Particle drag coefficients in turbulent fluids

An accurate estimation of particle settling velocities, and/or of particle drag coefficients, is required for modelling purposes in many industrially important multiphase processes involving the suspension of millimetre and sub-millimetre size particles in a liquid phase. It is known 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 processes that would benefit from a thorough understanding of this phenomenon, experimental data and reliable correlations are still lacking in the scientific literature, especially for the case of the above-mentioned intermediate size particles. This is probably due to the difficulties involved in the relevant experimentation. In this work, a new experimental technique for measuring average particle drag coefficients in turbulent media is presented. It is based on a direct measurement, by means of a suitable residence time technique, of the settling velocity exhibited by a cloud of particles. The experimental data obtained in a Couette-Taylor flow field are presented and discussed. These data confirm that free stream turbulence may significantly increase particle drag coefficients. At the highest turbulence intensities, drag coefficients more than 40 times greater than corresponding ones in the still fluid, were observed. A new correlation for the prediction of the influence of free stream turbulence on the drag coefficients of intermediate size particles is also proposed. (C) 1998 Elsevier Science Ltd. All rights reserved.