On the modelling of moisture transport in air and capillary porous materials

Abstract

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.

Accurate modelling of coupled heat and moisture transport problems in capillary porous materials and the interaction with surrounding air is important for many applications going from wetting and drying of soils to convective drying in industrial dryers. In this study the emphasis lies on the modelling of convective drying of porous building materials with a capillary or hygroscopic-capillary nature. An important aspect for the correct modelling of convective drying is the way the air boundary is implemented. Most modelling approaches today use convective transfer coefficients to model the impact of convective flow over a porous material. Often the heat and mass analogy is used to calculate a convective mass transfer coefficient (found in literature or from CFD calculations) from the convective heat transfer coefficient. A wrong estimation of transfer coefficients can however have a significant impact on the modelling outcome. This paper gives a short overview of the state of the art in conjugate heat and mass transport modelling for convective drying. In this review shortcomings of currently applied modelling approaches are highlighted. Secondly a finite volume coupled heat and moisture transport model is discussed. Recently this heat and moisture transport model was implemented in a commercial CFD package to model the coupled transport in porous materials and air. The material model is validated using a drying experiment on ceramic brick. This model is then used to study the importance of correct boundary conditions for convective drying modelling.