A general flowability index for powders

Abstract

In order to meet the need of industry for a universal index of flowability, we propose the slope value of the function linking material flow factor FF (Jenike's ratio of major principal stress Σ 1 to unconfined yield stress f) with inverse specific tension 1/T s (the reciprocal of tension T divided by its corresponding value of steady-state normal stress σ L). We find that for “regular” powders (those that show both a constant shear index, n, and angle of static internal friction, ψ), this value is nearly constant over a wide range of stress and may be used as a general index of flowability, ζ, which we express as a percentage, thus: ζ = [1−( n−1)sinψ][1−( n-1)sin 2ψ] 2 nsinψ × 100% The small minority of powders classed as “irregular” (those that show a variable n and ψ with increasing stress, temperature or dampness) may be given an instantaneous percentage flowability, ζ i, at the specified conditions ζ i = [1−( n−1)sinψ][1−sinψ][ G−( n−1) G2 nsinψ( G−1) [1−( n−1)sinψ][1−sinψ][ G−( n−1)G 2 nsinψ( G−1) × 100% where G 2 = 1 + n 2 cot 2 Δ and Δ is the major angle of internal friction. The resulting flowability scale ranges from over 90% for very free-flowing materials down to single figure percentages for severely cohesive ones. A classification table for 60 regular powders is presented.