Experimental studies of forced, combined and natural convection of water in vertical nine-rod bundles with a square lattice

Abstract

Experiments of upflow- and downflow-forced turbulent and laminar convection, natural convection and buoyancy-assisted combined convection of water are performed in uniformly heated, square arrayed, nine-rod bundles having P/D ratios of 1.25, 1.38 and 1.5. In the experiments, Re varies from 250 to 3 × 10 4, Pr from 3 to 9, Ra q from 5 × 10 5 to 3 × 10 8 for natural convection and from 10 7 to 7 × 10 8 for combined convection, and Ri from 0.03 to 300. The heat transfer data are correlated in the respective convective regimes, where the heated equivalent diameter is used as the characteristic length in all dimensionless quantities and water properties are evaluated at the mean bulk temperature. The forced convection data fall into two distinct regimes: forced turbulent and forced laminar; the Reynolds number at the transition between these regimes is a linear function of P/D. In the forced convection experiments the flow is hydrodynamically developing, but thermally fully developed. The natural convection data are correlated to within ± 10% in terms of Ra q and the combined convection data are correlated to within ± 15% by superimposing the Nusselt numbers, raised to the fourth power, of forced laminar and natural convection. For all P/D values, the transition from forced laminar to combined convection occurs at Ri = 2.0. A comparison with triangularly arrayed rod bundles shows that for the same flow area per rod, the rod arrangement negligibly affects Nu in both forced and natural convection regimes.