Research On The Mechanism And Characteristics Of Frost On The Surface Of Vertical Cryogenic Finned Tube

With the increasing proportion of natural gas in our country’s energy consumption,air temperature vaporizer(AAV)is widely used in the vaporization of liquefied natural gas(LNG).Cryogenic frost formation on the surface of an ambient air vaporizer significantly reduces its thermal performance due to the cryogenic characteristics of liquefied natural gas.In the present study,in order to explore the frosting mechanism and characteristic on the surface of the vertical cryogenic finned tube,a simulation study is carried out on the frost growth behavior of the cryogenic surface under forced convection.This study can provide a theoretical guidance for the operation of AAV under frosting conditions.First of all,a mathematical model coupled improved Lattice Boltzmann Method and Diffusion-limited Aggregation Method is developed to study frosting behavior of cryogenic plat under forced convection.In order to make the simulation more realistic,the improved Lattice Boltzmann Method is used by using porosity of frost layer to modify the flow fluid.After the proposed model is validated by the experimental data,the effects of cold surface temperature,air temperature,relative humidity and airflow velocity on the frost formation are discussed respectively.The results show that the air velocity has the greatest influence on the frost morphology,and the relative humidity and the air velocity are the significant factors affecting the growth of frost layer on the cold plat under forced convection.In the case of frosting for 45 mins,the thickness of frost layer will be increased on average by 11.89% respectively with a decrease of 20% on the relative humidity.A 1m/s increasement in the air velocity makes the thickness of frost layer decrease by 8.95%.Secondly,a heat and mass transfer model of frost formation on the cryogenic finned tube is established to study the growth of the frost layer formed on the surface of the cryogenic finned tube with a complex shape.Under the air supply mode of bottom air supply and top air exhaust,the growth of frost layer on the surface of a single vertical cryogenic finned tube is simulated with the use of Fluent software,and the effect of cold surface temperature on the growth of frost layer is analyzed.The results show that under the air supply mode of bottom air supply and top air exhaust,the growth rate of the frost layer at the same fin height of the eight fins is the same,and the growth rate of the frost layer at the tip of the fin is greater than that at the root of the fin.As for the effect of surface temperature on the thickness of frost layer formed on the surface of finned tube,the frost thickness decreases with the increasement of surface temperature.In the case of frosting for 60 mins,the thickness of frost layer decreased by 3.61% on average for every10 K increase in cold surface temperature.Finally,when humidity air across the finned tube,the growth process of the frost layer on the surface of the single cryogenic finned tube and the cryogenic finned tube bundle arranged in the form of 2×2 is studied,and the temporal and spatial variation of air velocity and air temperature around the fin tube is analyzed.The effects of air velocity,air temperature and relative humidity on the growth of frost layer formed on single finned tube are studied.The results show that the frost growth rate on the windward side of a single finned tube is greater than that on the leeward side.The higher the air temperature,the denser the frost layer on the surface of the finned tube;the increase of air velocity and air relative humidity result in the increasement thickness and density of the frost layer.In the case of frosting for 60 mins,the thickness of frost layer is increased on average by 5.47% with a decrease of 5K on the air temperature;the frost layer thickness is increased by 12.8% on average with a 1m/s increasement in the air velocity;20% increasement in the relative humidity of air make the thickness of frost layer decrease by 18.63%.As for the distribution of frost layer on the surface of each finned tube in the finned tube bundle,the thickness and density of frost layer on the surface of finned tube on the windward side of finned tube bundle are greater than those on the leeward side.In the case of frosting for 60 mins,the average thickness of frost layer on the finned tube on the windward side is 4.5mm,and the average thickness of frost layer on the finned tube on the leeward side is 3.8mm.