Abstract: The suppression pool is usually adopted in small pressurized water reactors. Under high pressure conditions in small reactors, unsteady under-expanded submerged steam jets are generated in the pool. During the condensation of an unsteady under-expanded jet, its internal pressure changes rapidly causing rapid transitions in the flow pattern. Meanwhile, expansion waves propagate inside the jet affecting its heat transfer and behavior. To study the problem, a visualization experimental facility was built. The facility can generate unsteady under-expanded submerged steam jets. A high-speed camera was used for visualization. Analysis of a series of images captured by a high-speed camera revealed that the flow pattern underwent transitions from an unstable underexpanded jet to a stable jet, then to bubbly flow, and finally to chugging throughout the entire discharge process. The transient heat transfer coefficient of the jet was extracted from the visualization images using an image processing method and was observed to oscillate with the periodic jet behavior. The oscillations became more intense with higher subcooling degree and higher nozzle exit pressure. Assuming that the oscillation of the heat transfer coefficient was caused by the propagation of expansion waves, a new transient heat transfer model was established by introducing the ratio of ambient pressure to nozzle exit pressure into the heat transfer model. Based on the transient heat transfer model, a criterion for the transition from the jet flow pattern to the bubbly flow pattern was developed using energy conservation. The predicted transition times were compared with those obtained experimentally confirming the reliability of the criterion. The results can provide a basis for transient safety analysis of small reactors.

Under-expanded jet,Direct contact condensation,Flow pattern transition,Transient heat transfer