(213b) Meso-Scale Nonequilibrium Characteristics in a Bubbling Fluidized Bed

To understand the meso-scale, far-from-equilibrium behavior in fluidization [1], we investigate, both numerically and experimentally, the nonequilibrium features in a pseudo 2D bubbling fluidized bed. In experiment, velocities of individual particles are measured by using a particle tracking velocimetry (PTV) method, and void fractions are obtained with Voronoi tessellation. A bimodal shape of probability density function (PDF) for particle vertical velocity is found in not only time-averaged but also time-varying statistics, which is caused by the transition between the dense and dilute phases and breaks the local-equilibrium assumption in continuum modeling of fluidized beds [1]. The results of time-varying radial distribution function and voidage distribution also confirm this finding. Moreover, analysis of voidage, velocity of particle, granular temperature and turbulent kinetic energy of particles shows that there is no scale-independent plateau over the interface, and it seems hard to find a scale-independent plateau to separate the micro- and meso-scales of fluidized beds, which require sub-grid meso-scale modeling for continuum or coarse-graining methods of gas-fluidized systems [2]. In numerical simulation, dense discrete particle method is used with the energy minimization multi-scale (EMMS) drag. Simulation results generally agree with the experiment.

References

1. W. Wang, Y. Chen, Mesoscale modeling: Beyond local equilibrium assumption for multiphase flow, Adv. Chem. Eng. 47(2015) 193–277.
2. Y. Tian, J. Geng, W. Wang, Structure-dependent analysis of energy dissipation in gas-solid flows: Beyond nonequilibrium thermodynamics, Chem. Eng. Sci. 171 (2017) 271–281.