Gas-phase velocity fluctuations in statistically homogeneous fixed particle beds and freely evolving suspensions using particle-resolved direct numerical simulation

Gas-phase velocity fluctuations in fixed particle beds and freely evolving suspensions are quantified using particle-resolved direct numerical simulation (PR-DNS). The flow regime corresponds to homogeneous gas-solid systems typically encountered in fluidized bed risers, with high solid to gas density ratio and particle diameter being greater than the dissipative length scales. The contribution of turbulent and pseudo-turbulent fluctuations to the level of gas-phase velocity fluctuations is quantified in flow past fixed particle assemblies. The simulations are then extended to freely evolving suspensions with elastic and inelastic collisions. It is found that for the parameter values considered here (solid volume fraction 0.1 and 0.2, particle to gas density ratio 100 and 1000, and coefficient of restitution in the range 0.7-1.0) the level of gas-phase velocity fluctuations in freely evolving suspensions differs by only 10% from the value for a fixed bed at the same solid volume fraction φ ∗Corresponding author Email address: [email protected] (S. Subramaniam) Preprint submitted to IJMF April 20, 2012 and mean slip Reynolds number Rem. Quantification of the Reynolds stress indicates that the second moments of the gas-phase velocity fluctuations are anisotropic, corresponding to unidirectional axisymmetric fluctuations. The anisotropy increases with Rem to a maximum that occurs in the range 10 ≤ Rem ≤ 40, and then decreases. In addition, the anisotropy decreases with increasing solid volume fraction for all cases considered in this study. The Reynolds stress is decomposed into isotropic and deviatoric parts, and their dependence on φ and Rem is quantified and explained.