Optical Measurement of Size Histories of Boron Particles in Ignition and Combustion Stages

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Abstract
An apparatus employing an Argon-Ion laser with light polarized perpendicularly to the scattering plane was constructed to measure the size histories of boron particles in the ignition and combustion zones. Three powder samples, having initial radius around 0.05 μm, 3.5μm, and 5.0 μm, were studied. Two kinds of measurement of relative light scattering intensities were performed: one is the relative scattered intensity at two different angles (backward and forward) to determine the size histories of particles with initial radius around 0.05 μm, and another is the relative backward scattered intensity to determine the size histories of particles with initial radius around 3.5μm and 5.0μm. The measured light scattering intensities were interpreted by Mie theory. It was found that the particle-size change is negligible during ignition, and particle radius decreases linearly with time in the combustion zone. Further considerations showed that the combustion of these kinds of boron particles is controlled by surface chemical kinetics. From analysis of the data, we have found that the chemical reaction rate,ω˙ on the boron particle surface during full-fledged combustion can be expressed as toω˙ = kpXo, in which k = 0.0625 ± 0.0125 (mole/cm2sec-atm), corresponding to a reaction probability 0.4 = 0.08 at T = 2500 K, and faring smoothly with earlier data at low temperature. These data provide a basis for calculation of clean-surface boron particle combustion.

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