Nonequilibrium Features in a Bubbling Fluidized Bed

The spatio-temporal evolution of the meso-scale structures, such as bubbles or clusters in a gas-solid fluidized bed, has been a hot area of research in recent decades [1, 2]. These dynamical heterogeneous structures can be characterized by nonequilibrium features. In the present work, the scale–dependent nonequilibrium features are investigated experimentally in a pseudo 2D bubbling fluidized bed. Velocities of individual particles are measured by using a particle tracking velocimetry (PTV) method, and void fractions are obtained with the 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. The results of time-varying radial distribution function and voidage distribution also confirm this finding. Moreover, the analysis of voidage, particle velocity, 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. Such scale-dependent nonequilibrium properties require sub-grid meso-scale modeling for continuum or coarse-graining methods of gas-fluidized systems [2, 3].

References:

[1] Li J, Zhang J, Ge W, Liu X. Multi-scale methodology for complex systems. Chem Eng Sci. 2004;59(8-9):1687-1700.

[2] Wang W, Chen Y. Mesoscale modeling: beyond local equilibrium assumption for multiphase flow. Adv Chem Eng. 2015;47:193-277.

[3] Tian Y, Geng J, Wang W. Structure-dependent analysis of energy dissipation in gas-solid flows: Beyond nonequilibrium thermodynamics. Chem Eng Sci. 2017;171:271-281.