Local Structure Effects on Heat Transfer in Very Low Tube-to-Particle Diameter Ratio Fixed Beds of Spheres

New fixed-bed radial heat transfer experimental data are reported for beds of perfect spheres for tube-to-particle diameter ratio (N) in the range of 1.38–6.29. These results are combined with a re-analyzed smaller data set for spheres in the range 1.13 ≤ N ≤ 6.4 from an earlier publication. Very strong local bed structure effects were found for the estimates of effective radial thermal conductivity and apparent wall heat transfer coefficient, giving maximum and minimum values at particular N, which departed from the overall trend. The extreme values often corresponded with fluctuations of the void fraction ε, but they could not be explained by the bulk void fraction alone. Local bed structures resulted in changes in the flow pattern. Standard correlations can predict only average changes and trends, due to their neglect of local structure, and may be considerably in error for any particular N.