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.