|M.Sc Thesis||Department of Chemical Engineering|
|Supervisors:||Prof. Emeritus Nir Avinoam|
|Dr. Olga Lavrenteva|
|Full Thesis text|
Air bubbles, injected into the viscous fluid sheared in a Couette device (stationary outer cylinder and a concentric rotating inner cylinder), tend to be trapped by Taylor vortices and maintain their vertical position defying buoyancy. Equal size bubbles eventually form a highly ordered string with equal distances between the inclusions. This behavior was first reported by Deng et al.1. The question: "why equal core bubbles tend to distribute uniformly" remained unanswered so far and the main goal of the study was to try to shed light on this question.
In the present research, the experimental results of Deng et al.1 were reproduced, detailed measurements of the process evolution were performed, a hypothesis of the inertial nature of the repulsing force between the bubbles was suggested and supported by a study of simplified theoretical model of the process. Separation distance between bubbles was measured and its temporal evolution was investigated.
It was demonstrated that the process under consideration has two time scales.
In a short time scale, the bubbles are getting entrapped in the secondary rotating flow of the Taylor vortices and are pushed to the center streamline of the vortex defying buoyancy. During a longer time scale the interaction between the bubbles in the flow drives the bubbles away from each other. Equal size bubbles eventually assume an ordered string with equal separation distances between all neighbors. This phenomenon repeats itself regardless of the number of bubbles in the flow. Hence, doublets will assume opposite positions, triplets will form a triangle, four bubbles will arrange in a square and so on.
The suggested explanation of the ordering of the bubbles is that the relative repulsion observed in the experiment is due to inertia effects on the scale of the bubbles.
This hypothesis is supported by study of the dynamics of two non-deformable particles moving in a linear flow of an inviscid liquid in 2D geometry. The force exerted by the flow was obtained by integrating the pressure force over the interface of the fluid particles. In addition, relative repulsion of the particle was confirmed by numerical simulation for the two deformable fluid particle embedded in a linear flow of an inviscid fluid for a planar geometry.
Hypothesis about inertia effects on the scale of the bubbles presents a novel feature in interaction between bubbles in Taylor vortices that was not studied previously.