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A predictive theory has been developed for rod bundle frictional pressure drop characteristics under laminar and transitional mixed convection conditions on the basis of the intraassembly and intrasubchannel flow redistributions due to buoyancy for a wide spectrum of radial power profiles and for the geometric arrangements of practical design interest. Both the individual subchannel correlations and overall bundle design correlations have been formulated as multipliers applied to the isothermal friction factors at the same Reynolds numbers. Standard and modified subchannel friction factors have been obtained to be used with spatial-average and bulk-mean densities, respectively. A correlating procedure has been proposed to assess the effects of interacting subchannel flows, developing mixed convective flow, wire wrapping, power skew, rod number, and transition from laminar flow. In contrast to forced convection behavior, a strong rod number effect is present under mixed convection conditions in bundle geometries. The results of this study are of design importance in natural circulation conditions because the mixed convection frictional pressure losses exceed the corresponding isothermal values at the same Reynolds numbers.

An experimental study has been conducted to validate the predictive models and correlations for laminar and transition flow frictional pressure loss in vertical rod bundles under mixed convection conditions. An experimental procedure has been developed to measure low differential pressures under mixed convection conditions in 19 heated rod bare and wire-wrapped assemblies. The proposed model has been found successfully to predict the effects of wire wrapping, power skew, transition from laminar regime, developing and Interacting flow redistributions, and rod number on the friction loss characteristics in bundle geometries over the bundle average Grq/Re number range of 6 to 18,500.