DEPOSITION PROCESSES 



Efficiency of impaction on spheres, cylinders, and strips has attracted 

 two distinct t}'pes of investigators — theoretical and experimental. There 

 have been large discrepancies between the conclusions of different 

 theoretical workers and again between wind-tunnel studies, but in more 

 recent papers substantial agreement is evident between theory and experi- 

 ment. Deposition on cyhnders has been studied by: Sell (1931), Glauert 

 (1946), Langmuir & Blodgett (1949), Johnstone et al (1949), Chen (1955), 

 Wong et al. (1956), La Mer & Hochberg (1949), and Landahl & Herrmann 

 ( 1 949). Deposition of cloud droplets on rotating multicylinders was studied, 

 both theoretically and in flight, by Brun et al. (1955). A theory of particle 

 deposition on cylinders, showing reasonably good agreement with the 

 observed data of Gregory (1951) and of Ranz & Wong (1952), has been 

 developed by C. N. Davies & Peetz (1956). Deposition mechanisms other 

 than impaction still await a coherent theory, and we must rely on experi- 

 mental values. (Fig. 10.) 



WIND-TUNNEL STLTDY OF IMPACTION 



The following account is based on work with a small, low-speed wind- 

 tunnel built at Rothamsted Experimental Station in 1949 (Gregory, 1951 ; 

 Gregory & Stedman, 1953) and includes some hitherto unpublished data. 



• Fig. II.— Diagram showing small wind-tunnel used in deposition study at Rothamsted 

 Experimental Station, elevation view. 



1-4, 'Perspex' working sections; b, bell-shaped intake; c, contraction to smooth flow; 



e, expansion and conversion from square to circular cross-section; h, paper-honeycomb 



straightener; m, motor; p, propeller; s, spore input; t, removable constriction to generate 



turbulence when required; x, trapping position. 



The wind-tunnel consists of a horizontal square duct (Fig. 11). The 

 two ends of the tunnel project through the end walls of a small building 

 which forms a laboratory traversed by the eight-feet-long working section 

 of the tunnel. The tunnel uses outdoor air which is passed through once 

 only and not re-circulated. A four-bladed wooden propeller absorbing 

 0-56 horsepower at 2,850 r.p.m. in the exit draws air down the tunnel. 

 An expansion section converts the 29 cm.-square working section to the 

 46 cm.-circular diameter at the fan. The flared intake-end is of 51 cm.- 

 square cross-section, contracts to a bell shape, and contains a paper honey- 

 comb 'straightener' to remove eddies and produce streamlined flow; 

 but if turbulent flow is needed a constriction is inserted in the first part 

 of the working section (indicated by dotted lines at t in Fig. 11). 



61 



