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Experimental research on sliding bubble diameter and velocity in a narrow rectangular channel under natural circulation condition YAN Meiyue1, YAN Changqi1, REN Tingting1,YANG Yongyong2, CHEN Kailun1, ZHANG Rui1, and YANG Kuan1 1.  Fundamental Science on Nuclear Safety and Simulation Technology Laboratory, Harbin Engineering University, Harbin 150001, China (yanmeiyue@hrbeu.edu.cn; changqi_yan@163.com ) 2.     China Nuclear Power Operation Technology Corporation, LTD, Wuhan 430074, China Abstract: Heat transfer enhancement by the behaviors of the bubbles sliding along the heating surfaceis studied by visual experiments by a high-speed digital camera to observe the bubble behaviors in subcooled flow boiling in a narrow rectangular channel under natural circulation. A sequence of image processing algorithms was used to deal with the original bubble images to get relevant bubble parameters, including bubble diameter and bubble velocity, etc. The experiments were performed at pressures ranging from 0.2 MPa to 0.4 MPa, with inlet subcooling ranging from 20 to 60 K and heat flux ranging from 100 kW/m2 to 300 kW/m2. We found that almost all the bubbles will slide along the heating wall. In each operating condition, the bubble diameter distribution approximately conforms to lognormal distribution while the bubble velocity distribution agrees well with normal distribution. Moreover, local thermal hydraulic parameters have significant influences on the stochastics and mean characteristics of the bubble. The present experiment data shows a good linear relationship between the mean velocity and mean diameter of sliding bubbles, and the correlation for the mean diameter was obtained by the multivariate analysis method. Besides, the experimental phenomenon shows that the bubble diameter and velocity oscillate simultaneously under natural circulation when the inlet subcooling is 40K and the heat flux exceed 200 kW/m2. Keywords: narrow rectangular channel; natural circulation; sliding bubble; bubble diameter; bubble velocity
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