Coupled dipole calculation of extinction coefficient and polarization ratio for smoke agglomerates

The mass specific extinction coefficient and the polarization ratio at 90 d egrees of simulated smoke agglomerates are computed for clusters with 3 to 1390 primary spheres and for size parameter from 0.05 to 0.6 using the coup led electric and magnetic dipole (CEMD) method. For the size parameter rang e of interest for postflame smoke, the specific extinction coefficient comp uted by CEMD is 21% to 36% greater than Rayleigh prediction, and for this s ize range for clusters with 100 to 1000 primary spheres, the range in the s pecific extinction coefficient is about 12%. Agreement between measured and predicted specific extinction coefficient would require selection of a ref ractive index outside the range of values widely used by the combustion com munity. The polarization ratio at 90 degrees is found to be a monotonic fun ction of primary sphere size and relatively independent of agglomerate size . Comparison with the limited experimental results indicates potential For polarization ratio as a diagnostic for primary sphere size. (C) 1999 by The Combustion Institute.