Influence of turbulence intensity on particle drag coefficients

The effect of turbulent eddies on the motion of the dispersed phase is often ignored by assuming a standard drag coefficient as applied to quiescent flow. Such an assumption can lead to large errors when predicting the dispersed phase concentration profile of an industrial flow under turbulent conditions. Recently, [G.L. Lane, M.P. Schwarz, G.M. Evans, Numerical modelling of gas-liquid flow in stirred tanks, Chem. Eng. Sci. 60 (2005) 2203-2214] developed a model relating the drag coefficient to fluid turbulence characteristics through the dimensionless group Stokes number. Their model has been successfully applied to predict gas holdups of a mechanically stirred tank over the range of St < 0.7. The present study focused on broadening the analysis by Lane et al. Specifically, the aim was to examine the effects of dispersed phase density and size on the applied drag force under turbulent conditions and to extend the much needed experimental data on particle drag coefficients in free-stream turbulence, as a function of solid particles characteristics. This was achieved by a systematic experimental approach using particles of different sizes and densities in two distinct turbulent flow fields generated by oscillating grids. The results indicated that the reduction in settling velocity is a function of both particle size and density and turbulent characteristics with maximum interaction between the continuous and dispersed phases occurring at low ratios of particle density to liquid density and high turbulence intensities. Richardson number, a dimensionless group, was employed to capture the effects of these parameters on the drag coefficient. (c) 2007 Elsevier B.V. All rights reserved.