Numerical Study Of Seepage And Heat Transfer In Saturated Soils Based On The Lattice Boltzmann Method

In recent decades, ground coupled heat pump has showed a rapid growth tendency due to its environmental friendliness and economic benefits. During the heat transfer process among heat exchanger with soil, the thermal seepage coupling mechanism is rather complex and the influence of groundwater seepage could not be neglected. In previous studies, mostly literatures focused on macro aspects to relevant experiments studies and numerical analysis. In this article, a new numerical simulation model in mesoscopic was established to reveal the structure of the soil and the influence of groundwater seepage on heat transfer performance.A numerical model was firstly introduced, named double-distribution-function Lattice Boltzmann Method (LBM) and basic principle and boundary schemes about it were declared. Similarly, natural convection in an enclosure square cavity was conducted to validate its feasibility and accuracy. Then a random generation growth method, quartet structure generation set (QSGS) was introduced to reconstruct the microstructure of soil. The two-dimensional geometry of reconstruct was quite similar to real structure of saturated soils. By adjusting the related parameters in this method, more detailed information of porous media could be characterized. Using this model, effective thermal conductivity of isotropic microporous media was analyzed in the case that groundwater flow was not considered. Based on that, a numerical study was performed for simulation of pore-scale single-phase saturated soil seepage and heat transfer process. Effects of various seepage pressure difference, porosity, particle size of porous media and the direction of the temperature gradient on flow and heat transfer were analyzed. The relationship between related hydraulic parameters and soil structure were also investigated according to the simulation results.The numerical results were as follows, a) Without considering the effects of groundwater seepage, the effective thermal conductivity of soil is approximately a negative exponential function about porosity of the soil, while a power function about that of the particle phase of soil. And it also indicates that it decreases monotonically with the particle average size. b) Under the influence of seepage, the average seepage velocity is positive correlation between the seepage pressure difference. The heat transfer mechanism is gradually converted from solid-fluid thermal conduction dominant to heat convection in the simulation area, while temperature rise rate slows down gradually. c) With the increase of soil porosity, soil pore connectivity tends to be better, thus seepage velocity increases accordingly. Alternatively, it enhances bridging between neighboring particles through pore water, the contact thermal resistance decreases distinctly, and groundwater advection is improved. In addition, seepage direction is consistent with the direction of heat conduction, thus the heat convection enhances greatly. Given the above, transfer performance gets better, d) When the soil particle size is larger, resulting local velocity spurts and the phenomenon of typical preferential flow occurs. With the reduce of soil particle size, the formation of intra-aggregate pores cause the effective thermal conductivity of soil to increase. In addition, heat diffusion resistance of porous media increases, average temperature of soil decreases accordingly. e) For the same position to the heat source, the average soil temperature of the driving potential about the flow and heat transfer in the same direction is higher than that reverse direction. With the increase of porosity, the temperature gap also will increase similarly.