VISUALIZATION STUDY OF BUBBLE SLIDING CHARACTERISTICS IN A SUBCOOLED FLOW BOILING NARROW RECTANGULAR CHANNEL UNDER NATURAL CIRCULATION CONDITION

Heat transfer enhancement by the motion of the bubbles sliding along the heating surface are wildly reported by many researchers, thus it is of great importance to quantitatively investigate the characteristics of sliding bubbles. A visualization study of subcooled flow boiling of water in a vertical single face-heated narrow rectangular channel under a series of natural circulation working conditions was conducted. Pictures of the bubble sliding behaviors were captured by a high speed camera simultaneously with thermal data. A sequence of digital image processing algorithms were applied to the original picture to extract bubble shape and location information, which post processing methods were adopted to obtain characteristic sliding parameters(including the distribution of the equivalent sliding bubble diameter and velocity, number density of the sliding bubbles). It is found that bubbles can be able to nucleate and grow while sliding on the heating plate after the ONB point; the bubble number density, average bubble sliding velocity and the average sliding diameter continue to increase along the test section; heat transfer in the flow channel are significantly enhanced along flow direction with relatively low local void fraction. The average bubble sliding velocity near the inlet is significantly smaller than the sectional average velocity of single-phase fluid of the flow channel, and then it exceeded near the outlet of the test section. The average bubble sliding velocity and diameter increase with increasing heat flux and decreeing local subcooling degree. The equivalent diameter of sliding bubbles and the bubble sliding velocity approximately follow normal distribution. The distribution of the bubble diameter and velocity both cover a wild range. The standard deviations of the probability density function of the sliding bubble diameter and velocity increase with increasing heat flux and decreasing subcooling degree.