The supersonic steam injector, one of the innovative technologies has been developed as a candidate of the most important component for next-generation nuclear reactors. The supersonic steam injector has functions of a passive pump without a large motor or turbo-machinery and a high efficiency heat exchanger. The performances of the supersonic steam injector as a pump and heat exchanger are dependent on direct contact condensation phenomena between a supersonic steam and a subcooled water jet. In the present study, the supersonic steam injector, which is made of visible transparent material and equiped with the measurement instrumentations for both axial/vertical temperature and velocity, is used to obtain the axial pressure distributions, the spatial temperature and velocity distributions in the supersonic steam injector. The visual observation on the interface of a water jet steam was made by a high-speed movie. The observation results reveal that there exist the condensation shock wave at the throat, the water jet surface fluctuation, and that the entrainment occurs on the water jet surface. The radial temperature and velocity distributions exhibit that the temperature and the velocity in the water jet are mostly symmetrical and higher at the downstream.
