Emittance of condensed oxides in solid propellant combustion products

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The spectral radiance of hydrogen-oxygen-water-alumina and RP-1-oxygen-magnesia rocket flames was recorded and the magnitude of continuum emission measured. This measurement, with knowledge of the flame geometry and the oxide particle-size distribution, mass fraction, and concentration, was converted into a quantity of radiation from each particle and a spectral hemispherical emittance calculated. The near-infrared emittance of liquid alumina particles 1 to 10 microns in diameter varied from about 10 −2 to 10 −1 as the temperature increased from the melting point to 2900°K. The emittance of solid magnesia particles of a similar size range at 0.59 microns wavelength was found to average 0.4 over a range of temperatures. Mie theory calculations were made for both the magnesia and the alumina. Using best available low temperature optical properties of magnesia, the emissivity was calculated and found to be about 3 orders of amgnitude below the experimental measurements. Calculations were also made for solid-phase alumina particles based upon available temperature-dependent optical properties valid up to the melting point of alumina. These calculations showed that solid alumina particles of the same size as the experimental particles had an average nearinfrared spectral emittance varying from about 10 −5 to 10 −4 as the alumina temperature changed from 1800°K to the melting point, 2320°K. Thus, these experiments indicate that a discontinuous jump in emittance of alumina occurs as the phase changes from solid to liquid. In addition to description of the experiments and results, some possible physical effects associated with radiant emission from condensed phase particles are discussed.