MESUREMENT OF SOLIDS CONCENTRATION PROFILE AROUND THE PERIPHERY OF AN IMMERSED TUBE IN A THREE-DIMENSIONAL GAS-FLUIDIZED BED

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Abstract
To adequately describe the heat transfer process at the surface of an immersed tube in a gas-fluidized bed, it is imperative to know the solids concentration at the tube surface. The knowledge of particle dynamics will also enable to establish the likely metal erosion and thus the heat exchanger life. However, determination of such parameters is not straightforward and here we describe the development of an image carrying fiber optic probe with a potential to generate such information. Using such a borescope in conjunction with a CCTV camera, TV monitor and video-cassette recorder, solids concentration profiles have been measured around the periphery of horizontal tubes (outside diameters 11 and 28 mm) immersed in an air-fluidized bed of 1.4 mm average diameter glass beads. In both the cases, it is found that the bulk bed voidage is considerably smaller than the voidage at the tube surface. The latter also varies with the location around the tube periphery. The smallest values are found for the downstream side, largest values for the equatorial sides, and intermediate values for the upstream side. These values are dependent on the fluidizing velocity and the tube diameter. Estimate are also given of the air film thickness at various locations between the tube and the particles.