Development of multidimensional mechanistic models of two-phase flow for the NPHASE CFD code

Full text of publication follows: This paper is concerned with the development of advanced multidimensional multiphase flow and heat transfer modeling and CFD simulation capabilities for application in nuclear reactor thermal-hydraulics. The accuracy of CFD predictions depends on the quality of closure models for local two phase flow phenomena under consideration. The majority of existing CFD codes use simplified closure laws, actually applicable only to dispersed bubbly/particle flows, where the interfacial interactions are limited to the drag and, possibly, virtual mass forces. In the present work, the recently developed models for slug and annular flows, as well as new models of subcooled boiling and CHF have been implemented in the NPHASE code and fully coupled with an advanced numerical solution algorithm. As a result of this, several generic closure laws, both local and mixed local/global (defined by specific mathematical operators and formulae) are available to model major flow regimes encountered in nuclear reactor systems. A fully coupled multidimensional multi-field mass/momentum flow solver that has been implemented in the NPHASE code allows for a tight coupling between the numeric and individual physical models of two-phase flow. Selected results of testing and validating the new models will be shown in the full paper. Specifically, the results of simulations will be discussed of two-phase flows for various flow regimes and nuclear reactor operating conditions. Furthermore, the final goal of the effort is to have the models of individual flow regimes linked together into a single scalable multidimensional model of a heated channel applicable over a wide range of conditions of interest in reactor thermal-hydraulic and safety analysis. (authors)