Particulate Stresses in Dense Disperse Flow

This paper reviews and discusses the progress as achieved to date in the matter of describing particulate stresses of different origins that occur in concentrated suspensions of solid particles. A few physical mechanisms of generating such stresses are explicitly recognized. First, these stresses appear as a result of direct momentum transport over a transient network of particles separated by thin lubricating films of the intervening fluid, so that a part of the apparent suspension viscosity must be attributed to its dispersed phase. Second, normal and tangential particulate stresses originate because of random fluctuations of particles caused by (1) their pair interactions as the particles are brought closer together and pass one another in mean shear flow, (2) the relative fluid flow and an external body force field as they interact with random fluctuations of suspension concentrations, and (3) random macroscopic flow patterns, such as bubbles rising in fluidized beds that produce a system of Reynolds-like stresses. The theoretical predictions pertaining to all these mechanisms are shown to be in good keeping with experimental evidence available for suspension shear flow and fluidized beds.