DISTINCTION BETWEEN UPWARD AND DOWNWARD FLOWS IN CIRCULATING FLUIDIZED-BEDS

A simple momentum probe was used to distinguish between upward and downward flow regions in risers with diameters of 66, 97 and 150 mm. Experiments indicated that a typical core-annulus flow structure existed only above the inflection point corresponding to the axial voidage profile. In the lower dense region, due to extensive solids turbulence and mixing, no clear core-annulus flow structure was observed. In the core-annulus flow region, the dimensionless core radius increased gradually with increasing distance from the inflection point. The variations in the thickness of the downflow region with gas velocity, riser diameter and solids circulation rate were investigated. Analysis of the results indicated that the dimensionless core radii almost coincide for different operating conditions and axial positions, but at the same cross-sectional averaged solids holdup. An empirical correlation was therefore developed to describe the variation of core radius, which showed a favorable agreement between predicted and experimental data obtained by present and literature studies.