| The air-heating finned-tube vaporizer uses ambient air as the heat source and is greatly affected by environmental factors such as temperature,humidity and wind speed.In actual operation,it is difficult to avoid frosting on the surface of the finned tube,which would decrease the heat exchange performance of the vaporizer and the degradation of the long-term performance.Therefore,it is vital to study the influence of the fin surface on frosting of the air-heating finned-tube vaporizer deeply,and search for reasonable and effective measures to improve the heat exchange performance,delay or even restrain frost growth of the vaporizer in industrial production and life.Based on theoretical analysis,this paper combines numerical simulation calculation with experimental research to study the influence on the heat transfer and frosting characteristics of air-heating finned-tube vaporizer of transfer tube with inner tube.The main contents and conclusions are as follows:(1)The heat transfer on the finned tube,the convective heat transfer on the single phase and the gas-liquid two phases in the tube,the enhanced heat exchange and the frost process on the surface of the finned tube are analyzed deeply to determine the appropriate empirical formula for the physical parameters of frost formation.It is proposed to install a tube in the inner side of the air-heating finned-tube vaporizer.The vaporizer is divided into three parts: preheating section,gasification section and heating section according to the transfer tube with inner tube,and the numerical model of heat transfer is established respectively.(2)The heat transfer process of air-heating finned-tube vaporizer is simulated by computational fluid dynamics(CFD)to obtain the distribution regularities of velocity,temperature,gas holdup,and heat transfer coefficient with different inner diameters(Φ6,Φ8,Φ10 mm).The results show that the inner tube has a disturbing effect on the fluid flow in the plate-fin heat transfer tube,which improves the heat exchange capability.Among the three specifications of heat transfer tube with inner tube,the Φ6mm-tube has the highest surface temperature and its gas holdup is 4% higher than that of the ordinary heat transfer tube,and the best heat transfer enhancement effect.(3)The experimental platform of transfer tube with inner tube of air-heating finned-tube vaporizer is built,and used liquid nitrogen as the working medium.Twelve working conditions are designed based on different inner diameters(Φ6、 Φ8、Φ10mm)of transfer tube with inner tube and inlet flow to obtain the temperature and thickness of frost layer at different measuring points on the surface of finned tubes,and the fluid temperature at the outlet of the finned tube.The results show that the transfer tube with inner tube can effectively delay the growth of the frost.Among the three tube specifications,the inner diameter of Φ6 mm has a better effect on delaying the growth of frosting.The time will be shortened when the cold fluid flow to the outlet by increasing the inlet flow of the finned tube.A large drop of fin surface temperature and a large driving force of phase transformation,which provides the condition for the growth of the frost,and accelerates the growth of the frost.Therefore,the frost on the fin surface can be effectively delayed by reducing the inlet flow of the medium reasonably.(4)According to the error analysis,the error between the simulated and experimental results of the outlet temperature of the heat transfer tube is 4.75%～6.46%,and the temperature error of the fin surface is within 5%,which verifies the accuracy of the assumptions used in numerical simulation,and provides a reliable basis for effectively improving the heat exchange performance and delaying frost of the vaporizer. |
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