Enhanced Nucleate Boiling in Microchannels

Abstract

In Chap. 5, two boiling mechanisms, nucleation and eruption boiling, have been visualized in microchannels. For channel diameters ranging from 100 to 150 pm, boiling initiates from bubble nucleation on the side walls and gradually turns into annular flows in the fully developed state; for channels smaller than 50 pm, on the other hand, boiling initiates from a sudden eruption of vapor at much higher temperatures and mist flow forms immediately. The eruption style boiling has also been observed in some previous research. It has been assumed to be due to the size effect, and hypotheses such as “evaporating space” have been proposed to explain this unusual boiling mechanism and the associated large amounts of wall superheat. However, this theory has not been experimentally confirmed because the reported critical size or space is inconsistent from various experiments, with a range from 50 to 700 μm [5.2–5.5]. In this chapter, a new theory is proposed and experimentally examined, a theory that relates the boiling mechanisms to the active nucleation sites in the channel walls.