The influence of bubble size on void fraction distribution in subcooled flow boiling at low pressure

1999 ◽  
Vol 26 (5) ◽  
pp. 607-616 ◽  
Author(s):  
J.Y. Tu
Keyword(s):  
Void Fraction ◽  
Bubble Size ◽  
Flow Boiling ◽  
Low Pressure ◽  
Subcooled Flow Boiling ◽  
Subcooled Flow ◽  
Void Fraction Distribution ◽  
Fraction Distribution

Numerical Simulation of Subcooled Flow Boiling Using a Bubble Tracking Method

2016 ◽  
Author(s):  
Tomio Okawa ◽  
Naoki Miyano ◽  
Kazuhiro Kaiho ◽  
Koji Enoki
Keyword(s):  
Numerical Simulation ◽  
Size Distribution ◽  
Void Fraction ◽  
Bubble Size ◽  
Flow Boiling ◽  
Bubble Nucleation ◽  
Bubble Size Distribution ◽  
Subcooled Flow Boiling ◽  
Tracking Method ◽  
Subcooled Flow

The process of bubble nucleation in subcooled flow boiling was visualized using a high speed camera to show that the bubble size can be significantly different between the nucleation sites. However, the bubble size is usually assumed constant in the numerical simulation of subcooled flow boiling. To explore the effect of the bubble size distribution on the void fraction in subcooled flow boiling, numerical simulations were performed using a bubble tracking method in which the size and position of each bubble are calculated individually using a Lagrangian coordinates. In the present simulation, the void fraction was greater when the bubble size distribution was taken into consideration. Since the bubble tracking method requires many correlations, further improvement is necessary. The present numerical results however indicate that the bubble size distribution should be taken in to consideration to evaluate the void fraction in subcooled flow boiling accurately.

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

2006 ◽  
Author(s):  
Hayato Kubota ◽  
Tatsuhiro Ishida ◽  
Tomio Okawa ◽  
Isao Kataoka ◽  
Michitsugu Mori
Keyword(s):  
Heat Flux ◽  
Void Fraction ◽  
Bubble Size ◽  
High Speed ◽  
Flow Boiling ◽  
Bubble Diameter ◽  
Nucleate Boiling ◽  
Vapor Generation ◽  
Subcooled Flow Boiling ◽  
Subcooled Flow

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.

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