Selective radiative preheating of aluminum in composite solid propellant combustion

A two-phase model has been developed to study aluminum (Al) particle preheating through selective radiation absorption in composite solid propellants. The two phases considered are one strongly absorbing particle (Al) phase and another weakly absorbing matrix (ammonium perchlorate (AP), binder and catalyst particle) phase surrounding the Al phase. Separate energy balance equations for the Al and matrix phases are developed. Both the matrix and the Al phase are assumed to be nonemitting, anisotropically scattering, absorbing media. The parameters identified which strongly influence Al preheating and melting are Al size, mass fraction, burn rate, and level of incident radiant flux. It was found that large Al mass fractions and small Al particle sizes promote lower Al temperatures. The effect of adding submicron iron oxide burn rate catalyst particles on aluminum preheating was also investigated. It was found that the addition of small amounts of catalyst can theoretically reduce Al temperatures significantly by dominating the optical properties. These results should prove useful to propellant formulators in trying to reduce the problem of unwanted Al agglomeration.