Electrostatics of the Granular Flow in a Pneumatic Conveying System

Abstract

The phenomenon of electrostatic charge generation and its effects on granular flow behavior in a pneumatic conveying system was studied. The main parameters used for quantitative characterization of the phenomenon were the induced current, particle charge density, and equivalent current of the charged granular flow. These were measured using a digital electrometer, Faraday cage, and modular parametric current transformer, respectively. Three different flow patterns corresponding to different electrostatic effects within the pneumatic conveying system were observed, and these were named the disperse flow, half-ring flow, and ring flow patterns. It was found that the induced current, particle charge density, and equivalent current increased with decreasing flow rates. Electrostatic effects generally become stronger with time, and this may lead to clustering behavior occurring even in the disperse flow regime. The effects of several factors such as pipe wall material, particle composition, relative humidity of the conveying air used, and the presence of an antistatic agent in the system were investigated and found to be important in determining the electrostatic charge generation characteristics and granular flow patterns observed.

On the electrostatic equilibrium of granular flow in pneumatic conveying systems

“…They found that granular cluster pattern differs with the voltage set [12][13][14] and is affected by humidity. 15 A similar phenomenon was observed 2 for larger particles (2.8mm) where three characteristic aggregation patterns (clusters, half-ring, and ring) were formed in a vertical pipe due to electrostatic effects in a pneumatic conveying system. Al-Adel et al 16 investigated the effect of static electrification on gas-solid flows in vertical risers and captured qualitative features of riser flows: core-annular particle distribution, annular particle downflow at low riser gas velocities, and annular upflow at high gas velocities.…”

“…Based on previous triboelectrification experiments in the pneumatic conveying system using PP granules and PVC pipes, 2,25 it can be deduced that positive charges are generated on the PP granule surface while equal amounts of negative charges are accumulated on the PVC pipe wall (Figure 2c). In the pipe cross section with grid level 20 ϫ 40 (Figure 2a), the electrostatic field vector (Figure 2d) at the point (i,j) from the point charge (l, k) can be calculated by the following:…”

“…The polarity of the induced current generated in the conveying system (Figure 3a-c) was found to fluctuate over both negative and positive values, and the mechanism was explained in our previous work. 2 Here, the averaged current values according to Eq. 18 at the three pipe sections are shown in Figure 3d-i.…”

“…This result seems reasonable because negative charges were found to be generated at the pipe wall by granule (PP)-wall (PVC) collisions. 2,25 Davies 27 studied charge generation on dielectric surfaces and found that PVC has fairly higher work function than PP and, hence, prefers to receive electrons as a result of such collisions. The establishment of a steady averaged current indicates a state of "electrostatic equilibrium.…”

“…In the pneumatic conveying system, the electrostatic charge generation mechanism of particles and pipe wall mainly depends on tribroelectrification. 2 The mechanical effect of particle-wall interactions gives rise to rough pipe wall and alters the electrostatic characteristics. For example, under strong mechanical impactions, the pipe wall may be in abrasion 29 and fine powders adhere to the pipe wall.…”