Radial Dispersion Coefficient Research Articles

Dispersion of tracer gas supplied at the distributor of freely bubbling fluidised beds

The dispersion of tracer gas, continuously injected beneath an isolated area of the distributor plate, has been determined from time averaged concentration measurements in freely bubbling three dimensional air fluidised beds. The concentration profiles at different levels and for a series of flow rates have been plotted dimensionlessly showing them to be substantially alike in shape, this plot has been well fitted with a simple expression [1]. The tracer dispersion at points within the bed has been successfully predicted using a single phase model, depending mainly on the radial dispersion coefficient. The radial dispersion coefficient has been found to be roughly constant at all levels in the bed (except near the bottom where it is high), but increases rapidly, from a value near molecular diffusion, with an increase in the excess gas flow ( U — U m.f.). Within the experimental range considered, it appears that the Peclet number is inversely proportional to the average bubble diameter.

Fluid flow in packed beds

Velocity profiles and turbulence parameters were measured with a hot-wire anemometer in the voids of a packing of rhombohedrally arranged spheres. Data were taken at Reynolds numbers of 4780 and 7010, based on particle diameter and superficial velocity. Turbulence energy spectra showed that eddy shedding did not occur inside the packing voids. Eddy diffusivities in the voids were much smaller than either the radial or axial dispersion coefficients. Therefore the dispersion coefficients are not directly, related to the eddy diffusivity in the flow region. The mean voidage over the whole cross-sectional area of the packing was kept constant. Under these conditions, the mean velocity still showed a maximum value within 1·5 particle diameters from the wall. Comparison of the mean velocity profiles and spectra inside the voids with similar data determined downstream from the packing indicated that there is no simple correlation between them.