Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

This study carried out at the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) concerns the bubbly flow numerical simulation. One of the determining features of the two-phase flow is the topology of the flow. Local interfacial area concentration, say ai, allows to determine the exchange surface for momentum, heat and mass transfers between liquid and vapor. In the case of bubbly flow the size of the inclusion, say D, (through the non-dimensional Eötvös number) determines essential liquidvapor momentum transfers such as the lift force (e.g. (Tomiyama et al. 1995)). The corresponding fields ai and D are functions of history, flow conditions, as well as local events such as coalescence, break-up and phase change. This justifies to model their evolution thanks to additional transport equations (in addition to momentum, mass and energy balance equations). Several approaches can then be distinguished, namely the population balance (Coulaloglou and Tavlarides 1977), the two-group interfacial area transport (Fu and Ishii 2002) or the moment densities (Kamp et al. 2001). The purpose of the present study is to examine the ability of the NEPTUNE_CFD code to predict bubble size distribution in complex flow configurations.
A series of experiments performed at FZD in the frame of the TOPFLOW project allows to determine the local structure of the two-phase flow including the bubble size local distribution. We focus more particularly on bubbly air-water flows around an asymmetric obstacle in the section of a vertical pipe (see (Prasser et al. 2006)). The presence of the obstacle induces a pronounced three-dimensional effect as well as coalescence events. We perform numerical simulations with the NEPTUNE_CFD code of these experiments. The NEPTUNE_CFD code (see (Guelfi et al. 2005)) is based on the two-fluid model formulation. It is developed in the frame of a research program supported by EDF, CEA, IRSN and AREVA_NP. In the present study, two different levels of description of the bubble size distribution in space and time are considered. At the first level of description, the bubble size distribution is determined thanks to an interfacial area equation allowing to define locally a single bubble size (monodisperse flow). At the second level we consider locally a spectrum of bubble sizes (polydisperse flow) with the help of a moment densities approach. We incorporate the model within NEPTUNE_CFD and present the resulting gain in accuracy of the simulation. Perspectives concerning improvement of the numerical modelling of bubble size distribution are discussed.