Study On The Influence Of Cold And Hot Fluid On The Finned Surface Frosting Of Air-heating Vaporizer

The air-heating finned-tube vaporizers is the heat exchange equipment to vaporize the cryogenic liquid inside the finned-tubes by absorbing the atmospheric heat.It is widely used in cryogenic engineering because of its advantages of simple structure,energy saving and environmental protection,low operati ng cost and so on.In actual operation,when the wall temperature of the vaporizer is below the freezing point of the moist air,the frost comes into being on the finned-tube surface,which leads to the reduction of gas-liquid two-phase boiling heat transfer inside the finned-tubes and the decrease of its vaporization capacity,seriously affecting the normal operation of the cryogenic system.Therefore,it is very important to explore reasonable and effective methods for delaying the frost formation in order to improve the heat transfer performance and operation stability of air-heating finned-tube vaporizers.In order to explore the basic scientific problems of the effect of cold and hot fluid inside and outside the finned-tube on the surface frosting,this paper is based on the theoretical analysis and uses the combination of numerical simulation and experimental research to the coupling heat transfer characteristics of the inner and outer field of a single finned-tube and the frosting process were studied as follows.Firstly,based on the theory of heat and mass transfer,the mechanism of convection heat transfer in finned-tube was deeply analyzed.For the single-phase and gas-liquid phases,the corresponding convective heat transfer correlations were selected respectively.At the same time,the form of convective heat transfer outside the finned-tube was determined,and the convective heat transfer coefficient K of the air side was obtained for frosting during operation.According to the theory of thermodynamics,the forming conditions and criteria of frost on the wall surface of air-heating finned-tube vaporizers are described.Secondly,the heat transfer characteristics of cold and hot fluid distribution for air-heating finned-tube vaporizer were numerically simulated with the software Fluent and based on Realizable k-? model.The temperature distributions of cold and hot fluids inside and outside the finned-tube and the finned wall were obtained,the effects of the inlet cryogenic fluid flowrate in the finned-tube and the air velocity outside the finned tube on the heat transfer characteristics were studied.The results show that under the condition of natural convection,the temperature of finned wall increases gradually along the direction of cryogenic fl uid(from bottom to top)and increases with the decrease of the inlet flowrate of the cryogenic fluid in the finned-tube.When the downward inlet air velocity outside the finned-tube is 2.0 m/s,the average temperature of the finned wall increases by about 17 K compared with the condition of natural convection.Finally,the surface frosting experimental table of air-heating finned-tube vaporizer was designed and built,and the growth rule of frost layer on the surface of the finned-tube was obtained by changing the inlet flowrate of the cold fluid in tube and the inlet velocity of heat fluid outside the tube,and the factors that affect frosting are studied.The results showed that the frost layer thickness increased at the fastest rate during the frost layer growth,then gradually slowed down and became stable,and the frost layer thickness on the finned surface was in the form of lower thickness and higher thinness along the tube length.Under the same environmental conditions,the thickness of the frost layer increases with the raise of the inlet flowrate of the cold fluid,and the faster the frost rate.The frosting on the finned surf ace began later when the downward inlet air velocity outside the tube increased,and the frost layer on the upper surface will melt and freeze.Therefore,appropriately reducing the inlet flowrate of the cold fluid in the finned-tube and increasing the inlet velocity of the hot fluid outside the finned-tube are beneficial to increase the temperature of finned wall and achieve t he purpose of delaying frost formation.The research results can provide reliable theoretical basis for the delay frosting of air-heating finned-tube vaporizer and promote its efficient utilization.