Wind Tunnel Experiments of Large Particle Reentrainment-Deposition and Development of Large Particle Scaling Parameters

Abstract

Reentrainment-deposition characteristics of lycopodium spores, timothy pollen, glass microballoons, glass spheres, and nickel spheres were examined in a series of wind tunnel experiments. Particle diameter varied from 18 to 34 (μm, and particle density varied from 1.0 to 8.9 g cm⁻³. Adhesion force distribution of each particle type was determined using the centrifuge method, and the force required to remove 50% of the particles had a range of more than an order of magnitude with lycopodium requiring the smallest force and glass spheres requiring the largest. The mean flow speed required to reentrain 50% of each particle type had a range of more than a factor of two with the same ordering from smallest speed to largest as found for adhesion force with the exception of nickel spheres, which required the second largest wind speed and the second smallest adhesion force for 50% removal. Threshold reentrainment flow speed, and distance traveled in a single reentrainment-deposition cycle were also examined for each particle type. Threshold reentrainment flow speeds were found to be inversely proportional to particle diameter rather than proportional to diameter, which is the case for coarse particles. Distance traveled by a particles in a single reentrainment-deposition cycle were found to be generally on the order of a few millimeters, implying that particle migration on the order of a few meters involves numerous reentrainment-deposition or rebound-deposition cycles. Threshold reentrainment flow speed and reentrainment distance measurements were used to develop large particle scaling parameters for threshold friction velocity based on particle diameter, and characteristic reentrainment-deposition length based on particle diameter and friction velocity.