Thermal Conductivity Prediction Theoretical Models For Composite-Porous Geomaterials

Thermal conductivity is a basic thermo-physical parameter that represents the ability of materials to conduction heat transfer.Therefore,whether this parameter is accurate or not directly affects the precision of foundational research which aims to understand the generation mechanism of heat and mass transfer related phenomenon,function and hazard in geomaterials that commonly exists in the field of energy and environment.However,compared to the research hotspots on heat and mass transfer multi-physics coupled-field analysis,the research on thermal conductivity determination method and technology is lagged due to the components and structures complexity of geomaterials.On this account,this work is carried out focusing on multi-component and porous structure characteristics of geomaterials to systematically investigate the thermal conductivity prediction models by using three different methods: theoretical analysis,laboratory test and numerical simulation.First,several basic models that used to predict the thermal conductivity of multi-component geomaterials are analyzed to study their accuracy,applied range and general variation law.All of these models are based on the mixing law models for investigating the effective property of composite material,in which five of them can be rewritten in a unified form.Nonetheless,this unified form developed by researchers can not be degraded to series model.To overcome this problem,a modification is performed in this dissertation and the complete degradation process analyses are then conducted.On these bases,a type of combined structure model that aims to predict the thermal conductivity of geomaterials is established.Both theoretical analyses and experimental results indicate that arithmetic mean combination among basic mixing law models has the best performance on the thermal conductivity prediction,which could be responsible for the phenomenon that more researches in existing literatures focus on this compound model rather than others.Second,inspired by using the homogenization method in multi-scale study,the highly simplified cylindrical unit cell model for predicting the thermal conductivity of porous geomaterimals is supplemented and improved when special attention is paid to the particle geometric characteristic of granular geomaterials.The results show that enhanced model increases the coincidence degree with experiments(The ratios of difference with experimental results decrease from 1.58 to 1.27)while complementary model achieves a full coverage of the porosity range [0,1] together with original Haigh's model.After that,a unified evolution conceptual model of soil-water-thermal conductivity characteristic curve(SWTCC)is proposed via analogy analysis on soil-water characteristic curve(SWCC).The whole shift evolution of proposed conceptual model demonstrates the thermal conductivity upward convex and concave trend with respect to degree of saturation,which is a promising tool to unify all relationships between thermal conductivity of different geomaterials and saturation.Further,the fractal geometry theory is introduced to describe the complex pore structure in geomaterials.Accordingly,an approximate criterion is firstly presented,based on which the size of a fractal REV of porous geomaterials can be obtained using their pore size distribution,i.e.the size of the cubic REV approximatly equals to five times of the maximum pore diameter.Meanwhile,taking the capillary bundle model in soil science for reference,a fractal capillary bundle model at pore scale is proposed to predict the thermal conductivity of composite-porous geomaterials,with the effect of pore size on air thermal conductivity,i.e.Knudsen effect,being considered.Above model clearly demonstrates the relationship between macroscopic thermal conductivity and components as well as microstructure parameters.Of important note,is that it provides an analytical method and approach which is different from those originated in the traditional Euclidean geometry space.Finally,the characterization and visualization on surface and interior microstructure of selected geotechnical samples are performed by using SEM and CT techniques.A 3D reconstruction using CT imanges is then adopted,based on which the finite element analyses on 1D steady heat conduction are carried out.Furthermore,comprehensive comparisons among the thermal conductivities obtained from theoretical predictions,experiments and simulations in this paper are conducted simultaneously.The results demonstrate that,owing to the differences in thermal conductivity of components,the pore structure and their spatial distribution as well as anisotropy have effect on the temperature distribution of geomaterials,which in turn leads to the convergence,divergency and turning of heat folw among different phases.These processes will change the conduction heat transfer ratio,finally causing the variation of thermal conductivity and resulting in significant complexity of the related research work on thermal conductivity of geomaterimals.