Symposium on Advanced Sensors and Modeling Techniques for Nuclear Reactor Safety

Pressure Drop Oscillations and Ledinegg Instability in Two-Phase Flow System

The (two-phase) flow instabilities are observed in nuclear power systems, ranging from low- pressure to the supercritical power systems. These flow instabilities can affect the safe operating condition and performance of the system efficiency, leading to mechanical vibrations and thermal oscillations, and can also lead to premature critical heat flux conditions. Some well-known flow instabilities have been investigated in the earlier studies, namely, density wave oscillations (DWO), pressure drop oscillations (PDO) and Ledinegg instability or flow excursion. It is to be noted that the PDO and Ledinegg instability are observed in low pressure, as well as, in supercritical systems. The stability analysis of PDO in a two-phase flow system, using three equation models, is carried out by MATCONT software. Moreover, using the same model, most prominent static instability (Ledinegg instability) is investigated in the two-phase flow system. For analysis of the Ledinegg instability and PDO, the steady-state internal pressure drop of the heated channel is obtained by using a polynomial fit. In the stability map, the parameter space is divided into two different regions, namely, stable and unstable. In order to investigate the above regions, the time evolution of the system is carried out for different perturbations. In the presence of the PDO instability, sustained oscillations of mass flow rate and pressure (inlet pressure) of the heated channel are investigated in the two-phase flow system. On the other hand, an excursion of equilibrium mass flow rate either to a higher or a lower mass flow rate depending on the sign of perturbation are identified in the system, due to the Ledinegg type instability. The physical significance of the above circumstances is that the longtime period oscillations due to the PDO may result in the damage of the channel, and also in case of flow excursion (Ledinegg instability), the channel may burn out.