Dependency of bubble column flow regime on bubble size distribution

The regime transition from homogenous to heterogeneous is one of the most important design parameters of bubble columns. The lateral lift force may have an important influence on this transition. As shown experimentally and by numerous direct numerical simulations the lateral lift force changes its sign in dependence on the bubble size. Recently the findings of Tomiyama et al. obtained for single bubbles in a linear laminar shear flow for a system with high Morton number (high viscosity) were also confirmed for low the viscid air-water system and turbulent conditions. The well-known correlation of Tomiyama et al. fits very well also for these conditions, provided the Eötvös number based on the major axis is used. With the Tomiyama correlation combined with the Wellek correlation for the bubble shape the critical diameter for the change of the sign of the lift force is about 5.8 mm for the air-water system. While the Wellek-correlation is valid for contaminated water deionized water was used in the new HZDR experiments. Replacing the Wellek- correlation by a correlation based on the observed bubble shape the critical diameter for the change of the sign is about 4.5 mm.
With a positive sign of the lift force coefficient – which is valid for bubbles smaller than the critical diameter a homogeneous bubbly flow is stabilized while larger bubbles destabilize the flow. Lucas et al. derived a stability criterion also for bubble size distributions that include small and large bubbles.
Experiments investigating the effect of the bubble size distribution were conducted in a high aspect ratio bubble column for air/purified water. The gas flow through the sparger groups was varied to modify the partial gas fraction of the small and large bubbles. Due to this variation, the stability criterion was manipulated from ‘strong’ negative to ‘strong’ positive. Measurements were done for different height positions in the column.
Completely different flow structures and profiles were observed by only changing the bubble size. Homogeneous flow characterized by flat profiles for gas volume fraction and liquid velocity were observed for a bubble size distribution with mainly small bubbles, while a center peak characterizing the heterogeneous regime occurs for the distribution with large bubbles. Applying the stability criterion of Lucas et al. these two situations correspond to ‘strong’ negative and ‘strong’ positive meaning homogeneous and heterogeneous flow regime, respectively. Beside these extreme cases also the transition region was investigated. Here the measurements are made difficult because coalescence changes the bubble size distribution along the column height resulting in a transient behavior. In any case, the lift force seems to be the key for a local criterion on the regime transition.