The influence of radiative heat feedback on burning rate in aluminized propellants

Metalized solid propellants have higher final flame temperatures and higher flame radiant intensities than nonmetalized propellants. However, the importance of radiative heat feedback in metalized propellant combustion is still unknown. In this study, radiative effects in aluminized propellants were examined by embedding optical fibers and microthermocouples in propellants to measure radiative and conductive feedback. Extinguishment by rapid depressurization was used to determine the condition and location of the thermocouple and optical fiber at the burning surface. Hemispherical reflectivity measurements were also made to determine propellant absorptivity. The reflectivity measurements showed that even a small amount of aluminum powder in a propellant increases the absorptivity significantly through multiple scattering in spite of the intrinsically high-bulk reflectivity of aluminum. The results of this study confirm previous findings that the effect of radiation on burning rate in nonmetalized propellants is negligible because of low incident flux from the flame and low absorptivity of the propellant. On the other hand, in aluminized propellants, radiative heat feedback is a significant fraction of the total feedback and is strongly dependent on pressure and metal loading. At 1 MPa with 20% A1 radiative feedback is 26% of the total heat feedback. It is concluded that while radiative feedback does not dominate the burning rate its effect is not negligible in aluminized propellants, at least in the pressure range 0.5-10 MPa. These results suggest that radiative feeback should also be considered in propellants containing other metals.