Study On Characteristics Of Coupled Heat And Moisture Migration In Multilayer Porous Structures

The human residential environment is a significant problem that is currently focused social and technological attentions upon. Moisture accumulation in building enclosed structures is a menace to the service performance of building structures and the residential environment. The characteristics of heat and moisture migration are systematically studied in this dissertation by taking the moisture migration and moisture accumulation in building enclosed structures as an engineering background. The numerical simulation and experimental approaches to the process of transient coupled heat and moisture transfer in multilayer porous structures are developed for the purpose of revealing the mechanism and characteristics and the inverse problem about system parameters identification in the process of heat and moisture migration.The causes of moisture accumulation in wall are discussed after the process of moisture migration is analyzed. Two typical examples analyses are conducted for the average conditions both of winter and summer climates under vapor diffusion and air leakage. A further discussion is set forth to determine the main factors that the moisture accumulation in wall is affected. It is found that the factors are the relative humidity at the wall surface, indoor temperature, permeability, air pressure difference and rate of the infiltrative or exfiltrative airflow.The simulation of the 1-D transient coupled heat and moisture transfer is further improved, in the aspects of modeling, boundary conditions assuming and numerical schemes utilizing. The transient mathematical equations are addressed for the coupled heat and moisture transfer by taking account of moisture accumulation procedure. An analytical method by means of the transfer function is proposed to predict the transient distributions of temperature and moisture content at different interfaces in walls. A numerical analysis approach based on an efficient finite-difference method is developed to deal with the procedure of coupled heat and moisture transfer in a multilayer wall with nonlinear boundary conditions considered. Transient boundary conditions are assumed for both heat and moisture transfer at external surfaces. The error due to the boundary conditions are taken as constant is evitable by the approach. An efficient finite-difference approximation scheme is used to deal with discontinuity of material properties of the multilayer wall.A technique for identifying total parameters (TITP) is employed in the inverse problem of heat and moisture migration research in porous structures, accordingly the basic principle and implementary approach is presented to obtain the temperature and humidity dependent thermal and humid physical parameters. It is indicated theoretically that the applications of pulse spectrum technique (PST) to the inverse problem are feasible. The preliminary experimental research is conducted to the process of heat and moisture transfer in building elements, hence an experimental technique is developed for the indirectly measurement of mass diffusivity and thermo-mass diffusivity of the moist porous media.