Modeling two-phase flow in pipe bends

In order to have a better understanding of the interaction between the two-phase steamwater coolant in the outlet feeder pipes of the primary heat transport system of some CANDU reactors and the piping material, themalhydraulic modelling is being performed with a commercial computational fluid dynamics (CFD) code-FLUENT 6.1. The modeling has attempted to describe the results of flow visualization experiments performed in a transparent feeder pipe with air-water mixtures at temperatures below 55 degrees C. The CFD code solves two sets of transport equations-one for each phase. Both phases are first treated separately as homogeneous. Coupling is achieved through pressure and interphase exchange coefficients. A symmetric drag model is employed to describe the interaction between the phases. The geometry and flow regime of interest are a 73 deg bend in a 5.9 cm diameter pipe containing water with a Reynolds number of similar to IE5-IE6. The modeling predicted single-phase pressure drop and flow accurately. For two-phase flow with an air voidage of 5-50%, the pressure drop measurements were less well predicted. Furthermore, the observation that an air-water mixture tended to flow toward the outside of the bend while a single-phase liquid layer developed at the inside of the bend was not predicted. The CFD modeling requires further development for this type of geometry with two-phase flow of high voidage.