Vapour transport to and from surfaces

In early wind tunnel experiments, Chamberlain (1953) liberated elemental 131I vapour and measured deposition on various surfaces. Metal surfaces, and also cellulose-based filter paper, were found to act as perfect sinks for the vapour, deposition being controlled by diffusion through the boundary layer over the surface. In Fig. 6.1, the results for deposition to a flat plate covered with filter paper are compared with the Polhausen equation 

To investigate mass transfer of a radioactive vapour for which all surfaces are perfect sinks, Chamberlain (1966, 1974) used a vapour of 212Pb (ThB). This was generated by decay of 220 Rn (thoron) in a dust-free atmosphere and carried with a flow of filtered air into a wind tunnel (Fig. 6.2). Experiments with real and artificial grass swards in the tunnel are described below. In experiments with bean plants (Vida faba) in the tunnel, the deposition of 212Pb to individual leaves was measured, and analysed in terms of the Sherwood number, using the maximum chord of the leaves as the characteristic length, L. 

The wind speed at the level of the top of the bean plants was 2ms-1, causing fluttering of the leaves, but the Sherwood numbers were only 

Sh = 0.66Re° 5Sc°33 line B, theory, turbulent flow, Sh = 0.037Re° Sc0 33. 

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