Liquid backmixing in bubble columns

Abstract

A noninvasive Computer Automated Radioactive Particle Tracking (CARPT) technique is used to quantify liquid backmixing in bubble columns of three different diameters over the range of gas velocities that span the bubble, transitional and churn turbulent flow regimes. The obtained axial dispersion coefficients (eddy diffusivities) are much larger than radial values and exhibit a stronger dependence on gas velocity. Column averaged axial diffusivities differ from the lumped effective axial dispersion coefficients, calculated from correlations, especially in smaller columns. Hurst's rescaled range (R/S) statistical analysis of the fluctuating velocities indicates different mixing mechanisms in the radial and axial direction. For the radial direction Hurst's exponent is about 0.5 indicating a diffusion type mixing mechanism. For the axial direction, Hurst's exponent is larger than 0.5 indicating the persistence of long range effects due to random turbulent eddies.