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 (d₃₂) 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 (λ₁ = 0.6328 μm and λ₂ = 1.064 μm). 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 d₃₂ = 0.97 μm was obtained, which agrees well with other reported values. The values of n obtained for molten AhO3 at 2680 K (n_mλ1 = 1.65 and n_mλ2 = 1-64) were significantly less than the values which have been reported for solid AhOs at temperatures just below the melting point of 2320 K (n_sλ1 = 1.82 and n_sλ2 = 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 μm and for temperatures from 2320 to 3000 K.