Thermally induced flow oscillation in vertical two-phase natural circulation loop

Abstract

To study the two-phasenaturalcirculation during a hypothetical small break loss of coolant accident in an LWR, simulation experiments have been performed using a freon-113 boiling/condensation loop. In quasi-steady state, the flow became relatively stabilized and certain regular patterns of flowoscillations were detected with ranges of periods in 8–35 s and 2.5–4 min. To explain the nature of these oscillations, a preliminary analysis with the general expression of the characteristic equation was introduced. The characteristic equation which is a function of the residence times in each flow region can be used to obtain the stability criteria. A simplified analytical approach showed that the short-period oscillations were representative of manometric oscillations, which can also be confirmed by simplifying the characteristic equation. The longer period oscillations infer the density wave oscillation which has the oscillatory period close to the residence time of a fluid around the loop.