An experimental study of monopropellant combustion in small volume

Combustion of liquid nitromethane in a cylindrical chamber with a volume of 108 mm³ and diameter of 5 mm was experimentally investigated. The meso-scale combustor utilized a vortex combustion concept, which has been demonstrated to be useful for stabilizing combustion of gaseous hydrocarbon air mixtures in small volumes. The monopropellants were injected tangentially from the backend of the cylindrical combustor and the combustion products exited the chamber tangentially at the other end. Stable combustion of nitromethane was not found achievable at atmospheric pressure, although combustion can be self-sustained by enhancing the kinetic rates of the reaction with the addition of a small amount of oxygen (air). Pressurization of the combustor has an equivalent effect as oxygen enrichment on flame stabilization due to the high sensitivity of nitromethane kinetic rates to pressure. Stable combustion in the vortex combustor was achieved at pressures higher than 260 psi, although various hydrocarbon compounds such as CH₄, C₂H₂, and C₂H₄ are found in the FT-IR spectra of the combustion products at low pressures suggesting incomplete combustion due to insufficient flow residence time. Complete combustion was achieved at chamber pressures above 350 psi. The chemical power input was ∼230W for all the cases investigated.