Heat transfer experiments of forced turbulent and laminar and combined laminar downflows of water were conducted in uniformly heated, triangularly arranged, seven-rod bundles having P/D ratios of 1.25, 1.38, and 1.5. In the experiments Re ranged from 148 to 2.85 × 104, Pr from 5 to 9, Raqfrom 6 × 104to 8 × 107, and Ri from 0.001 to 10. The data in the forced turbulent and the laminar flow regimes are in good agreement with the upflow correlations (within ± 15%). Also, the value of Reynolds number at the transition between these two regimes, which increased linearly with P/D ratio, is the same as that for upflow. In the laminar flow regime, the flow entering the heated section, except for the P/D ratio of 1.25, was developing hydrodynamically. However, the variation in Nu with axial location was small, suggesting that the flow in the heated rod bundles could have been developed thermally. The transition from forced laminar to combined flow occurred at Ri = 0.1, which is an order of magnitude lower than that for upflow. The combined flow data are correlated by superimposing the correlations for forced laminar and natural laminar flows as:
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