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Inertial impaction of heavy molecules

Published online by Cambridge University Press:  20 April 2006

J. Fernández De La Mora ,
B. L. Halpern  and
J. A. Wilson
J. Fernández De La Mora
Affiliation:
Departments of Mechanical and of Chemical Engineering, Yale University, New Haven, Connecticut 06520
B. L. Halpern
Affiliation:
Departments of Mechanical and of Chemical Engineering, Yale University, New Haven, Connecticut 06520
J. A. Wilson
Affiliation:
Departments of Mechanical and of Chemical Engineering, Yale University, New Haven, Connecticut 06520
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Abstract

The transition from diffusion-dominated to inertia-dominated behaviour in the transport of condensable heavy molecules carried in a continuum subsonic He jet that impinges on a solid surface is studied experimentally. The Stokes number S, or ratio between the heavy-molecule relaxation time and the fluid-dynamic time, is varied in the interval 0 [lsim ] S [lsim ] 1 by changing the jet Mach number at a constant value of the Reynolds number. Although the heavy species departs considerably from equilibrium at all but the smallest values of S, the helium jet is always near equilibrium conditions. At values of S of order unity the observed rate of deposition at the stagnation point asymptotes to a value some six times greater than in the diffusion region (where S → 0), implying that the process is governed by the large inertia of the heavy species, very much like in aerosol impactors. As a result, it is argued that the concept of pressure diffusion is unsuitable to explain the observed behaviour. An approximate theoretical description of the transport process is given for the region S [Lt ] 1 where the kinetic problem is amenable to a hydrodynamic treatment. Finally, the analogy with the inertia-dominated behaviour of aerosols is used to assess the relative merits of various aerodynamics schemes aiming at separating isotopes.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 149 , December 1984 , pp. 217 - 233
Copyright
© 1984 Cambridge University Press

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