Euler-euler Large Eddy Approach And Numerical Simulation Of Dense Gas-solid Two-phase Flow

With the development of computer technology and computational method, numerical simulation has become one of the most promising methods for studying gas-solid flows. Improving methermatical models and numerical simulation methods has important significance to further study of the complex mechanism and effect factors of gas-solid flows.Large eddy simulation (LES), which includes both theories from the direct numerical simulation (DNS) and Reynolds average numerical simulation (RANS), is an advanced numerical simulation method, which can be applied to the flow of higher Reynold and more complex geometry compared with DNS, and give more information of fluctuations compared with RANS. Currently, LES has been successfully applied to single phase flow, but its application to gas-particle flows still needs further invetigation.Transport equations of gas-solid flows are filtered by means of the volume fraction weighted average method (Faver method). The filerted conseveration equations for gas phase and solid phase are derived, and Euler-Euler two-fluid large eddy simulation of dense gas-solid flows LESg-θ-LESp is presented. For gas phase and solid phase, the sub-grid scale modeling is based on the Smagorinsky form. The filtered solid pressure is modidifed to replace the the terminal velocity by the minimum fluidized velocity of particles. The effect of collision of particles is considered by the kinetic theory of granular flow. The effect of meso-scale of particles on solids thermal conductivity is considered. The filtered boundary condidtion for solid velocity and granulare tempratture is presented. Flow behavior of gas and particles is performed by means of LESg-θ-LESp with two sub-grid scale models of gas and solid phases.The LESg-θ-LESp model is applied to simulate hydrodynamics of gas-solid two-phase flows in a bubbling fluidized bed. The model predicts the process of bubbles grow-up, coalescence and break-up. The time-averaged velocity of gas phase and particles, and solid concentration are obtained. The simulated time-averaged particle velocity and fluctuating velocity of particles are agrrement with Yuu et al. experimental data. The predicted concentration of particles agrees with Taghipour et al experimental results. Larger bubbles are formed without the consideration of sub-grid scale turbulent viscosity of particles. With the increase of coefficient Cs, more and smaller diameter bubbles are formed, the time-averaged granular temperature is decreased, and the particles concentration is increased in the center region, and decreased near the walls. The relative velocity is decreased in the center region and increased near the walls. With the increase of particles concentration, the sub-grid scale particles viscosity and thermal conductivity are decreased, and the sub-grid scale solid pressure is increased.The LESg-θ-LESp model is used to simulate the hydrodynamics of gas-solid two-phase flows in a riser. The model predicts the core-annular structure of particles flow. The time-averaged particles concentration and velocity are obtained and agreed with Knowlton et al. experimental results. With the increase of particles concentration, both filtered and sub-grid scale time-averaged particles viscosity and thermal conductivity are increased, the filtered time-averaged solid pressure is first increased and then decreased. The sub-grid scale time-averaged solid pressure is increased. In the centre region, with the increase of coefficient Cs, the time-averaged particles concentration is increased; the time-averaged gas velocity, time-averaged particles velocity, time-averaged granular temperature and particles fluctuation velocities are decreased, opposite to the near-wall region. The effective time-averaged particles viscosity and thermal conductivity are increased with the increase of coefficient Cs, while the effective time-averaged solid pressure is decreased. With the correction of drag coefficient, the time-averaged particles concentration, granular temperature and pressure drop increased, the gas and particle velocities are decreased, and the effective time-averaged particles viscosity, particles thermal conductivity and solid pressure is increased.The LESg-θ-LESp model is applied to simulate the hydrodynamics of gas-solid two-phase flows in a chemical looping combustion (CLC) reactor. The particles concentration is low in the center and high near the walls. It is low at the top and high at the bottom. The results illustrate the inherent core-annular pattern of particles flow in the air reactor. The flow structures are similar in the riser and bubbling fluidized bed respectively. The heterogeneous chemical reaction is applied to simulate the catalytic combustion reaction of CH₄ and NiO without oxygen carrier circulation in the fuel reactor. With the processing of time, NiO is reduced to Ni, the mass fraction of Ni, CO₂ and H₂O are increased in the fuel reactor. For lowing of temperature and reducing of mass fraction of NiO, the chemical reaction rate becomes low, which leads to the decrease of production rate of Ni, CO₂ and H₂O, and the increase of the mass fraction of CH₄ in the material bed. Additionally, the turbulence intensity is found more violent during the fluidization process for the volume of gas phase is expanded. The mass fraction of CH₄ in the free space region is higher than that at the top of the material bed, while the mass fraction of CO₂ and H₂O are opposite in the free region.