Mesoscopic Study On Natural Convection In Cavity With Porous Media And Fluid By Lattice Boltzmann Method

Fluid flow and heat transfer problem in porous media is widespread in all over the natural world and the areas of production activities of society, in which, the problem of natural convection in porous media/fluid cavity, such as building heating and ventilation engineering, electronics cooling, solar collectors, the drying process, fuel cells, geothermal engineering and refrigerated cooling devices and so on. Therefore, study on the mechanism of flow and heat transfer in the cavity has important scientific significance and practical value.In this paper, from the view of mesoscopic point, the problem of natural convection in porous media/fluid cavity was calculated by lattice Boltzmann method. Combined with X-CT constructing real porous media cavity, the phenomenon in the real porous media/fluid cavity was studied. The porous materials of sieve, sponge, plaster, brick and others were chosen as analog samples and scanned and imaged digitally with X-CT by one cross-section to another. Scanned images were filtered, sharpened, binarized and others by Matlab and reconstructed real porous media/fluid cavity model and imported the data file which the cavity model was transformed into lattice Boltzmann model to simulate. Meanwhile, compared the real porous media/fluid cavity restructured based on X-CT with the cavity reconstructed in lattice Boltzmann model, results showed that the reconstructed model was feasible and effective.For calculation of real porous media/fluid cavity model, the couple distributed thermal lattice Boltzmann model which could simulate natural convection in porous media/fluid cavity was established. In two-dimensional model, flow and temperature field were discreted by D2Q9 and D2Q5 lattice model separately. While the three-dimensional model, discreted separately by D3Q19 and D3Q7 model, then the flow and heat transfer were coupled by thermal buoyancy. Through building equilibrium distribution function and relaxation time in solid to achieve coupling of fluid/solid heat transfer, simulating the real porous media/fluid cavity model. Compared the results calculated and obtained in the typical operating conditions with multiple documents solution and the experimental data by particle image velocimetry technology separately, results showed they were in good agreement and validated simulation by lattice Boltzmann method effective and accuracy, study on the mesoscopic mechanism of natural convection in porous media/fluid cavity is establishing foundation.The restructured complex cavity partially filled with real porous medium based on X-CT, the problem of natural convection in porous media/fluid cavity is studying from the view of gap scale by lattice Boltzmann method based on C++. The basic characteristics of flow and heat transfer in porous cavity were obtained and to analyze the discipline of flow and heat transfer in porous composite cavity, discuss the various parameters (thickness of porous media and height, section porosity, Ra and Pr number, etc) on the influence of flow and heat transfer in cavity, explore the changes affect of slip effect at the interface in porous media/fluid layer region and study the variation of interface slip coefficient. Through study of natural convection in porous media/fluid cavity, found that Ra has a significant impact on natural convection, as Ra increases, the flow and heat transfer become more severe and sufficient; Pr has an important impact on it, when Pr increases, the flow become more slowly but heat transfer sufficient and will be steady; thickness of porous media has an impact on it, flow and heat transfer become weakened with the increase of thickness. The velocity slip and stress jump coefficient decrease slowly with Ra at the interface in porous media/fluid cavity; the change of thickness may lead to the coefficients less change obviously; while near the wall, the coefficients changed significantly, but at other region they remain unchanged.This paper mainly studied the parameters (thickness of porous media and height, section porosity, Ra and Pr number, etc) on the effect characteristics of natural convection and the interface slip effect in porous media/fluid cavity, and got the variation of effects of all parameters, which laid a foundation for the research of natural convection in the cavity.