Shock tube measurements of species time-histories in monomethyl hydrazine pyrolysis

Abstract

The first measurements of NH 2 and NH 3 time-histories in monomethyl hydrazine (MMH) pyrolysis were performed behind reflected shock waves in a shock tube using laser absorption techniques. An improved measurement of MMH using IR laser absorption is also presented. MMH concentrations of ∼1% in Ar were employed, over the temperature range 941–1252 K, at pressures near 2 atm. NH 2 was measured at the peak of the overlapping A ˜ A 1 ← X 2 B 1 ( 090 ← 000 ) Σ P Q 1 , N 7 doublet lines at 16739.90 cm −1 (597.4 nm). NH 3 and MMH were measured using direct absorption of CO 2 laser lines at 9.22 and 10.22 μm, respectively. These measurements were then compared to a current comprehensive MMH pyrolysis mechanism based on the work of Sun et al. (2009) and Zhang et al. (2010). Based on the measurements of NH 2 and NH 3, it was possible to measure rate coefficients for two key reactions in the MMH pyrolysis system: (1) CH 3 NHNH 2 → CH 3 NH + NH 2 (2) CH 3 NHNH 2 + NH 2 → CH 3 NNH 2 + NH 3 These rates combined with the measured overall MMH decomposition rate strongly imply that Reaction (1) is the dominant MMH decomposition channel. The following rate coefficients (2 atm, 900–1300 K) were uniquely determined: K 1 = 1.50 × 10 58 * T - 12.84 exp ( - 39580 / T ) s - 1 K 2 = 3.70 × 10 14 exp ( - 2620 / T ) cm 3 mol - 1 s - 1 Based on the MMH measurement, the value of the CH 3 decomposition channel is 0–20% of the NH 2 channel, and a value of 1.64 × 10 58 * T −12.84 exp(−39580/T) s −1 is recommended for the overall unimolecular decomposition of MMH. Further analysis of the NH 2 measurements indicate that the rate of the following reaction used in the Princeton mechanism should also be significantly increased: (4) CH 3 NNH + NH 2 → CH 3 NN + NH 3 The changes to the MMH pyrolysis mechanism recommended in this work result in greatly improved agreement between measured and modeled NH 2, NH 3, and MMH time-histories over the entire range of the study.