A Combustion Study of Metal Powders in Contact with Sodium Nitrate

Abstract

Combustionphenomenaofmetal powdersin contact with NaNO, used as propellantshave been studied in order to find out burning rate variations at different oxidiser levels. Commonly used metals like Mg, AI,Zr and B(a metalloid)have been consideredfor the presentstudy. It isfound thatthe burningrate of the propellants largely depends on mixture ratio of the fuel and oxidiser. The burning rate of Mg-NaNO, and Zr-NaNO, propellants decreases with increasing concentration of NaNO, in contrast to the energy contentofthepropellants. On theotherhand the burningrate as wellas the energy contentof AI-NaNO,and B-NaNO, propellants increases with increasing NaNO, 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 phaseandat the surface with a delay in ignition due to the oxide coating ofthe metal particlesin presence of higher concentrationofmolten 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 combustionat all mixture ratios. This causes an increase of burning rates with increasingoxidisercontentof the mixture upto the stoichiometric ratio. The burning surface temperature data and the photographs of partially burnt samples also support the suggested mechanism.