Development, calibration and validation of a mathematical model for the low-pressure-vaporization of the water in porous media

This paper reports the development, calibration and validation of a mathematical model of the low-pressure-vaporization process in different types of porous media. This model aims to improve models developed by other authors in this area, namely by considering some aspects that have not been addressed, such as: the vapor pressure is not equal in the whole chamber; the interfacial vapor pressure is not equal to the saturation pressure; the surroundings have a significant influence on the vaporization phenomena in all zones of the porous media; and the first stage of the process is not characterized only by diffusion vaporization. For that purpose, an experimental calibration procedure was made involving the definition of several LPV parameters: vaporization coefficient of the porous medium εPM, vaporization coefficient at the liquid–vapor interface within the porous medium εi,PM, free surface vapor pressure Pv,FS, vapor pressure of the free layer Pv,FL, interfacial vapor pressure Pv,i, volume of the vaporization layer VVL and volume of the free layer VFL. These parameters were represented by a set of multivariable functions and determined from a series of experiments in different types of porous media, each one characterized by a different initial water temperature. Thus, in this work an enhanced physical model of the LPV in porous media is proposed, concerning two distinct stages. The experimental set-up comprises two main components: the vacuum chamber (VC) and the depressurization system (DS). Part of the VC volume is occupied by an open container for porous media, which is isolated from the VC walls.The results obtained show that the multivariable functions determined through the vaporization coefficient have a good agreement with experimental results (e.g., R2 varies within 0.9581–0.9903) and also that this parameter describes well all regimes of the second stage of the LPV process. The importance of the capillary effect and of the surface tension forces on the superheating degree and, consequently, on the LPV evolution was demonstrated, as well as the effect of the amount of water initially contained in the porous medium. From the validation based on the measured water temperature evolutions, deviations within ±5% are found, which confirm the good agreement between experimental and simulated values. Thus, it is possible to conclude that the calibrated mathematical model describes well the experimental results and it is a good model of reference to various studies and applications.