Two-phase flow regimes during the rewetting and refilling of hot horizontal tubes

The development of accurate mechanistic models for the refilling and rewetting of hot horizontal tubes or rod bundles requires knowledge of the relative phase distribution (ie, flow regimes) encountered during such processes. Characterization of the relative phase distribution is essential in evaluating the interfacial transport of mass, energy, and momentum and hence is important in the development of mathematical models of the refilling and rewetting phase of the hypotheticall loss of coolant accident (LOCA) in nuclear power reactors.Experiments on the refilling and rewetting of hot horizontal tubes were carried out. The relative phase distribution near the advancing quench front was obtained by analyzing the simultaneous measurements of transient void fraction and transient circumferential surface temperature distribution taken at the same cross section of the tube. The void fraction was measured by using a gamma densitometer, and subminiature thermocouples were used to measure wall temperatures. The results showed clearly that three different flow regimes can be encountered: stratified, inverted annular, and annular. For moderate refilling flow rates [up to 900 kg/(m2·s)], the propagating quench front was preceded by a stratified refilling front. For higher flow rates, stratification effects were less pronounced, and the advancing refilling front approach an inverted annular flow pattern. However, for low tube wall heat capacity and at low inlet flow, annular flow was encountered. The effects of inlet flow rate, initial wall temperature, inlet subcooling, and the wall heat capacity on the phenomena are analyzed, and the implication of the results on the physical modeling are discussed.