Phase Change of Water in a Hygroscopic Porous Medium. Phenomenological Relation and Experimental Analysis for Water in Soil

Abstract

The phenomenological relation of non-equilibrium liquid–gas phase change in a porous medium is described at the macroscopic level using the difference in chemical potentials between the liquid and its vapor. The experiments conducted consisted in lowering the partial pressure of water vapor in the pores of a hygroscopic soil and analyzing the return to equilibrium by two measurements: the macroscopic temperature and the partial pressure of vapor. The central hypothesis of the study is that the characteristic time associated with thermal equilibrium is much lower than the characteristic time associated with mass transfers. From these measurements, it is possible to determine the relation that links phase change rate to the logarithm of the ratio of partial vapor pressure divided by the equilibrium pressure (RH). The representation of this relation according to RH reveals two regimes in the return to equilibrium. The characteristics of these regimes are analyzed according to water content, temperature, and total gas phase pressure.