The helically coiled tube of heat exchanger is used for the evaporator of prototype fast breeder reactor “Monju”. This report aims at the grasp of relation between two-phase flow phenomena and heat transfer characteristics of forced convective boiling of water inside helically coiled tube, especially focusing on oscillation of transition point. A transparent double tube made of glass is used as a heat exchanger. Water flows up inside helicial tube and the high temperature oil flows down in the outside tube. The oscillation of the transition point was observed, that is mainly caused by intensive nucleate boiling near the dryout point and evaporation of thin liquid film flowing along the surface. Fluctuations of local heat flux, local temperature, pressure and flow velocity can be explained with relation to visualized flow situations.
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