Quantification of Solid Mixing in Bubbling Fluidized Beds via Two-Fluid Model Simulations

Abstract

The influence of particle density and fluidization velocity on the mixing behavior of Geldart B particles was investigated in bubbling fluidized beds for various operating conditions using a two-fluid model approach. The simulation results proved that the predicted solid flow pattern for a wide range of Geldart B particles differs from particles of Geldart B/D classification at identical fluidization numbers. Quantitative analysis of the predicted solid velocity variances revealed that the solid axial dispersion rate is higher than the lateral one. In detail, a Peclet number of 1.54 and 3.92 was predicted for axial and lateral mixings, respectively. Also, the solid dispersion and diffusion coefficients were linearly correlated with the excess gas velocity for a range of particle properties. Finally, the characteristic global particle mixing times were computed. Thereby, the characteristic mixing time can be correlated with the particle Froude number, expanded bed height, and solid dispersion coefficient with high accuracy.

Capability of the TFM Approach to Predict Fluidization of Cohesive Powders

“…Micromixing is associated with mixing due to the fluctuation of solid particles’ velocity, quantified by granular temperature. 27 On the other hand, macromixing is attributed to solids mixing due to the motion of bubbles in the fluidized bed, quantified by the solids velocity variance. 27 More information about the mixing can be found in our previous works.…”

“…Therefore, the variance of the hydrodynamic velocity is of high relevance to quantify macromixing. 33 In this regard, the solids velocity variance tensor was computed using the following equation in each computational cell 27 where v ® and v ( t k ) represent the time-averaged and instantaneous solids velocity in the computational cell, respectively. ⊗ denotes the dyadic product.…”

“…In this section, the effect of particle cohesiveness was evaluated on the mixing quality at the particle level (micromixing) and global level (macromixing). Micromixing is associated with mixing due to the fluctuation of solid particles’ velocity, quantified by granular temperature . On the other hand, macromixing is attributed to solids mixing due to the motion of bubbles in the fluidized bed, quantified by the solids velocity variance .…”

“…On the other hand, macromixing is attributed to solids mixing due to the motion of bubbles in the fluidized bed, quantified by the solids velocity variance . More information about the mixing can be found in our previous works. − …”

MP-PIC Simulation on Solid Dispersion in a 350 MW CFB Boiler

“…Differing from the CFD-DEM model, the two-fluid model (TFM) is also applied. ,− Hernández-Jiménez et al used the TFM to study solid dispersion by setting two types of solid phases and evaluating the mixing completion level according to the relative volume fraction of two solid fluids. They found that it took little time to reach complete mixing with smaller particle sizes.…”