Analysis Of Heat Transfer Mechanism Of Thermal Insulation Concrete Based On Mesoscopic Scale

In recent years,the rapid development of China's construction industry has brought about a series of resources,energy consumption and environmental problems.In response to the call of building energy conservation and sustainable development of the construction industry in China,the research,development and promotion of various green building materials have become hot spots.However,for this kind of concrete,the thermal insulation ability still needs to be further improved in order to be truly recognized by the market and widely promoted.Therefore,on the premise of meeting requirements of its strength,finding an effective way to improve its thermal insulation performance is one of the key issues for this kind of concrete which should be studied urgently.Thermal insulation concrete(TIC)is a kind of multi-phase heterogeneous material.Due to the addition of thermal insulation aggregate,the mode of its heat transfer is more complicated.It is difficult to epurate the heat transfer mechanism of TIC from macroscopic indexes.As a bridge that connecting macroscopic working performance and microscopic characterization of materials,the research on the mesoscopic scale of TIC can analyze its heat transfer mechanism from the perspective of the composition structure and internal temperature field of TIC,which can provide effective supplement to macroscopic test and microscopic characterization conclusions.The effective thermal conductivity of concrete is an important index to reflect its thermal insulation ability.Based on the mesoscopic scale,this paper proposes an improved mesoscopic random aggregate model,and uses the model to study various influencing factors of the effective thermal conductivity of TIC of added glazed hollow beads(GHB)and reveal the heat transfer mechanism of TIC.The main research contents and conclusions are as follows:(1)The image analysis method is adopted to analyze the mesoscopic structure of TIC.Combined with the current situation of concrete simulation calculation,it is equivalent to discrete phase of coarse aggregate and GHB,and the mortar layer.According to the test results,the mesoscopic scale parameters are determined.Firstly,the discrete aggregates are randomly distributed based on Monte Carlo probability and statistics principle.Mesoscopic models of three different shapes of shapes are established.The shape of the coarse aggregate is circular,elliptical and polygonal respectively.Secondly,in view of the grid malformation problem caused by the small size of individual particles and the large number of particles per unit volume in the modeling process of GHB,an improved background grid method based on digital image processing is proposed.(2)Based on the mesoscopic model of thermal insulation concrete of GHB established in this paper,APDL language is used for programming to simulate the steady heat conduction test of the TIC.The validity of the model is verified by comparing the experimental and simulation results.It is found that when the volume fraction of coarse aggregate is 35% and the volume fraction of GHB is 25%,the effective thermal conductivity of TIC is 0.400 W/(m·K).And the effective thermal conductivity of experimental is 0.412 W/(m·K)The numerical calculation is close to the experimental results,which verifies this model is reliable.(3)The mesoscopic random aggregate model is used to study the influence of different factors on the effective thermal conductivity of TIC,including: the shape of coarse aggregate,the volume fraction of coarse aggregate,the gradation of coarse aggregate,the volume fraction of GHB and the gradation of GHB.The results show that the shape and the gradation of coarse aggregate have little influence on the effective thermal conductivity;When the volume fraction of GHB is 0% and 15%,the thermal conductivity of TIC will increase with the increase of the volume fraction of coarse aggregate.When the volume fraction of GHB is 25%,the effective thermal conductivity will decrease as the volume fraction of coarse aggregate increases;The effective thermal conductivity decreases significantly with the increase of the volume fraction of GHB.When the volume fraction of GHB is 5%,the effective thermal conductivity of TIC decreases by 71.29%;It is found that the effective thermal conductivity decreased with the decrease of particle size of GHB.(4)Comparing the isotherm diagram and heat flow diagram of TIC withcommon concrete,the mechanism of heat transfer of TIC is found: Before adding thermal insulation aggregate,the thermal conductivity of concrete is controlled by the thermal conductivity of discrete coarse aggregate,which forms a stable large heat flow channel while serving as a load bearing framework.The thermal insulation aggregate can be regarded as a "machine of thermal resistance" in the heat flow channel.After adding a large amount of GHB,the original large heat flow channel inside the concrete is continuously blocked and compressed,and heat transfer is blocked;At the same time,the thermal insulation aggregate is uniformly distributed.So that the thermal resistance of the concrete is increased,and the effective thermal conductivity of the concrete is reduced.At the same time,because the thermal insulation aggregate is uniformly distributed,the thermal resistance of TIC is increased,which reducing the effective thermal conductivity of TIC.(5)Based on the above analysis of influencing factors and the mechanism of heat transfer performance of TIC,combined with the existing calculation model of thermal conductivity of concrete,a second-order thermal conductivity model considering the fineness of insulation aggregate is established,which can accurately calculate the effective thermal conductivity of TIC.