Experiments of heat transfer at low flow rate are performed in a 5×5 square arrayed rod bundles. The diameter of the rod is 10mm with a pitch of 13.3mm, length of the test section is about 3 meters. Inlet Reynold number ranges from 2000 to 30000, Bo * ranges from 4×10−6 to 5×10−3. The rods are heated using a DC power, the heat flux ranges from 30 to 300 kW/m2. The experiment is aimed to investigate the buoyancy effect of mixed convection in rod bundles. The experimental data shows that similar with mixed convection in circular channels, buoyancy force has great effect on heat transfer at mixed convection regime in rod bundles. But the buoyancy effect appears at higher Bo* conditions. The spacer effect have also been investigated at both turbulent forced convection regime and mixed convection regime. The reconstruction of heat transfer downstream of spacers is different at different flow regimes, a reasonable explanation was provided.

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