In a vertical rod bundle with forced upflow and a nonuniform radial power profile, the potential for flow reversal into the low power side of the bundle exists due to buoyancy forces. In this paper a criterion is developed to predict the onset of flow recirculation in rod bundles. The criterion is validated with experimental data for flow recirculation in water-cooled bare rod bundles and with the results of a standard multichannel rod bundle code. A criterion for the onset of mixed convection is also given. Thus, the boundaries between the forced convection, mixed convection, and recirculation regimes are identified. Results are expressed on a flow regime map in terms of the dominant dimensionless parameters (the Reynolds number and a buoyancy parameter B*) and as a closed-form equation.

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