Evaluation of lubrication force on colliding particles for DEM simulation of fluidized beds

“Lubrication force” arises from hydrodynamic pressure in the interstitial fluid being squeezed out from the space between two solid surfaces. In the previous DEM simulations of gas-solid flows this force has not been explicitly taken into account since it may introduce the famous “Stokes Paradox”, which postulates that: Two solid surfaces can never make contact in a finite time in a viscous fluid due to the infinite “lubrication force” when the distance approaches zero at the last moment of contact. It is easy to imagine that lubrication effect is critical in liquid-solid systems, but it may not be negligible even in gas-solid systems of light and small particles. Although the lubrication theory has been well established in liquid-solid systems, its application in gas-solid systems should be used with caution because the assumptions adopted in the classical lubrication theory are only valid for highly viscous systems. In the present study, these assumptions are examined and semi-theoretical expressions for lubrication force are proposed based on numerical analysis. The paradox of contactless collision due to infinite lubrication force is effectively avoided by considering surface roughness, non-continuum fluid effect and van der Waals force. The coefficient of restitution is defined as a criterion for evaluating the significance of lubrication effect in collisions of particles in fluidized beds. For demonstration the lubrication effect was evaluated for beds of FCC particles and GB (glass beads), with diameters ranging from 25 to 100 μm and initial approaching velocity from umf to ut. The calculated restitution coefficient ranged from 0 to nearly 1 and clearly showed that lubrication force plays a significant role during a close encounter of two particles even in gas-solid systems.