Flow pattern and void fraction fluctuation in oscillatory two-phase flow

Safety of large-capacity boiling channel systems were highly advanced in these three decades. But low-mass flux and low-pressure operation of recent advanced compact boilers brought about high possibility of unstable flow condition. Many existing works focused mainly on the stable flow, and therefore discussions on the effect of the flow oscillation on the safety issues, e.g., CHFs are essential as was pointed out by Ozawa et al.[1]. Aiming at the fundamental understanding of the possible mechanisms of these CHF problems, this paper focuses the discussion on the experimental and analytical investigations about the effect of oscillatory flow on the flow pattern and void fraction behavior. Then, the results are summarized as follows: (1) The flow pattern transition at local point was different under oscillatory flow condition from the steady condition, and a large bubble and thus slug flow is temporarily formed in the gas injection area during the flow reversal state. (2) Void fraction fluctuation and void propagation velocity very well predicted by the simulation based on the drift flux model. (orig.)