Effect of Particle Size on Combustion of Magnesium-Sodium Nitrate Propellants

Abstract

The Combustion of Mg-NaNO₃, propellants at different mixture ratios has been studied to establish the effect of particle size on the burning rate characteristics. Results indicate that compositions containing NaNO₃, of finer particle size (50 μm) give higher burning rate at high fuel content of the mixture than at the stoichiometric ratio, whereas the compositions containing NaNO₃ of coarser particle size (250μm) show increased burning rate with increasing oxidiser content (maximum at the stoichiometric ratio). Thermal decomposition results indicate that condensed phase heat release as well as reactions in the vapour phase are responsible for the variations in the burning rate. Decomposition products of finer size NaNO₃, formed before Mg particles acquire sufficient energy for ignition, react exothermically with the Mg particles and lead to condensed phase heat release. This energy release, which is larger at higher fuel contents: is seen as causing higher burning rates with low pressure sensitivity. Increasing oxidiser content reduces the heat release in the condensed phase due to oxide coating of the metal particles with the higher concentration of the molten oxidiser leading to formation of metal agglomerates which eject from the surface and cause lower burning rate with high pressure sensitivity. In contrast, the decomposition products of coarser size NaNO₃ diffuse out along with the metal particles for the reaction to occur in the vapor phase. This causes an increase of burning rate with increasing oxidiser content of the mixture up to the stoichiometric ratio with a pressure dependence. Further, the burning rate was found to increase with decreasing particle size of Mg due to the faster rate of heat transfer with the increase in surface area of the particles. The burning surface temperature data obtained during static tests at different mixture ratios and particle sizes also support the suggested mechanism.

Key words:

• Mg-NaN0₃ propellants
• particle size effect
• burning rate
• combustion
• thermal decomposition
• metal agglomeration