Void Fraction Evolution in Subcooled Flow Boiling Under Low-Pressure and Low-Flowrate Condition

Abstract

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.