Dependence of Vapor Void Fraction on Fundamental Bubble Parameters in Subcooled Flow Boiling

Conference · Sat Jul 01 04:00:00 EDT 2006

OSTI ID:20997017

Kubota, Hayato; Ishida, Tatsuhiro; Okawa, Tomio; Kataoka, Isao ^[1]; Mori, Michitsugu ^[2]

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1. Osaka University, 1-1, Yamadaoka, Suita-shi, Osaka (Japan)
2. Tokyo Electric Power Company (Japan)

A visual study of water subcooled flow boiling was conducted to clarify the mechanism of triggering the net vapor generation (NVG). The test section was a transparent sapphire grass tube of 20 mm in inside diameter; a high-speed camera was used to capture the behavior of vapor bubbles. In the present experiments, the vapor void fraction in the heated tube was expressed as the function of the following bubble parameters: nucleation site density, frequency of bubble release, bubble lifetime, and bubble size. Among these four bubble parameters, the bubble size had a particularly strong influence on the vapor void fraction: the void fraction was approximately proportional to the forth power of mean bubble diameter. Consequently, mean bubble diameter should be large enough for the vapor void fraction to increase rapidly with the wall heat flux. In low flowrate experiments, bubbles generated at nucleation sites were relatively large at the onset of nucleate boiling (ONB). The heat flux at ONB hence appeared the reasonable approximation of that at NVG. Whereas, in high flowrate experiments, bubbles were small at ONB and much higher heat flux was necessary to obtain large bubbles. Thus, the heat flux required to trigger NVG was much higher than that at ONB. It was concluded in the present experimental conditions that accurate evaluation of mean bubble diameter was of significant importance in predicting the onset of net vapor generation. (authors)

• Research Organization: The ASME Foundation, Inc., Three Park Avenue, New York, NY 10016-5990 (United States)
• OSTI ID: 20997017
• Country of Publication: United States
• Language: English

Related Subjects

42 ENGINEERING
APPROXIMATIONS
BUBBLES
CAMERAS
DENSITY
HEAT FLUX
LIFETIME
NUCLEATE BOILING
NUCLEATION
PROGRAMMING LANGUAGES
SAPPHIRE
TUBES
VAPORS
VELOCITY
VOID FRACTION
WATER