Sliding bubble dynamics and the effects on surface heat transfer

Abstract

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

“…By changing from rising bubbles to sliding ones, Maxworthy [9] performed one of the first studies of the dynamics of a bubble sliding under an inclined surface, explaining that sliding bubbles differed from free rising bubbles in that they only experienced a predominant buoyancy force [9,[26][27][28][29]. Depending on the angle of inclination of the upper wall and the bubble size, several behaviors of bubbles can be found: (i) bouncing for low angles of α < 5° [29], (ii) sliding for intermediate angles 5°< α < 80° [26,27,30], or (iii) steady bouncing of constant amplitude for high angles [18,31]).…”

“…Particle Image Velocimetry PIV under three different angles of inclination (α=20°, 30°, and 40°) [26][27][28]. It has been shown that an increase of α and bubble db increased the bubble velocities [9,30,32]. In addition, the spatial and temporal evolution of the flow structures consisted of two distinct regions: (i) a near wake moving in close association with the bubble, forming a recirculation region after which fluid separated from this high-velocity region and was drawn towards the inclined surface.…”

“…Evolution of oxygen concentration fields in the near wake of a confined bubble db=4.54± 0.15 mm for 10º < α < 60º at different positions X' of10,30 and 60 mm at Ar=800.…”

On the Mechanism of Bubble Induced Forced Convective Heat Transfer Enhancement

“…Donnelly et al (2009Donnelly et al ( ), (2012 conducted experiments of bubble sliding in natural convection, which corroborated that assumption. Similarly, the bubble causing the greatest disruption in a forced convection thermal boundary layer is the one with the highest Reynolds number (large relative velocity and bubble diameter); this corresponds to the bubble just departing the injection or nucleation site.…”