Experimentation and Modeling of Subcooled Flow Boiling from the Perspective of Large Bubble Formation

Abstract

The authors’ previous studies are summarized to discuss that mechanistic or semi-mechanistic prediction from the perspective of large bubble formation (LBF) is possible for the occurrences of the onset of significant void (OSV) and the departure from nucleate boiling (DNB) in subcooled flow boiling. From the analogy with the flow regime transition from bubbly flow to slug flow in gas-liquid two-phase flows, it may be assumed that the global bubble coalescence leading to large bubble formation takes place in subcooled flow boiling when the local void fraction exceeds the critical value about 0.3. In addition, the commercial CFD codes are a satisfactory tool to calculate the local void fraction accurately. From these facts, it is discussed that the CFD codes can be used to carry out mechanistic and conservative prediction of the DNB heat flux even under the complex situations such as the flow channel of complex geometry with mixing vanes, inclined channel, and rolling and/or pitching motion.