BUOYANTLY-DRIVEN 2-PHASE COUNTERCURRENT-FLOW IN LIQUID DISCHARGE FROMA VESSEL WITH AN UNVENTED GAS SPACE

This paper details experiments and analyses regarding the phenomenon o f liquid discharge into a gaseous atmosphere from the bottom of a vess el with an unvented, upper gas space. The primary goal is the developm ent of a simple model that predicts the rate of liquid discharge under the prevailing unvented condition. A literature survey of previous wo rk on this phenomenon Yielded only simple experiments and analyses tha t were limited in scope. Experiments were subsequently undertaken with an air-water system, using a larger volume and a wide range of drain line diameters. In addition to flowrate data, visual information was a cquired regarding the physical mechanism possibly governing the preval ent flow regimes. The governing physical mechanism is identified as th e stability of a gas-liquid interface, perturbed by buoyancy, at the d rain line entrance. G.I. Taylor's fundamental analysis of interfacial stability lead to the determination of criteria for flow regime transi tion among the three prevalent flow regimes, corresponding to so-calle d small, medium, and large diameters. Also, analysis of the growth of interfacial instabilities lead to the application of flooding models f or drainage rates within each regime. The models for moderate and larg e diameters were then compared against data, which confirmed their suc cess in predicting discharge rates under the unvented condition. The m otivation for this effort, besides the basic scientific significance o f studying such a fundamental phenomenon, was its numerous application s, one of which is commercial nuclear reactor systems. Specifically, t he phenomenon prevails in liquid coolant discharge from a PWR pressuri zer, with an unvented steam volume, into a steam atmosphere existing i n the adjoining hot coolant leg. Such a phenomenon could occur as part of a transient, or severe accident, scenario, entailing saturated con ditions and steam production in the normally subcooled primary heat tr ansport loop. The developed model was implemented in the Modular Accid ent Analysis Program (MAAP), a computer code designed to predict react or system behavior in response to postulated off-normal conditions, in cluding severe accident scenarios.