Calculation Of Dynamic Heat Transfer Characteristics Of LNG Ambient Air Vaporizer Under Frosting Conditions

With the call of energy saving and emission reduction all over the world,LNG has been used more and more widely in China,and the dependence of LNG on foreign countries in China is getting higher and higher.LNG Ambient Air Vaporizer,as a gasification device of LNG,has many problems in its design and use.Because LNG is-162 ?,there will be a great temperature difference between the air and the ambient air in the process of entering the Ambient Air Vaporizer and gasifying.The existence of water vapor in the air will produce frost on the cold wall,which seriously deteriorates the heat transfer effect of the Ambient Air Vaporizer.In this paper,the dynamic heat transfer of frosting in LNG Ambient Air Vaporizer AAV is studied.Firstly,through the study of physical model,a prediction model of heat transfer characteristics based on the principle of energy conservation and mass conservation is established,and the unsteady heat and mass transfer process of Ambient Air Vaporizer under frosting conditions is described.The numerical calculation of the mathematical model is realized by Fortran computer language.Distribution rules of physical parameters inside and outside the tube along time and space length are obtained respectively.The influence rules of external factors on heat and mass transfer effect of Ambient Air Vaporizer are analyzed.Frosting and heat transfer condensation of Ambient Air Vaporizer under different operation time,different space position and different number of finned tubes are obtained.Fruit.The results show that the higher the temperature of cold wall,the smaller the thickness of frost layer and the smaller the thermal resistance of frost layer;when the temperature is large enough,the thickness of frost layer and the thermal resistance of frost layer will decrease to 0;and the thickness of frost layer along the length of the tube will not decrease uniformly;the accumulation of frost is mainly in the liquid phase and the two phase section,but less in the gas phase section.The direct effect of frosting is to deteriorate the effect of heat transfer outside the tube,reduce the coefficient of heat transfer outside the tube obviously,and the coefficient of heat transfer outside the tube without frosting is 2.28 times higher than that under frosting.Frosting not only causes the change of the coefficient of heat transfer outside the tube,but also has an obvious influence on the coefficient of convective heat transfer inside the tube and the coefficient of heat transfer from the air side to the LNG fluid side,and one by one.Increasing thermal resistance can also significantly inhibit the heat transfer coefficients of the frosting section of the two tubes,and the temperature of the fluid in the tube and the outer wall of the finned tube are also greatly affected by frosting.Under the condition of no frost,it can be concluded that the temperature of the fluid in the tube without frost is obviously different from that in the tube under frost condition;when the ambient temperature is higher,the proportion of the three-phase flow in the finned tube will reach equilibrium in a relatively short time,and when the ambient temperature is 30 C,the outlet temperature of the finned tube will not decrease because of the running time;when the ambient temperature is lower,the outlet temperature of the finned tube will not decrease because of the running time.At the same time,the proportion of the three-phase section in the tube will not keep balance,and the outlet temperature will decrease with the increase of operation time.The mass and energy transfer effect of the Ambient Air Vaporizer is different in different longitudinal positions.The larger the number of fins is,the larger the heat transfer coefficient is,but the greater the maximum frosting thickness is,the less the influence is,with the increase of operation time.The more fins,the earlier the frost thickness of Ambient Air Vaporizer decreases.In this paper,the heat transfer characteristics of AAV heat transfer tube are calculated.The research results can provide technical support and theoretical basis for optimum design of heat transfer tube structure and performance of Ambient Air Vaporizer in China.It has important academic significance and engineering application value.