Abstract
Understanding of an onset of boiling plays an important role in the design of boiling equipment. The onset of boiling condition in downward flow is complex due to stagnant and/or reverse bubble caused by buoyancy force on thermalnonequilibrium flow field. The purpose of this study is to clarify the characteristics of onset of boiling under downward flow. We conducted heat transfer experiments with different heating length under both upward flow and downward flow. The onset of boiling condition is estimated by using the pressure drop of test section. In the case of long heating length, low heat flux condition, the thermal equilibrium quality on onset of boiling condition had almost constant value regardless of mass flux. In the case of shot heating length, high heat flux condition, the thermal equilibrium quality on onset of boiling condition was drastically decreased with an increase of mass flux. These characteristics were classified into two groups; the thermal dominant condition and the thermal-fluid dominant condition. The modified Bowring’s correlation and modified Levy’s correlation were in good agreement with the experimental results for each condition.
