Void Fraction Evolution in Subcooled Flow Boiling Under Low-Pressure and Low-Flowrate Condition
NURETH-14 - 2011 September 25-30

Presented at:
2011 September 25-30
Toronto, Canada
Session Title:
A1-2 Boiling and Condensation Fundamentals

Tomio Okawa (Osaka University)
Yosuke Yamagoe (Osaka University)
Rouhollah Ahmadi (Osaka University)


Visualization of bubble behavior in water subcooled flow boiling was carried out to investigate

the triggering mechanism of the net vapor generation under a low-pressure and low-flowrate

condition. Flow direction was vertical upward and a hydrophilic surface was used as the heated

surface. In the experiments under high liquid subcooling, bubbles were lifted off the heated

surface immediately after the nucleation and collapsed in the subcooled bulk liquid. Since the

condensation rate was nearly equal to the vaporization rate, the vapor void fraction did not

increase significantly. When the liquid subcooling was low enough, bubble coalescence took

place to produce large bubbles sliding up the vertical heated surface. It was considered that the

large sliding bubbles mitigated the condensation to contribute to a rapid increase in the vapor

void fraction with an increase in the thermal-equilibrium quality.

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