Supersonic steam injector is a passive jet pump which operates without rotating power source or machinery and it has high heat-transfer performance due to the direct contact condensation between supersonic steam flow and subcooled water jet at the mixing nozzle. Since the supersonic steam injector has a quite simple and compact structure, it has been considered to apply to the safety system for the Next-generation nuclear power plant. There are various researches about the formulation and modeling of operating, flow structure and heat transfer characteristics of both vapor and liquid flow. However, there are few models which are capable of evaluating heat and momentum exchange at the boundary layer between supersonic steam flow and water jet. Since heat and momentum exchange is considered to have a major impact to operating characteristics of a supersonic steam injector, it is necessary to formulate the model which simulates such complex phenomena at the boundary layer with high accuracy. The objective of the present study is to investigate the relation between the thermal characteristics and interfacial behavior between the flows to develop a model which is able to assess the heat and momentum transfer characteristic and the flow structure of the supersonic steam injector in detail. In the present study, a visible test section of water jet-centered supersonic steam injector was adopted to conduct visualization of the water jet with high speed video camera. In addition, special measurement instrumentations of temperature and pressure were applied to obtain radial distribution of temperature and pressure in the mixing nozzle of the injector. There were large velocity and temperature gaps between the water jet and the supersonic steam flow which indicated the existence of large momentum and heat exchange at the boundary layer of the flows. It was clarified that there was pressure gradient which was considered to stabilize the water jet in the mixing nozzle from calculation of radial distribution of total pressure gradient. From the visualization measurement, it was also clarified the existence of a complex wavy behavior on the surface of the water jet. The wave velocity was estimated by the image processing technique and the cross-correlation method. It was found that there was a relation between the wave velocity and heat transfer characteristics in the supersonic steam injector. It is suggested that the enthalpy ratio between the liquid subcool enthalpy and steam condensation enthalpy as well as the Jacob number between both flows could be an indication factor for the effect of the wavy behavior on the condensation.

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