In a fixed or fluidized bed at low particle Reynolds numbers, the overall or effective Sherwood and Nusselt number has been found by many investigators to be much less than unity. The limiting value of the particle Sherwood or Nusselt number based on local concentration or temperature differences is shown to be equal to or greater than unity. An analytical model was established using realistic packed bed geometries to allow for diffusion in the flow direction, channeling due to nonuniformities in bed voidage and different particle sizes, and inaccuracies in the experimental measurements. The predicted values of the effective Sherwood and Nusselt numbers are found to agree closely with experimental measurements for gases and liquids. Diffusion is shown to be the primary mechanism for the fall-off in the effective bed characteristics.

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