A new model for bubble coalescence and breakup in poly-disperse bubbly flows

A basis model was proposed in this work, which considers bubble coalescence and breakup due to different mechanisms, including coalescence due to turbulence, laminar shear, wake entrainment and eddy capture, and breakup due to turbulent fluctuation, laminar shear and interfacial slip velocity. The basis model was implemented into an efficient 1D Test Solver, which is developed specially for bubbly flows in a round vertical pipe. For the first step, the new model was validated with the high quality experimental data for the case of air-water mixtures, which were obtained at the TOPFLOW facility. Simulation results showed that at relatively low superficial gas velocities, the initial bubble size was small and had a narrow distribution, and coalescence dominates the evolution; with the increase in the superficial gas velocity, large bubbles appear near injection position, and result in a much wider bubble size distribution, and breakup became predominant. The radial air volume fraction was redistributed from the injection position at wall to the whole cross section, and the stable peak remains near the wall for small bubbles while migrating towards the pipe centre for large bubbles. The comparison of simulation results with experimental data showed encouraging agreements, which affirmed the feasibility and capability of the basis model and indicated also the requirements for adjustments and improvements.