Graduate Thesis Or Dissertation
 

Heat transfer studies in fine particle fluidized beds

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  • Two new experimental techniques are presented which enable the direct measurement of heat transfer coefficients in fine particle gas fluidized beds. The first technique uses fine electrically heated Alumel wires to probe fluidized beds of fine particles. The wires varied in diameter from 50.8 to 813μm while the bed material consisted of uniformly sized sand, glass, aluminum and polyethylene particles from 105 to 754μm. The wires are loose and hence are free to move around and sample the cross-section of the bed. By measuring the voltage and current flow in the wire the temperature and hence heat transfer coefficient can be determined. The heat transfer coefficients obtained varied from 262 to 2100 W/m².K and these were successfully correlated by a new equation. The heat transfer coefficients obtained for fine wires in fine particle fluidized beds are greater than for fine wires in air alone. The significance of this enhancement in heat transfer is discussed. The second technique uses the change in magnetic properties of a low Curie-point ferrite to track the changing temperature of a cold sample of particles injected into a hot fluidized bed. The hot fluidized bed is surrounded by a detector coil which is connected to an electronic sensor. This coil-sensor arrangement is capable of detecting small changes in the magnetic properties of the material in the fluidized bed. A model was developed to take into account the effects of particle size distribution, mixing, equipment lag, and changing magnetic permeability on the output response of the ferrite sensor. By comparing the data and model the heat transfer coefficient could be evaluated. The size of particles studied varied from 125μm to 355μm while the heat transfer coefficients obtained varied from 70 to 230 W/m².K. The heat transfer coefficients evaluated using this technique are greater than the previously reported values but lie below those predicted for a single particle in air alone. A qualitative discussion is presented that supports the argument that the true fluid-particle heat transfer coefficients for fine particle fluidized systems lie above the correlation for single spheres in air.
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