During full power operation of Pressurized Water Reactors (PWR), heat transfer phenomena of subcooled nucleate boiling may occur on the surface of the fuel rods. Despite high subcooling, this behavior results from the high heat flux up to 100 W/cm2 where vapor bubbles condensate when they are detached from the rod surface. In case of an accident with disturbance of cooling during transition from bubble to film boiling the critical heat flux (CHF) can be reached.

This paper outlines the experimental investigation of heat transfer during subcooled flow boiling on a capillary tube. To investigate the heat transfer processes under these boiling conditions, a test facility for flow boiling with access for optical measuring methods was constructed. The temperature is measured with a thermocouple inside the tube while boiling bubbles are generated on the outside. For different subcooling and flow velocity the heat flux is increased in a range from zero up to approximately 115 W/cm2. The major aims of these investigations are to generate a database for modeling of these dependencies in computational fluid dynamic (CFD) codes and enhance the knowledge of phenomenological effects of subcooled flow boiling. This provides a contribution for the prediction of the critical heat flux with simulation codes.

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