CFD modelling of the non-isobaric evaporation of cryogenic liquids in storage tanks

A new CFD model relevant to non-isobaric cryogen evaporation has been developed. It considers the heat influx, from the surroundings to the liquid and vapour phases, and the heat transfer between the phases. The phase change is modelled considering two contributions: an interfacial energy balance far from the tank wall, and by direct wall-to-liquid conduction near the wall. The model provides well resolved liquid and vapour temperature and velocity profiles, as well as pressure build-up and evaporation rates. It shows that below the vapour-liquid interface, liquid natural convection is dampened by thermal stratification, which is driven by the increase of the interfacial temperature due to increase in pressure. The pressure build-up is mainly governed by the heating of the vapour, particularly at the beginning of the evaporation. The model was used to analyse liquid velocity profiles for two different sets of experimental data for evaporation of liquid nitrogen by optimizing empirical parameters, namely the wall heat partitioning fraction and the overall heat transfer coefficients. The results show that a large fraction of the vapour heat ingress is not transferred to the vapour phase bulk. Instead, it is transported through the walls and transferred to the liquid at the liquid-vapour-wall contact point, driving phase change. Heat conduction from the walls to the interface has a significant effect, even for low heat fluxes, and cannot be neglected.