Holographic interferometric study of heat transfer to a vapor bubble sliding along a downward facing heater surface

Mechanistic modeling of heterogeneous boiling requires understanding of bubble nucleation, growth and departure from the heater surfaces. Often due to the nature of flow or heater geometry, bubbles upon leaving their nucleation sites, slide along the heater surface before eventual lift-off. In such cases understanding of bubble growth and its effect on heat transfer from the surface becomes very important part of mechanistic models. In this work an attempt is made to study the heat transfer associated with single vapor bubbles sliding along downward facing heater surfaces. Heater surface is made of polished silicon wafer, with a set of foil-heaters attached at the back. The surface can be tilted to a required angle of inclination. By controlling the power to these heaters the surface can be maintained at different wall superheats. Experiments were conducted with PF-5060 as test liquid, for liquid subcoolings ranging from 0.2 to 1.2°C and wall superheats from 0.2 to 0.8°C. Single bubbles were generated on an artificial cavity at the lower end of the heater surface. High-speed digital photography was used to measure the size of the bubbles and obtain the bubble growth rate. Temperature field in the liquid around the sliding bubble was measured non-intrusively using holographic interferometry. Heat transfer into the sliding vapor bubble is obtained from the interferometry fringes. Results show that for the range of parameters considered the bubbles continue to grow, with rates of growth decreasing with increasing liquid subcooling. Heat transfer measurements show that condensation occurs on most of the bubble interface away from the wall. Condensation accounts for less than 12% of the heat transfer from the bubble base. Results also show an increase in the area averaged heat flux values of about one to two orders of magnitude, with the presence of the vapor bubbles compared to that of natural convection. The heater surface showed no drop in temperature as a result of sliding bubbles in this study. A simple model for transient conduction in the liquid layer beneath the bubble showed that even with liquid layer thickness below 10 microns, the temperature drop will be negligible. Based on the steady state conduction through the layer, to sustain the heat flux through bubble base, an upper bound for the liquid layer thickness may be fixed at about 100 microns. Recommendations for future work are proposed for developing a general model for wall heat flux partitioning during bubble sliding motion including the effect of liquid subcooling, wall superheat and heater surface angles of inclination.