Experimental visualization of sliding bubble dynamics in a vertical narrow rectangular channel

Visual investigation on sliding bubble dynamics in a narrow rectangular channel was carried out in this paper, and the flow direction was vertically upward with deionized water as working fluid. The test section mainly consists of a polyethylene block, optical quartz glass, heating plate and other auxiliary components. The cross section of the narrow rectangular channel is 2 mm × 8 mm, and it was fabricated in an optical quartz glass for observation of the sliding bubble dynamics from the wide side and the gap side of the narrow rectangular channel using a high speed digital camera. The results from the present experimental study show that bubbles always slide along the heating surface after departing from the nucleation sites, and the sliding bubble diameter increases during the sliding process. The phenomenon on bubble lift-off from the heating surface is not observed with low heat flux in the isolated bubble region. What is more, the sliding bubble is spherical according to the observation from the wide side of the channel, but it is elongated in the direction normal to the heating surface according to the observation from the gap side of the channel. During the process of bubble sliding along the surface, the upstream and downstream contact angles are almost equal. It is also found that there is an increase in sliding velocity at the initial moment, and the increase in sliding velocity decreases gradually as the time increases, and the sliding velocity reaches a constant one finally. The sliding bubble velocity is less than that of local liquid velocity on the streamline through the mass center of the bubble at the initial moment. However, it will exceed the local liquid velocity with increasing time, and the shear lift force acting on the sliding bubble pushes the bubble against the heating surface in this case, which is mainly the reason for bubble sliding along the heating surface.