Shock tube study of the pressure dependence of monomethylhydrazine pyrolysis

Monomethylhydrazine (MMH, CH₃NHNH₂) pyrolysis was studied behind reflected shock waves using a laser absorption method. NH₂ concentration time-histories in MMH/argon mixtures were measured over the temperature range of 1100–1400 K and pressures 0.3–5 atm. The MMH pyrolysis mechanism developed by Sun et al. (2009) [9], with the update by Cook et al. (2011) [11], was used to simulate the NH₂ time-histories and to compare with the experiment. The rate constant of the reaction: MMH = CH₃N·H + NH₂(1a) was determined based on the NH₂ time-history measurements. Pressure dependence of k_1a was observed at 0.3–5 atm. The measured reaction rate constants follow a pressure dependence trend close to the theoretical results by Zhang et al. (2011) [10] based on transition state theory master equation analysis, and reducing their theoretical results by ∼40% leads to a close match with the current data. With the experimentally determined k_1a, the simulation results match closely with the NH₂ time-histories at early times. Utilizing the later times of the NH₂ time-histories, we find that a good fit is achieved with the reaction rate constant of k₂ = 1.5 × 10¹⁴ exp (−755/T) cm³/mol/s for the reaction: NHNH₂ + H = NH₂ + NH₂ (2). Simulation results using the modified mechanism, with the updated k_1a and k₂, match well with the measured NH₂ time-histories in the current study.