It has been proposed to introduce a steam injector into boiling water reactors as a feed water heat exchanger and a safety injection pump. In the present paper, the heat transfer characteristics in the steam injector were examined. The nozzle size of the water jet in the steam injector that was used in the present experiments was 5 mm. The length of the mixing section of the water jet and the steam flow in the injector was 53 mm. Subcooled water was supplied to the water nozzle. Saturated steam at approximately 0.1 MPa was also supplied to the mixing section of the steam injector. Water jet velocities tested in the present experiments were in the range of 9.7 ∼ 21 m/s. The velocities corresponded to the Reynolds number of 5.9×104 ∼ 1.5×105. Radial and axial temperature distributions of the water jet in the steam injector were measured. Velocity distributions of the water jet were also measured. From the measured temperature and the velocity distributions, heat exchange rates from the steam flow around the water jet to the water jet were derived. The obtained results indicated that the heat exchange rates were greatly larger than those of usual turbulent flow in a pipe. A flow state of the water jet was also visually examined. The results of the visual observation revealed that the interface between the water jet and the steam flow was very wavy. It was supposed that the wavy motion on the water jet surface created the effective-large-internal circulation flow in the water jet, which resulted in the tremendously effective heat transport into the center portion of the water jet. From the pictures of the water jet surface recorded by a high speed video camera, characteristics of waves on the surface; the wave height, the wave velocity, the wave length and the wave frequency, were obtained. The heat transfer of the water jet in the steam injector was correlated with the wave characteristic properties. The heat transfer of the water jet was also analyzed by using the commercial CFD code of STAR-CD. When the wavy interface was introduced into the STAR-CD code analysis, the radial heat transport was drastically improved. This analytical result also supported that the tremendously more effective radial heat transport than that of the usual turbulent flow was caused by the wavy motion of the water jet surface.

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