CFD simulation of concurrent-up gas-solid flow in circulating fluidized beds with structure-dependent drag coefficient

Meso-scale structure existing in the form of particle clusters or strands in concurrent-up gas-solid two-phase flow has been extensively corroborated in experimental research. However, its significant effects on interfacial drag coefficient are seldom taken into account in current computational fluid dynamic (CFD) simulations by using the two-fluid models. The energy-minimization multi-scale (EMMS) approach, in which the heterogeneous structure is described by the so-called multi-scale resolution and energy-minimization method, is adapted in this study for investigating the dependence of drag coefficient on structure parameters. The structure-dependent drag coefficients calculated from the EMMS approach are then incorporated into the two-fluid model to simulate the behavior of the concurrent-up gas-solid flow in a riser. Simulation results indicate that the simulated solid concentration with the Wen and Yu/Ergun drag correlations is rather dilute, leading to a more homogenous structure; while the dynamic formation and dissolution of clusters can be captured with the drag correlations derived from the EMMS approach, and the simulated outlet solid flux and voidage profile in both radial and axial directions are in reasonable agreement with experimental results, suggesting the feasibility for the EMMS approach to be used as a sub-grid closure law for drag coefficient. (C) 2003 Elsevier B.V. All rights reserved.