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"Thermal Analysis of Aluminum Particle Combustion in a Simulated Solid Propellant Flame"

R. B. White, S. W. Dean, M. L. Pantoya, D. A. Hrschfeild, W. Gill and W. W. Erikson

Accepted to the Journal of Propulsion and Power, April 2007

Aluminum particles have been shown to provide performance enhancements when used in propellant systems due to the large amount of heat released during aluminum combustion and the ensuing formation of aluminum oxide. The contribution of aluminum particles and their combustion products to the heat transfer characteristics of impinging flow geometries is not well understood. Experiments were performed to elucidate the role of Al particle combustion on the heat transfer characteristics of a propellant flame. Measurements of temperature and heat flux delivered to an impinged surface were made. A particle-laden oxygen-acetylene flame was used to simulate the burning behavior of a solid propellant in a controlled, reproducible manner. Yttria Stabilized Zirconia (YSZ), Alumina-Titania (AlTi), and Aluminum (Al) powders were examined in order to compare and quantify the heat flux contribution of each type of powder. The main focus was Al, so inert powders YSZ and AlTi were used to compare reacting, non-reacting and melting powders. Copper coupons captured and quenched reacting Al particles from the flame. A scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and a differential scanning calorimeter (DSC) were used to examine the degree of completion of Al oxidation. The area encompassed by the aluminum melting endotherm on the DSC plot was used as quantification for available Al energy for the oxidation reaction. Oxygen-acetylene ratio and standoff distance are varied and the change in available Al energy was observed. This allowed for a calculation of Al reaction percentage as a function of flame and environment conditions. Correlations were made between particle properties in the flow and heat flux delivered to an object's surface.

Results show that flames seeded with particles which remain in the solid phase during the combustion process experience minor enhancement to heat flux (1.9%) compared to gas-only flames. Flames seeded with AlTi particles, which melt in the flame zone, show an 80.2% increase in heat flux over gas-only flames. Reacting Al particles provide the greatest increase in heat flux, yielding a 232.7% gain over gas-only flames. DSC results show that Al consumption percentages for the flame with 2.5 oxygen-fuel ratio (OFR) are an average of 8.6% higher than those for the 1.5 OFR flame. Al reaction percentages increase steadily as standoff distance from the torch nozzle increases, with one limitation discussed later in the text.