Critical Evaluation of Liquid and Vapor Transport During Evaporation From Porous Media

Experiments have demonstrated "enhanced" vapor fluxes from porous media relative to predictions based on Fick's law. Theoretical models were proposed since the early 50's offering various mechanistic and phenomenological enhancement factors to reconcile apparent discrepancies between experiments and predictions made based on macroscopic diffusion theory. Some of the assumptions and experimental setups are critically reevaluated considering dominance of capillary-induced liquid flow extending from primary drying front to vaporization surface. The extent and existence of hydraulic connections through the unsaturated zone above the drying front has been predicted theoretically and demonstrated experimentally using high resolution x-ray tomography of sand columns during evaporation. We also clarified and tested the dominant role of vapor diffusion during stage 2 evaporation in a series of experiments with prescribed vaporization depths (using hydrophobic layers to disrupt liquid continuity and fix vaporization plane). The present critical reevaluation provides new insights regarding primary mechanisms controlling evaporation from porous media, and emphasizes the extended role of capillary flows that reconcile observations with prediction without the need to invoke unobservable local thermal gradients and similar "enhancement" factors.