Comparison and uncertainty quantification of two-fluid models for bubbly flows with NEPTUNE_CFD and STAR-CCM+

The nuclear industry is interested in better understanding the behavior of turbulent boiling flows and in using modern computational tools for the design and analysis of advanced fuels and reactors and for simulation and study of mitigation strategies in accident scenarios. Such interests serve as drivers for the advancement of the 3-dimensional multiphase Computational Fluid Dynamics approach. A pair of parallel efforts have been underway in Europe and in the United States, the NEPTUNE and CASL programs respectively, that aim at delivering advanced simulation tools that will enable improved safety and economy of operations of the reactor fleet. Results from a collaboration between these two efforts, aimed at advancing the understanding of multiphase closures for pressurized water reactor (PWR) application, are presented. Particular attention is paid to assessment and analysis of the different physical models implemented in the CFD tools respectively used in the NEPTUNE and the CASL programs, for application to turbulent two-phase bubbly flows. The experiments conducted by Liu and Bankoff (Liu, 1989; Liu and Bankoff, 1993a,b) are selected for benchmarking, and predictions from NEPTUNE_CFD and STAR-CCM+ codes are presented for a broad range of flow conditions and with void fractions varying between 0 and 50%. Comparison of the CFD simulations and experimental measurements reveals that a similar level of accuracy is achieved in the two codes. The differences in both sets of closure models are analyzed, and their capability to capture the main features of the flow over a wide range of experimental conditions are discussed. Finally, a parametric sensitivity study for the set of closures used in STAR-CCM+ is included to serve as a preview of how uncertainty quantification methods can provide insights into interactions between closures of different phenomena. In conclusion, it is seen that, the multi-CFD-code approach and uncertainty analysis of a set of closures in a particular CFD code, are both of great value in assessing the limitations and the level of maturity of multiphase hydrodynamic closures, and can serve as aids in further improving them.