The effect of polymeric binder on composite propellant flame structure investigated with 5 kHz OH PLIF

High speed (5 kHz) planar laser-induced fluorescence (PLIF) and high resolution imaging are used to probe the flame structure and to image coarse ammonium perchlorate (AP) particles on the surface of deflagrating bimodal composite propellants formulated with various binders. Three binder systems are examined: hydroxyl-terminated polybutadiene (HTPB), polybutadiene acrylonitrile (PBAN) and dicyclopentadiene (DCPD). A comparison of coarse AP particle behavior and flame structure is presented for each propellant over a pressure range of 1–6.4 atm. Individual AP particle ignition delay, burn time, and flame heights are quantified. Both jet-like diffusion flames and lifted, overventilated flames were observed for the propellants examined. The average diffusion flame height is observed to increase gradually over the pressure range studied. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) was also performed on the AP/binder systems. It was found that both AP/HTPB and AP/PBAN react together exothermically while AP/DPCD reacts independently, with a quick binder pyrolysis proceeding AP decomposition. Propellants formulated with HTPB, PBAN, and DCPD were found to possess a similar diffusion flame structure, but those with DCPD were measured to have a significantly higher burning rate. Propellants formulated with DCPD were observed to routinely eject coarse AP crystals from the burning surface, which results in the higher burning rate. Based on experimental observations, it is argued that either fast binder pyrolysis or poor adhesion to coarse AP particles is the mechanism responsible for the particle ejection.