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Annals of Occupational Hygiene Advance Access originally published online on July 7, 2004
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Ann. occup. Hyg., Vol. 48, No. 5, pp. 483-490, 2004
© 2004 British Occupational Hygiene Society
Published by Oxford University Press

Particle Deposition in Industrial Duct Bends

THOMAS M. PETERS and DAVID LEITH*

Department of Environmental Sciences and Engineering, The University of North Carolina, Chapel Hill, NC 27599, USA

Received 30 September 2003; in final form 6 January 2004; published online on 7 July 2003

A study of particle deposition in industrial duct bends is presented. Particle deposition by size was measured by comparing particle size distributions upstream and downstream of bends that had geometries and flow conditions similar to those used in industrial ventilation. As the interior surface of the duct bend was greased to prevent particle bounce, the results are applicable to liquid drops and solid particles where duct walls are sticky. Factors investigated were: (i) flow Reynolds number (Re = 203 000, 36 000); (ii) particle Reynolds number (10 < Rep{infty} < 200); (iii) particle Stokes number (0.08 < Stk < 16); (iv) bend angle ({theta} = 45°, 90°, 180°); (v) bend curvature ratio (1.7 < R0 < 12); (vi) orientation (horizontal-to-horizontal and horizontal-to-vertical); and (vii) construction technique (smooth, gored, segmented). Measured deposition was compared with models developed for bends in small diameter sampling lines (Re < 20 000; Rep{infty} < 13). Whereas deposition measured in this work generally agreed with that estimated with models for particles <30 µm (Stk < 0.7), it was significantly lower than that estimated for larger particles. As the flow around larger particles became increasingly turbulent, the models progressively under-represented drag forces and over-estimated deposition. For particles >20 µm, deposition was slightly greater in the horizontal-to-horizontal orientation than in the horizontal-to-vertical orientation due to gravitational settling. Penetration was not a multiplicative function of bend angle as theory predicts, due to the developing nature of turbulent flow in bends. Deposition in a smooth bend was similar to that in a gored bend; however, a tight radius segmented bend (R0 = 1.7) exhibited much lower deposition. For more gradual bends (3 < R0 < 12), curvature ratio had negligible effect on deposition.

Keywords: bend; elbow; deposition; droplet; duct; impaction; transport; ventilation


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