(254b) Phase Change Transient Model for Predicting Frost Formation in Ambient Air Vaporizer

Natural gas is continuously getting attention as an eco-friendly and sustainable energy source mainly composed of methane. Natural gas is liquefied into liquefied natural gas (LNG) in order to facilitate transportation and then regasified to natural gas (NG) to be supplied to the user at the terminal. LNG regasifiers used in terminals include ORV, SCV, AAV, etc. ORV uses seawater as a heat source and SCV burns fuel to provide heat. Among them, Ambient Air Vaporizer (AAV) is LNG regasification technology using air in the atmosphere as a heat source. The ORV increases the temperature of seawater and affects the marine ecosystem, and SCV burns the fuel, which adversely affects the atmospheric environment. On the other hand, AAV uses the atmospheric air directly, so it is eco-friendly and offers reasonable price and high operating efficiency. However, in the process of transferring thermal energy from air to LNG, the phenomenon of condensation of water vapor contained in the air into frost on the AAV surface occurs due to cryogenic temperature of LNG. This frost acts as a thermal resistance that interferes with the heat transfer between the air and the LNG, thereby lowering the heat transfer efficiency between the LNG and the air. As time goes on, the frost accumulates more and more. As a result, after a certain period of time, the heat transfer efficiency of the AAV becomes very low and the target NG output temperature is not reached. Thus, the AAV will alternate between operation and defrosting. Therefore, for the design of AAV and its application to LNG regasification of AAV, it is essential to understand how frost is formed in AAV. However, research on AAV and studies on frost growth in AAV are lacking. Therefore, this study presents a model for frost growth using CFD, and applies this model to AAV to analyze the effect of frost growth on AAV performance on AAV.
The proposed model is an Eulerian-Eulerian multiphase flow model of ice and moist air simulating frost formation and growth using CFD. At below freezing point, the water vapor contained in the moist air is phase-changed to Ice, and the generated ice accumulates without flowing. That is, mass transfer with phase change from air to frost occurs over time, changing the thickness and density of the frost and affecting the flow of moist air. Therefore, this model can observe the thickness, density, and shape of the frost with time, and also the effects of frost on the flow of air and its effect on heat transfer.