Ignition and combustion of boron particles in hydrogen/oxygen combustion products at 30 to 150 atmospheres

The ignition and combustion of crystalline boron particles is studied at high pressures and temperatures in the combustion products of nitrogen diluted premixed hydrogen/oxygen mixtures at nearly constant pressure conditions. Particle ignition and combustion are monitored optically by measuring combusting particle emissions. The methodology is used to measure the ignition delay and combustion times of {dollar}sim{dollar}24 micron boron particles over a range of pressures (30-150 atm), temperatures (2440, 2630, 2830 K), excess O{dollar}sb2{dollar} concentrations (5, 11, 20%), and with two proposed ignition enhancers (CO{dollar}sb2{dollar}, HF). Several particles sizes are investigated at one condition ({dollar}sim{dollar}12, {dollar}sim{dollar}24, {dollar}sim{dollar}48 {dollar}mu{dollar}m) to determine particle scaling laws.; Although boron has been observed previously to exhibit a two-stage ignition process at lower pressures and temperatures, only a single continuously increasing luminous stage is observed here. Boron particle ignition delays for {dollar}sim{dollar}24 {dollar}mu{dollar}m are of the order of 1-2 milliseconds, and are reduced with increased pressure, decreased particle size, and increased temperature but increase with increasing ambient oxygen concentrations. Combustion times for {dollar}sim{dollar}24 {dollar}mu{dollar}m particles are between 1-5 msec, but drop significantly between 2440 K and 2600 K, decreasing by a factor of at least two, and are reduced with increased pressure. Both ignition delays and combustion times obey a {dollar}dsp1{dollar}-scaling law (linear) as particle diameter increases, suggesting processes are kinetics-controlled. The two tested ignition-enhancing agents show no signs of accelerated ignition and 5% HF was found to increase ignition delays.; Measured ignition delays are compared to predictions from two ignition models showing generally good agreement in the average sense when convective heating is handled appropriately; however, the models under-predict the measured decrease in boron particle ignition delays with increasing pressure and do not predict the sharp decrease in ignition and combustion times as temperature is increased between 2440 K and 2600 K.; The results of this study demonstrate that boron particle lifetimes at elevated pressures are sufficiently short to make these particles suitable for additives to propellants and that smaller 1-10 {dollar}mu{dollar}m sized particles, which have still shorter lifetimes, may be appropriate for addition to explosives.