Pergamon PH: SOO21-8S02(96}OO277-7 J. ..1.,,,,01 Sd.. Vol. 27. Suwl I. W. 5409-5410, 1996 Copyrighl 4d 1996 ElseVIer Science Ltd Printed in 0 rea1 Bntam , All nghll reserved S15.00 +0.00 DISTRIBUTION OF AEROSOLS IN THERMAL BOUNDARY LAYERS P. L. GARCIA-YBARRA and J. L. CASTILLO Dept. Ffsica Fundamental, UNED, Apdo. 60141, 28080 Madrid (Spain). KEYWORDS Thermophoresis, Boundary Layer, Aerosol Distribution, Aerosol Deposition The distribution and deposition rate of aerosols in laminar boundary layers around solid bodies are affected by the presence of temperature gradients. For the range of submicron aerosol particles and due to the large mass disparity between aerosol particles and gas molecules, the Schmidt number Sc= vlD (carrier gas momentum diffusivity over aerosol particle diffusivity) and the thermal diffusion factor, ar (relative importance of thermophoresis with respect to Fickian diffusion transport) become very large but the ratio, a= arISe, remains of order unity. Then, thermophoresis becomes the main diffusive transport with Fickian diffusion playing a secondary role (Rosner, 1980, Rosner et al., 1992). Inside the self-similar laminar boundary layer around a wedge, the mass fraction distribution of a dilute aerosol, 'P is governed by a transport equation of the form (Garda- Ybarra and Castillo, 1996) Se· 1 'P' +[/ + a (In6)']'P' +a(Jn6t 'P = 0 with primes denoting derivatives respect to the boundary layer similarity variable, 71 . The first terms accounts for the Fickian transport. The second term stands for the addition of advective transport (with/being the usual Blasius function) and thermophoretic drift (where 8 == TIT.. is the dimensionless temperature distribution, being T.. the mainstream , temperature). The sum / +a(Jn 8) accounts for the particle velocity component normal to TJ . Moreover, the third term comes from the compressibility (non-zero divergence) of the associated thermophoretic velocity field. In the large Schmidt number limit, Sc » I, the solution of this second order ordinary differential equation becomes a singular perturbation problem which has being studied by taking Se- I as the smallness expanding parameter. Depending on the coefficients of the first derivative term, the solutions may exhibit different qualitative behaviors. Whereas the Blasius function, f, takes always positive values and , vanishes at the wall, the thermophoretic contribution, a(ln6) , vanishes at the outer edge and has the same sign throughout the boundary layer. But, it can either be positive or negative, leading to two completely different situations which have been analyzed in the present work. , When a(Jn8) > 0, (a wall colder than the mainstream) thermophoresis drives the aerosol particles toward the wall. Thus, cold walls act as thermophoretic precipitators which collect particles from particle-laden gases. The boundary layer can be separated in an outer S409