Numerical Simulation of Gas−Solid Dynamics in a Circulating Fluidized-Bed Riser with Geldart Group B Particles

Abstract

Circulating fluidized-bed (CFB) risers with Geldart group B particles have found significant application in combustion reactions. The present work attempts to study the solids flow dynamics in a CFB riser that is operated with group B particles, using computational fluid dynamics (CFD) techniques. The key feature in the present study is that the various closure schemes in the CFD model have been evaluated against data from non-invasive experimental techniques: computer automated radioactive particle tracking (CARPT) for solids velocity field and computed tomography (CT) for solids holdup. Since solids flow in a riser is multiscale in character, in addition to the measured averaged solids velocity profiles and solids fraction profiles in the experimental section, mean granular temperature profiles have also been compared. Two flow regimes (viz., fast fluidization and dilute phase transport) have been considered in this study.

Prediction of Radial Distribution Function of Particles in a Gas−Solid Fluidized Bed Using Discrete Hard-Sphere Model

“…Although results from the present simulations and from eqs 37, 39, and 40 increase with the particles' concentrations, eq 39 gives some unreasonable values of the self-pair radial distributions of light and heavy particles at the low concentrations, that is, similar to the case of binary particles presented in the previous section. It is also noted that the self-pair radial distribution functions at contact, g o, 11 and g o,22 from eqs 37 and 40, are in agreement with present simulations, while the calculated mutual pair radial distribution functions g o,ij from eqs 37 and 40 are higher than that present simulations. Considering the different densities of particles in the binary mixture, we modify eq 37 and propose following equation for pair radial distribution function at contact…”

A second‐order moment method applied to gas–solid risers

“…The time-averaged mean and fluctuating solid velocity fields have been quantified using the computer automated radioactive particle tracking (CARPT) technique. Predictions by Vaishali et al 59 are, however, under-predicted distribution of concentration in the riser. The parameters used in the present computations are given in Table 3.…”

“…Figure 14 shows the radial profile of concentration of particles at the superficial gas velocity and solid mass flux of 3.9 m/s and 33.7 kg/m 2 s. Simulations show the distinctly higher concentration near the wall and also higher radial gradients. The discrepancy between the simulations predicted by Vaishali et al 59 using KTGF and experiments from Bhusarapu et al 40 is most likely because of the fact that the granular temperature models fluctuations in solid velocity under the assumption that the fluctuating kinetic energy has a Maxwellian distribution (inherent in the kinetic theory approaches). Measured concentration of particle increases from the center, reaches a maximum and then decreases.…”

“…This is due to the high gradient in the axial solid velocity. The simulations from Vaishali et al 59 are also shown in. Thus, the fluctuating motion is principally directed along the main axis of flow.…”

“…The parameters used in the present computations are given in Table 3. Simulations predicted by Vaishali et al 59 are also given in using the commercial FLUENT 6.2 code. Experiments indicate that three distinct slopes in the profiles can be identified.…”