Kinetic modeling of the ignition delays in monomethylhydrazine/oxygen/argon mixtures

Ignition delay times for monomethylhydrazine (MMH or CH ₃NHNH₂)/O₂/Ar gaseous mixtures have been modeled by a reaction scheme containing 70 species and 373 equilibrated elementary reactions. For many reactions, the rate constants had to be estimated or adjusted because rate constants are available for only a few reactions. The basis for a comparison with reality is measurements of ignition delay times behind reflected shock waves performed over a range of temperatures, pressures, and composition. Good agreement between measured and calculated ignition delay times was obtained for lean and less dilute mixtures. A least-squares analysis of the computed ignition delays provides power dependences of the concentrations and an activation energy very similar with those obtained experimentally. The relative importance of the different reactions has been clearly shown by performing different sensitivity analyses. The reaction begins with the scission of the N-N bond in MMH, but elimination reactions from MMH also have to be considered, especially at high initial temperature. The NH₂ radicals formed react with MMH to give CH₃NNH₂ radicals and products. These radicals react with O₂ to produce methyldiazene (CH₃N₄NH) and HO₂ radicals. These two species appear to be very important for this specific chemistry. In particular, HO ₂ radicals react with MMH to give CH₃NNH₂ radicals and H₂O₂, this last species playing a major role through its thermal decomposition that produces hydroxyl radicals.