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.
Experimental study of flow pattern and heat transfer associated with a bubble sliding on downward facing inclined surfaces
Abstract During boiling, heat transfer can be significantly enhanced by motion of the bubbles sliding along the heater surface. In the present study, sliding bubble dynamics and associated flow pattern and heat transfer over a downward facing inclined heater surface were experimentally investigated in the subcooled and nearly saturated high performance liquid PF-5060. The heat transfer surface was a polished silicon wafer of a width 49.5 mm and a length of 185 mm. It is found that bubbles change shape from a sphere at the initial position to an elongated spheroid or ellipsoid at the upper end of the inclined plate. Between the sliding bubble and the heater surface a wedge-like liquid gap and thin liquid film are observed to form. The angle and the length of the wedge are found to be a function of plate inclination angle θ and bubble size. Holographic interferometry was used to obtain non-intrusively the temperature field in the liquid. In subcooled liquid, the formation and shedding of vortices in the bubble wake are the typical flow patterns associated with sliding bubble. The enhanced convection lasted during most of the interval between two consecutive bubbles accompanied with an increased temperature gradient in the liquid very close to the surface and a wall temperature drop. Contributions to heat transfer due to bubble sliding motion have been quantified. For bubble duration less than 1% of the total time at a location heat transfer rate is increased for about 10%. At inclinations close to vertical, bubbles hopped along the heater surface. Series of bubbles generated at higher frequency induced changes in flow pattern in bulk liquid at small Rayleigh numbers.
The EU Legal Framework for the Enforcement of Consumer Law
The European Union (EU) is special in that it is a multi-level system in which, principally, consumer law can be adopted at the EU level but it is enforced at the level of the Member States. Nevertheless, the EU has over time developed an ever denser legal framework for the enforcement of consumer law by the Member States. It is this legal framework that this contribution focuses upon.