Visualization study of bubble behavior in a subcooled flow boiling channel under rolling motion

Boiling heat transfer equipment in a vessel can be affected by the additional force which is generated by the rolling, swing and heaving motion of the vessel. Bubble behavior is very important for the research of boiling phenomenon. Bubble behavior under rolling motion condition is experimentally studied by using a high speed camera. The experiment is conducted in a subcooled flow boiling rectangular channel, and the cross section size of the channel is 2 mm x 40 mm. Two types of bubbles with large discrepancies in sliding and condensation behaviors can be observed in the captured images. The first type bubbles disappear quickly after generation and the slide distance is only a few times of bubble maximum diameter, while the second type bubbles can survive a longer time after leaving the nucleation site and slide for a long distance with the flowing fluid. Bubble characteristics under rolling motion are separately studied for different type bubbles based on the above reasons. The results show that the lifetime, maximum diameter, nucleation frequency and sliding velocity of the first type bubble are periodically fluctuated and the period is same with the rolling motion. The fluctuation intensity of the bubble lifetime and maximum diameter can be enhanced by the increase of the rolling amplitude. The peak value of bubble lifetime, maximum diameter, and nucleation frequency appears when the rolling platform plate rolls to the maximum positive angle, while opposite trend can be observed in the variation of bubble sliding velocity. In view of the characteristics of the second type bubbles, lifetime and maximum diameter are not measured. And the variation of nucleation frequency and sliding velocity of the second type bubbles under the effect of rolling motion is same with the first type bubbles. Furthermore, the effects of additional force, variation of local pressure and flow rate oscillation on bubble behavior are analyzed. The results indicate that the fluctuations of the bubble parameters can be generated by the variation of local pressure caused by rolling motion even no influential flow rate fluctuation occurs. The effect of the acceleration variation vertical to the heated surface on bubble behavior is unclear and need more researches in the future work. (C) 2014 Elsevier Ltd. All rights reserved.