Research On Heat Transfer Performance Of Unsaturated Porous Building Materials Based On Lattice Boltzmann Method

The proportion of building energy consumption in the total social energy consumption is rising rapidly,and how to reduce building energy consumption has become an urgent problem to be solved.The key to reducing building energy consumption is to reduce the energy consumption of building envelope.Most of the materials used in building envelopes belong to porous media,and due to the complex external heat and humidity environment,the internal pores of the materials often contain moisture.In addition,most scholars assume macroscopic continuum in their simulation calculations,and it is difficult to determine the parameters in the model or approximate them with experiments and empirical formulas,which will cause the model parameters to deviate from reality.Therefore,in order to study the heat transfer performance of under unsaturated porous building materials,this paper conducts a study based on the lattice Boltzmann method,the specific contents are as follows:(1)Taking aerated concrete as an example,the structural form,pore distribution and other characteristics of porous building materials are measured and analyzed to obtain the basic parameters of their structure.Based on these basic parameters,the geometric model of porous building materials is constructed using an improved random structure generation method,and the different phases inside the material are defined by the phase function matrix.(2)The lattice Boltzmann method is introduced,and the lattice Boltzmann numerical model of two-dimensional solid-gas-liquid threephase heat conduction in porous media is established.The whole process is realized by M language programming.Then,the aerated concrete samples under different humidity conditions were obtained by the weighing method,and their effective thermal conductivity was measured respectively.The model is validated by the obtained experimental data.The results show that the relative error between the simulated calculated value and the experimental data is about 7.6%,and the model can simulate the heat transfer of unsaturated porous building materials well.(3)The influence of important influencing factors such as moisture content,porosity and pore size distribution on the heat transfer performance of unsaturated porous building materials was simulated and analyzed.The results show that the increase of the effective thermal conductivity of unsaturated porous building materials with different porosity is different in different moisture content ranges,and the moisture content has a more significant effect on the effective thermal conductivity of materials with high porosity.At the same time,the effective thermal conductivity of the material with low solid-gas thermal conductivity increases more greatly with the increase of moisture content.The effect of pore size distribution on the effective thermal conductivity of unsaturated porous building materials is small.When the pore distribution is uniform and the proportion of connected pores in the number of pores is large,the effective thermal conductivity is slightly larger.(4)The numerical simulation model involves the three-phase thermal conductivity of solid,liquid and gas and the proportion of each phase.The proportion of each phase is reflected by moisture content and porosity.The heat transfer of unsaturated porous building materials was further studied by means of temperature nephograms.The results show that the temperature field is distorted in areas with dense pore connectivity or distribution;with the increase of moisture content,this change gradually slows down to a gentle change.