Numerical Investigation On Turbulent Flow And Mass Dispersion Through Porous Media With High Porosity

As the 21st century,the resources and the environment have become an increasingly primary and prominent problem.Because of its high combustion efficiency and low pollution, the filtation combustion technique of the gas or liquid fuel in high porosity porous media has become an international research hotspot and has gained more and more practical applications. To understand the working mechanism of the porous media burner and of the porous media internal combustion engine,in this thesis,turbulent flow behavior and species transport processes in high porosity porous media are studied by numerical simulation.The main objective is to gain insights into the characteristics of turbulent flow and fuel-air mixture formation in porous media more systematically,and furthermore to promote theoretical research and practical applications in this field.In this thesis,the following aspects have been studied and explored.1.Three representative macroscopic turbulent model for the flow in porous media are introduced and comprehensively analyzed.The characteristics of turbulent flow in open channels with a porous bed are numerically studied by using two leading turbulent models. Numerical results were compared with the experimental data from thr literature and found that the calculated results from the P-dL macroscopic turbulent model were closer to the experimental data than those from the A-L model.2.To understand essential influences of turbulence on transport and combustion processes in porous media with high porosity,a two-dimensional pore network model for random porous media structure is presented,which consists of a number of cylindrical pores and parallelepiped connecting throats.A standard k-εmodel is employed to simulate the turbulence effect in the microscopic pore-level flow field.Computational results obtained from the pore-network model are transformed into information of the macro flow field through a volume average approach over the entire computation domain.The results are then used to deduce the value for the unknown coefficient in the macroscopic model.Comparisons with available data in the open literature seem to indicate good agreement.3.To understand the working mechanism of the porous medium(PM) internal combustion engine,effects of a porous medium heat regenerator inserted into a combustion chamber on the turbulent flow characteristics and fuel-air mixture formation are studied by numerical simulations.A simplified model for the random structure of the porous media is presented,to simulate turbulent flows in the porous media and spray/wall interactions,energy equation and spray model are modified.Effects of PM structure and porosity and spray injection and spray cone angle on the turbulent flow and fuel air mixture formation and homogenization process are studied with emphasis.Calculation results show great influences of PM structure and porosity on the turbulent flow in the porous media,as well as influence of the flow field inside the PM on the flows in the entire combustion chamber.The spray cone angle affets directly the level of turbulent kinetic energy in the clear fluid region and the spray/PM interaction process as well as the level of turbulent kinetic energy in PM.The variation in spray injection pressure can modify the velocity of fuel droplets.With increasing spray injection pressure,the velocity of fuel droplets to increases and the level of turbulent kinetic energy also increases accordingly, consequently,the fuel vapor becomes more homogeneous and distributed over a wider area.4.One of the main difficulties in the flow and combustion PM with high porosity is the co-existence of the turbulent fluctuation in time and the disturbance in space.The disturbance in space is namely dispersion.The dispersion is a transport phenomenon that is caused by the numerous and irregular pores in the medium.A simplified model for the cylindrical periodical structure of the PM is presented,the diffusion process of methane and air in the PM is studies numerically.The computational results obtained from the microscopic computation domain are employed to derive the longitudinal mass dispersion coefficient and transverse mass dispersion coefficient through a volume average approach.The results are analyzed and compared with the correlative experimental data and empirical formulas in the literature.The results are then used to deduce the value for the unknown coefficient in the macroscopic model.Comparisons with available data in the open literature show good agreement.This provides a useful approach for studying dispersion and turbulent diffusion effects in porous media.