Mechanism Analysis Of Particle Concentration Radial Distribution In Gas-solid Two-phase Downer

The radial distributions of the local holdup and velocity in downer directly determine the yield of the desired product.The ideal radial distributions of the solid holdup and the void fraction（corresponding to the volume fraction of the gas phase）are well matched/uniform while the radial gradients are fairly low.It is believed to be advantageous for improving the contacting/reaction of the two phases.However,the actual radial gradient of the solid holdup in the downer is obviously non-uniform.It is more complicated than the radial non-uniform structure in the riser.In this paper,the theory of aerodynamics and field theory were used to analyze and quantify the non-uniform distribution of the particle concentration in the gas-solids downer.Based on theoretical analysis and experimental data presented in the literature,the expression F_K-J=-ρ_g（ν_p-ν_g）（（?）ν/（?）r）r of Kutta-Joukowski transverse force in downer was obtained;the distribution of Kutta-Joukowski transverse force in the downer was given.The influences of operating parameters on the transverse force distribution characteristics were analyzed.In addition,based on the radial distribution features of the particle concentration gradient and the relationship of the Kutta-Joukowski transverse force to the concentration gradient force,the expressions of the concentration gradient force,F_ρ=K（dρ/dr）A,and that of the concentration gradient force coefficient K=（-ρ_g（ν_p-ν_g）（（?）v/（?）r）r）/（dρ/dr+（d²ρ/dr²）r）were presented.Furthermore,based on the published experimental data on the riser,the radial distribution of particle concentration gradient in riser was also analyzed.The more accurate calculation method to the concentration gradient force in the riser and the expressionsoftheconcentrationgradientforcecoefficient K=（-ρ_g（ν_p-ν_g）（（?）v/（?）r）r）/（dρ/dr+（d²ρ/dr²）r）were presented.It is pointed out that the concentration gradient force in the riser is a function of particle velocity gradient,the slip velocity and the gas phase density.The distributions of concentration gradient force coefficients K in both the riser and the downer were given.The influences of operating conditions on the distribution characteristics of concentration gradient force coefficients were analyzed.The results show that the transverse force of local Kutta-Joukowski was the main factors for the non-uniform radial distribution of the particle concentration in the downer or riser.The vector direction of the force is from low density zone towards high density zone;while its valu was mainly determined by the particle physical property,the particle flux,the superficial gas velocity,the radial/axial position.Based on the experimental data,the empirical correlation of Kutta-Joukowski transverse force radial distribution in the downer was obtained by regression.The calculated value was in good agreement with the experimental value.Finally,the particle concentration gradient force is introduced and quantified.This force balances the Kutta-Joukowski transverse force in the downer and riser.The empirical correlations of these two forces were established.Based on the analysis results,the empirical correlation of K in riser was also determined.