Particle segregation in aerosol flow

• Theoretical assessment of aerosol homogeneity.
• Calculation of segregation index along a laminar flow tube.
• Representative samples better extracted from tube centerline.

It is shown how the uniformity of a polydisperse aerosol can be assessed by means of a segregation index, defined in terms of the mean and the mean square of n(Dp;r→)dDp, the number concentration of particles with diameter in the range (Dp,Dp+dDp)(Dp,Dp+dDp) in a volume element centered at r→. The segregation index is zero for a uniform aerosol and attains positive values for any other situation, in such a manner that the larger the value of the index, the worse the aerosol uniformity. As an illustrative example, numerical calculations have been performed for a polydisperse aerosol undergoing simultaneous diffusion and electrostatic dispersion in a laminar flow tube. In this case, the segregation index, zero at the tube entrance (uniform aerosol), increases steadily as the aerosol approaches the tube outlet. Some practical implications for aerosol sampling from a duct are discussed.