Particle Diffusivity in Fully Developed Turbulent Pipe Flow of Dilute Suspensions
Ni, Shimao
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https://hdl.handle.net/2142/70140
Description
Title
Particle Diffusivity in Fully Developed Turbulent Pipe Flow of Dilute Suspensions
Author(s)
Ni, Shimao
Issue Date
1986
Department of Study
Mechanical Engineering
Discipline
Mechanical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Mechanical
Abstract
Information on particle diffusivity is essential in the formulation of the constitutive relations needed for the solution of conservation equations governing multiphase flow. Particle diffusivity is also a key parameter in the modeling and scaling of systems involving multiphase flow. The present investigation is concerned with the diffusivity of small particles in steady, fully developed, turbulent horizontal pipe flow of dilute air-solid suspension under the influence of gravity and electrostatic forces.
An upgraded continuous flow test loop for air-solid suspensions with a cyclone-venturi particle feeder, an improved isokinetic sampling system, a redesigned optical density probe system, and a Faraday cage were used to measure local particle mass flux, particle phase density, and average particle charge-to-mass ratio. The local particle diffusivity was then calculated with the aid of the conservation equations for mass and charge. For alumina particles of an equivalent volume diameter of 31 (mu)m, pipe Reynolds numbers of 7.21 x 10('4), 1.41 x 10('5), and 2.15 x 10('5), solid-to-air mass ratio up to 0.81 and particle charge-to-mass ratio up to 10('-4) Coul/kg, the gravitational and electrical forces have only a weak influence on the particle diffusivity. Fluid-solid interaction is the dominating mechanism that controls particle diffusivity. The present data reveal that the particle diffusivity is not uniformly distributed over the pipe cross section while uniform distribution has been commonly assumed in the literature. A more important finding is that the product of the particle diffusivity and the inverse relaxation time is nearly constant for a given pipe Reynolds number and material properties of the particles and the fluid. A method for evaluation of the constant is proposed. This phenomenological relation should be useful for the study of the dynamics of fully developed, turbulent pipe flow of dilute gas-solid suspensions.
The Lagrangian equation of creeping motion of a small spherical particle in unsteady, nonuniform flow recently given by Maxey and Riley has been examined. Errors were unearthed and corrected. When the equation was applied to small and heavy particles in isotropic turbulent flows together with the incorporation of Csanady's cross-trajectory modification, the analytical predictions of particle diffusivity show good agreement with the experimentally determined data in the core region.
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