STUDY ON OPERATING CRITERIA OF THE SUPERSONIC STEAM INJECTOR

Supersonic steam injector (SI) is a passive jet pump with a venturi structure which operates by condensation of the supersonic steam flow in a subcooled water jet. There are advantages that a SI which operates without electric power has high heat-transfer performance and discharges water at high pressure. Therefore the SI is expected to be applied to the safety system which is able to condense steam efficiently and inject water into a core reactor when severe-accident occurs in a nuclear power plant. According to the previous studies about a SI, there are two conditions about the definition of a SI, one is formation a water jet in the mixing nozzle, the other is stopping drainage from the drain. The 1-dimension analysis model, which the steam condensed completely, at the mixing nozzle, is developed by Narabayashi et al. (1996). Abe et al. (2011) quantified the interfacial behavior of water jet with high resolution observation in the mixing nozzle and evaluated correlation between the interfacial behavior and heat and momentum transfer characteristics. However the operating limit of a SI which is taken account of flow structure at the diffuser is not cleared. The objective of the present study is to reveal the effects of two-phase flow behavior in the diffuser on the operating limits of the SI. In the present study, static pressure distributions along flow direction were measured in the SI with change about the back pressure. The discharge pressure was measured and compared with the value of discharge pressure predicted by 1-dimention analysis model. On the other hand, the discharged flow in the diffuser was observed with a high speed camera. According to the measurement of pressure, a significant pressure rise in the diffuser was measured as the back pressure increased. In addition, the value of the discharged pressure just before the SI becomes inoperative by the back pressure rise was defined as the maximum discharge pressure. It is suggested that this maximum discharge pressure is one of the indicators which predicts the operating limit of the SI. From visualization result of the diffuser, it was confirmed that there is propagation of white color. The void fraction in the discharge flow was estimated at about 2% from the velocity of the propagation. From the experimental results, effect of two-phase flow behavior of discharged water jet on the operating limits of a SI is discussed.