The generality of the standard 2D TFM approach in predicting bubbling fluidized bed hydrodynamics

Abstract

Hydrodynamic simulations of a pseudo-2D bubbling fluidized bed were carried out and compared to experiments conducted over a wide range of flow conditions. The primary purpose of this study was to assess the generality of the standard 2D Two Fluid Model (TFM) closed by the Kinetic Theory of Granular Flows (KTGF) which is regularly used in the literature to simulate bubbling fluidized beds. Comparisons of the bed expansion ratio over wide ranges of fluidization velocity, bed loading and particle size showed systematic differences between simulations and experiments, indicating that the generality of this modelling approach is questionable. More detailed flow velocity measurements collected via Particle Image Velocimetry (PIV) showed that the model greatly over-predicts flow velocities in the bed. Subsequent 3D simulations showed this over-prediction to be the result of 2D simulations neglecting the wall friction at the front and back walls of the pseudo-2D bed.

The effect of frictional pressure, geometry and wall friction on the modelling of a pseudo-2D bubbling fluidised bed reactor

“…The smallest affordable grid size of 2.5 mm was used in a uniform structured mesh. As was previously shown by the authors [6], this grid size is sufficiently fine for all but the finest particle sizes considered in this study.…”

“…One of the most cited studies about the validation of the 2D TFM approach in a bubbling fluidized bed [10] used a pseudo-2D experimental setup and found good comparisons with regard to bed expansion ratio and local solids volume fraction profiles measured with an optical probe. In our previous study [6], bed expansion ratios over a range of fluidization velocities were evaluated and it was confirmed that the simulated bed expansion ratios mirrored experimental observations almost exactly.…”

“…For example, it has recently been found that the friction on the large front and back walls of a pseudo-2D fluidized bed has a very large influence on the solids velocity observed in the unit. When simulating a pseudo-2D unit on a 2D plane using the standard TFM, it was found that the mean solids velocities occurring inside the reactor can be over-predicted by a factor of four [6]. This very large discrepancy was attributed to the neglected friction between the particles and the large front and back walls in the 2D simulation and this will be further studied in this work over a range of fluidization velocities, particle sizes and bed loadings.…”

Use of α -shapes for the measurement of 3D bubbles in fluidized beds from two-fluid model simulations

“…energy production equals energy dissipation. This assumption is often used in dense, slow moving fluidized beds where the local generation and dissipation of granular temperature far outweigh the transport by convection and diffusion [44]. Lastly, the solids pressure was…”

“…Regarding the particle-wall interaction, the proper selection of the wall boundary condition for the solids phase is not straightforward for EulerEuler models and different formulations can be found in the open literature: no-slip, partialslip and free-slip, e.g. [44,45] conditions. Although it seems that the partial-slip is the most realistic approach, the selection of a proper specularity coefficient is not trivial and needs to be done carefully.…”