Numerical investigation of instabilities in the two-fluid model for CFD simulations of LWRs

Paper in proceeding, 2015

We present a two-fluid framework for simulation of adiabatic gas-liquid flow. The aim of the investigation is to confirm and analyze phase instabilities and meso-scale flow patterns for the vapor phase arising due to instabilities in the two-fluid model. For this purpose, the solver is applied to a set of two-dimensional, periodic problems with initially flat velocity and void fraction distributions. We demonstrate the occurrence of such instabilities and we analyze the temporal development of the void fraction field. The instabilities are shown to emerge from the initially uniform distribution of void, via a numerically unstable but non-physical distribution leading to the appearance of meso-scale structures. The importance of the equation discretization schemes is evaluated and it is shown that the lower order schemes postpone the emergence of the instabilities. Furthermore, horizontally confined systems of different widths are studied and it is shown that the instabilities do not occur below a certain system width with the current model formulation and conditions. We also investigate different formulations of the void fraction equation and we show that not all the proposed formulations are able to capture the meso-scale structures. The presented results and analysis propose that the appearance of mesoscopic structures and void instabilities in a typical two-fluid model can be pronounced and thus need to be recovered in order to accurately model the liquid-vapor flow in nuclear reactors.