A SIMPLE DESCRIPTION OF SOME INERTIA EFFECTS IN THE BEHAVIOR OF HEAVY-PARTICLES IN A TURBULENT GAS-FLOW

he effects of some inertia parameters and the influence of non-linear drag in the behaviour of heavy particles in a turbulent gas flow are i nvestigated by examining the properties of their streamwise fluctuatin g motion. The particles are suspended in a stationary homogeneous turb ulent gas flow, under the influence of an external body force field, w hich implies a non-zero mean slip velocity between the two phases. Two kinds of methods, the results of which are compared, are used. The fi rst method is based on a simplified kinematic simulation of the fluid velocity fluctuation in the streamwise direction, according to a known turbulence spectrum. The second method is a classical Lagrangian trac king technique. The presence of an external body force held makes the particle turbulent response depend on three dimensionless parameters, among them the conventional Stokes number St and the particle Reynolds number <(Re-p)over bar>, based on the mean relative velocity between the fluid and the particle, are found to be the most influential ones for heavy particles. A particle response diagram is built, leading to an approximate expression for the particle-to-fluid turbulent intensit y ratio in terms of a modified Stokes number which embodies the effect s of both St and <(RE(p))over bar>. Numerical predictions are provided in the case of an upward vertical flow, in a large range of particle mean Reynolds numbers exceeding unity. A significant influence of the drag non-linearity can be observed. The main conclusion is that the ef fect of non-linear drag may surpass the well-known crossing-trajectory effect, in causing the particle mean-square velocity to increase with increasing mean relative velocity, contrary to what is generally expe cted for small inertia particles. (C) 1996 Elsevier Science Ltd.