Research On Effect Of Moisture Distribution And Transfer On Thermal Conductivity Of Porous Building Materials

The thermal conductivity of building materials is one of the most important thermal parameters of cooling and heating load of building and energy consumption calculation.There are moisture contents in most of the porous building materials,the thermal performance will change when moisture exists.However,in both the calculating methods and softwares for cooling and heating load of building currently,the physical property of building materials is often considered as constant,the neglected influence of moisture variation on heat conduction of material will lead to the calculation error of heat transfer.Therefore,the researches on the quantitative relationship between the moisture content of a porous material and the internal heat transfer as well as the correction of thermal conductivity are significant for energy efficiency analysis and load calculation of building.The internal moisture content,distribution characteristics and transfer characteristic are important parameters that could influence the heat transfer of materials.These parameters are not only influenced by structure characteristics of materials porosity,but also the heat and moisture transfer environment.The climate varies enormously around China and the structures of materials are different.The thermal performance of materials would be obtained accurately only by the reveal of the relationship between moisture content,moisture transfer and heat conduction,as well as the quantitative calculation of it.Considering the existential state of moisture content in building materials,including the static distribution and moisture transfer,the influence mechanisms of which on internal thermal performance of materials are different.In view of the above problems,this thesis aims to reveal the quantitative relationship between moisture and thermal conductivity of materials,and propose the correcting computational method for thermal conductivity of building materials with moisture content.Based on the theoretical analysis,experimental measurement and numerical calculation,the internal structure characteristics of porous materials was studied,in which the static and transfer distribution of moisture were explored,the effect of moisture distribution process on heat conduction inside the materials was revealed and the changing relationship of thermal conductivity and moisture was obtained.Considering the impact of static moisture on heat conduction in porous materials,only the heat transfer of material solid skeleton and moist air is considered in the coexistence model of solid and gas currently,the moisture adsorption and moisture condensation inside the material is ignored,thus,the coexistent model of solid,liquid and gas for heat transfer is proposed based on the replacement of the gas with liquid,which is caused by the state change of moisture inside the materials.Due to the feature that pore structure characteristics of material is twisting and connected,the capillary tube structure with a certain degree of tortuosity is established based on fractal theory,then a calculation model of thermal conductivity in porous materials that a three phase coexist.The calculation model is employed to obtain the quantitative influence relationship between thermal conductivity of material and static moisture distribution factors which include porosity,pore size distributions,moisture content and fractal structure parameter.The research results show that the change rate of the thermal conductivity decreases at first and then increases with the increase of porosity.There is an obvious change when the porosity is less than 0.1,the lower the saturability is,the changing characteristic is more obvious.The thermal conductivity would be decreased by 10% when the minimum and maximum porosity ratio dropped one order of magnitude.The thermal conductivity decreases with the increase of tortuosity,fractal dimension of tortuosity and area,this trend is more obvious when the area fractal dimension is higher 1.8.Considering the impact of moisture transfer on heat conduction,the coupled heat and moisture transfer governing equations without moist phase transition is established by the analysis of heat and moisture transfer mechanism of porous material,and the vapor partial pressure and temperature gradients are chosen as driving potential.To solve the problem that current macro moist diffusion theory employed in the moisture condensation inside the material can not reflect the moisture condensation theory,the coupled heat and moisture transfer governing equations with moisture phase transition is established based on the coupled heat and moisture transfer governing equations without moisture phase transition through introducing into moisture phase transition heat and moisture sources,combined with the classical evaporative condensation theory.Based on the interaction of mositure phase transition inside porous materials and temperature and moisture field,a loop iteration calculation method is proposed and the additional thermal conductivities caused by moisture migration and phase transition are obtained.Based on the above models and large amount of calculation,it is found that the change of additional correction of thermal conductivity is more sensitive while the change of additional correction with vapor permeability coefficient is stable when the thermal conductivity of material is less than 0.2 W/(m·K).With the decrease of the temperature difference between indoor and outdoor,the correction amplitude of the additional thermal conductivity caused by moisture migration increases.When the temperature difference is 10℃ and the vapor partial pressure of two sides increases from 1000 Pa to 4000 Pa,the thermal conductivity of normal concrete,clay brick and aerated concrete increases about 2%,10% and 35% respectively.Under the condition of building heating period and air-conditioning period for residential buildings,the quantitative influence of the static moisture and transfer moisture on thermal conductivity in each layer material of external thermal insulation envelope is analyzed.It is found that the thermal conductivity of normal concrete increases by about 8% ~ 10% caused by static moisture content under the two conditions.Under the condition of heating in severe cold and cold zones,the influence of heat transfer on thermal conductivity of materials could be ignored.In the hot summer and cold winter,hot summer and warm winter zones,the additional correction of concrete is-10%~15% under the condition of air-conditioning.In conclusion,comprehensive correction of thermal conductivity of normal concrete during the heating period and air-conditioning period are 8%~9% and-5%~25% respectively.