(260d) Fine-Grid Simulations of Gas-Solids Flow In a Circulating Fluidized Bed

Abstract

It
has been known for more than a decade that fine heterogeneous structures in the
form of clusters and streamers increase the slip velocity in gas-driven flows
of small particles [1]. We have shown the necessity to include sub-grid scale
closures with coarse-grid simulations of gas-solids flows in order to
accurately predict solids segregation and flux profiles [2] as well as axial
pressure gradient profiles [3] in circulating fluidized beds.

Fine-grid
simulations of a small-scale bubbling fluidized bed of Geldart type A material
showed that a grid size in the order of few particle-diameters is essential to
predict the correct bed expansion due to the formation of small bubbles that
were not resolved with coarse-grid simulations [4]. In the present study, grid
refined computations are conducted on a much larger riser section of a
circulating fluidized bed to demonstrate accurate predictions of continuum
(two-fluid) models with respect to gas pressure drop and solids hold-up
measurements. A major hurdle in conducting these transient simulations is the
requirement to obtain long time-averaged numerical data to compare with
experimental measurements. Simulations with a maximum of about one million
computational cells, translating into a grid size of 1 mm with a grid-size
ratio of one, were conducted in this study and showed clear improvement in the
two-fluid model (using a standard homogeneous drag law) predictions of the gas
pressure drop vertical profiles.

References

1.  
Agrawal
K, Loezos PN, Syamlal M, Sundaresan S. The role of meso-scale structures in
rapid gas-solid flows. J. Fluid Mech. 2001; 445: 151-185.

2.  
Benyahia
S. On the Effect of Subgrid Drag Closures. Ind. Eng. Chem. Res. 2010;
49: 5122-5131.

3.  
Benyahia
S. Analysis of Model Parameters Affecting the Pressure profile in a Circulating
Fluidized Bed. AIChE Journal. Accepted for publication, doi:
10.1002/aic.12603.

4.   
Wang
J, van der Hoef MA, Kuipers JAM. Why the two-fluid model fails to predict the
bed expansion characteristics of Geldart A particles in gas-fluidized beds: A
tentative answer. Chem. Eng. Sci. 2009; 64: 622-625.