Hydrodynamical interactions between particles and liquid flows in biochemical applications

The interactions between a turbulent flow field and discrete particles have numerous applications in biochemical engineering. On the one hand, flows have a strong influence on the particle motion, from which consequences for heat and mass transfer, mixing or even damage to particles are derived. On the other hand, the presence of the discontinuous (solid) phase is regarded as altering the turbulent field (two-way coupling). At present, no fully explained mechanism of this turbulence alteration is offered in the literature. However, the two-way coupling can no longer be considered when the particle concentration becomes sufficiently high. The dominant mechanism affecting the flow is then the particle-particle interaction. Until now, no clear definition of a demarcation between hydrodynamic (fluid-particle interaction) and viscous (particle-particle interaction) influences in liquid-solid or liquid-solid-gas systems has been given in the literature. In this paper we present first a description of the forces acting on a particle in a flow and the most relevant parameters linked to the response of a particle to turbulent stimulations. Some illustrations are given for common biochemical applications. The second part is concerned with the action of the particles on the turbulence, the main trends observed and their significance in such applications being focused on. It is also demonstrated here that the transition between the hydrodynamic and the viscous regimes is located between 20% and 30% in solid volume concentration.