Abstract
The characteristics of bubble behavior have been in particular interest for decades due to its significant contribution to understanding the mechanism of heat transfer. In the present work, visualized experiment is conducted to study the bubble characteristics in subcooled flow boiling of a narrow rectangular channel under natural circulation. The experiments were performed at pressures of 0.2 MPa, with inlet subcooling ranging from 20 to 60 K and heat flux ranging from 100 kW/m2 to 300 kW/m2. A high-speed digital camera is used to capture the pictures of bubble behaviors. A sequence of image processing algorithms is used deal with the original bubble images to get relevant bubble parameters. We observe the whole process of a single sliding bubble lifetime and found most of bubbles slide along the heating surface after detaching the nucleation sites. Five typical sliding bubble growth paths are observed in the present experimental conditions. According to the analysis of the experimental data, it can be found that the liquid subcooling and wall superheat are the main factors that affect the bubble size during sliding in narrow rectangular channel under natural circulation condition. Due to the difference of driving force, the sliding velocity of bubble in forced circulation is always larger than that in natural circulation. At the same time, the bubble velocity changes significantly at different heat flux and shooting location.
Highlights
Flow boiling under natural circulation has been found in many industry applications including nuclear reactor, External Reactor Vessel Cooling (ERVC) (Ha et al, 2004), Passive containment cooling System (PCCS) (Park et al, 2008)
From a visualized experiment in a narrow rectangular channel conducted by Xu et al (Xu et al, 2010, 2013), we found that the maximum bubble dimension in the direction normal to the heating surface is >1 mm in the flow direction, while most bubble diameters are in the range of 0.09–0.4 mm, and the upstream and downstream contact angles were almost equal during sliding process
Because of the restriction of narrow channel, there will more sliding bubbles generated on the heating surface
Summary
Flow boiling under natural circulation has been found in many industry applications including nuclear reactor, External Reactor Vessel Cooling (ERVC) (Ha et al, 2004), Passive containment cooling System (PCCS) (Park et al, 2008). In the studies concerning about narrow rectangular channel, more attention has been attracted in sliding bubbles for their great improvements on the capacity of heat transfer. Study on sliding bubble behaviors is helpful to understand the mechanism of heat transfer in a narrow rectangular channel under natural circulation. In the experiment of subcooled flow boiling of water by Prodanovic et al (Prodanovic et al, 2002), bubble slid for the length of several bubble diameters and migrated toward the bulk fluid. It was found that bubble sliding is the only way for bubble departure from the nucleation site in microchannel in the study carried
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