Abstract
Understanding the inverted annular flow (IAF) boiling regime is crucial for various engineering disciplines. The IAF is a post-dryout, or post-CHF film boiling regime in forced convective flow in tubes or channels. The flow structure of the IAF is characterized by the liquid core present in the channel center, which is separated from the surrounding heating source by a vapor film envelope. It has been studied extensively as one of the film boiling regimes since 1950s. In the core reflooding phase of the light water reactors, in the event of the loss-of-coolant-accident (LOCA), one of the dominant heat transfer modes is film boiling. During that process, various flow regimes are generated in the flow channel, including the IAF. The IAF appears immediately downstream of the pre-CHF flow regime and initiates at the quench front. It gradually progresses to an inverted slug and dispersed flow as it travels downstream. For the improved safety of engineering systems, including modern and advanced nuclear reactors, proper utilization of heat transfer and drag coefficients of IAF for the postulated LOCA is crucial. The present study extensively reviews the existing correlations proposed for IAF heat transfer and drag. In addition, the flow regime transition criteria applicable for IAF, based on the liquid jet-breakup concept, are reviewed. IAF correlations utilized in modern reactor safety analysis codes will also be revisited.
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