Two-phase flow models coupled with developed steam-water table look-up method for 1D fast transient flows in Europlexus

The physics of the two-phase flow models involved on fast transient phenomena has a predominant importance in nuclear studies and in other industrial fields, where fluid-structure interaction occurs. Focusing only in the aspect concerning the fluid, in this work we give some comparisons between two-phase flow models for fast transients, in order to understand the main theoretical differences and the related physical aspects due to the different thermodynamic non-equilibria involved. In the presented models, an EoS as a function of specific volume (v) and specific internal energy (e) of the fluid is always needed in order to close the system of the governing flow equations, based on a conservation law. According to this specific need, it has been decided to tabulate the physical magnitudes, such as pressure, temperature and speed of sound, in a (v, e) domain and pick them during the simulation, when they are required. To do that, a proper algorithm has been developed. This one uses a bicubic interpolation of the EoS provided by IAPWS-IF97, creating a tabulation, that is fully coupled with the proposed models. A version of the steam-water look-up tables, already available and used for the simplest models, have been also extended to cover metastable vapor applications. To accomplish this goal, the construction of the spinodal curve on the vapor side has been performed, in order to find the limit of the admissible mechanical states. Then, the new domain in (v, e) has been constructed, widening the range of applicability of the new steam-water tables. The developed look-up table procedure is consequently valid to be coupled with simpler model, as HEM and HRM, as well as for the forthcoming Six-equation model, not available at that moment. Its use is compared to the iterative method, so called ”model EAU”, already existing on EUROPLEXUS code, in order to show a direct and accurate way to calculate the thermodynamic properties. Finally, the new Steam Water Look-up Tables, already validated, are tested using the Homogeneous Equilibrium Model (HEM) and the Homogeneous Relaxation Model (HRM) on EUROPLEXUS code for some industrial application cases.