Fluid flow and heat transfer in the entrance region of rod bundles

Asymmetry in the form of three types of eccentricities are studied. It is found that the flow is affected only in the immediate vicinity of the displaced rods. Every where else the symmetry of the flow is maintained. The fully developed as well as the developing f.Re values decrease as the eccentricity increases. Maximum decrease in f.Re values occurs at the geometry which has the maximum pressure drop. Significant changes in the development length from the symmetric case are noted only for whole bundle eccentricities.Heat transfer in the combined entrance region of symmetric and asymmetric bundles is solved. It is found in symmetric bundles the fully developed and the developing local bundle Nusselt number increases with the peripheral rod radius to a maximum after which it decreases. The maximum bundle Nusselt number occurs at a higher peripheral rod radius than the one at which the maximum pressure drop occurs. In the case of eccentric bundles, large displacement in rods leads to decrease in the fully developed and the developing local bundle Nusselt number. However, small eccentricities in bundle with peripheral rod radius smaller than the one at which the maximum bundle Nusselt number occurs, lead to slight increases in the bundle Nusselt number. Even small eccentricities affect the entire temperature field of the bundle.The effect of mixed convection in the entrance region of symmetric bundles has been studied. It was found when buoyancy aids the flow the Nusselt number increases and when buoyancy opposes the flow the Nusselt number decreases. When the mass flux through the bundle is fixed, the pressure drop ($-$dp/dz) becomes positive for aiding flows. When buoyancy is aiding the flow, the pressure recovery is able to match the buoyancy forces for smaller Gr/Re ratios only. The percentage increase in Nusselt number in buoyancy aided flows depends on whether the peripheral rods are closer to the central rod or to the channel wall. The largest increases occur when the peripheral rods are crowded near the central rod. The smallest increase occurs at a peripheral rod radius which has the largest forced convection Nusselt number. (Abstract shortened with permission of author.