Fluidization and Surface-to-Bed Heat Transfer Coefficient in Fluidized Beds of Very Fine Ni and Ni-Alloy Powders

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An attempt is made to study the fluidization and the surface-to-bed heat transfer coefficient in a fluidized bed of very fine (∼10 μm) hydrogen-absorbing alloy (HAA) powder. Considering the difficulty in handling HAA powder fluidized directly by hydrogen, experiments were carried out using helium and nitrogen. In addition, two different types of nickel powders with similar size range were also employed for the investigation using hydrogen as well as helium and nitrogen as fluidizing gases.Bed pressure drop and voidage were measured with variation of gas velocity. It is found that the three powders are similar in fluidization characteristics, although some differences can be seen in shape and surface roughness of the primary particles.Surface-to-bed heat transfer coefficients were measured by immersed probes with cylindrical surfaces heated electrically. The measurement results were compared with predictions by a very simple model based on the application of the equation for single-phase flow, in which the dimensionless Nusselt, Reynolds and Prandtl numbers describe the effective properties of gas-solid suspension calculated by empirical correlations in literature. It is shown that the predictions agree with the experimental very well when modification is made to the effective velocity.
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