Optical Constants of Al2O3 Smoke in Propellant Flames

An in situ light scattering and extinction technique was developed to determine the optical constants (n - ik) and mean optical size (d(32)) of molten Al2O3 smoke particles in propellant flames. Direct transmittance and bidirectional transmittance and reflectance measurements were made using scattered laser light on aluminimized solid propellant flames at visible and near infrared wavelengths (lambda(1) = 0.6328 mu m and lambda(2) = 1.064 mu m). The optical properties of the molten Al2O3 smoke combustion product were obtained from the light scattering and extinction measurements by inverse solution of the radiative transfer equation. A mean optical size of d(32) = 0.97 mu m was obtained, which agrees well with other reported values. The values of n obtained for molten Al2O3 at 2680 K (n(m,lambda 1) = 1.65 and n(m,lambda 2) = 1.64) were significantly less than the values which have been reported for solid Al2O3 at temperatures just below the melting point of 2320 K (n(s,lambda 1) = 1.82 and n(s,lambda 2) = 1.81) indicating that a substantial decrease in n occurs upon melting. This decrease in n can be attributed to the expansion that takes place upon melting and is in good qualitative agreement with the predictions of the Lorentz-Lorenz equation. The value of k obtained for molten Al2O3 at 2680 K was 0.006 +/- 0.004 (at both wavelengths), which is in reasonable agreement with other reported values. A dispersion analysis was also performed to fit this and other data over the spectral region from 0.5 to 5.0 mu m and for temperatures from 2320 to 3000 K.