Interaction of bubbles rising inline in quiescent liquid

- Numerical study on the interaction of two inline bubbles rising in stagnant liquid.
- Trailing bubble interacts with wake of the leading bubble and accelerates beyond terminal velocity.
- At low Reynolds (Re) numbers, inline configuration is stable and bubbles collide.
- At higher Re values, trailing bubble deviates from rectilinear path due to vorticity development.
- The onset of path deviation depends on the Reynolds and Eotvos numbers of the bubbles.

In the buoyant rise of two bubbles arranged in an inline configuration, the trailing bubble tends to accelerate beyond its terminal velocity, due to its interaction with the wake of the preceding bubble. It is demonstrated that several different interactive behaviours could be obtained due to this acceleration. Firstly, at low values of Reynolds numbers (Re=ρlVtD/μ⩽35), the inline configuration was found to be stable, and the two bubbles would collide and coalesce due to the velocity difference between the two. Secondly, at higher Re values (Re > 50), vorticity development around the trailing bubble causes it to deviate away from the inline configuration, thus preventing the occurrence of a head-on collision between the two bubbles. The deviation of the bubble was found occur if a certain critical velocity is exceeded by the trailing bubble. The value of the critical velocity was found to decrease with increasing Re values. Further, an increase in Eotvos number (Eo=ρlgD2/σ) tends to increase the critical velocity, indicating that enhanced bubble deformability actually improves the path stability of the trailing bubble. The interaction of bubbles can therefore significantly influence the tendency of the bubbles to collide and coalesce.