Study On The Transfer Laws Of Coupled Heat Moisture And Air Transfer For The External Insulation Wall

Moisture accumulation inside the wall will reduce its thermal performance. It is recognized that the transfer laws of heat and moisture transfer inside the wall are very important to correctly calculat the energy consumption. Currently, most of the coupled heat and moisture transfer research was carried out under no pressure difference condiction. there was few research on coupled heat and moisture transfer under pressure difference condiction, thus further research is needed. In this paper, taking the external insulation wall for example, a theoretical model of coupled heat moisture and air transfer is built. The model is discreted by finite difference method, and a program is developed to solve the equations. The transfer laws of coupled heat moisture and air transfer are studied through experiment and numerical simulation, the influence of coupled heat moisture and air transfer on characteristics of heat and moisture of the wall is analyzed, the main conclusions are as follows:According to the principle of energy and mass conservation the model is built considering liquid water transfer, water vapor diffusion, air infiltration and phase transition. The model consider air moisture content and temperature as drive potential. It is easy to solve the model with the coefficient having a clear physical meaning as well as to solving.The equations of coupled heat moisture and air transfer are discreted by using finite difference method. Exterior connections method is used in grid division. The backward difference method is used to discrete the unsteady item. The first centered difference method is used to discrete the diffusion item. The harmonic mean method is used to calculate the interface transfer coefficient. A MATLAB program is established according to the discreted equations.A device for the transfer laws of coupled heat moisture and air transfer is set up. The distribution of the temperature and relative humidity of the internal nodes of the wall is test under pressure difference conditions of △P=0 and △P>0. The results show that the moisture transfer will increase and the wall will be tend to wet when the air transfer and water vapor diffusion are in the same direction, or the moisture transfer will decrease and the wall will be tend to dry. The air transfer have a greater influence on moisture transmission with a smaller effect on temperature transmission.The distribution of the temperature and moisture of the internal nodes of the wall under pressure difference conditions of △P=0 and △P>0 is calculated by the MATLAB program. The results compared with test and simulation show a good consistency and the program have a high numerical simulation accuracy, and verify the feasibility of the model and the reliability of the program.The heat and moisture characteristics of the wall is calculated under pressure difference conditions of △P=0,△P>0 and △P<0 in cold area and hot summer and cold winter area during five years. The characteristics are calculated according to the model established in this paper by the program established in MATLAB. The results show that in cold area of Taiyuan, the wall will be tend to wet under pressure difference conditions of △P<0 in Winter and △P>0 in Summer. In hot summer and cold winter area of Shenzhen, the wall is more wet under pressure difference condition of △P>0 than the wall under pressure difference condition of △P=0 in Summer.