Distribution of the volume fraction of several discrete particle sizes of a mixture of particles were measured in a CFB cold flow model with cross section area 0.3 m #xD7; 0.45 m and height 8 m. The corresponding circulating mass flows of the discrete phases were measured, too. The free parameters were the amount of solids (30, 50, 70 kg) and the superficial gas velocity (3, 4, 5m/s). Circulating mass flow of sand was practically zero at gas velocity 3m/s. In the theoretical modelling the transportation of solids was assumed to be the sum of time mean convection and diffusion type mixing. Using the measured data the axial diffusion coefficient and particle/gas friction coefficient correlations were calibrated to give the best possible fit between the measured and calculated values. It became clear that the bigger particles ( 0.5 mm) had remarkable time mean velocities downwards and the transportation of these particles was only due to the intensive turbulent mixing. Excellent agreement was obtained between the measured and calculated results. Erosion probes were placed at the elevations 0, 2,4,8 m. All probes experienced severe erosion were in the free flow zone outside the wall boundary layers. The probes at 4,6, 8 meters elevations had practically no erosion within 40 mm distance from the vertical walls. Some attempts were made to measure the descending wall flows which have been generally regarded as characteristic for CFB systems. No experimental evidence on the existence of massive descending time mean wall flows was found.
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