Steam injector (SI) is a passive jet pump with a converging-diverging structure. A SI operates without an electrical power source by direct contact condensation of a supersonic steam flow and a subcooled water jet in a mixing section. Furthermore, there are advantages that a SI has high heat-transfer performance and discharges water at high pressure. Therefore a SI is expected to apply to the safety system that is able to condense steam efficiently and inject water into a core reactor when severe-accident occurs in a nuclear power plant. However it is not cleared about the operating range of a SI which is taken account of the discharged flow structure.

The objective of the present study is to reveal the influence of two-phase flow behavior on operating limits of a SI. The test section of the SI is made by transparent material to observe flow structure in it. The pressure distributions along the flow direction and the discharge pressure were measured by changing the inlet steam pressure and a load on exit of the SI. At the same time, the discharged flow at the diffuser was observed with a high speed camera.

From the observation results, it was confirmed that a boundary which the flow structure changed in the discharged flow. The area from the throat to this boundary is seemed a two-phase region that steam which has not been condensed completely in the mixing nozzle remains. It was found that this boundary moved to upstream as the load on the exit increased and significant pressure rise occurred at the position the boundary reached. Additionally white propagations toward downstream were observed. This propagation is seemed a pressure wave propagation. The velocity of the propagation was estimated by image processing. Assuming a pressure wave propagates at sonic speed, void fraction at the discharged flow was estimated by existing homogenized model.

From the above, the influence of two-phase flow in discharged flow on the operating limits of the SI is discussed.

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