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 different time constant. A new reentrainment model is presented to explain the time-dependence of the reentrainment flux, which is based on the adhesive strength distribution, surface renewal of the powder layer, and the time-delay of the reentrainment. The reentrainment flux increases with time elapsed in an accelerated flow, while it decreases in a steady state flow. Air acceleration has a significant effect on the reentrainment flux not only in the accelerated flow but also in the steady-state flow. Furthermore, the critical (incipient) reentrainment velocity decreases with increasing air acceleration. The experimental results agreed well with the results calculated based on the new model.