Fluid dynamics in gas-liquid stirred tank reactors: Experimental and theoretical studies of bubble coalescence

This work deals with studies of bubble coalescence in turbulent flows of gas-liquid stirred tank reactors. Coalescence plays an important role in determining bubble size distribution, on which flow and mass transfer in such reactors largely depend. The main objective of this work is to obtain understanding of some of the basic phenomena related to bubble coalescence in turbulent flows, particularly regarding mechanisms of bubble collision. This objective is approached through laboratory works combined with computational fluid dynamics (CFD). Measurements using a high-speed video imaging technique show that once bubbles collide, the coalescence usually occurs very fast, in less than 2 milliseconds. The measured drainage/coalescence time is shorter than that obtained from film drainage theory for coalescence in a stagnant liquid. Flattening of the bubble surfaces prior to collision was not observed in either the measurements or the simulations. New mechanisms of bubble collision, namely trapping of bubbles in stationary vortices and large turbulent eddies, are described in the present work. Additionally, a new model is proposed for buoyancy-induced collision. A large stationary vortex was revealed experimentally using particle image velocimetry (PIV) at the leeward side of each of the tank baffles. The vortex rotates rapidly, causing a local pressure gradient that can drive bubbles toward the vortex axis. Both the increase of the local hold-up and the removal of the fluid between the bubbles result in a very intense coalescence in the vortex. The PIV measurements also reveal the presence of large turbulent eddies with a vorticity on the same order as that of the stationary vortex. The average size of these eddies is 2-4 times larger than that of bubbles. Their average lifetime is longer than bubbles' eddy-capture time, and much longer than the coalescence time mentioned above. These facts suggest that bubbles can be trapped into the large eddies as they are trapped in the stationary vortex. Results of both the measurements and simulations show that buoyancy-driven collision occurs mainly from the side. For bubbles in the range of 0.5 to 5 mm, there is an attractive force between bubbles towards a low-pressure region at the edge of the bubbles, and a repulsive force in front of and behind the rising bubbles. A model describing the transverse lift force due to the proximity of other bubbles is introduced in the framework of Eulerian-Lagrangian modelling.