The effects of aluminum particle size on aluminized propellant combustion

The goal of this thesis was to: test current ideas regarding the complex behavior of aluminized propellant combustion, extend understanding to conditions not previously clarified, and demonstrate the applicability of the results in the tailoring of propellant formulations to specific applicational needs. The study focused on ammonium perchlorate (AP), hydrocarbon binder, aluminum (Al) formulations. All formulations had 11% polybutadiene (PBAN) binder and 89% solids (i.e., 18% Al and 71% AP). The primary propellant formulations were Al and fine AP (fAP) particle sizes. The values of these variables were chosen according to current practical standards and ideas on how to tailor the Al behavior. Bimodal size distributions were used for AP and Al. For bimodal Al formulations, the coarse Al particle size was 30μm (nominal) and the fine Al particle size was 0.1μm (nominal), and for bimodal AP, the cAP particle size was 400μm (nominal) and the fAP particle size was either 82.5μm or 10μm (nominal). The effects of the Al particle size, Al c/f ratio, fAP particle size and the AP c/f ratio were examined for monomodal and bimodal aluminized propellants. This experimental investigation challenges conventional thinking with regards to the mechanisms involved with Al combustion and provides guidelines in formulating propellants with minimal losses in performance.; The results showed the existence of an intense aluminized burning region (ABR) very close to the propellant surface with ultra-fine Al (UFAl) and 3μm Al that encouraged heat feedback to the flame front and to the propellant surface in the form of radiation and conduction. The high burning rates observed with UFAl were part of a continuum of reducing the size of the Al particles, which also lead to the presence of a dense ABR close to the propellant surface. It was also shown that major modification to the burning rates could be achieved by moderate amounts of UFAl and/or significant reduction in the AP c/f ratio. A detailed investigation was carried out on the nature of the gas phase flame complex with the goal of identifying (mapping) where the gas phase temperature is high enough to melt the Al₂O₃. The results showed the amount and degree of modification to the burning rates was found to be dependent upon the ability to ignite the Al through leading edge flames (LEF) and/or the matrix flame.; The use of UFAl significantly reduced the total production of the Non-Smoke Combustion Residue (NSCR) and increased the Smoke Combustion Residue (SCR). This phenomenon was found to be very dependent on the Al c/f ratio, AP c/f ratio and fAP size. The amount of agglomeration was found to be dependent upon the Al c/f ratio and pressure to the extent that large agglomerations were eliminated from the collected NSCR. The UFAl was found to burn to submicron residuals and SCR, this suggested the UFAl agglomerated significantly. In addition, the presence of the ABR close to the propellant surface was thought to disrupt and alter the conventional burning of the 30μm Al. This disruption in the 30μm Al burning was thought to be significant and responsible for large reductions in the collected NSCR.; The results from the investigation demonstrated that manipulation of bimodal Al distribution could effectively tailor both burning rates and residual oxide production.