Dynamics of long bubbles propagating through cylindrical micro-pin fin arrays

The dynamics of two-phase flows confined within complex and non-straight geometries is of interest for a variety of applications such as micro-pin fin evaporators and flow in unsaturated porous media. Despite the propagation of bubbles in straight channels of circular and noncircular cross-sections has been studied extensively, very little is known about the fluid dynamics features of bubbles and liquid films deposited upon the inner walls of complex geometries. In this work, we investigate the dynamics of long gas bubbles and thin films as bubbles propagate through arrays of in-line cylindrical pins of circular shape in cross-flow, for a range of capillary and Reynolds numbers relevant to heat transfer applications and flow in porous media, different pitch of the cylinders and bubble lengths. Three-dimensional numerical simulations of the two-phase flow are performed using the open-source finite-volume library OpenFOAM v.1812, using a geometric Volume of Fluid (VOF) method to capture the interface dynamics. Systematic analyses are conducted for a range of capillary numbers Ca = 0.04 - 1, Reynolds numbers Re = 1 - 1000, streamwise pitch of the cylinders sx = 0.125R - 2R, with R being the radius of the pin fins, and initial bubble length Lb = 2.5R- 12R. The simulations reveal that when bubbles propagate through pin fin arrays, they tend to partially coat the cylinders with a thin liquid film and to expand in the cross-stream direction within the gap left between adjacent cylinders. The liquid film deposited on the cylinders is significantly thinner than that reported for straight channels and similar geometrical constraints. As the streamwise distance between the cylinders is decreased, the flow configuration tends towards that for a straight channel, whereas larger distances cause the bubble to expand excessively in the cross-stream direction, and to eventually arrest when sx > 2R. Inertial effects have a strong impact on the bubble shape and dynamics when Re > 500, triggering time-dependent patterns that lead to bubble fragmentation and much thicker liquid films.