Deposition of Unattached Radon Progeny from Enclosure Flows

Numerical predictions of spatial distribution and local deposition velocities of unattached ²¹⁸Po and ²¹²Pb to the surfaces of square two-dimensional enclosures under laminar natural convection flow are reported. The Grashof number (Gr) characterising the flow is chosen to be 1.6 x 10¹⁰, in the middle of the three orders of magnitude range of Gr values found for natural convection flows in rooms. Deposition velocities are calculated for various locations on the enclosure surface for two different thermal boundary conditions. For the cases considered, the surface-averaged mean deposition velocity was found to be in the range (2-4) x 10^-4m.s^-1 for ²¹⁸Po and (1-3) x 10^-4 m.s.^-1 for ²¹²Pb. The production and decay of ²¹⁸Po affects the spatial distribution of its concentration, the magnitude of its deposition velocity and its dependence on location compared with that of ²¹²Pb, which practically behaves as a stable species with respect to the timescale of convective motion. The local deposition velocity is observed to depend strongly upon the position of the surface with respect to the flow. An order of magnitude difference between the maximum and minimum values of the deposition velocities for both species is seen. The results have significance for assessing the health risk associated with indoor exposure to ²²²Rn and ²²⁰Rn decay products and for investigating mass transport aspects of the more general problem of the interaction of air pollutants with indoor surfaces.