On the origin of heterogeneous structure in dense gas–solid flows

Abstract

The formation and evolution of flow structures in dense gas-fluidized beds with ideal collisional particles (elastic and frictionless) are investigated numerically by employing the discrete particle method, with special focus on the effect of gas–particle interaction. It is clarified that heterogeneous flow structures still exist in systems with elastic and frictionless particle collisions, even though the particles in the emulsion phase are more loosely packed. The flow structures in various flow regimes, including uniform, bubbling and turbulent fluidization, can be reproduced from these simulations. Systems with strong particle fluctuating motion but weak gas suspension display an emulsion-bubble structure. On the contrary, systems with weak particle fluctuation motion tend to produce uniform structures. If these two interactions are equally important, the system features complex flow patterns resembling those displayed in the turbulent fluidization. Energy analysis has revealed that the fluctuating motion of particles corresponds to the existence of heterogeneity in ideal collisional systems. The flow regime transition is actually the macro-scale expression of the altering degree of dominance of particle–particle and particle–fluid interactions. Particularly, it is also found that non-linear drag has the “ phase separation” function by accelerating particles in the dense phase and decelerating particles in the dilute phase producing the fluctuating motion and thereby triggering non-homogeneous flow structure formation.