EXPERIMENTAL AND NUMERICAL INVESTIGATIONS OF SOLIDS MIXING IN A GAS FLUIDIZED BED

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Abstract
Solids mixing in a bubbling gas-fluidized bed was Investigated using two Independent methods. In the first method, distributions of solids mean velocity and bulk density, and dispersion coefficients were measured, employing the single particle mode of the particle tracking technique. This technique tracks the position of a single radioactive particle, dynamically identical to bed particles, by using a bank of scintillation detectors around the bed. Time-differentiation of the instantaneous tracer positions then yields the local velocities which were ensemble-averaged locally after long experimental runs to yield the mean velocity distribution. Lagrangian autocorrelation functions were obtained to determine the radial and axial dispersion coefficients, which were found to differ by an order of magnitude. These experimental data formed the basis of a numerical model of convective-diffusive mixing, formulated to predict the time-dependent concentration distribution for a given set of initial conditions. The second method employed the multiparticle tracking mode in which the downward migration and mixing of a quantity of radioactive particles Introduced at the top of the bed was monitored by the radiation detectors. The variation of the detector outputs with time served as an Indicator of the distribution of the radioactive swarm. When the results were compared with the predicted detector outputs using the model with independently obtained velocity and dispersion coefficient distributions, good agreement was found, especially at higher fluidization velocities.