Dynamic behavior of a deformable bubble rising near a vertical wire-mesh in the quiescent water

Abstract

Because wire-meshes have a great potential application for designing high-efficient heat and mass transfer systems, a deformable bubble of Re ~ 800 rising in still tap-water near a vertical mesh of different porosity is studied using a high-speed shadowgraphy apparatus including two mutually perpendicular projections. We focus on the effect of the boundary conditions with three types of meshes (Mesh-L, Mesh-M, and Mesh-S) and initial wall distance s* on the dynamic behavior of the bubbles rising by the presence of the meshes. The dynamic parameters, shape oscillation, and energy components are discussed, compared to the cases of the free-rising bubbles and that near impermeable walls. Results show that the near-mesh bubble always travels within a three-dimensional spiral trajectory at different initial wall distances, similar to the corresponding free-spiraling bubble. The mesh-bubble collisions only happen on the Mesh-L and Mesh-M, at the smallest initial distance s* = 0.11, due to their lower pressure drop before and after the mesh as a liquid through it and causing the stronger added mass effect on the bubbles. Compared to the no-slip wall, the mesh, as a slippery boundary condition of non-zero wall-normal velocity, has insignificant interference on the bubble wake, suppressing the symmetric vortex shedding and then the transition of the bubble path from spiraling into zigzagging. The interference of the mesh can lead to the transition and obvious fluctuation in the characteristic frequencies of the lateral migration, velocity, and symmetric shape-oscillation. In terms of energy, the horizontal momentum components of a near-mesh rising bubble are transferred to each other to maintain the spiral motion. The absence of bubble-mesh collisions may explain why just the energy dissipation ED generated by Mesh-S changes slightly and maintains at a lower level at s* < 9.05, while ED caused by other walls or meshes cannot achieve this low level until s* > 4.58. It is expected that our findings will be helpful to predict or modify the near-mesh gas void-fraction distribution and to improve mass, heat transfer in related engineered systems.