Sliding bubble dynamics and the effects on surface heat transfer
An investigation into the effects of a single sliding air bubble on heat transfer from a submerged, inclined surface has been undertaken. Existing literature has shown that both vapour and gas bubbles can increase heat transfer rates from adjacent heated surfaces. However, the mechanisms involved are complex and dynamic and in some cases poorly understood. The present study utilises high speed, high resolution, infrared thermography and video photography to measure two dimensional surface heat transfer and three dimensional bubble position and shape. This provides a unique insight into the complex interactions at the heated surface. Bubbles of volume 0.05, 0.1, 0.2 and 0.4 ml were released onto a surface inclined at 30 degrees to horizontal. Results confirmed that sliding bubbles can enhance heat transfer rates up to a factor of 9 and further insight was gained about the mechanisms behind this phenomenon. The enhancement effects were observed over large areas and persisted for a long duration with the bubble exhibiting complex shape and path oscillations. It is believed that the periodic wake structure present behind the sliding bubble affects the bubble motion and is responsible for the heat transfer effects observed. The nature of this wake is proposed to be that of a chain of horseshoe vortices.
Hydrodynamics and gas-liquid mass transfer around a confined sliding bubble
Abstract An experimental investigation of gas-liquid mass transfer in the wake of a confined air bubble sliding under an inclined wall in a 2D Hele-Shaw cell is reported. A colorimetric technique based on an oxygen-sensitive dye was used to visualize the oxygen transfer. Bubble velocities, shape eccentricities, interfacial areas and, for the first time, the instantaneous spatio-temporal distribution of oxygen concentration fields in the bubble wake, have been investigated for upper wall inclination angles of 10° ≤ α ≤ 60° and Archimedes numbers of 783 ≤ Ar ≤ 3221. Image processing has allowed, through a specific approach, a quantification of mass transfer. The calculation of the mass flux allowed the deduction of the liquid-side mass transfer coefficient kL. Experiments reveals that, at low angles of inclination, bubble velocities decelerates, shape eccentricities increased, and the instantaneous spatial and temporal distribution of oxygen concentration fields illustrated two distinct regions underneath the sliding bubble: a single vortex loop enclosing the near wake where oxygen is transferred, and a far wake containing oxygen in the form of a single long strip. When inclination angles and bubble sizes were increasing, velocities were increasing, the vortex elongated gradually until it disappears at high angles where total mass fluxes increased. This increase of bubble velocities has increased liquid-side mass transfer coefficient kL allowing a scaling law between the Sherwood number and the modified Archimedes number Ar.sin(α) to be proposed.
Experimental study on the magnetic convection in a vertical cylinder
Abstract The flow of paramagnetic fluid inside a cylinder placed in a bore of a superconducting magnet was studied experimentally. Single-phase closed thermosyphon configuration was employed. The lower side wall of the cylindrical enclosure was heated while the upper side wall was cooled with the thin adiabatic interface in between. The experiment was carried out with an aqueous solution of glycerol. The magnetic susceptibility of the working fluid was increased by adding Gd(NO 3 ) 3  × 6H 2 O and was measured by a magnetic susceptibility balance. The encapsulated liquid crystal slurry (KWN-2025, Japan Capsular Product Inc.) dispersed in the working fluid was illuminated in the middle height horizontal cross-section of the enclosure to visualize the temperature field. The color images of flow mode were taken by a digital camera. The average heat transfer rates were also measured. Depending on the Rayleigh number, different spoke patterns were observed. The number of angular structures (spokes) increased with increase not only in the Rayleigh number but also in the strength of magnetic field. The heated fluid was repelled by the magnetic field, while the cooled fluid was attracted. The magnetic field enhanced the heat transfer rate.