The effect of particles on the film drainage and bubble coalescence in a slurry bubble column

Understanding bubble coalescence in slurry columns and how it is affected by the presence of particles is of great significance to a variety of engineering applications. Despite decades of research, high-resolution data on the film drainage process in a bubble column are scarce, which prevents a precise description of the phenomenon and the derivation of reliable models for further analyses. The existing work on bubble coalescence in the presence of particles either focuses on experimental or analytical studies under nearly hydrostatic conditions with very low approach velocity (up to 0.1 mm/s), or is limited to a mesoscopic scale, for example, by acquiring void fraction and bubble size distribution changes in the column. The present work aims to fill the gap in-between and provide insights into the film drainage process at the microscopic scale under bubble column hydrodynamic conditions. By coupling the volume-of-fluid (VOF) and multiphase particle-in-cell (MP-PIC) methods with a chimera mesh approach in OpenFOAM, a high resolution of the interface and fluid flow field is realized and meaningful results on the effect of particles are achieved. In the investigated parameter range and condition, the presence of particles in the liquid is shown to affect majorly the film drainage process, while have negligible effects on the bubble rise and approach velocity. The influence of particle number concentration is found to be complex and multimodal in co-axial coalescence. At sufficiently low concentration, particles are pushed out from the film and do not alter the drainage and coalescence rate noticeably. As the concentration increases, first a physical blocking effect then a slight promotion because of the drainage changing from axisymmetry to asymmetry is observed. The drainage process is greatly retarded by a conjunct motion, where the bubbles rotate along the colliding interfaces. Furthermore, no dimple formation is observed at high concentrations, which is typical at low particle load or in pure liquids. As the film is thinned down to a critical thickness in the conjunct stage, the interface becomes wavy and instable leading to film rupture. The presence of particles captured in the thin film affects its stability greatly. Both particle size and density are shown to have a dual effect on the coalescence time. Increasing of them leads to first suppression then promotion of coalescence. The results on the effects of number concentration, particle size and density agree with the observations of the previous literature.