Oxidizer coarse-to-fine ratio effect on microscale flame structure in a bimodal composite propellant

The microscopic flame structure of a composite propellant is expected to change significantly as the particle size distribution is varied. In this paper we report observations of the diffusion flame structures in burning ammonium perchlorate (AP) composite propellants with varying ratios of coarse to fine AP. The coarse-to-fine ratio (C/F) was varied between 1:16 (mostly fine AP) to all coarse AP. Five kHz OH planar laser-induced fluorescence (PLIF) was used for in situ imaging of the highly transient and microscale flames above single and groups of AP particles at atmospheric and elevated pressures. Jet-like diffusion flames are observed for all propellants above coarse AP crystals at 1 atm. At elevated pressures, both jet-like and lifted arched inverted overventilated diffusion flames (IOF) were observed. Jet-like flames were seen more frequently for propellants with low C/F ratios, and were rarely seen for propellants with the highest C/F ratios. On the other hand, lifted IOF were seldom seen for the low C/F ratio propellants but were more frequently observed for higher C/F ratio propellants. Differences in the flame structures are postulated to be due, in part, to the dissimilar local burning rates and flame temperatures between the fine AP/binder matrix and coarse particles. For the first time, the diffusion flames from multiple coarse particles were observed to merge (group combustion), especially for higher C/F ratio propellants. As a consequence, flame height is affected by this group combustion of clustered coarse AP particles that behave similarly to a larger single particle. The ignition delay and lifetime for single AP particles in a composite propellant were measured at 1 atmosphere (atm). The particle lifetime and ignition delay for coarse AP particles vary as a function of C/F, indicating that nearby coarse particles can significantly affect coarse particle ignition and combustion. The data obtained is useful for comparison to numerical simulations of AP composite propellant combustion.