Effect of breakup and coalescence kernels on polydispersed bubbly flow in continuous casting mold

Previous studies on polydispersed bubbly flow in continuous casting (CC) mold were mainly attentive to the modeling of shear-induced turbulence, interfacial force, and bubble swarm effects. The current study reveals the coalescence and breakup mechanisms of bubbles in CC mold. Then, a calibration factor of coalescence kernel is proposed based on local gas holdup to harmonize the imbalance between coalescence and breakup rates. The effect of bubble coalescence and breakup models on the Sauter mean diameter, gas holdup, and liquid velocity were studied and against with the experimental data. The results show that the Turbulent and Liao models underestimate the bubbles coalescence rate and the Luo coalescence model overestimates the bubbles coalescence rate. Meanwhile, turbulence shear and surface instability overestimate the bubble breakup frequency, and turbulent impact is the most important mechanism used to describe bubble breakup phenomenon in the CC mold. Furthermore, the turbulent fluctuation, buoyancy driven, and wake entrainment should be considered to define the bubbles collision frequency, while the viscous shear and eddy capture mechanisms could be neglected. The Luo-Prince model could accurately predict the bubble size distribution and the fan-shaped distribution of bubble clusters, which comprehensively considers three collision mechanisms. Finally, the combination form of calibration factors could effectively describe the influence of local gas holdup on bubble collision frequency. The proposed calibration factor can predict bubble size distribution more accurately in the CC mold. The relative error of mean bubble diameter prediction is reduced from 63.53 % to 17.86 %.