion System

Some seemingly unrelated natural and industrial processes such as the formation and fall process of rain and snow,the combustion process,the sewage treatment process and so on,reveal the same scientific problem,that is,the changing of droplet size during these processes.Droplet size is related to the interaction between droplets,and the collision between droplets or the action of gas applies to droplets are the main factors affecting the droplet size.Therefore,the research on this aspect has certain engineering value and practical significance.Based on the gas-liquid separation process in two-phase flow,the mechanism of dynamic behavior of droplet collision in gas-liquid system and the multi-scale gas-liquid separation process based on droplet group collision are studied in this paper.The colliding process of the two droplets was studied firstly.The internal force and external force caused by the droplet collision and the evolution of interface after the collision were studied through theoretical analysis and numerical simulation methods.Taking the droplet sizes of 30μm,40μm,and 50μm,the relative collision velocity of 1m/s,2m/s,and 4m/s,and the eccentricity of 0.2,0.5,and 0.8 as example,the droplet collision dynamics in a gas-liquid two phase flow were simulated and analyzed,including the evolution of interface and the distribution of velocity and pressure.Based on the research results of two droplets collision,the collision process of group droplets was studied.The Population Balance Model(PBM)was adopted,and a gas-liquid separation case in a corrugated plate was carried out,and the influence of droplet diameter,relative collision velocity,and plate distance on the droplet size distribution and separation performance were obtained.The research of this work establishes a relationship between the microscopic characteristics of droplet collision causing the evolution of interface and the macroscopic characteristics causing separation performance,and therefore,a multi-scale research was achieved.It can be known form the research results that the deformation degree and oscillation period increase with the increase in droplet diameter and relative collision velocity.However,the influence of collision on the velocity field distribution is not obvious if the droplet size is smaller than 40μm and the relative collision velocity is less than 1m/s.In this case,there is no obvious vortex generating and the velocity field and pressure filed are symmetrical.On the other hand,if the droplet diameter is greater than 40μm and the relative collision velocity is greater than 1m/s,the vortex caused by the collision present in the velocity field,and the amount and magnitude of the vortex increase with the increase in velocity.As a result,a small part of the gas cannot be discharged in time during the droplets contact process,which causes the phenomenon of sudden increase of local pressure.It is found that eccentricity has a significant influence on the evolution of droplet interface through the study of the eccentricity collision process of droplet with eccentricity of 0.2,0.5 and 0.8.The eccentricity causes the droplet rotation because the centrifugal force generates during droplet movement.The rotation makes the larger tensile deformation degree and the longer oscillation period.The time when the velocity field becomes stable decreases with the increase in eccentricity.An approach between droplets collision in the microscopic domain and gas-liquid separation in the macroscopic domain was established through Population Equilibrium Model.The effects of droplets collision in the microscopic domain,and the phase content and plate spacing in the macroscopic domain on separation efficiency and pressure drop were obtained.The corrugated plate is beneficial to promote the coalescence of droplets and to increase separation efficiency.The droplet size distribution in corrugated plates is mainly affected by initial droplet size and gas-liquid velocity.In a certain range,increasing the flow velocity promotes the collision and coalescence,and thus improves the separation efficiency.However,the increase in flow velocity causes the increase in relative collision velocity,which causes the breakup of the droplet,and thus decreases the separation efficiency.Therefore,a reasonable flow velocity should be selected in the separation process.Through the study of droplet interaction in gas-liquid dispersion system from different aspects,the physical essence of droplet collision in gas-liquid flow is revealed.The research results can expand and enrich the basic research of droplet collision in gas-liquid flow,and provide the basis for the further study of droplet collision process in gas-liquid two-phase.