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.