Impacts Of Mesostructures On The Dynamic Coupled Heat And Moisture Transfer In Organic Closed-cell Thermal Insulation

Organic closed-cell thermal insulation material is a common thermal insulation material with low thermal conductivity.However,when there is a high temperature and humidity gradient,the phenomenon,such as penetration,condensation and accumulation of moisture are easily occurred in the material,which affect the thermal performance of the system.In the traditional method of analyzing the coupled heat and moisture transfer,there are problems that the effective transmission coefficient is difficult to be determined and the impacts of the related structures are not clear.In order to solve such key problems,this thesis proposed a multi-scale model to establish the influence of the internal structure of organic thermal insulation materials on its dynamic heat and moisture transfer process.In this thesis,the Voronoi diagram division method and QSGS are used to construct a grid unit that can describe the mesostructure of the organic closed-cell thermal insulation,phenolic.Then,the relations between the mesostructure and effective transmission coefficients are analyzed by LBM double distribution model.Based on the simulation data,a database is derived and neural network algorithm is used to realize prediction of effective transmission coefficients in mesoscopic scale.Furthermore,the mesoscopic effective transmission coefficients are introduced into the macro-scale dynamic coupled heat and moisture control equation,and the influences of the mesostructure on the dynamic coupled heat and moisture transfer process are analyzed through the multi-scale model.Finally,in order to provide a reference for the performance optimization of organic closed-cell insulation materials,multiobjective optimization method and compromise optimization method are used to obtain the theoretical optimal structure.From the above studies,it is found that: i)Among the mesoscopic effective transmission coefficients and parameter range of this study,the total porosity has the greatest influence strength on the effective thermal conductivity(2.68).The total porosity and volumetric ratio have the greatest influence strength on permeability(1.38,1.66).Other structural parameters also have a certain degree of influence on the effective thermal conductivity(0.05～0.54)and permeability(0.14～0.72).As the total porosity decreases,the influence of volumetric ratio,relative deviation ratio and average pore diameter gradually decreases.ii)In the process of dynamic coupled heat and moisture transfer,With the increase of structural parameters the main factor that affects the heat transfer process is still the total porosity(13.4%～-8.9%).The volumetric ratio(9.6%～-5.8%)and average pore size(-4.5%～7.8%)is the second and the relative deviation ratio(2.7%～-1.9%)is the smallest.The most important factor affecting the moisture transfer process is the volumetric ratio(40.0%～-26.4%);the the total porosity(-6.3%～20.1%)is the second;the relative deviation ratio(4.9%～-3.8%)and average pore size(6.2%～-7.3%)is the smallest.Take the application of phenolic in cold storage as an example,when the total porosity is constant,it can clearly indicate the impacts of varying the volumetric ratio,relative deviation ratio,and average pore diameter on the moisture transfer process,and in turn affects the heat transfer process.The impact increases by 11.0% with the increase of the total porosity.iii)By taking the thermal conductivity and the moisture content as the objectives,combining multi-objective optimization and ε-constraint method,the optimal structure characteristics of the organic closed-cell insulation material is obtained as:the optimal performance is usually derived when the total porosity is maintained at the maximum.When the total porosity is 0.95,the volumetric ratio needs to be controlled at about9;the recommended value of the average pore diameter is between 80μm and 90μm;and the relative deviation ratio should be 0.2 to 0.3.