INTERACTIONS BETWEEN UNEQUAL PARALLEL BUBBLES RISING IN A NON-NEWTONIAN FLUID

Abstract

The interactions between three unequally sized parallel bubbles rising in a non-Newtonian liquid with power-law rheology are investigated numerically using the volume of fluid (VOF) interface capturing method. The VOF interface capturing method is used within the simulation software ANSYS Fluent. Using a piecewise-linear method, the geometric reconstruction approach is used to capture the interface between the two-phases. The continuum surface force model is used to calculate surface tension. Four distinct configurations of larger (DL) and smaller (DS) bubbles are considered in terms of their relative size. The diameters of the three bubbles (i.e., D1, D2, and D3) are varied individually to achieve these configurations. The critical horizontal interval (sc) for bubble coalescence is obtained by varying the initial bubble spacing in the simulations and observing the subsequent motion and interaction. The behaviour of the bubbles in different configurations is captured through the plots of rise velocity for the coalescing bubbles, and the plots of horizontal and vertical bubble spacing for the cases where merger is not observed. The results indicate a substantial increase in the critical interval when compared to equally sized bubbles that can be attributed to the characteristic flow field in the vicinity of the bubbles. The critical interval is also observed to be affected by the different configurations of bubbles. While lateral coalescence is observed for small bubble intervals, the coalescence is observed to occur in the wake of the larger bubble as the spacing approaches the critical interval. Beyond the critical interval, a repulsive effect leads to a gradual increase in the horizontal bubble interval similar to equally sized bubbles. This increase, however, subsides eventually but is compensated by a continuous increase in the vertical distance, thereby avoiding coalescence. Eventually, the effect of shear flow index (n) on bubble rise behaviour is also studied for a particular configuration. For lower values of n, the larger bubble experienced breakup, resulting in four separate bubbles that did not merge. However, as the shear flow index (n) increased, breakup was not observed, and a critical spacing could again be determined below which coalescence was observed.