Ignition and combustion characteristics of boron particles under reduced pressure

The ignition and combustion processes of boron particles are crucial to achieving high combustion efficiency in solid-fuel ramjet engines, particularly under reduced static pressure conditions in the secondary combustion chamber. This study carried out ignition and combustion experiments on amorphous boron particles with an average size of ∼3 µm, using a Hencken multi-diffusion flat flame burner under controlled pressures (0.3–1 atm) and temperatures (1900–2200 K). Optical measurements were utilized to qualify the ignition time, which increases with decreasing pressure. Then, the ignition and combustion models of small-size boron particles under reduced pressure were established, for which the Langmuir layer was introduced to calculate heat and mass transfer between particles and surrounding gases in the transition regime. Results showed that the Langmuir layer significantly lowered heat transfer rate, increasing the ignition time. A comparison of the heat fluxes of evaporation, heterogeneous reaction, and heat convection for the ∼3 µm boron particle demonstrated that the ignition process was limited by heat convection, while the combustion process is dominated by the heterogeneous reaction near the particle surface.