CFD simulation of hydrodynamics in downer reactors

Abstract

A k-e-Θ-k p model was proposed to describe the complicated two-phase flow in downers, taking into account the fast and dense gas-solid flows in downers, transport phenomena caused by turbulence mechanism and collisions between particles, i.e., a k-e turbulence model for gas phase, a kp turbulence model, and a kinetic theory description of solid stresses characterized by pseudo-temperature of particles (Θ) for solid phase. A FORTRAN code was developed to solve the steady-state 3D flow fields of two-phase flows numerically. To validate the reliability of the k-e-Θ-k p model, the gas-solid flows in (1) the developing and fully developed regions of a downer (140 mm ID) and (2) solids jet development in the entrance region of a downer (92 mm ID) were simulated. The predicted results had satisfactory agreement with a large quantity of experimental data, i.e., axial and radial distributions of local solids fraction, local particle velocity, and pressure. In particular, the above model and corresponding simulations were also implemented successfully using CFX 4.2/4.3, a commercial software developed by AEA Technology Company. Furthermore, the CFX code was applied to predict the radial flow structure in a downer of 418mm ID, which was validated by the ongoing experiments in the corresponding experimental setup. Further analysis of the simulated data also showed that the diffusion coefficient of particles is strongly correlated with the local solids fraction. A lower solids fraction corresponds to a larger radial diffusion coefficient. With the increase of solids fraction, the diffusion coefficient decreases sharply when the solids fraction is less than 0.05, and then levels off. This means that the local solids fraction has an important effect on the dispersion behavior of particles. This predicted result gives reasonable agreement with the experimental results in the literature.