Oke, OS; (2016) Numerical investigation of homogeneous expansion and lateral solid mixing in gas-fluidized beds. Doctoral thesis , UCL (University College London).

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Abstract

This research project is concerned with the numerical investigation of the behaviour of homogeneous and bubbling gas-fluidized beds. We started by investigating the homogeneous expansion of gas-fluidized beds using the Richardson & Zaki (1954) equation. We modelled the stable expansion of gas-fluidized beds of different diameter, accounting for enduring contacts among particles and wall effects. We solved the model numerically to obtain the bed expansion profiles, back-calculating from them the values of the expansion parameter n. To validate our model, we carried out fluidization and defluidization experiments, analyzing the results by means of the Richardson & Zaki equation. We obtained a reasonable agreement between numerical and experimental findings; this suggests that enduring contacts among particles, which are manifestations of cohesiveness, affect homogeneous bed expansion. The results showed that homogeneous gas-fluidized beds do not consist of particles floating freely; rather they are made up of particles in sustained frictional contacts. We then investigated the process of lateral solid mixing in bubbling fluidized beds, adopting the Eulerian-Eulerian modeling approach. To quantify the rate at which solids mix laterally, we used a lateral dispersion coefficient (Dsr). The values of Dsr obtained numerically are larger than the experimental ones, within the same order of magnitude. The overestimation has a twofold explanation. On one side, it reflects the different dimensionality of simulations (2D) as compared with real fluidized beds (3D), which affects the degrees of freedom of particle lateral motion. On the other, it relates to the way frictional solid stress was modelled. To investigate how sensitive the numerical results are on the constitutive model adopted for the frictional stress, we ran the simulations again using different frictional models and changing the solid volume fraction at which the bed is assumed to enter the frictional flow regime (Φmin). We observed that Dsr is quite sensitive to the latter. The results show that accurate prediction of lateral solid dispersion depends on adequate understanding of the frictional flow regime, and accurate modelling of the frictional stress which characterizes it. We further examined the influence of simulation dimensionality in numerical results. We ran 3D CFD simulations using the same powder, the same operational conditions and the same computational setup as in the previous 2D simulations. The 3D simulations predicted Dsr more accurately than the 2D simulations, the simulation dimensionality appearing to be an important factor. To analyse further the role of frictional stress models, we ran 3D DEM simulations. The simulation results agreed reasonably well with the empirical data, but their accuracy depended on the values used for the collision parameters; also, the 3D CFD simulations matched the empirical data more closely. Altogether, we thus concluded that the simulation dimensionality plays a dominant role in predicting Dsr accurately.

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