Single-bubble dynamics during pool boiling under low gravity conditions

Results of an experimental study on growth and detachment mechanisms of a single bubble on a heated surface conducted during the parabola flights of the KC-135 aircraft are described. An artificial cylindrical cavity 10 mum in diameter was etched in the center of a silicon wafer. The wafer was heated on the back side, and the wall superheat was controlled. Degassed distilled water was used as the test liquid. Bubble growth time, bubble size and shape from nucleation to liftoff were measured under subcooled and saturation conditions at system pressures varying from 0.101 to 0.115 MPa. The wall superheats were varied from 2.5 to 8.0degreesC. Significantly larger bubble diameters and longer bubble growth periods than those at Earth normal gravity were measured. Bubble diameters as large as 20 mm at liftoff were observed as opposed to about 2.5 mm at Earth normal gravity. Consistent with results of numerical simulations, it is found that for the same wall superheat and liquid subcooling the bubble liftoff diameter can be approximately related to the gravity level through the relation D-d proportional to g(-0.5) and the growth period as t(g) proportional to g(-1.05). The effect of wall superheat and liquid subcooling on bubble liftoff diameter is found to be small. However, the growth periods are found to be very sensitive to liquid subcooling at a given wall superheat. Small accelerations along the heater surface can lead to sliding motion of the bubble prior to liftoff. At the same gravitational acceleration the liftoff diameter of sliding bubbles is smaller than that of nonsliding bubbles.