CFD investigation of bubble breakup and coalescence in a rectangular pool-scrubbing column

Pool scrubbing refers to the process of retention of harmful particles or gaseous components by bubbling in liquid media, which is characterized with high injection velocity and complex bubble dynamics. A comparative study of bubble breakup and coalescence models is performed to investigate the bubble size evolution in a pool-scrubbing column. The bubble size distributions along the column height are obtained by using a CFD (Computational Fluid Dynamics) - PBM (population balance model) coupled method, where the flow field is described by a two-fluid model while bubble size change by the discrete PBM. Various combinations of bubble breakup and coalescence models are investigated, and the bubble size distribution and Sauter mean diameter are compared with the literature data. The results show that under the pool-scrubbing conditions, bubble breakup is dominant compared to coalescence and bubble size decreases continuously, especially in the injection zone. The breakup rate predicted by three breakup models available in the official release of OpenFOAM foundation software is found to differ by several orders of magnitudes, and their daughter size distributions are completely different as well. Some model combinations are able to predict the bubble size more satisfactorily, but there are still biases. For example, the fraction of bubbles is either overestimated or underestimated in certain size ranges compared to the experimental values. Proper description of the breakup rate and daughter bubble size distribution needs further efforts. Additionally, the mechanism governing the breakup is investigated by implementing the breakup model of Liao, which accounts for the effects of turbulence fluctuation, velocity shear in the bulk and eddies as well as interfacial friction. It revealed that both turbulence and interfacial friction contribute to bubble breakup in the pool scrubbing column, and the calibration of each mechanism under different conditions is necessary for future study. Moreover, the assumption of binary or multiple breakup has an obvious effect on the simulation results. Future efforts are needed to model the breakup of gigantic bubbles in the injection zone and turbulence modulation caused by large deformable bubbles.