Review of the empirical correlations for the drag coefficient of rigid spheres

Empirical correlations relating the drag coefficient of spheres to the Reynolds number are qualitatively reviewed with the focus on the presumed mathematical expressions used in the regression procedure to determine the parameter constants. By putting the shape factors to one in general correlations valid for spheres and non-spheres indirect empirical correlations for spheres are obtained and these are reviewed likewise. It is concluded that the total drag experienced by a sphere in an infinite flowing medium is exactly the sum of Stokes's law and Newton's law. The resulting Stokes-Newton equation is proposed to be the new landmark for the relation between the drag coefficient and the Reynolds number. The values calculated using this new landmark correlation are cross-compared with the estimates of the direct and indirect correlations and with the data set of the historical Standard Drag Curve. The retained Stokes-Newton correlation is confirmed by the fluid dynamics of diluted fluidization at the extreme point of entrainment of an individual particle. (C) 2019 Elsevier B.V. All rights reserved.