A COMBUSTION STUDY OF METAL POWDERS IN CONTACT WITH SODIUM-NITRATE

Combustion phenomena of metal powders in contact with NaNO3 used as pr opellants have been studied in order to find out burning rate variatio ns 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 mixt ure ratio of the fuel and oxidiser. The burning rate of Mg-NaNO3 and Z r-NaNO3 propellants decreases with increasing concentration of NaNO3 i n contrast to the energy content of the propellants. On the other hand the burning rate as well as the energy content of Al-NaNO3 and B-NaNO 3 propellants increases with increasing NaNO3 concentration. Thermal d ecomposition 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 rate s. 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 concentrat ion of molten oxidiser, thus provide vapour phase combustion leading t o low burning rates. In contrast, decomposition of the propellants con taining Al and B occurring after the metal particles acquire sufficien t energy for ignition due to the presence of protective nature of thei r oxide layers around the metal particles leading to vapour phase comb ustion at all mixture ratios. This causes an increase of burning rates with increasing oxidiser content of the mixture up to the stoichiomet ric ratio. The burning surface temperature data and the photographs of partially burnt samples also support the suggested mechanism.