Shuji Matsusaka

Electrostatics of particles

Abstract In powder handling, each particle collides with another particle or a wall, and consequently becomes charged up to a certain value. Such contact charging is experienced in various fields. In the present review, the basic concepts of contact charging are summarized; in particular, the effect of the contact potential difference and the initial charge on the charge transfer is described in detail. Furthermore, the variation of the particle charging caused by repeated impacts on a wall is formulated. This theory is extended to the particle charging in gas-solids pipe flow, where each particle has a different amount of charge; the distribution of the particle charge is also analyzed theoretically. In addition, the method of measuring important electrostatic properties, the technique of detecting particle charging and the application of particle charging are described.

Electrification of an elastic sphere by repeated impacts on a metal plate

Impact electrification between an elastic sphere and a metal plate has been studied experimentally. To find out how charge transfers between the contact bodies, the voltage profiles at the impact are measured under various experimental conditions using a digital oscilloscope, and simultaneously the contact deformation of the sphere is visualized with a high-speed camera. The initial charge on the sphere and the transferred charge are obtained from the integrated voltage with respect to the elapsed time of the impact process. The variation of the electrification by repeated impacts is analysed by taking account of the initial charge and charge relaxation with elapsed time. Furthermore, the relationship between the transferred charge and the contact area as a function of the impact velocity is investigated based on the electrification theory and Hertz analysis of elastic contact deformation.

Micro-feeding of fine powders using a capillary tube with ultrasonic vibration

Micro-feeding of fine powders has been studied experimentally by use of a vibrating capillary tube whose diameter is varied from 0.58 to 1.26 mm. Ultrasonic vibration of 20 kHz was generated by a piezoelectric transducer and applied to the tube so as to discharge micron or sub-micron particles. The mechanism of the powder flow in the vibrating capillary tube is deduced, such that a thin layer of particles near the inner wall acts as a lubricant through their micro-vibrations. Therefore, the inner powder easily passes through the tube by the force of gravity. The powder flow rate, velocity and packing fraction of particles in the capillary tube were also obtained. Furthermore, a new factor corresponding to the viscosity of fluid was introduced in characterizing the powder discharge. From the experiments using various fine powers, it was found that continuous operation of micro-feeding was possible even at a rate of milligrams per second.

Micro-feeding of a fine powder using a vibrating capillary tube

Micro-feeding of a fine powder was experimentally investigated by use of a capillary tube (≤ 1.6 mm in diameter) as an powder exhaust pipe that was vibrated under conditions of less than 760 Hz and 30 μm in amplitude. It was found that only the particles near the inside wall vibrated at random and almost all of the inner particles were transported downwards as a lump of powder. Particles of 10 μm in diameter could be discharged continuously at a constant rate as small as 0.2 mg/s and the powder flow rate increased with the vibration frequency. It was also found that the critical frequency for powder discharge was nearly inversely proportional to the square root of the vibration amplitude. However, there was a maximum flow rate that depended on the capillary diameter d and the kinds of powder. The maximum flow rate was found to be proportional to d” where n = 2–2.5.

The contact potential difference of powder and the tribo-charge

Abstract The contact potential difference (CPD) against Au (reference material) was measured directly for resin coated particles and solid wall materials. Furthermore, tribo-charging between these particles and the wall materials has been examined based on the work function estimated by the measured CPD-values. As a result, it was found that the polarity of particle charge could be estimated by the CPD-values both for metals and polymers. The relationship between the saturated tribo-charge and the CPD-values was found to be almost linear for resin coated sample powders as far as the coating ratio of resins was relatively small, while it deviated gradually from the linear relationship as the coating ratio increased. It was also found that the saturated tribo-charge of sample powders was almost independent of their charge relaxation time-constants τ r as far as τ r ⩽ 10 2 s. For τ r ⩾ 10 3 s, however, the tribo-charge depended on the time-constant. By use of the work function estimated by the CPD-values and two different time-constants for charging and charge relaxation, tribo-charging of particles against a solid wall could be analyzed quantitatively.

Particle Reentrainment from a Fine Powder Layer in a Turbulent Air Flow

ABSTRACT Particle reentrainment from a fine powder layer was investigated both in a steady-state flow and in an unsteady-state (accelerated) flow. Experiments were conducted in a rectangular channel, where a powder layer of fly ash was placed. The average air velocity was increased at a constant rate in the range of 0.01–0.6 m/s2 up to a certain velocity and, thereafter, it was maintained at the velocity. The reentrainment flux was measured automatically by an electrostatic method. Microscopic observation showed that small aggregates were reentrained randomly from the surface of the powder layer, and then the reentrainment gradually progressed through the depth of the powder layer. Through these processes, surface renewal of the powder occurred. The experimental results also showed that the distribution of adhesive strength (wall shear stress) was approximated by a log-normal distribution. Further, the time-delay of the reentrainment was found to be represented by two simple exponential functions with dif...

Simultaneous phenomenon of particle deposition and reentrainment in charged aerosol flow -- effects of particle charge and external electric field on the deposition layer

Abstract Formation of a particle deposition layer has been investigated as a simultaneous phenomenon of particle deposition and reentrainment in a turbulent aerosol flow. Aerosol particles passed through a corona charger were pneumatically transported into a glass tube equipped with a ring-type electrode. The deposition layers formed on the tube wall under various conditions were then quantitatively analyzed with particular attention to the effects of particle charge and external electric field on the deposition layer. The charged particles formed a filmy deposition layer around the electrode independent of the polarity of the particle charge and applied voltage. The mass of the deposition layer increased with elapsed time and became constant after an equilibrium state of particle deposition and reentrainment had been achieved. The time dependence was represented by an exponential equation; the time constant decreased with increasing particle charge and/or applied voltage, and the equilibrium mass of the deposition layer increased with particle charge. Furthermore, it was found that an appropriate arrangement of electrodes to control external electric field eliminates the filmy deposition layer.

Reentrainment of deposited particles by drag and aerosol collision

Abstract The reentrainment of deposited particles in turbulent aerosol flow has been studied theoretically and experimentally. The moments of forces, i.e., particle adhesion, gravity, aerodynamic drag, and aerosol collision, acting on a small aggregate adhering to a wall are calculated as a function of particle diameter. The analytical solutions indicate that the collision of an aerosol particle larger than several micrometers plays an important role in the reentrainment, whereas, the effect of the aerodynamic drag dominates for sub-micron particles. Furthermore, the critical velocity of aerosol flow for the reentrainment is calculated on the basis of a moment balance. The critical velocity decreases with increasing particle diameter. Experiments were conducted using alumina particles of size 3.3– 10.3 μm in mass median diameter. The particles were fully dispersed into airflow and fed into a glass tube. The variation in the state of the particle deposition layer was observed through a digital video camera, and the critical velocities for no particle layer formation were obtained under various conditions. Although the experimental data on the critical velocities deviate somewhat from the theoretical values, the trends are in reasonable agreement. In particular, it was found that the inertial collision by aerosol particles is efficient for the removal of the particle deposition layer.

Control of electrostatic charge on particles by impact charging

The control of electrostatic charge on particles in gas–solids pipe flow has been studied experimentally and theoretically. Alumina particles of 3.3 μm in count median diameter were dispersed in airflow and pneumatically transported in the dilute phase. Five different materials were used for the transport pipes, and the relationships between the particle charge and the pipe length were obtained. The polarity and the amount of particle charge were found to depend on the pipe material and the length. In order to control the particle charge, a system combining two different pipe materials was proposed depending on the particle-charging characteristics. The charge controlled by this method was in good agreement with the theoretical calculation. Furthermore, it was found that the distribution of particle charge as well as the average can be controlled.

Measurements of powder flow rate in gas-solids pipe flow based on the static electrification of particles

Abstract A New measuring method of powder flow rate in gas-solids pipe flow was developed and investigated both theoretically and experimentally. The method is based on the static electrification of particles by their impaction on the inside wall. The powder flow rate was successfully calculated by use of the electric currents generated from the wall to the ground. It was found that the effect of initial charge of particles on the measurement could be eliminated by using two metallic pipes whose inner-surfaces were coated with different materials. It was also found that the powder flow rate and the mean particle charge could be measured simultaneously.