Tribo-electrification of particles under sudden change of fluid flow in the junction between branch pipe and straight pipe

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An experimental investigation of the tribo-electrification of glass beads fed by an ejector has been conducted by measuring both the current generated at the pipe wall and the specific-charge measured by a Faraday cage under sudden change of fluid flow in the junction between a stainless-steel branch pipe and a stainless-steel straight pipe. In measuring a current per unit mass, for Dp,50⩾206μm at the branch pipe section the current has a positive value as expected by the contact potential difference between glass beads and stainless-steel. On the other hand, for Dp,50⩽105μm at the branch and straight pipe section, the current has a negative value couldn’t be explained solely by the contact potential difference. In measuring a specific-charge by the Faraday cage, the specific-charge has a negative highest value at the ejector. The negative specific-charge decreases along the particle flow direction. Therefore, an “unusual” charge-transfer, which couldn’t be explained solely by the contact potential difference, was confirmed also by the Faraday cage. Although the charge-transfer between the beads and the inclined stainless plate with high impact speed has been examined, the sign of the current is positive for all data. It was found that the “unusual” charge-transfer in this study couldn’t be caused by the high speed impaction. An negative current in air by using a stainless steel needle detected at the ejector for Dp,50=51μm while an positive current in air detected at the branch and straight pipe. The reason is suggested that the ion balance in the air does not keep between the ejector and the branch pipe due to both the adsorption of some negative ions on the pipe wall and the decrement of negative charge of particles. Therefore the “unusual” charge-transfer consists of not only the ionization caused by the self-discharge but also an adsorption of ions on the inner wall of the pipe.