Pore network study of slow evaporation in hydrophobic porous media

A three-dimensional pore network model is developed in this paper to disclose the slow evaporation characteristics in various hydrophobic porous media. The constructed network is composed of large cubic pores connected by small throats of square cross section. During evaporation in the network, the two-phase flow is analogized as quasi-static imbibition, and water vapor transport is described as the diffusion process. Based on the developed model, parameter studies are performed to elucidate the effects on evaporation of the pore size distribution (PSD) and the aspect ratio (i.e. throat-to-pore ratio). In the wide PSD network, the drying induced two-phase displacement is a random process, and the impact of the aspect ratio is not significant. Two-phase displacement in the narrow PSD network is also a random process when the aspect ratio is high; but for the lower aspect ratio, the drying fronts are stably receded. In the case of the high aspect ratio, the evaporation rate with respect to network liquid saturation can be characterized by four periods in both the narrow and wide PSD networks: i.e. an initial drying period, a constant rate period, a falling rate period, and a receding front period, similar to that in the hydrophilic case.