PARTICLE DRAG COEFFICIENTS IN TURBULENT FLUIDS

An accurate estimation of particle settling velocities, and/or of part icle drag coefficients, is required for modelling purposes in many ind ustrially important multiphase processes involving the suspension of m illimetre and sub-millimetre size particles in a liquid phase. It is k nown that the settling velocity of particles in a turbulent fluid may be significantly different from that in the still fluid, depending on turbulence and particle characteristics. Despite the wide range of pro cesses that would benefit from a thorough understanding of this phenom enon, experimental data and reliable correlations are still lacking in the scientific literature, especially for the case of the above-menti oned intermediate size particles. This is probably due to the difficul ties involved in the relevant experimentation. In this work, a new exp erimental technique for measuring average particle drag coefficients i n turbulent media is presented. It is based on a direct measurement, b y means of a suitable residence time technique, of the settling veloci ty exhibited by a cloud of particles. The experimental data obtained i n a Couette-Taylor flow field are presented and discussed. These data confirm that free stream turbulence may significantly increase particl e drag coefficients. At the highest turbulence intensities, drag coeff icients more than 40 times greater than corresponding ones in the stil l fluid, were observed. A new correlation for the prediction of the in fluence of free stream turbulence on the drag coefficients of intermed iate size particles is also proposed. (C) 1998 Elsevier Science Ltd. A ll rights reserved.