Multiphase Modelling of Water Evaporation and Condensation in an Air-Heated Porous Bed

Abstract

Moisture transport as well as evaporation and condensation are key mechanisms in the energy management of many processes occurring in different porous media. The role of moisture varies significantly among several applications such as food drying, concrete heating, deep geological barriers, and smouldering combustion of wet fuels. Currently, there is a lack of numerical models able to accurately predict these processes within a complex porous matrix. A one-dimensional numerical model was developed to predict the transient and spatial movement of evaporative and condensation fronts in an air heated inert porous bed partially saturated with water. The results showed that by introducing simple calibration constants in the effective thermal properties, the model was able to accurately predict experimental results. The numerical results revealed that temperatures rapidly increase to boiling conditions because of vapour condensation. Moreover, the model identified that the temperature plateau occurred because all the available energy was used to evaporate water. Evaporative cooling was also predicted and showed a rapid temperature decrease when air was initiated. When the air supply was initiated along with the heater, evaporative cooling resulted in a temperature plateau lower than air-off conditions. This was caused because vapour was carried forward much faster by the forced air, resulting in lower condensation and consequently lower energy released. Overall, this work provides a valuable representation in space and time of the key moisture transport processes in the gas phase as well as key phase-change and heat transfer processes occurring in porous media.

Keywords: Evaporation, condensation, Boiling, Phase-Change, porous media, Evaporative Cooling