Brucato A

Particle drag coefficients in turbulent fluids

Abstract 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.

Solids Suspension in Three-Phase Stirred Tanks

The ‘pressure gauge technique’ recently developed for the quantitative assessment of the mass of solid particles suspended at any agitation speed is extended to the case of three-phase stirred tanks. As a result, curves of the fraction of suspended solids at various gas flow rates, versus agitator speed, are presented. The difficulties involved in the extension of the technique to three phase systems are addressed and discussed. The experimental results show that the presence of the gas phase causes a significant increase of the agitation speed required to attain complete suspension of solids and lowers the degree of suspension at all agitation speeds below it. The experimental data obtained are shown to be well fitted by the Weibull functions previously adopted for two-phase systems. Correlations for the estimation of the influence of particle size, particle concentration and gas flow rate on the suspension degree are presented and discussed. Finally, the fractional suspension dependence on particle size is found to be very different when very large particles are dealt with.