A generalized mass transfer law unifying various particle transport mechanisms in dilute dispersions |
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Authors: | Abhijit Guha |
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Institution: | (1) Aerospace Engineering Department, University of Bristol, University Walk, Bristol, BS8 1TR, UK |
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Abstract: | A generalized mass transfer law for dilute dispersion of particles (or droplets) of any sizes suspended in a fluid has been
described, which can be applied to turbulent or laminar flow. The generalized law reduces to the Fick’s law of diffusion in
the limit of very small particles. Thus the study shows how the well-known and much-used Fick’s law of diffusion fits into
the broader context of particle transport. The general expression for particle flux comprises a diffusive flux due to Brownian
motion and turbulent fluctuation, a diffusive flux due to temperature gradient (thermophoresis plus stressphoresis) and a
convective flux that arises primarily due to the interaction of particle inertia and the inhomogeneity of the fluid turbulence
field (turbophoresis). Shear-induced lift force, electrical force, gravity, etc. also contribute to the convective flux. The
present study includes the effects of surface roughness, and the calculations show that the presence of small surface roughness
even in the hydraulically smooth regime significantly enhances deposition especially of small particles. Thermophoresis can
have equally strong effects, even with a modest temperature difference between the wall and the bulk fluid. For particles
of the intermediate size range, turbophoresis, thermophoresis and roughness are all important contributors to the overall
deposition rate. The paper includes a parametric study of the effects of electrostatic forces due to mirror charging. The
present work provides a unified framework to determine the combined effect of various particle transport mechanisms on mass
transfer rate and the inclusion of other mechanisms not considered in this paper is possible. |
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