Ignition and combustion phenomena of laser-ignited boron agglomerates at elevated pressures

Boron agglomerates having nominal initial diameter of 534 μm suspended at the tip of SiC fibers were ignited by a focused CO₂ laser in a quiescent O₂/N₂ (20/80) environment. A near-infrared two-color pyrometer and a broadband radiometer were used to measure the surface temperature and the radiation emitted by the burning agglomerates. From the temporal outputs of the pyrometer and radiometer, the ignition delay time, the burning time and the surface temperature of the boron agglomerates were obtained. High speed cinematography (1000 f.p.s) was also used to observe the phenomenological events; i.e., the flame structure, and to correlate with the output obtained from the pyrometer and the radiometer. In the range of pressures (1.7 to 20.4 atm) investigated in the present study, three distinct combustion regimes were observed. In regime I (1.7 to 5.1 atm) partial or incomplete burning of the boron agglomerates occurred. In regime II (6.8 to 13.6 atm) steady-state, full-fledged burning was observed, in which the burning time decreases with increasing pressure. In regime III (above 13.6 atm ) violent combustion with frequent particle ejections was observed. The surface temperature of the burning boron agglomerates in regime I is consistently below the boiling temperature of boron oxide (2316 K). In regime II, the agglomerate surface temperature is in the vicinity of 2400 K, whereas, the surface temperature is above 2400 K in regime III. Due to very high surface temperatures attained, and corresponding fragmentation and shattering of boron agglomerates in regime III, it appears almost certain that the agglomerate melting does occur at higher pressures. Limited investigation of the effect of oxygen mole fraction on burning time showed that increasing the oxygen mole fraction has the same effect as that of increasing pressure; i.e., decrease burning time. Qualitative studies of flame structure from high-speed cinematography showed that the flame surrounding the boron agglomerate at all pressures consists of two distinct zones: a luminous inner zone surrounded at its edge by a green envelope (BO₂). The size and luminosity of the inner zone increases with increasing pressure. The presence of boric oxide (BO₂) at all pressures investigated, imparts significance to its inclusion in the modeling of boron agglomerate combustion.