Maximum Electrostatic Charge of Powder in Pipe Flow

概要

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In dry powder processes, particles collide against each other and onto walls, and normally, electrostatic charge is generated. Such process is called triboelectrification or tribo-/contact-/impact- charging of particles. In this paper, the amount of maximum (or saturated, or equilibrium) charge on a particle in a powder flow of a pneumatic conveyer with a metal pipe was discussed. A theoretical discussion involving the space charge effect on the basis of 'charge relaxation model, ' in which the maximum charge is limited by the gas break down, is given. The result showed two regions, i. e., the region in which the space charge does not have any effect, and the other region where the space charge dominates the amount of the charge. The charge was proportional to the particle size to the power of 1.5 in the region of no space charge effect, and the power of 3 in the other region. An approximated analytical formula was empirically found, for the whole range, including these two regions. The maximum charge as a function of particle size pipe size and volume fraction of the powder, was given as: q/C=1.1×10-4 (dp/m)3/√[{Φ(D/m)}2+{17.1(dp/m)1.5}2], when the relative dielectric constant of the particle is 2. A comparison showed that the theory gives a certain overestimation to the data cited from literature. Additional detailed discussions are given in the paper.
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