Utilization of Nonequilibrium Phase Change Approach to Analyze the Nonisothermal Multiphase Flow in Shallow Subsurface Soils

The prediction of coupled nonisothermal multiphase flow in porous media has been the subject of many theoretical and experimental studies in the past half a century. In particular, the evaporation phenomenon from the shallow subsurface has been extensively studied based on the notion of equilibrium phase change between liquid water and water vapor (i.e., instantaneous phase change). One of the frequent assumptions in equilibrium phase change approach is that liquid water is hydraulically connected throughout the vadose zone. Furthermore, classical soil-water retention curves (e.g., van Genuchten model), which have been extensively used in the literature to model evaporation process, are only valid for high and intermediate saturation degrees. Although these limitations have been addressed and improved in separate studies, they have not yet been rigorously incorporated in the numerical modeling of nonisothermal multiphase flow in shallow subsurface of in-field soils. Therefore, the aim of this study is to investigate the coupled heat, liquid, and vapor flow in soil media through the Hertz-Knudsen-Schrage (HKS) phase change model and by incorporating a water retention model which captures the soil-water characteristics from full to oven-dried saturation degrees. A numerical model is developed and validated against the in-field experimental data. Reasonable agreements between the calculated and measured values of water contents at all depths, as well as the temperature, and cumulative evaporation are observed. Results also confirm that the contribution of the film flow in overall mass flow in the medium is required for accurate modeling and cannot be ignored.