6 - Interaction between particles and fluids

Abstract

The interaction between particles and fluids is important in many industrial processes. The size distribution based on the sedimentation behavior of particles in fluids and the applicable size is the settling or Stokes diameter. The simplest case to consider is the settling of a single homogeneous sphere, under gravity, in a fluid of infinite extent. Experiments have been carried out to determine the relationship between settling velocity and particle size under these conditions and a unique relationship between drag factor (CD) and Reynolds number (Re) has been found that reduces to a simple equation, known as the Stokes equation, at low Reynolds number. Settling behavior is extremely complex in the high concentration region and several equations may apply according to the range of porosity considered and the presence or absence of flocculation. The determined particle size decreases with the addition of dispersing agents and it is suggested that the size so determined is floc size. The technique is therefore a useful and simple one for determining floe size.