Research On Coupled Heat And Moisture Transfer Characteristics Of Multilayer Walls In Hot And Humid Climate

In hot and humid climate of south China, the heat and moisture transfer through wall has an important impact on the hygrothermal performance of building envelope, energy consumption and indoor environment. Therefore, in this dissertation, based on the heat and moisture transfer through porous media, the characteristics of heat and moisture transfer through multilayer wall subjected to hot humid climate of south China are systematically studied by taking the moisture migration and accumulation in building enclosed structures as an engineering background.First, the detailed analysis and discussion on the laws of heat and moisture transfer through multilayer wall was conducted. Based on the Budaiwi model, coupled heat and moisture transfer model was established by considering phase change and liquid tranfer within wall. The mass and energy exchange between wall and air was taken as boundary conditions in this model and the heat produced by vapour phase change occurring within wall was taken as a part of energy balance. The discontinuity between material and air or the interface between different materials was avoided by using air humidity ratio and temperature as driving potentials. Therefore, the heat and moisture transfer process within materials can be linked with desorption/adsorption process at the material surface which make the analysis process easier.To ensure the numerical stability, the governing equations were discreted by finite volume method with implicit forward differences for time. The linear equations were obtained from the non-linear equations using coefficients updating method. The solution of linear equations was obtained by Gauss-Jordan elimination technique. A compution program was developed. The accuracy of the numerical model was analyzed. To evaluate the correctness of the model, the prediction of the model was compared with the experimental test data. The prediction results have good agreement with the experimental results. For the interface close to outdoor side, the average deviation of temperature and relative humidity is 1.1K and 4.2%, respectively. For the interface close to indoor side, the average deviation of temperature is 0.95K and the average deviation of relative humidity is 5.7%.An experimental setup was built to test the coupled heat and moisture transfer in a multilayer construction. Taken cement mortar-red brick-cement plaster multilayer wall under natural climatic boundary conditions as an example, the temperature and relative humidity was measured in the external walls of an air-conditioned room in Changsha during January and July. The experimental results showed that:(1) Whatever in summer or winter, the temperature and humidity ratio within the wall was coupled strong and the effect of temperature on humidity ratio was more important.(2) Whatever in summer or winter, the temperature and humidity at the interface between the cement mortar and red brick were seriously affected by the variation of outdoor temperature and humidity. The temperature and humidity at the interface between the red brick and cement plaster had small variations; they were affected by the variation of indoor temperature and humidity.(3) The relative humidity at the interface between cement mortar and red brick was likely to reach saturation in summer and winter during the period of air-conditioning. In addition, the relative humidity was kept at higher than 80% for a long-term.(4) In summer, the temperature and humidity at the interfaces of the wall with foamed plastic wallpaper were generally higher than those of the wall without treatment. In winter, the temperature at the interfaces of two walls was basically the same, but humidity at the interfaces of the wall without treatment were slightly higher than that of the wall with foamed plastic wallpaper.(5) Whatever in summer or winter, rainy day climate has a great influence on the moisture content in the wall, which can cause to relative humidity at the interface between the cement mortar and red brick close to saturation for a long-term.(6) Whatever in summer or winter, solar radiation intensity has a great impact on the distribution and variation of temperature and humidity inside the wall.Regarded the hygrothermal model established in this dissertation as main model, the hygrothermal model neglecting solar radiation as sub-model 1 and the well-kn(?)n conduction equation as sub-model 2. Results calculated by main model are compared with the results calculated by the sub-model 1 and sub-model 2. Through these comparisons the effects of solar radiation and moisture transfer on hygrothermal performance of multilayer wall are obtained.According to the work of Motakef and EI-Masri, taken the vapour condensation as a vapour sink, water source and heat source, coupled heat and moisture equations for wet zone were established. The determination of wet zone for steady stage and unsteady stage are provided. Closed-form analytical expressions for the condensation rate, moisture content and the time at which critical moisture content value is reached are obtained. The effect of condensation on material thermal conductivity is obtained according to the moisture content distribution.Detailed analysis on hygrothermal performance of several typical walls of south China is conducted using the codes developed in this dissertation. Finally, the determination method of minimum thermal resistance to prevent mould growth is proposed according to internal moisture production rate, air change rate, indoor and outdoor temperature.