Development And Application Of Natural Convection Evaporation Model Of LNG Droplet

For long-distance and large-scale transportation of natural gas,the natural gas is usually liquefied to-163? LNG(liquefied natural gas),and then transported by LNG ship.Before the LNG tank is put into operation,it is necessary to replace the high condensation point gas in the liquid cabin with BOG(LNG vapor)before spraying the cooling down to ensure the safety of loading.LNG is often used as refrigerant in large LNG tank,and the purpose of cooling down is achieved by evaporating and absorbing heat in BOG.The risk coefficient of cooling down is high and the technology is difficult.However,there is a lack of droplet evaporation model for LNG droplet evaporation in BOG,which is suitable for LNG spray chamber cooling down process prediction.Therefore,based on the principle of energy conservation of gas-liquid interface,a numerical model of natural convection heat transfer and evaporation of LNG single droplet and non evaporation constant temperature and cryogenic sphere in LNG vapor was established by using computational fluid dynamics(CFD)method.Firstly,the model was used to simulate the natural convection heat transfer characteristics of a cryosphere with a temperature difference of 190 K to 10 K and a diameter of 2.5 mm to 0.1 mm.On this basis,the characteristics of natural convection heat transfer and evaporation of LNG droplet in its vapor under the same conditions were studied.The influence of temperature difference and diameter on the characteristics of evaporation and heat transfer was analyzed,and the "blowing effect" was quantitatively studied by comparing with the characteristics of cryogenic sphere heat transfer,and the dimensionless correlation was established.On this basis,the calculation model of natural convection evaporation of LNG droplet which can be used in engineering calculation was established,and the correction factor of Lee model was proposed.Finally,the application of LNG single droplet natural convection evaporation model in tank cooling down was discussed,and some valuable results were obtained.The results are as follows:(1)The temperature boundary layer of droplet evaporation increases with the increase of angle ?,slowly thickens from 0 ° to 90 ° and rapidly thickens from 90 ° to 180 °.The average thickness of temperature boundary layer increases linearly with the decrease of temperature difference and decreases with the decrease of diameter.(2)The local vapor velocity,the local heat flux and the local Nusselt number of the droplet decrease slowly and then rapidly along the direction of the incoming flow,slowly decrease from 0 ° to 90 ° and rapidly decrease from 90 ° to 180 °.The mean steam velocity,mean heat flux,mean Nusselt number,heat transfer and mass evaporation rate all decrease linearly with the decrease of temperature difference.With the decrease of diameter,the average steam eruption speed and average heat flux experienced a two-stage process of increasing first slowly and then rapidly.They increased slowly from 0 ° to 90 ° and rapidly from 90 ° to 180 °.The average Nusselt number,heat transfer and mass evaporation rate decrease with the decrease of diameter.(3)The "blowing effect" makes the temperature boundary layer thicker,which leads to the decrease of heat flux,Nusselt number and heat transfer.The "blowing effect" decreases linearly with the decrease of temperature difference,and basically remains unchanged with the decrease of diameter.When the temperature difference is 300 K,the "blowing effect" is about 29.2 %;When the temperature difference is less than 25 K,the "blowing effect" is less than 5 %,which can be ignored.(4)The established dimensionless correlation can directly calculate the Nusselt number of natural convection of LNG droplet,On this basis,the calculation model of natural convection evaporation of LNG droplet which can be used in engineering calculation was established.By using the model,the mass evaporation rate of LNG droplet can be directly obtained.The modified Lee model can be applied to the calculation of natural convection evaporation of LNG droplet in a certain range.(5)The cooling down process of a single droplet in a tank was simulated.When the tank volume remains unchanged,the larger the droplet size is,the lower the initial temperature of the tank is,and the shorter the cooling down time of the tank is.The cumulative superposition of "blowing effect" reduces the temperature drop rate of the tank and greatly prolongs the completion time of cooling down.The smaller the diameter is,the stronger the blowing effect is,the initial temperature of liquid tank has almost the same effect on "blowing effect".The research in this paper will provide a reference for the further establishment of a more comprehensive droplet evaporation model under unconventional conditions,and provide technical support for the prediction and risk control of the LNG spray cooling down process,and further improve the safety and economy of the liquid chamber cooling down operation.