Analysis Of Heat Transfer Mechanism Of Nanofluid Phase Change Process In Porous Media Based On LBM

With the development of science and technology, it is setting higher requirements for boiler and metallurgy equipment, such as water wall of blast furnace. As an effective heat transfer mode, phase change heat transfer is widely used in heat exchanger. Heat pipe is a highly effective heat transfer device, transporting thermal energy through the refrigerant phase-change heat transfer. So heat pipe working medium has dominate influence on heat transfer characteristics. Generally, the study of heat pipe can be classified into two aspects: heat pipe in the heat transfer medium of the nanofluid in this subject and the study of porous media in thermal pipe wick. The nanofluid viscosity, thermal conductivity, thermal diffusivity and convective heat transfer coefficient are growing increasingly significant compared with pure fluid, which make it become a new promising phase change heat transfer medium. Porous medium has the characteristics of low density,high porosity and large specific surface area, and has many excellent properties, Therefore,it is of vital importance to study heat transfer characteristics of phase change in porous media. In this paper, the numerical simulation is adopted to analyze the porous medium and nanofluids. From the mesoscopic point of view of using the lattice Boltzmann method(LBM), the author stimulates different stages of heat transfer in the process of nanofluids in porous media flow to explore the mechanism of flow and heat transfer. The main researches are listed as follows:(1) Adopting the coupled double distributed LBM mode proposed by Guo, and taking water and air as the reference, I simulated the heat transfer velocity field and temperature field of different nanofluids natural convection in partially filled with porous medium cavity. In this way, I instigated the effects of different porosity, non dimensional thickness D and Rayleigh number and Darcy number on heat transfer. It indicates that there is a critical Rayleigh number for different Ra and?respectively, and when Ra is different, the effect of heat transfer will show an opposite tendency. When the thickness of the porous layer is very thin or very thick, the heat transfer intensity is more sensitive to the thickness.Simultaneously, the Da and porosity also exert important influence on heat transfer.(2) Based on the model of nanoparticles proposed by LI, and combined with the pseudo potential model of Zeng, I built a lattice Boltzmann model of nanofluid bubble flow boiling in tube. Additionally, I investigated the effects of different spacing and different lateral acceleration on the bubble growth process and heat transfer by choosing 3%Al2O3-water nanofluids flow boiling process of bubbles growth, fusion coalescence and separation process to numerical simulate and analyze. It indicates that the decrease of bubble spacing and the increase of lateral acceleration can strengthen the effect of heat transfer.(4) By using a two-dimensional partially filled with porous medium structure model,which is available to porous wick constructed by the stochastic collocation method with a lattice Boltzmann model can describe the liquid phase transition process in porous media.Via Simulating, it is found that the boiling heat transfer is stronger than that of pure liquid when the nanofluid are as the heat transfer medium, and in a certain range, the size of the nanoparticles decreases, the heat transfer ability of the fluid increases.