Heat Transfer in Fixed Beds at Very Low (<4) Tube-to-Particle Diameter Ratio

New heat transfer measurements are reported for packings of full and hollow cylinders in tubes over a tube-to-particle diameter ratio ( N ) range of 1.8−5.6. Both high and low thermal conductivity packings were used. These results are analyzed in conjunction with previously-reported data for spheres in the range 1.13 < N < 6.4 and data for cylinders and rings in the range 5.2 < N < 6.9 to provide a comprehensive picture of heat transfer in the region of very low N (< 4) and a comparison to data at higher N . Single-phase heat transfer correlations are critically evaluated with regard to their dependence on N, to determine whether their range of applicability extends to N < 4. The effective radial thermal conductivity k r and wall heat transfer coefficient h w depend on N less strongly for full and hollow cylinders than they do for spheres. For spheres, there is evidence of high rates of radial heat transfer as N approaches unity, and the bed behaves as a single pellet string, but for 2 ≤ N ≤ 4, k r is highly dependent on the specific value of N .