On the fluidization characteristics of fine powders
Powders belonging to group A of the Geldart classification have been studied in detail using the bed collapse technique. To obtain the correct properties of the powders a model is developed, which takes into account the system configuration for both one- and two-valve experiments. An experimental apparatus has been assembled and used to validate the model using glass ballotini in the size range 22-106 mum. The system allows simultaneous measurements of total bed height, using a digital camera, and pressure transients at various positions along the vertical axis. The powders have been sieved to obtain 6 size ranges and the fluidization properties have been measured. Mixtures of particles have been prepared and the effect of the size differences has been investigated. The experimental results allow the construction of plots of the void fraction vs inlet gas superficial velocity, which show a continuous transition around the minimum bubbling point, with no sudden contraction of the dense phase voidage immediately above the minimum bubbling point for all powders. The plot of 6d vs Ud yields a characteristic curve, which within the experimental uncertainty has overlapping values for systems below and above the minimum bubbling point. The minimum bubbling point was affected by the structure of the distributor plate, with a coarser sinter leading to premature bubbling. The experimental data were used to test drag force correlations reported in the literature. It was found that none of the correlations commonly used in CFD simulations describes accurately the experimental 6d and Ud measurements. Modifying the literature equations it was possible to correlate the experimental 6d and Ud curve and use this information to predict the minimum bubbling point using a CFD model recently developed at UCL. The experimental results measured in this study show a strong dependence with voidage, which has allowed the formulation of constitutive equations for the characteristic dimension of the CFD model. The resulting simple correlation of the model parameter was used to obtain a new minimum bubbling criterion and predictions are compared to an extensive database of literature values.