Secondary flows drive triboelectric powder charging in pneumatic conveying

TL;DR

This study investigates how carrier-flow dynamics govern triboelectric charging of powders in pneumatic conveying, testing whether flow-pattern design can control charging. Using direct numerical simulation with the pafiX solver, the authors compare square-duct flows that exhibit secondary flows to channel flows, isolating the impact of flow structures on charging rates and cross-sectional charge distributions. They find that duct flows accelerate charging and produce more uniform cross-sectional charging due to increased wall collisions and enhanced mixing, though near-equilibrium charging in channels can still emerge in corners. Additionally, charging alters particle distributions and turbulence, increasing wall accumulation and reshaping Reynolds stresses and energy spectra, highlighting the potential for flow-pattern-based control to mitigate hazards and optimize powder handling.

Abstract

Highly resolved simulations reveal the fundamental influence of a carrier fluid's flow dynamics on triboelectric powder charging. We found that particles transported through a square-shaped duct charge faster than in a channel flow caused by secondary flows that led to more severe particle-wall collisions. Specifically, particles with a Stokes number of 4.69 achieve 85 % of their equilibrium charge approximately 1.5 times faster in duct flow than in channel flow. Also, charge distribution is more uniform in a duct cross-section compared with a channel cross-section. In channel flow, particles are trapped near the walls and collide frequently due to limited movement in the wall-normal direction, causing localized charge buildup. In contrast, duct flow promotes better mixing through secondary flows, reducing repeating collisions and providing uniform charge distribution across the cross-section. Upon charging, electrostatic forces significantly reshape particle behaviour and distribution. Once the powder achieves half of its equilibrium charge, particles increasingly accumulate at the wall, leading to a reduced concentration in the central region. These changes in particle distribution have a noticeable impact on the surrounding fluid phase and alter the overall flow dynamics. These findings open the possibility for a new measure to control powder charging by imposing a specific pattern.

Figures (16)

• Figure 1: Normalized stream-wise velocity contours and cross velocity vectors of fluid in channel (a) and duct (b) flows' cross-section. The lower left half of the channel and lower half of the duct are shown. Arrows represent the direction of the secondary flows. The confined area shows fluid acceleration to the corners due to coinciding vortices.
• Figure 2: Dimensions of channel and duct flow containers. Dashed lines on the duct container show the bisectors.
• Figure 3: Temporal evolution of the average powder charge normalized by the equilibrium charge (a). The charging rate of powder (b). Lines represent channel flow, and dashed lines represent duct flow. Colors indicate Stokes number; ( ) $St\,=\,37.50$, ( ) $St\,=\,18.75$, ( )$St\,=\,9.38$, ( )$St\,=\,4.69$.
• Figure 4: Rate of average particle-wall collisions (a), the charge transferred during one particle-wall collision, impact charge, over time (b). Lines represent channel, dashed lines represent duct flow. Colors indicate Stokes number; ( ) $St\,=\,37.50$, ( ) $St\,=\,18.75$, ( )$St\,=\,9.38$, ( )$St\,=\,4.69$.
• Figure 5: Frequency of wall collision for channel (a) and duct (b) flows. It shows the percentage of particles and the corresponding number of collisions with the wall until the average powder charge reaches half of the equilibrium charge, $q^*_{\textup{avg}}=0.5$. The histograms exclude particles that did not collide with a wall yet. Colors indicate Stokes number; ( ) $St\,=\,37.50$, ( ) $St\,=\,18.75$, ( )$St\,=\,9.38$, ( )$St\,=\,4.69$.