Effective particle diameters for simulating fluidization of non‐spherical particles: CFD‐DEM models vs. MRI measurements

Abstract

Computational fluid dynamics—discrete element method (CFD‐DEM) simulations were conducted and compared with magnetic resonance imaging (MRI) measurements (Boyce, Rice, and Ozel et al., Phys Rev Fluids. 2016;1(7):074201) of gas and particle motion in a three‐dimensional cylindrical bubbling fluidized bed. Experimental particles had a kidney‐bean‐like shape, while particles were simulated as being spherical; to account for non‐sphericity, “effective” diameters were introduced to calculate drag and void fraction, such that the void fraction at minimum fluidization (εmf) and the minimum fluidization velocity (Umf) in the simulations matched experimental values. With the use of effective diameters, similar bubbling patterns were seen in experiments and simulations, and the simulation predictions matched measurements of average gas and particle velocity in bubbling and emulsion regions low in the bed. Simulations which did not employ effective diameters were found to produce vastly different bubbling patterns when different drag laws were used. Both MRI results and CFD‐DEM simulations agreed with classic analytical theory for gas flow and bubble motion in bubbling fluidized beds. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2555–2568, 2017

Analysis of the effect of small amounts of liquid on gas–solid fluidization using CFD‐DEM simulations

“…Rolling friction is not accounted for in the present simulations. A Hertzian contact model was used; details of the contact mechanics equations are provided elsewhere …”

“…As described elsewhere, “effective particle diameters” were used in this work, such that the minimum fluidization velocity and the void fraction matched experimental results for dry systems with nonspherical particles, while spherical particles were used in simulations. In this technique, different particle diameters were used for calculations related to contact mechanics, void fraction, and drag force to mimic the behavior of nonspherical particles while still modeling particles as spherical.…”

“…We also vary aspects of the model to test the sensitivity of the model. Uncertainties in this model above and beyond those associated with traditional CFD‐DEM are: The use of effective particle diameters to study nonspherical particles: the effects of this modeling technique are not studied in this article, but have been studied thoroughly and validated against experiments previously The assumption of zero contact angle: this was assumed because finite contact angle would allow for liquid to bead on the surface of particles, thus requiring a model which tracks the position of liquid on the surface of particles, which would add significant computational expense.…”

“…Simulations were carried out in a fluidized bed matching that used in dry fluidization experiments by Boyce et al Similar dry simulations were conducted to validate the CFD‐DEM model used here with effective particle diameters . Parameters for the simulations are given in Table .…”

“…In this article, we present a CFD‐DEM model, which accounts for both capillary and viscous forces from pendular liquid bridges and simulates liquid bridges forming at a finite rate. We use this model to simulate fluidization behavior in a bed which has previously been characterized in a dry state using magnetic resonance imaging (MRI) and CFD‐DEM . In simulations, we investigate the effects of surface tension and viscosity on minimum fluidization velocity assessed using defluidization–fluidization curves, as well as hydrodynamics and liquid bridging behavior under bubbling fluidization conditions.…”