Coarse-Grid Simulation of Gas-Particle Flows in Vertical Risers

Abstract

Continuum model equations for unsteady gas-particle flows in devices such as fluidized beds and circulating fluidized bed risers contain unstable modes whose length scale is of the order of 10 particle diameters. Yet, because of limited computational resources, these flows are routinely simulated by solving the discretized version of continuum models over coarse spatial grids. These simulations resolve the large-scale flow structures but not the finer scale structures. In most industrial applications involving large devices, it is impractical to resolve all the fine-scale structures, and therefore the effects of the unresolved structures must be addressed through suitable subgrid models. Using gas-particle flows in a wide and very tall vertical channel as an example, we have demonstrated in this study that the results obtained in coarse-grid integration of the microscopic equations for gas-particle flows change appreciably if subgrid corrections to account for the effects of unresolved structures are included. The addition of a simple time-averaged subgrid closure for the effective drag coefficient and particle phase viscosity and pressure led to a qualitative change in the simulation results. Our simulations also revealed a lack of separation of time scales between the resolved and unresolved structures. This led us to formulate a simple stochastic subgrid closure for the drag coefficient and investigate its consequence. The addition of a stochastic correction made quantitative, but not qualitative, changes to the simulation results.

Grid independence behaviour of fluidized bed reactor simulations using the Two Fluid Model: Effect of particle size

“…An increasing body of literature focusing on the subject is already available (e.g. [8][9][10][11]). However, the additional modelling included in this filtered approach introduces a substantial degree of uncertainty simply due to the complex nature of the subgrid clustering phenomena.…”

Development, Verification, and Validation of Multiphase Models for Polydisperse Flows

“…However, in order to simulate large scale gas-particle flows in fluidized beds and risers, grids much larger than 10 particle diameters are used in order to keep the total computational time within reasonable bounds (Sundaresan, 2000). Coarse-grid simulations which ignore the consequences of the sub-grid scale flow structures tend to overestimate the fluid-particle drag force Andrews et al, 2005;Igci et al, 2008). In light of these observed differences between fine-and coarse-grid simulations of continuum models there has been a substantial amount of research devoted to the development of filtered two-fluid model equations, which are obtained by filtering the microscopic two-fluid model equations (Andrews et al, 2005;Igci et al, 2008).…”

Grid independence behaviour of fluidized bed reactor simulations using the Two Fluid Model: Detailed parametric study

“…Firstly, TFM-KTGF simulations are used to derive industrial scale filtered models (e.g. [4][5][6]) which are enjoying significant research attention at present. Secondly, as shown in the first part of this work [7], larger particle sizes have much less stringent grid size requirements than smaller particle sizes and industrial sized beds using large particles (~500 µm) can already be simulated with current computational capacities.…”