Experimental investigation on bubble sliding during subcooled flow boiling in microchannel

Visualization experiments were carried out to investigate the sliding bubble dynamics during subcooled flow boiling in rectangular microchannel. Deionized water was used as the working fluid, and the bubble sliding process was visualized and recorded by a high speed camera. The cross section of the rectangular microchannel is 1 mm × 1 mm, and it was fabricated in a thin copper plate. The results showed that single bubble always slid along the channel wall before moving into the main flow, and the bubble sliding is the only way for bubble departing from the nucleation site in microchannel. Moreover, it was also found that the bubble sliding process could be divided into two stages based on the characteristics of bubble sliding velocity, namely the slow sliding stage and the accelerated sliding stage. The bubble sliding velocity in the initial stage is much smaller than that in the accelerated sliding stage. The boundary between these two stages is named as the onset of the accelerated sliding (OAS). It was noted that the characteristics of bubble sliding in both stages are significantly different. In slow sliding stage, the sliding bubble size and the contact diameter keep nearly constant and the bubble sliding velocity fluctuates slightly. In accelerated sliding stage, the sliding bubble size and the contact diameter decrease continually along with the rapid increase of the bubble sliding velocity. It was also observed that the bubble lift-off from channel wall only occurred at the end of the accelerated sliding stage. In addition, the effects of the operating conditions on the bubble sliding were investigated. It was demonstrated that the operating conditions have significant influences on the sliding bubble dynamics.