A Study on Flow in a Rod Bundle Influenced Strongly by Buoyant Force

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The experimental study of the thermal plumes in a rod bundle has been performed to measure the turbulent heat flux between subchannels and to provide information on the thermal plume behavior at the gap. An Ar-ion laser was used to measure velocity and temperature simultaneously without disturbing the flow field. A fiber LDV was made and inserted into a glass pipe with a Nichrome film heater on its outer surface, through which the laser beam was introduce into the fluid. The velocity information was obtained from the light scattered from SiC scattering particles and the temperature was estimated from the change of fluorescence of Rhodamine B solution. The turbulent heat flux at the gap center was measured under the conditions of Re=50-400 and Gr_<DELTAT>=300-1300 for various power skews. The experimental results show that the turbulent heat flux obtained experimentally was for smaller than the predicted values. Therefore, for application to the sodium cooled reactors it was expected that the turbulent heat flux might be negligibly small. The turbulent heat flux increased with Re and Gr_<DELTAT>. Negative turbulent heat flux, that is, counterflux heat transfer was observed. The change of temperature and velocity with time showed that the phase difference between velocity and temperature brought the counterflux. The phase difference is considered to be a reason for smaller turbulent heat flux than the predicted values. The simultaneous measurement system for velocity and temperature used in the present study operated quite well and is expected to display its ability not only in rod bundles but also in general thermal fluid study.