Particle coagulation in steady turbulent flows: Application to smoke aging

Abstract

Particle coagulation depends both on the local particle volume concentration and on the collision rate between particles. Turbulent flows can have two effects on the coagulation of dispersed particles: (1) vary the particle volume concentration and (2) increase the collision rate between particles. The dispersed particles may also change the turbulent characteristics of the flow, if their volume concentrations are so large that gravity and particle inertia are of the same magnitude as the turbulent forces. In this work only dilute dispersions (volume concentration less than 0.05) are considered; effects of the dispersed particles on the turbulent flow will be at most described by a small perturbation correction. The collision rate between particles in a turbulent flow is primarily determined by the magnitude of the particle size relative to the Kolmogorov microscale of turbulence. The collision rates are presented as a function of the local turbulent characteristics (energy dissipation and Kolmogorov microscale). The following subjects are briefly discussed: (1) the effects of particle size distribution (polydispersity) on coagulation rates; (2) the effects of interparticle potential on coagulation rates; and (3) viscous interaction effects. Finally, an application of the coagulation concepts in turbulent flows is presented: namely, the coagulation (“aging”) of smoke particles in the plumes originating from flaming fires. Prediction of “smoke” aging is desirable because of its significant effect on the response of smoke detectors.