In the steam generator of the prototype FBR (Fast Breeder Reactor) in Japan, heat exchange tubes of helical coil type are utilized. The gas-liquid two-phase flows in the helical coil tube have different characteristic from straight tubes due to the effects of centrifugal acceleration in the curved tubes. In our study, the interfacial structure of the gas-liquid two-phase flows in the helical coil tube is visualized to provide the flow pattern map. Simultaneously, the pressure loss and its local fluctuation are measured in order to investigate the dynamic characteristics of the two-phase flow appearing in the helical coil tube. The result reveals that the bubbly flow regime extends and the stratified flow vanishes compared the gas-liquid flow in a horizontal straight tube. Moreover, the slug flow has asymmetric structure due to the effect of centrifugal acceleration. On the contrary the pressure loss is basically not remarkably different from the straight tube except the fact that the pressure has a high fluctuation component.

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