Development And Application Of Single Droplet Evaporation Mode Of Moving Methane In Methane Vapor

In order to achieve the strategic goals of"carbon peak"and"carbon neutral"as soon as possible,the demand for natural gas as an efficient and clean energy source is increasing year by year.The safest and most economical way to transport or store natural gas is to cryogenic liquefy it into LNG（Liquefied Natural Gas）and transport it using LNG carriers.In order to prevent safety accidents caused by excessive temperature difference inside and outside the liquid tank during precooling,LNG is generally used as refrigerant for spray precooling of the liquid tank,and before precooling,LNG vapor is used to replace the gas with higher condensation point in the tank.However,there is currently a lack of droplet evaporation models suitable for moving single methane droplets in methane vapor.Therefore,based on the theoretical model of flow and heat and mass transfer at the gas-liquid interface,using the computational fluid dynamics（CFD）method,and based on the principle of energy conservation at the gas-liquid interface,a numerical model suitable for forced convection evaporation of moving methane droplets in their vapor was established.The model was first applied to simulate the evaporation process of a moving methane droplet in its vapor with a temperature difference of 190 K,particle size from 1 mm to 0.1mm,and Re from 13.5 to 4486.4.The flow field characteristics,resistance characteristics and heat and mass transfer characteristics of methane droplets were obtained and compared with the ball.The effects of different particle sizes and Reynolds numbers on droplet evaporation and heat transfer were analyzed.Finally,a preliminary investigation of the application of the kinematic methane single droplet evaporation model in liquid chamber pre-cooling was carried out,and results with reference value were obtained.The main research results were as follows:（1）The surface pressure p of the moving droplet and the vapor evaporation velocity v₂change in the same trend.That was,both decrease first and then increase with the increase of the angleφof the incoming flow.The larger the Re,the greater the change of p and v₂.The smaller the particle size,the larger p and v₂were.The separation characteristics of droplet show a piecewise change with the increase ofφ.The change trend of p and separation characteristics of the ball is consistent with that of the droplet.However,due to the"blowing effect"of the droplet,there is steam ejecting from the surface.Therefore,compared with the low temperature ball,the separation angle of the droplet under the same condition was smaller than that of the low temperature ball,and the larger the Re was,the closer the separation point was.（2）With the increase ofφ,the differential pressure drag coefficient C_dof droplet and ball first decreases to the minimum valueC_dminand then increases slowly.C_ddecreased as the Re increases until it reachedC_dmin,and then increased as the Re increased.The pressure resistance coefficient C_d-dof the droplet under the same condition was greater than that of the ball.The total drag coefficient of droplet and ball decreases with the increase of Re,and the total drag coefficient of droplet surface C_dt-dwas smaller than that of ball.The maximum and minimum decreased of C_dt-dby 55.52%and 0.59%compared with C_dt-b.A dimensionless correlation formula was established for the total drag coefficients of droplet and ball,which were C_dt-d=9.8Re^-0.51、C_dt-b=31.4Re^-0.62,respectively.（3）The dimensionless boundary layer thickness h₂of droplet and ball increases with the increase ofφ,and the larger the Re,the smaller the initial h₂.The thickness of the boundary layer affects the convective heat transfer between droplet and ball.The Nu of droplet and ball decreased first and then increased with the increased ofφ.When the particle size was the same,the larger the Re was,the larger the Nu was.When Re was the same and particle size was different,Nu was basically the same.Due to the"blowing effect"of liquid droplets,the boundary layer thickness of liquid droplets was greater than that of ball,which hinders heat transfer,so the Nu of liquid droplets under the same condition was less than that of ball.The dimensionless correlation between the droplet Nu and Re was Nu=1.64Re^0.30（13.5<Re≤89.7）,Nu=0.68Re^0.47（89.7<Re≤4486.4）.（4）Simulation of the pre-cooling process of the moving spraying droplets in an LNG storage tank was conducted,combined with the heat exchange dimensionless correlation equation,yielded that the larger the spraying rate,the larger the Nu and the larger the heat exchange,thus the faster the temperature inside the tank droplets and the shorter the pre-cooling time when it reached 110 K.The larger the nozzle radius and the greater the number of nozzles,the smaller the pre-cooling finish time.The research in this paper further promotes the study of the evaporation characteristics of moving methane droplets in its vapor,and also provided some technical support for the safety of spray pre-cooling of LNG storage tanks.