HEAT-TRANSFER IN FIXED-BEDS AT VERY-LOW (LESS-THAN-4) TUBE-TO-PARTICLE DIAMETER RATIO

New heat transfer measurements are reported for packings of full and h ollow cylinders in tubes over a tube-to-particle diameter ratio (N) ra nge of 1.8-5.6. Both high and low thermal conductivity packings were u sed. These results are analyzed in conjunction with previously-reporte d 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 pictur e 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 criti cally evaluated with regard to their dependence on N, to determine whe ther their range of applicability extends to N < 4. The effective radi al thermal conductivity k(r) and wall heat transfer coefficient h(w) d epend on N less strongly for full and hollow cylinders than they do fo r spheres. For spheres, there is evidence of high rates of radial heat transfer as N approaches unity, and the bed behaves as a single pelle t string, but for 2 less than or equal to N less than or equal to 4, k (r) is highly dependent on the specific value of N.