Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Bubble columns are widely used in industrial processes. The performance in case of chemical reactions between liquid and gaseous substances depends on characteristic parameters as interfacial area density, turbulence level, or kLa-values. Transitions between the homogeneous and the heterogeneous operating regime dramatically change such characteristic parameters and are for this reason subject of many investigations. Many of them tried to correlate the transition with the gas superficial velocity Jg or equivalent integral parameters, but there is no general valid critical value for the transition. Lucas et al. (2005) discussed the influence of the bubble size distribution on the stability of a homogeneous bubbly flow. Depending on the sign of the lateral lift force, which changes with the bubble size (Tomiyama et al., 2002), it can stabilize (positive lift force coefficient – small bubbles) or destabilize (negative lift force coefficient – large bubbles) the flow. Basing on a linear stability analysis finally a criterion was obtained for the stability of a homogeneous bubbly flow in dependence on the bubble size distribution. Indirectly it also depends on Jg since usually bubble sizes increase with increasing Jg caused by larger size of the injected bubbles and by increased coalescence. Lucas et al. (2007) discussed the complex relations between local and global instabilities that may be quite complex basing on CFD-simulations. Akbar et al. (2013) showed the influence of injection and bubble size on the flow structure in a rectangular bubble column.
The bubble size distribution clearly has an influence on the transition between the homogeneous and heterogeneous regime in bubble columns which can be explained by effects of the lateral lift force. It interacts with other phenomena like inlet induced instabilities and coalescence processes. A detailed investigation on these effects is presented in this contribution.