FLOW VISUALIZATION AND WAKE CHARACTERIZATION OF A SLIDING BUBBLE USING PARTICLE IMAGE VELOCIMETRY

Both vapour and gas bubbles are known to increase heat transfer rates from adjacent heated surfaces, a phenomenon attributed to the associated wake behaviour. Although the wake structures of free rising bubbles have been studied extensively, few studies have been performed on the sliding bubble wake, a case that occurs in many practical engineering applications, such as shell and tube heat exchangers. This study investigates the wake structures present behind a single bubble, sliding on an adiabatic surface, inclined at 30° to the horizontal. Bubble volumes of 0.05, 0.1, 0.2 and 0.4 ml were examined. The velocity and vorticity of the wake structures were measured parallel and perpendicular to the test surface using the Particle Image Velocimetry technique. These two planar flow field measurements were used to infer the three-dimensional wake structure. Analysis of the vorticity and velocity data suggests the wake structures are hairpin vortices, shed alternately from each side of the bubble rear. This finding is supported by the literature associated with the wakes of freely rising bubbles and bluff bodies, such as spheres. This improved understanding of the wake structures will allow for future optimisation of the heat transfer enhancement effects offered by sliding bubbles.