Bubble swarms in power-law liquids at moderate Reynolds numbers: Drag and mass transfer

The motion of and mass transfer from swarms of mono-size non-circulating spherical bubbles in clean power-law liquids have been numerically investigated at moderate Reynolds and Peclet numbers. A simple sphere-in-a-sphere cell model has been used to account for hydrodynamic interactions between neighbouring bubbles. A wide range of values of the gas hold-up has been considered so as to evaluate the detailed drag and mass transfer characteristics of dense swarms of bubbles and of almost single bubbles rising in power-law liquids. Extensive numerical results have been obtained to elucidate the effects of the Reynolds number (Re), Schmidt number (Sc), power-law index (n) and gas hold-up (Φ) on the drag coefficient and Sherwood number over the ranges of conditions: 0.6 ≤ Φ ≤ 10−6, 1 ≤ Re ≤ 200, 1 ≤ Sc ≤ 1000 and 0.6 ≤ n ≤ 1.6. The drag coefficient (Cd) decreases, whereas, the average Sherwood number (Shavg) increases with the decreasing values of n. However, the effect of the power-law index is found to be more significant on the rising velocity of swarms of bubbles than that on the rate of mass transfer. Based on the present numerical results, predictive correlations have been proposed for drag coefficient and Sherwood number which can be used to estimate the free rising velocity and mass transfer coefficients respectively in a new gas-liquid application involving power-law continuous phase.