Simulation of Oscillatory Flow Induced by Flashing Instability Using the ASYST System Analysis Code

Two-phase flow instabilities can lead to continuously evolving long-term behaviors involving various non-negligible phenomena. Prediction of instability consequences is therefore a challenge which requires comprehensive and robust models capturing critical physics consistently beyond steady states. As thermal hydraulic codes for reactor system analysis are typically developed with anticipated capability covering a wide spectrum of reactor transients, validation of these code in capturing flow instability is of practical interest. The current work tests the performance of a code, the Adaptive System Thermal-hydraulics Version 3 (ASYST VER3), in reproducing periodic limit-cycle oscillations induced by flashing instability on a low-pressure natural circulation facility. With prescribed periodic boundary conditions, single-channel simulations are conducted to investigate the code capability in correctly modeling two-phase phenomena. Discrepancies between the simulation and experiment are checked, leading to further analysis of potential modeling defects. In general, this work presents the simulation practices and validation attempts of a thermal hydraulic code, ASYST, against a benchmark dataset composing of periodic flow oscillations from flashing instability.