Mechanism and calculation of heat transfer of LNG-AAV under frosting condition

During the operation of LNG AAV (Ambient Air Vaporizer), frosting often occurs, which may deteriorate heat transfer and thereby affect the performance of LNG AAV. How the frost growth on the surface of the finned tubes changes with time and affects the heat transfer of the AAV was examined to provide theoretical guidance in the design and operation of LNG-AAVs. Based on the equations of mass and energy conservation and the empirical formulas of physical parameters for the frost layer, the closed equations were established to calculate the thermal resistance of the frost layer on the cold surface with time. Considering the influence of the frost thermal resistance, the dynamic process of mutual coupling of the heat and mass transfer changes with time inside and outside the LNG-AAV was calculated, the operation of the LNG-AAV under the frosting condition was simulated, and finally the distribution of physical parameters along the length of the finned tube at different time was achieved. Research results show that the frost layer can cover 80% of the surface area of the finned tubes, which means that the influence of the frost layer on the heat transfer of the LNG-AAV cannot be ignored. In most cases, the frost layer reduces the heat transfer efficiency of the finned tubes, even by 85%, except the local areas where the heat transfer is enhanced because the frost layer can be treated as fins, and the total heat transfer coefficient ranges in 80-220 W/(m²·K) after frosting. Moreover, the AAV will reach another steady operation in a short period of time after frosting when the surrounding environment is suitable.