| Air-heating cryogenic finned-tube vaporizer is one of the low temperature heat exchangers,which is using ambient air as heat resource to transfer energy to cryogenic liquid in convective way so that it could achieve the purpose of vaporizing cryogenic liquid.The equipment performance is greatly affected by the ambient factors,such as temperature,humidity,wind direction and wind velocity,then frost formed on the finned-tube surface might cause some problems of low-efficiency heat transfer,uncompleted gasification and unstable gas supply.Therefore,it’s significant to study the law of air flowing along the surface of finned-tube vaporizers,and find out air distribution on the surface of finned-tube and interrelationship between air distribution and frosting rate on finned-tube surface,which has theoretical meaning and practical application value to improve heat exchanging efficiency of air-heating cryogenic finned-tube vaporizer and restrain frost growth on the finned-tube surface.Based on theoretical analysis,the distribution of flow field on the surface of air-heating cryogenic finned-tube vaporizer and the frosting process were studied combining numerical simulation with experiments.The main points and results are:(1)The heat transfer process of ambient cryogenic finned-tube vaporizer was analyzed.According to the different phases of low temperature medium in the finned tube,the heat transfer of finned tube was divide d into three regions of single liquid phase,gas-liquid two-phase and single gas phase,of which the single liquid phase and single gas phase regions were considered as forced convection heat transfer,and the gas-liquid two-phase region belonged to boiling heat transfer.Based on whether frosting or not on the surface of finned tube,the analysis of the heat transfer process was carried out under two kinds of working conditions.Under the frost-free condition,the model of natural convection in infinite space was used to calculate the heat transfer process,and under the frosting condition,heat conduction of frost was calculated in the form of thermal resistance,that is,the heat transfer resistance for the surface of finned-tube including the heat conduction resistance of frost and the convection transfer resistance between the flowing air and frost.Base on the above,the convective heat transfer on the surface of the finned tube was calculated.(2)Computational Fluid Dynamics simulation was used to study the distribution of temperature field and velocity field on the process of air natural convective heat transfer in ambient cryogenic finned-tube vaporizer,and the varying rules of air temperature and velocity and impact of these changes on heat transfer of finned-tube surface were analyzed.The results show that: for single liquid phase region,the air flowing is obvious on the surface of the finned-tube,natural convection heat transfer mainly concentrates on the area near wall,and the intensity of natural convection heat transfer on the finned-tube surface becomes stronger with being closer to the entrance region of the finned-tube.Comparing with the single liquid phase region,the air flowing in the single gas phase region is not obvious,and the intensity of natural convection is weak.(3)The experiment equipment of air-heating cryogenic finned-tube vaporizer was established.A gasification experiment of cryogenic liquid in the finned-tube was conducted under frosting condition and it was investigated that the interaction effect between the gasification of cr yogenic liquid and the frost growth on the finned tube surface.The results show that the working state of the vaporizer under frosting condition consists of two states: pre-cooling state and stable state.The former is that the cryogenic liquid gasifies rapidly when entering into the vaporizer and contains two heat transfer regions of gas-liquid two phase and single gas phase.The latter is that the cryogenic liquid gasifying in the vaporizer goes through three heat transfer regions of single liquid phase,gas-liquid two phase and single gas phase.The frosting on the finned-tube surface is relatively serious in entrance region while there is no frost at the finned tube in outlet region.It is concluded that the ambient finned-tube vaporizer can be designed according to working states and heat transfer regions to weaken the effect of frosting on the heat transfer and improve the heat transfer efficiency of the vaporizer.(4)In addition,the experiment was conducted to investigate the air flowing characteristics on the finned-tube surface under the frosting condition,and the interaction between air flowing and frost process was also discussed.The results indicate that frosting on the finned-tube surface appear early at the fin top of finned-tube in the entrance region.Along the radial direction of the finned-tube,the frosting closer to the fin root occurs later,and frosting on the base tube surface appears latest.Frost thickness gradually decreases from the fin tip to the fin root.Frost grows rapidly at the beginning,and the average frost growth rate reaches 0.16mm/s.In the middle period,the frost growth tends to be gentle,and the average frost growth rate decreases to 0.07mm/s.In the later period,there is no frost growth and frost crystals come into falling down.In the initial frosting period,the heat transfer intensity of flowing air for the finned-tube(0-300mm)surface gradually turns to be weak,and the heat transfer intensity of flowing air for the finned-tube(600-900mm)surface varies from weak to strong and then begins to descend.In the later frosting period,the heat transfer intensity of flowing air for the whole finned tube presents a stable state. |
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