Micro-macro Characterizations On Flow And Heat Transfer In Geomaterials

The seepage and heat transfer characteristics in geomaterials are one of the key issues involved in a large number of major engineering practice,such as large hydropower engineering,nuclear waste disposal,geothermal energy exploitation,oil or gas energy underground storage and CO2 geological storage.As a typical discontinuous and heterogeneous material,the geomaterial presents that its macroscopic properities of seepage and heat transfer are closely related to its micro-structure characteristics with a complex coupling effect with environment and geotechnical deformation.Based on the micro-structure of materials,the seepage and heat transfer characteristics and its microscopic effects of geomaterials are studied by micromechanical method and mesoscopic numerical simulation.The effective seepage and thermal conductivity models of geomaterials based on meso homogenization method are established,the lattice Boltzmann method for numerical simulation of mesoscopic flow and heat transfer is developed,and the influence of the microstructure on seepage and heat transfer process is elucidated,which reveal the micro-macro mechanism of seepage and heat transfer characteristics of geomaterials.The main contents of this paper are as follows:(1)Based on the micro-structure of geomaterial,the uniform expression of meso homogenization on seepage and heat transfer characteristics of soil is deduced based on micromechanics method and the basic solution of ellipsoidal matrix-inclusions problem.With consideration of the micro-porous structure,pore distribution,and pore water distribution mechanism,an effective thermal conductivity model of unsaturated bentonite is established using homogenization techniques.The model reflects the effects of matrix shape,pore shape,pore bimodal distribution,porosity and saturation of bentonite on the thermal conductivity of unsaturated bentonite.A homogenization-based effective thermal conductivity model is alose proposed for unsaturated compacted bentonite-sand-based buffer materials with the consideration of the microstructure of the mixture,which is approximated with pores and sand particles of spheroidal shape and random orientation embedded in homogeneous bentonite matrix.The proposed estimate is dependent on the sand content,saturation and the aspect ratios of pores and sand particles.The two micromechanical models are validated by using a variety of bentonite test data,indicating that the proposed models not only have a more explicit physical mechanism and fewer model parameters than traditional component analysis model,but also show a better predictive performance.(2)The thermal lattice-Boltzmann method is established using the multi-relaxation scheme.and the seepage process of the rock fractures with different shear displacements is simulated.The results show that shear displacement can induce significant nonlinear flow behavior under high Reynolds number condition and Forchheimer equation can be used to describe the nonlinear relationship between the flow rate and the hydraulic gradient.The formation and evolution of eddy flow in the process of velocity increase are described.The eddy aperture and advection flow aperture are used to quantitatively describe the microscopic mechanism of the decrease of effective convection area and the decrease of transmissivity capacity in fractures.The convective heat transfer process of rough fractures is simulated by thermal lattice model,which indicates that the convective heat transfer characteristics of water flow through fractures are largely affected by flow velocity,especially when Re = 1?50.Compared with parallel plate model,the roughness of the fractures increases the contact area between fluid and fracture wall,which effectively increases the convective heat transfer capacity.Under different roughness conditions,the average heat transfer coefficient of fracture are little different,but the local heat transfer coefficient shows some disparity and its variation trend and amplitude are consistent with the variation characteristics of fracture morphology,especially under the condition of high flow velocity.(3)With a wavelet analysis technique,the three-dimension fracture roughness is decomposed into primary roughness(i.e.the large-scale waviness of the fracture morphology)and secondary roughness(i.e.the small-scale unevenness).A 3D Lattice Boltzmann method is adopted to predict the flow physics in rock fractures numerically created with and without consideration of the secondary roughness,respectively.The simulation results show that the primary roughness mostly controls the pressure distribution and fracture flow paths at a large scale,whereas the secondary roughness determines the nonlinear properties of the fluid flow at a local scale.As the pressure gradient increases,the secondary roughness enhances the local complexity of velocity distribution by generating and expanding the eddy flow and back flow regions in the vicinity of asperities.In addition,the nonlinear properities of fluid flow are further studied based on the fracture roughness element model and an empirical model for parametric expression of the Forchheimer's coefficients is established,which reflects the influence of the geometrical parameters of the rough fracture on the nonlinear flow behaviors.