Experimental Study on Bubble Sliding and Lift-Off Behavior in a Narrow Rectangular Channel

Abstract

Visual investigation on bubble sliding and lift-off from the vertical and inclined (upward and downward facing) heating surface in a narrow rectangular channel was carried out in this paper. 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 2×8 mm2, and it was fabricated on an optical quartz glass for observation of bubble sliding and lift-off from the heating surface by the wide side and the gap side of the narrow rectangular channel using a high speed digital camera. In upflow boiling for vertical and inclined downward facing heating surface, the bubbles departing from the nucleation sites always slid along the heating surface, and the bubble lift-off phenomenon was not observed with low heat flux in the isolated bubble region, and the sliding bubble diameter increases during the sliding process. It is also found that the sliding velocity is less than that of local bulk liquid velocity at the initial moment of bubble sliding. However, it will exceed the local liquid velocity with increasing time. In upflow boiling for inclined upward facing heating surface and vertical downflow boiling, the bubble tends to lift off from the surface, and the bubble velocity increases after lift-off from the heating surface, and the condensation phenomenon of lift-off bubble will occur when the bulk flow is subcooling, which results in the change of bubble shape. Base on the experimental results, the influences of different flow mode and heating surface inclined mode on the sliding and lift-off of bubble are analysed.