EFFECTS OF A NONCONDENSABLE GAS ON THE MICROBUBBLE EMISSION BOILING

Experiments on subcooled pool boiling were conducted to study the effects of a noncondensable gas on heat transfer performance and bubble behaviors in the regime of microbubble emission boiling (MEB). Snapshots of bubble behaviors were taken with the aid of a high-speed video camera (Photron: Fastcam SA5). The experimental results show that the noncondensable gas can extend the transition process from nucleate boiling to MEB and enlarge the emitted microbubbles from coalescing bubbles in MEB. A vapor film with a violently waving interface that for a long time covers the heating surface is observed for subcooled boiling of nondegassed water rarely found in the MEB of degassed water. In addition, it is also found that the noncondensable gas can weaken the inertial shock of liquid caused by condensation and may result in a collapse of a vapor bubble. Therefore, according to these findings, it can be considered that the noncondensable gas can tend to inhibit the collapse of a vapor film, resulting in a reduction of the bubble collapse frequency. This may be for this reason that the noncondensable gas could deteriorate the heat transfer performance in the regime of MEB.