Boron particle ignition and combustion at 30-150 ATM

The ignition and combustion of boron particles is investigated at high pressures and temperatures produced by the combustion (unsteady fast deflagration) of nitrogen-diluted hydrogen/oxygen mixtures. A novel device is described to inject particles after sufficient delay for combustion and gas mixture transients to equilibrate, permitting particle ignition at high temperature and nearly constant pressure conditions. Particle injection is calibrated at high pressure with aluminum particles showing agreement with independent determinations. Using this technique, the ignition and combustion times of ~24 micron crystalline boron particles are measured over a range of pressures (30-150 atm), temperatures (2440, 2630, 2830 K), and excess O₂ concentrations (5, 11, 20%). Although boron has been observed elsewhere to exhibit a two-stage ignition process at lower pressures and temperatures, only a single stage is observed here. Boron particle ignition delays are reduced with increased pressure, decreased particle size, and increased temperature. Combustion times drop significantly between 2440 and 2600 K, decreasing by a factor of at least two. Two proposed ignition-enhancing agents, CO₂ and HF, show no signs of accelerating ignition and 5% HF actually increases ignition delays. Measured ignition delays are compared to predictions from two ignition models, one incorporating a convective heating model, the other detailed surface chemistry, showing generally good agreement for ignition delays except for an underprediction of the measured decrease in particle ignition delays with increasing pressure. This study demonstrates that boron particle lifetimes at elevated pressures are sufficiently short to make particles smaller than 20 μm suitable for high-speed air-breathing propulsion applications.