Mathematical modelling and numerical simulation of heat and moisture transfer in textile assemblies

Abstract

In this thesis, we study heat and moisture transport processes in textile assemblies in both one-dimensional and three-dimensional settings. At first, our problem is described by a fully dynamic multi-phase multi-component flow model which covers heat/moisture convection, conduction, diffusion, phase change and moisture absorption in fibrous porous media. The model is based on a previous work with a significant modification to take into account the air resistance to moisture transfer. To maintain physical conservations, a splitting semi-implicit finite volume method is proposed for solving the system of nonlinear convection-diffusion-reaction equations, in which the calculation of liquid water content absorbed by fiber is decoupled from the rest of the computation. The numerical results show good agreement with the experimental measurements. As we know, in many sweat transport systems, the moisture concentration (or sweat) as well as moisture flux are relative small and the air concentration reaches a steady state very quickly. Therefore, secondly, based on the fully dynamic model, a quasi-steady state model is introduced. An analytic form of the air concentration is obtained in terms of the mixture gas (vapor and air) concentration (or pressure) and temperature. The new model is described in the form of a single-component flow with an extra air resistance (permeability), involving only the vapor concentration (or pressure), temperature and water content. Numerical results show that the proposed quasi-steady state model is realistic and less complicated. Thirdly, we concern the numerical study of heat and moisture transfer in three-dimensional clothing assemblies, based on the previous fully dynamic model. A finite volume method (FVM) based on non-orthogonal structured meshes has been used here, and the normal flux on a cell edge (face) with the normal flux being continuous across the edge is carefully approximated. Four types of clothing assemblies are investigated and comparisons with experimental measurements are also presented. Finally, we apply our fully dynamic model to a more complicated system (i.e. a garment-air gap-human skin system under flash fire). In this case, thermal radiation is taken into account since a huge temperature gap between the garment and flash fire exists. Generally, in hot environments, human body sweats a lot in order to reduce skin temperature by evaporating the sweat on the skin surface to prevent skin burns. From the numerical results, it does validate that sweat evaporation could effectively keep the skin temperature low. Moreover, certain amount of free water on the fiber also can provide good protection for human body.