A pseudo-Stokes representation of the effective drag coefficient for large particles entrained in a turbulent airstream

摘要

Recently published data by Littman et al., Powder Technol., 77 (1993) 267 and Powder Technol., 84 (1995) 49, for the effective drag coefficient in the vertical pneumatic transport of 1 and 2 mm glass and 1.99 mm rapeseed particles are represented by the pseudo-Stokes equation, C_d = 24/Re_(pc). The effects of freestream turbulence are represented using Lee's (Int. J. Multiphase Flow, 13 (1987) 247) correlation for the apparent turbulent kinematic viscosity of the fluid felt by the particles in a suspension flow. These results not only confirm the importance of freestream turbulence in lowering the drag coefficient for those particles significantly below that on the standard drag curve but suggest that the turbulence intensities are high enough to largely or completely eliminate the particle wake. These low drag coefficients increase corresponding slip velocities above terminal suggesting that solids residence times in the pipe are much higher than predicted by use of the standard drag curve.