Dual-stage ignition of boron particle agglomerates

The ignition characteristics of boron particle agglomerates prepared by drying 2–3 mm water slurry droplets of crystalline or amorphous boron powders were investigated by introducing suspended agglomerates into a flow of high temperature oxidizing gases (oxygen/nitrogen/water vapor) obtained by a combination of combustion and electrical heating. Monitoring of the agglomerate luminosity, spectra, and temperature revealed a dual-stage ignition process. The first stage ignition was observed at flow temperatures as low as 715 K and was followed by agglomerate quenching if the oxygen concentration in the flow was below some critical value. As the oxygen concentration of the flow was increased beyond this critical value, the second stage ignition leading to full-fledge combustion was observed. The ignition characteristics of boron particle agglomerates were theoretically investigated by developing a numerical model that considers the oxygen concentration gradient inside the agglomerate in order to simulate the ignition process. The model results accurately captured the qualitative behavior of the first stage ignition and quenching as well as the dual-stage ignition processes. The model identified the closure of pores in the agglomerate due to the build-up of the boron oxide layer, which effectively seals off the large interior surface area of the agglomerate from the oxidizer, as being the mechanism responsible for the two-stage ignition phenomenon. The critical flow conditions in terms of flow temperature and ambient oxygen concentration for the second stage ignition were determined by this model. The experimental result for the critical ambient oxygen concentration for the second stage ignition in dry flow was determined to be 70%, which is fairly close to the model prediction of 79%.