Numerical Simulation Of The Thermal Properties And Heat Transfer Process Of Foamed Concrete

It is urgent to develop green buildings,since the total energy consumption of buildings accounts for about 20 percent of the national total energy consumption.The development of high durability,low energy consumption,high fire insulation building materials meets the urgent need of the society of the relation of green building insulation and energy saving,reduce building energy consumption and carbon emissions of the society.As a kind of inorganic insulation material with high fire resistance,foam concrete is widely used in building roof insulation,floor heating insulation and exterior wall insulation.It is an important basis for optimizing the performance of foamed concrete and quantitatively evaluating the thermal performance of buildings to study the influencing factors of the thermal performance of foamed concrete and obtain its heat transfer process under the action of heat and humidity coupling.In this paper,Portland cement and sulphoaluminate cement are used as cementitious materials to develop foam concrete with different dry densities.The calculation and analysis model of thermal conductivity of foamed concrete is proposed.The parametric pore numerical model of foamed concrete and the two-dimensional and three-dimensional meso-models of real foamed concrete are established.The COMSOL software platform was used to simulate the transient and steady-state heat transfer as well as the heat transfer process under the action of wet and heat coupling.The main research is as follows:(1)Foam concrete with dry density of 300?1200kg/m~3 was prepared used ordinary Portland cement and high belite sulphoaluminate cement.The quantitative effects of dry density,porosity and other factors on compressive strength and thermal conductivity of two types of foamed concrete are studied.The results are as follows: With the increase of dry density,the strength of foamed concrete increases linearly,porosity decreases,and thermal conductivity index increases.Compared with Portland cement foamed concrete,high-belite sulphoaluminate cement foamed concrete developed faster in the early stage and had similar strength in the late stage.When the dry density is relatively small,the strength of sulphoaluminate cement foamed concrete increases rapidly.When the dry density is greater than 1200kg/m~3,the strength of the two types of foamed concrete increases roughly the same.(2)A new method to establish a two-dimensional parameterized pore model of foamed concrete is proposed.The real image of foamed concrete section was identified and processed by intelligent algorithm and image processing method,and the two-phase mesoscopic model of real foamed concrete was established and imported into COMSOL finite element software.Based on the XCT image of foamed concrete,the real three-dimensional solid and gas phase models are established,and a new method which can be used in finite element calculation based on this model is proposed.(3)The thermal insulation mechanism was studied based on the transient heat transfer simulation of foamed concrete.Based on the steady state heat transfer simulation of foamed concrete,the effective thermal conductivity was calculated according to Fourier's law,and compared with the experimental results and other thermal conductivity calculation models.The result shows that the results obtained by numerical simulation are in good agreement with the experimental results.(4)Based on the fractal theory,the fractal dimension of the cellular structure of foamed concrete is calculated by the Box-counting method and the Sand Box method,and the fractal dimension of the cellular structure of foamed concrete with different dry density is obtained,which proves that the cellular structure of foamed concrete has good fractal characteristics.A calculation model for the thermal conductivity of foamed concrete considering the integral dimension,tortuousness fractal dimension,porosity and other microscopic parameters is proposed according the parallel structural models of solid and gas phases.(5)Based on basic heat and mass transfer theories such as Fick's law and Darcy's law,a temperature-humidity coupling model with temperature and relative humidity as driving potential was established.COMSOL software was used to simulate the heat and moisture transfer process of single-layer wall and composite wall of foamed concrete under the action of heat and moisture coupling.Compared with the data in the literature,the proposed model has high accuracy.