Particle diffusivity in fully developed, turbulent, horizontal pipe flow of dilute air—solid suspensions

Abstract

The diffusivity of small particles in steady, horizontal pipe flows of dilute air—solid suspensions under the influence of gravity and weak electrostatic forces was studied. An isokinetic sampling system, a density probe with randomized bifurcated fiber optics and a Faraday cage were used to measure distributions over a cross section of particle mass flux, particle phase density and the locally averaged particle charge-to-mass ratio. The local particle diffusivity was then determined by using the mass and charge conservation equations. For alumina particles of equivalent volume diameter of 31 μm, pipe Reynolds number of 7.21 × 10 4, 1.41 × 10 5 and 2.15 × 10 5, a solid-to-air mass ratio up to 0.4 and a particle charge-to-mass ratio up to 1.6 × 10 −4 C/kg, the fluid—solid interaction is the dominating mechanism that controls particle diffusivity. The gravitational and electrical forces have only a very minor influence. The data reveal that the particle diffusivity is not uniformly distributed over the pipe cross section, contrary to the assumption of uniform diffusivity commonly used in the literature. However, it was found that the product of particle diffusivity and inverse relaxation time is almost a constant over the cross section. A simple formula for the approximate evaluation of the constant is given.