Reza Azadi - Meso-scale bubble dynamics

Meso-scale bubble dynamics: Experiments

The flow of dispersed gas bubbles in a viscous liquid can create a bubbly, slug bubble, or elongated bubble flow regime. Slug bubble flow, characterized by bubble sizes equal to the hydraulic diameter of the channel, is a transition regime with a complex local flow field that has received little attention in the past. In this study, the dynamics of this flow regime in a square capillary with a cross-sectional area of 3 × 3 mm2 was studied. The main geometric parameters of the flow field, such as film and corner thickness, and volume fraction were calculated for different flow conditions based on a semi-empirical approach. Using velocity fields from particle image velocimetry (PIV), combined with the analytical equations derived, local mean variation of the film and corner flow thickness and velocity were analyzed in detail. Analysis of the results reveals a linear relation between the bubble speed and the liquid slug velocity that was obtained using sum-of-correlation (SOC) PIV. Local backflow, where the liquid locally flows in the reverse direction, was demonstrated to occur in the slug bubble flow, and the theoretical analysis showed that it can be characterized based on the bubble cross-sectional area and ratio of the liquid slug and bubble speed. It was shown that backflow is only contributed to the channel corners and its speed can increase to the bubble speed. However, there is no evidence of reverse flow in the liquid film for the flow conditions analyzed in this study.

You can find the published work here: (Physics of Fluids)

Main components of the flow cell

The experimental setup with its main elements annotated.

Meso-scale bubble dynamics: Simulations

The motion of long bubbles in tubular capillaries has typically been described by bulk characteristics. However, the dynamics of slug bubbles in square capillaries are more complex due to a corner flow and a thin film flow. The physics can be correctly explained by elucidating local 3D features of the two-phase flow field. To this aim, an experimental study based on particle tracking velocimetry (PTV) and a numerical simulation based on the volume-of-fluid method were conducted to investigate the dynamics of slug bubbles rising in a flowing square capillary with a cross-sectional area of 3×3 mm2. To precisely analyze the phases’ interaction, interfacial flow data was mapped onto a radial-tangential coordinate system on a central and diagonal plane. The simulated interface topology and velocity fields show a good agreement with the experimental PTV data on the central plane, with an absolute error of less than 1.2% for terminal bubble speed. Tangential speeds show their maxima occurring in the channel corners, where pressure is maximum. The thin liquid film flow that occurs where the bubble approaches the wall applies noticeable shear stress on the channel walls, where high and low-pressure regions are generated. Structures of vortices inside the bubble were identified using iso-surfaces of the Q-criterion, and their cores were detected based on the parallel vector method. Results reveal a dominant vortex ring adjacent to the liquid film flow and two oblique vortex tubes close to the bubble’s nose.

You can find the published work here: (Physics of Fluids), (International journal of heat and mass transfer)

(a) Distribution of the pseudo-Lagrangian velocity magnitude, normalized by the terminal speed of the bubble, on the surface of a confined bubble with D ̃_G = 1.11 wc and u ̃_G = 0.65 mm/s (Multimedia view). (b) Distribution of the normalized pseudo-Lagrangian velocity magnitude on five different planes parallel to the xy (or XY) plane, with equal spacing of 0.25 wc between them. The pseudo-Lagrangian velocity streamlines and vectors are superimposed onto the first and second rows of the plots in (b), respectively. Z =0 shows the bubble’s centroid position, and the gas/liquid interface is displayed in a black thick closed curve on each slice. The density of the streamlines and velocity vectors were manipulated for visualization purposes.

IJHMT.mp4