A study of liquid entrainment in oscillatory flows

In a large break loss of coolant accident (LOCA) in a pressurized water reactor (PWR), a break may occur on one of the cold legs. The emergency core cooling system (ECCS) must provide sufficient coolant to the core to remove decay heat and prevent the cladding from exceeding 1477.6 K. During reflood, flow to the core is most often gravity driven, resulting in an oscillatory delivery of coolant to the core. These oscillations are attributed to vapor generation in the core and the dynamic response of the downcomer water level. The amount of liquid entrained can be significant because in an accident scenario, the progress rate will be slowed and it can take longer to fully recover the core. Most reflood experiments have been conducted with constant forced reflood rates, and have not explored in detail the effects of oscillations on liquid entrainment and rod bundle heat transfer. The few studies that have been conducted for oscillating flows indicate enhanced entrainment of liquid at the quench front. At the NRC-PSU Rod Bundle Heat Transfer (RBHT) Test Facility, an electrically heated 7×7, 3.66m long rod bundle array has the capabilities to perform both constant and oscillatory forced flooding rate experiments. For varying oscillation periods and inlet subcoolings, these system parameters’ effect the liquid entrainment has been analyzed. Using a frequency analysis and signal processing technique, the percent liquid entrainment has been determined in one up- or down-stroke for oscillatory reflood. These findings, which are important to rod bundle heat transfer during reflood transients, have been explained based on the physics of the problem.