Study On Coupled Heat And Moisture Transfer Characteristics In Building Envelope With Freezing And Thawing Process

The heat and moisture transfer in building material has a very important impact on thermal insulation,energy consumption and indoor environment comfort in severe cold regions.Building envelope is a typical multi-layer porous material structure,and its pores are filled with moist air,liquid water or ice.This implies that heat,air and moisture transfer inside the building material is a complex and challenging process coupled with heat and moisture.At present,the research on the heat and moisture transfer characteristics of building envelopes under freezing/thawing conditions is far from enough.Therefore,it is necessary to systematically study the coupled heat and moisture transfer process of porous building materials under freezing/thawing conditions,to evaluate the physical parameters,the heat and moisture fields and thermal and moisture performance of porous building materials in cold regions,and to reveal the mechanism of the influence of the freezing/thawing process on the heat and moisture transfer of building porous materials.This not only can effectively guide the engineering design and construction of building insulation and moisture protection,but also enrich and improve the theory of heat and moisture transfer of building envelope.It has important theoretical value and practical significance for the design and optimization of building envelope.In this paper,three kinds of common building porous materials,such as lime cement mortar,EPS insulation board and red brick for construction,and two kinds of external thermal insulation systems composed of them are taken as research objects.The combinations of theoretical research and experimental research,numerical simulation and test analysis are adopted to study the coupled heat and moisture transfer in building envelope with freezing and thawing process.The main innovations and research results are as follows:?1?The investigation of porosity and permeability of porous materials is a challenge due to its complex pore microstructures.In this paper,an improved fractal model of porosity is proposed based on the pore filling method,in which the facts of microscopic observation are considered.The model is verified by two-dimensional and three-dimensional experiments,and the results show that the improved model can accurately describe the porosity and the pore volume distribution of fractal porous materials.A probability density function of pore size without conventional restrictions is also proposed based on the improved model of porosity,which makes the scope of application of the model of tortuosity fractal dimension much wider.An improved model of permeability is finally proposed based on the improved model of porosity and the probability density function.By comparing the predicted values of the new permeability model with the experimental data,it is confirmed that the permeability model proposed in this paper could accurately predict the permeability of fractal porous materials.In addition,several experiments have been carried out to measure the physical parameters of porous materials,and a non-steady-state method of constant power planar heat source is designed.The thermal conductivity of wet porous materials is determined by this method,and the effective thermal conductivity model is verified.?2?In view of the coupled heat and moisture transfer in building material with freezing and thawing process,a mathematical model is proposed by using the temperature gradient and the water content gradient as the driving potentials of the transport process.The effects of the phase transition of vapor,water and ice on heat and moisture transfer are simultaneously considered in the present model.The energy equation takes into account not only the influence of the latent heat of freezing?or thawing?and evaporation?or condensation?,but also the energy change caused by moisture transfer.The treatment method of water content gradient at the interface of multi-layer structure wall is proposed,which overcomes the problem of discontinuity of water content between interfaces.?3?Based on the analysis of the current three mainstream solutions of coupled equations,a variable step multitridiagonal matrix algorithm?VSMTDMA?is designed in this paper,which reduces the computational time cost while satisfying the accuracy of the solution.Under the natural conditions of no ice,a micro-experiment room is designed to verify the reliability of the proposed model applied to heat and moisture transfer of the multi-layer wall.In addition,the model is verified experimentally with respect to the temperature,relative humidity,and frozen moisture content?ice,water and vapor?.A comparison is made among the data using the proposed model,the data using the conventional approach,and the experimental data.It is found that the data predicted by the conventional approach without considering the liquid water freezing deviates greatly from the experimental data,and the data using the proposed model that considers the freezing factors is closer to the experimental data.The comparison between the model prediction and the test data shows a better consistence of the proposed model than the conventional approach especially in the condition of freezing in building material,which indicates that the proposed model can improve the prediction accuracy of the coupled heat and moisture transfer in building material with freezing and thawing process.?4?For the EPS board cast-in-place concrete exterior insulation system?S1?and the EPS board thin plastering exterior insulation system?S2?,which are widely used in China,the proposed model is applied to investigate the thermal and moisture performance of them under the natural climate conditions of Harbin?a severe cold region?for ten years.The results show that most of liquid water is concentrated in the concrete layer and freezing/thawing and temperature drop mainly occur in the insulation layer.Water content and ice content cause the heat transfer coefficient of the two insulation systems compared with that of their dry state to increase by an average of 24.1%and 20.8%,respectively,over the ten years,which leads to the heat transfer coefficient of the system S1 to exceed the specified value in the Chinese national standard.The annual energy consumption of the two insulation systems gradually decreases over time.The annual energy consumption of the two insulation systems with freezing/thawing is on average 17.24kWh?m^-2and11.05kWh?m^-2higher than that without freezing/thawing,respectively.In addition,the thickness of the insulation layer of the two insulation systems under the premise of meeting the specified heat transfer coefficient is determined.The influence of the vapor barrier layer and the air layer on the heat and humidity characteristics of the two insulation systems is studied.