Microbubble Return Phenomena During Subcooled Boiling on Small Wires

An experimental investigation was conducted to explore the characteristics of subcooled boiling on microwires of 25 and 100 lm diameter. Microbubbles were observed to return to the wire surface after detachment, with two types of bubble return identified, i.e., isolated bubble return, and bubble return with liquid–vapor trailing jets. The former mode of bubble return occurred when isolated small bubbles (of less than 50 lm diameter) were generated from bubble collapse, while in the latter mode, a larger bubble (of up to 200 lm in diameter) at the end of a liquid–vapor jet issuing from the wire departed and then returned to the wire surface. The numerical simulations conducted show that the isolated bubble return is caused by large temperature gradients in the vicinity of the wire which lead to Marang- oni flows and result in a strong thrust force driving the bubble back to the wire. Existence of large temperature gradients close to the microwire surface was demonstrated by experimental measurements, confirming numerical predictions. The numerical model accounts for the influence of noncondensable gas on the vapor saturation temperature as well as the interfacial condensation coefficient. The pres- ence of noncondensable gas facilitates bubble return