Spatial dependence of local heat transfer coefficients in liquid metal sub-channel flows

CFD code, FLUENT was applied to investigate the spatial dependence of local rod surface heat transfer coefficients of liquid sodium and liquid lead flows in a triangular sub-channel. As a result of high thermal conductivities in liquid metals, their thermal boundary layers are thicker than ordinary fluids. Their developments are then dependent on the available void spaces inside the sub-channel. This means that local surface convective heat transfer coefficients are not uniform around the fuel rod, resulting non-uniform surface temperature distributions in the azimuthal direction. Our studies show that higher local heat transfer coefficients are observed at surfaces whose facing channel widths are narrower. The ratios of maximum-to-minimum heat transfer coefficients are 2.66, 1.92, 1.57 for pitch-to-diameter ratios of 1.1, 1.2, 1.375 respectively. Such spatial dependence is quite apparent for low P/D's. (authors)