A Combustion Study of Metal Powders in Contact with Sodium Nitrate

ABSTRACT

Combustion phenomena of metal powders in contact with NaNO₃ used as propellants have been studied in order to find out burning rate variations at different oxidiser levels. Commonly used metals like Mg, Al, Zr and B (a metalloid) have been considered for the present study. It is found that the burning rate of the propellants largely depends on mixture ratio of the fuel and oxidiser. The burning rate of Mg-NaNO₃ and Zr-NaNO₃ propellants decreases with increasing concentration of NaNO₃ in contrast to the energy content of the propellants, On the otherhand the burning rate as well as the energy content of Al-NaNO₃ and B-NaNO₃ propellants increases with increasing NaNO₃ concentration.

Thermal decomposition results indicate that propellants containing Mg and Zr at low oxidiser content have higher condensed phase/surface heat release before the metal particles acquire sufficient energy for ignition and hence provide surface mode of combustion leading to high burning rates. At higher oxidiser contents, they have a lower heat release in the condensed phase and at the surface with a delay in ignition due to the oxide coating of the metal particles in presence of higher concentration of molten oxidiser, thus provide vapour phase combustion leading to low burning rates. In contrast, decomposition of the propellants containing Al and B occurring after the metal particles acquire sufficient energy for ignition due to the presence of protective nature of their oxide layers around the metal particles leading to vapour phase combustion at all mixture ratios. This causes an increase of burning rates with increasing oxidiser content of the mixture upto the stoichiometric ratio. The burning surface temperature data and the photographs of partially burnt samples also support the suggested mechanism.

Key Words:

• Metal-Oxidiser propellants
• effect of NaNO₃
• burning rate
• combustion
• thermal decomposition
• metal agglomeration