A literature review on mechanisms and models for the coalescence process of fluid particles

The flow in the wake of single and two interacting air bubbles freely rising in water is studied experimentally using digital-particle-image-velocimetry in combination with high-speed recording The experiments focus on ellipsoidal bubbles of diameter of about 04–08 cm which show spiraling, zigzagging, and rocking motion during their rise in water, which was seeded with small tracer particles for flow visualization Under counterflow conditions in the vertical channel, the bubbles are retained in the center of the observation region, which allows the wake oscillations and bubble interaction to be observed over several successive periods By simultaneous diffuse illumination in addition to the light sheet, we were able to record both the path and shape oscillations of the bubble, as well as the wake structure in a horizontal and vertical cross section The results show that the zigzagging motion is coupled to a regular generation and discharge of alternate oppositely oriented hairpin-like vortex structures Associated with the wake oscillation, the bubble experiences a strong asymmetric deformation in the equatorial plane at the inversion points of the zigzag path The zigzag motion is superimposed on a small lateral drift of the bubble, which implies the existence of a net lift force This is explained by the observed different strength of the hairpin vortices in the zig and zag path; a seemingly familiar phenomenon was found in recent numerical results of the sphere wake flow For spiraling bubbles the wake is approximately steady to an observer moving with the bubble It consists of a twisted pair of streamwise vortex filaments which are wound in a helical path and are attached to the bubble base at an asymmetrical position The minor axis of the bubble is tilted in the tangential plane as well as in the radial plane toward the spiral center Due to the pressure field induced by the asymmetrically attached wake two components of the lift force exist, one that causes the lateral motion and the other a centripetal force that keeps the bubble on a circular path A mechanism is proposed to explain the reason for one bubble to spiral or to zigzag Experiments with two simultaneous released bubbles show that bubble interaction is strongly triggered by the wake dynamics Once a bubble is captured in the wake of a rocking bubble, it accelerates and rises via successive jumps until they collide The jumps are explained by the upwards induction effect of the ring-like heads of the hairpin vortices being shed from the leading bubble The final collision and repulsion thereafter abruptly enlarges the wake for a short moment, which is suggested to be one major contribution to the amplification of turbulence production in bubbly flows

Structure and dynamics of the wake of bubbles and its relevance for bubble interaction

Rise velocity of a swarm of large gas bubbles in liquids

Influence of slurry concentrations on bubble population and their rise velocities in a three-phase slurry bubble column

Bubble interaction in low-viscosity liquids

The dynamical behavior of a single and a pair of spherical-cap bubbles was investigated while they were held stationary by means of a downward liquid flow in a vertical channel. No significant differences were found in behavior such as bubble geometry, rising velocity, or liquid velocity profile around a bubble, for free-rising bubbles in a stationary liquid pool and for the present bubbles held stationary in a water channel. The fluid dynamical properties of the wake behind spherical-cap bubbles were classified by Reynolds number, as a laminar wake (10 500). The laminar wakes of the preceding bubbles had a decisive effect on the rising velocity of the trailing bubbles, in which an additional velocity component caused by the wakes of the preceding bubbles was correlated well with the wake velocities. The turbulent wakes had a limited effect on the acceleration of the trailing bubbles and often caused them to break up.

Motoyoshi Kamojima

Papers

Wake behavior and its effect on interaction between spherical-cap bubbles