Experimental Investigation of Bubble Departure Mannersat Inclined Heated Wall

The visual experimental facility of inclined heated surface was built and the visual experiments in subcooled flow boiling were performed. The results show that there are mainly three manners of bubble departure at one nucleation site, including bubble free departure, bubble collision departure and bubble disturbance departure. Bubble free departure means the objective bubble grows to a specific size and departs from the nucleation site without any influence of other bubbles. It is believed the reason of bubble free departure is that the bubble cannot maintain a static state and continue to grow at the critical size. The bubble will slide along the heated surface after departure. Bubble collision departure means the objective bubble touches other sliding bubbles and merges into a new bubble which will depart from the nucleation site immediately. Bubble disturbance departure means the objective bubble is disturbed by other sliding bubble, but does not recombine with the sliding bubble, and gradually leaves the nucleation site. The minimum irrelevant number rule was established for the quantification of bubble detachment characteristics in order to reduce the interference of randomicity. The minimum irrelevant numbers of bubble free departure and bubble collision departure are 25 and 34 respectively. The statistical errors of bubble departure characteristic quantities are guaranteed within ±5% by the minimum irrelevant number rule. Under the minimum irrelevant number rule, the test experiments show that with the increase of heat flux on the heating wall, the bubble free departure diameter increases, the bubble collision departure diameter decreases, the bubble growth time and waiting time decrease, and the bubble departure frequency increases. These indicate that in these experimental conditions, the bubble growth time of bubble collision departure is fully smaller than that of bubble free departure. When collision departure happens, the growing bubble departs earlier owing to the push of sliding bubble, which causes the bubble growth time becomes shorter. This suggests that the bubble waiting time of bubble collision departure is almost larger than that of bubble free departure. In the process of bubble collision departure, a fast-moving sliding bubble knocks into the growing bubble and the newly formed bubble departs from the nucleation site quickly. The collision of sliding bubble and the movement of newly formed bubble disturb the boundary layer near nucleation site seriously, which causes the nucleation site spends a longer time nucleating a bubble renewedly. These results show that the bubble departure frequency of bubble collision departure is fully larger than that of bubble free departure. The reducing effect on bubble growth time is stronger than the delayed effect on bubble waiting time during the process of bubble collision departure, which indicates that bubble collision departure shortens bubble departure cycle and quickens bubble departure process.