Bubble coalescence and breakup mechanism in a vertical pipe with an obstacle

One of the major issues in numerical study of bubbly flows is the prediction of bubble size. Among other physical processes coalescence and breakup represents a huge challenge. A variety of investigations on bubble coalescence and breakup in turbulent flows has been motivated, and a remarkable diversity of mechanisms has been observed. However, knowledge on the superposition of multiple mechanisms remains insufficient. The purpose of this work is to investigate the effect of a sudden change of flow fields on coalescence and breakup events. Turbulent bubbly flows in a vertical pipe with a ring obstacle are analyzed with ultrafast x-ray computed tomography and a generalized population balance model. The results show that coalescence and breakup are in equilibrium before the obstacle leading to a nearly unchanged bubble size distribution (BSD) and quasi fully-developed flow, which is destroyed by the presence of the obstacle. Immediately behind it, both the coalescence and breakup rates are enhanced due to intensified turbulence and recirculation, and as a result, the BSD becomes broader. At first, the increase in coalescence exceeds that in breakup such that the mean bubble size increases. After about ten millimeters downstream from the obstacle (L/D≈0.2), breakup overtakes coalescence and the BSD shifts to the direction of small bubble size. At L/D≈0.4, the bubble size before the obstacle is almost recovered, but it decreases further downstream. The predicted and measured BSDs agree well with each other upstream and downstream, where the bubble coalescence and breakup is majorly controlled by turbulence. Difficulties are encountered in capturing the drastic change in the region directly downstream of the obstacle in particular the rapid increase in coalescence rate. The test of turbulence single-phase flow through a sudden expansion reveals that it is difficult for a RANS model to reproduce the turbulence structure in the recirculation zone reliably, which directly affects the performance of coalescence and breakup models. Furthermore, coalescence resulting from other mechanisms such as wake entrainment or laminar shear is shown to have a minor contribution, and turbulence is dominant. Further investigations concerning bubble coalescence and breakup, turbulence as well as its effect on interfacial momentum transfer are necessary for the obstructed flow.