Combustion behavior and thermophysical properties of metal-based solid fuels

Abstract

Two metal-based solid fuels (magnesium-based and boron-based) have been studied to determine their combustion behavior and thermophysical properties. The burning rate for the magnesium-based (Mg/PTFE/Viton A) solid fuel was found to increase monotonically with ambient pressure and to follow the Saint Robert's law in both air and nitrogen environments. The fuel, however, burned 10% slower in air than in nitrogen. The slower burning rate in air is postulated to result from the entrained oxygen which competes with fluorine to react with magnesium. Because of the lower heat of formation of MgO vs MgF2, the near-surface heat release is reduced when the oxygen is present, thus reducing the burning rate. This reasoning is also supported by results obtained from the companion pressure deflagration limit (PDL) and ignition tests, which show that the combustion of the magnesium-based fuel has a higher PDL and a longer ignition delay time in air than in nitrogen. Results from the study of ignition and combustion characteristics of boron-based solid fuels show that boron can significantly reduce the ignition delay times of poly(BAMO/NMMO). In determining the thermophysical properties of fuel samples, a subsurface temperature-measurement method was developed to quantify the temperature dependence of the thermal diffusivities of fuel samples. Results show good agreement with those obtained with the laser-flash method.