RECENT DEVELOPMENTS ON THE CFD-MODELLING OF SUBCOOLED BOILING IN FUEL ASSEMBLIES OF WATER COOLED REACTORS

摘要

Many efforts are being spent towards the CFD simulation of the forced subcooled boiling in fuel assemblies of water cooled nuclear reactors, the ultimate strategic target being the prediction of the Critical Heat Flux (CHF). The current approaches to the CHF prediction issue are characterized by some conservatism, being usually based on empirical correlations, experimental databases, and thermo-hydraulic modelling at system and sub-channel scale. Those limitations are expected to be removed by the CFD modelling of boiling flows, being such approach inherently local, three-dimensional and geometry-independent. On the other hand, the state-of-the-art CFD codes with multiphase flow modelling capabilities are not yet sufficiently mature for addressing a problem as tough as a multiphase fluid flow characterized by the simultaneous presence of different flow regimes. However, in general they can manage - more ore less successfully - individual flow regimes, like for instance the bubbly flow, which constitutes a first, indispensable, step forward. The present paper constitutes a review of some literature documenting the most recent modelling and code development activities conducted with the purpose of predicting subcooled boiling bubbly flows in the heated channels of nuclear fuel assemblies. The simulation of that flow regime represents the current front of advancement of CFD codes development, while the prediction of the flow regimes subsequently encountered towards the CHF is still beyond the current capabilities.