Soutenance de Christopher Madec

Multiphase flow involving gas, liquid, and solid particles plays a vital role in catalytic fluidized beds, subsea gas release, and even in avalanches and volcanic eruptions. Bubble rise and interactions is a fundamental ingredient for many such industrial processes and geophysical phenomena. Here, the canonical problem of the rising motion of a single bubble and bubble-bubble interactions is investigated in a Hele-Shaw cell containing a granular suspension through simple experiments and theoretical models.

In a Newtonian liquid, it is shown that the rise velocity $v_b$ of a single bubble of diameter $d_b$ evolving between two plates separated by a thickness $h$ increases with $d_b$ up to a maximum value for $d_b \gg h$, when the Reynolds number Re = $v_bd_b/\nu_f(h/d_b)^2$ is very small. In the case of a neutrally-buoyant non-Brownian suspension, bubble speed is demonstrated to be faster in suspensions than in particle-less liquids of the same effective viscosity. By carefully measuring this bubble speed increase at various particle volume fraction and via velocity field imaging, this new bubble dynamics is linked to a reduction in the bulk dissipation rate. A good match between our experimental data and computations based on Suspension Balance Model illustrates that the underlying mechanism for this dissipation-rate deficit is related to a non-uniform particle distribution in the direction perpendicular to the channel walls due to shear-induced particle migration.

The second part of this thesis deals with the dynamics of bubble-bubble interactions and bubble chains rising in a Newtonian liquid. When several bubbles interact with each other, they first show a steady approach, they accelerate and undergo sharp shape changes until they come into contact. By considering the neighboring bubble's influence on the pressure field and the reduction in the dissipation, it is possible to qualitatively describe the bubble merging process. After merging, the bubble chain remains intact and their dynamics, when compared to the single bubble rise, is slower. In addition, spatial reorganizations within the same train of bubbles are observed.

These results provide a promising avenue for studying bubble-bubble merging and bubble(s)-particle interaction in the context of complex multiphase flow.