Mechanism Investigation On Flow And Heat Transfer In Porous Media On Pore Scale

Transport phenomena in porous media have been investigated for many years for applications in petroleum engineering, food, medicinal materials, agricultural and the evaporation or the capillary pumped process in many kinds of heat exchangers. So it is very important to further investigate on the energy transport in the porous media.The lattice Boltzmann method is intrinsically a mesoscopic approach based on the evolution of statistical distribution of lattices and has achieved considerable success in simulating fluid flows and associated transport phenomena. It was proved that the lattice BGK method was consistent with the Navier-Stokes equation for fluid flow through Chapman-Enskog expansion. The most important advantages of LBM are the easy implementation of multiple interparticle interactions and complex geometry boundary conditions.Based on the background of Capillary Pumped Loops (CPLs), the purpose of this thesis is to investigate heat and mass transfer inside a porous medium on the pore scale by using the LBM and porous media renconstruction technique, especially, the heat transfer mechanisums with flow and evaporation in a CPL's porous wick, optimize the design of some thermal control devices with porous structure, such as CPLs. The investigation is focused on the following aspects.1.2D,3D stochastic porous media generationQuartet structure generation set and mathematic statistic method are introduced to generate random porous media respectively. In the former, the random porous meida is generated by some special rule and the grains grow directly inside a computational domain. In the latter, the 2D segment image of a real CPL porous wick is obtained by Scanning Electron Microscope (SEM), and then some mathematic characteristic, such as porosity, average pore diameter, autocovariance function can be obtained by image analysis. Finally, a 3D random porous media whose statistic characteristic consists with the real porous wick can be generated using some mathematic rule.2. Pore scale investigation of mass transfer with phase chage inside a porous mediumA novel 2D multiphase lattice Boltzmann model with phase change is proposed to predict the growth, the merge of the bubbles during the phase change. The mass transfer curves during the phase change are also calculated. Combine the multiphase LBM with the 2D porous structure, as well as introducing a special external force term in the present LBM, a pore scale lattice Boltzmann model is proposed to predict the evaporation process of a two-dimensional porous medium, which can be used to investigate the growth and the merge of bubbles inside a porous medium.3. Investigation on the three-dimensional multiphase conjugate conduction inside the porous wickBy a spatially varying relaxation time in the present lattice Boltzmann model, A D3Q7 lattice Boltzmann multiphase conduction model is proposed, and combined with the porous media reconstruction technique, the conduction process during the evaporator start-up is investigated by the present LBM. The influence of different factors including the porosity of the porous wick and the heat load are analyzed. Especially, an equation which describes the time when the wick reaches the saturated state is obtained according to the results. Furthermore, the effective conductiviy wich is one of the most important parameters of the porous media is also predicted by the presnt LBM, and the influences of the porosity and the average pore diameter are also discussed.4. Pore scale numerical investigation of the evaporatorA novel double-distribution thermal lattice Boltzmann model including 2D and 3D is developed to analyze the coupled heat and mass transfer process. The whole evaporator including the cover plate, the vapor grooves and the porous wick is reconstructed, together with the LBM, the heat transfer with flow and evaporation in the porous wick can be easily investigated on the pore salce, which reveals the physical iperational mechanisums of the evaporator. Some new phenonmen phenomenon wich can not be observed quit well by traditional CFD is obtained using our neo LBM.5. Analytical investigation of CPL systemA transient model based on some physical parameters which is predicted by the LBM is developed to predict the thermal and dynamic behavior of a CPL. This model can be used to explain the start-up process. The influences of different factors including the heat load and the temperature oscillations in the reservoir are analyzed.