Shock tube investigation of ignition in methaneoxygennitrogen dioxideargon mixtures

The ignition of CH 4O 2NO 2 mixtures highly diluted with Ar was examined in the region behind a reflected shock wave in a single pulse shock-tube. The measurements covered temperatures of 1200 to 1800°K, pressures of 3 to 20 atm, total gas concentrations of 3.3 − 4.5 × 10 −5 mol/cc and fuel-oxidizer mixture equivalence ratios of 0.38–1.60. The induction times ranged from 13 to 580 μsec for a maximum dwell time of about 700 μsec. When this system was compared to a CH 4O 2Ar system, it was found that NO 2 addition shortens the induction time. The ignition temperature was also lowered by about 200°C by the addition of NO 2. The dependence of induction time on concentration for an oxidizer ratio range of 25–75% NO 2 is correlated by the relationship: τ6.45 × 10 −15 exp(28700/ RT) [ CH 4] −0.32 [ O 2] −0.20 [ NO] −1.30 [ Ar] +1.00 sec A product analysis showed that during preignition methane decreases substantially when NO 2 is present and much larger quantities of CO are formed than when no NO 2 is involved. It was also found that a mixture of CH 4NO 2Ar produced pressure and temperature oscillations. It is suggested that an important step when NO 2 is present is its decomposition to produce NO and O. The reaction between CH 4 and O becomes an important contributor to chain branching. Other aspects of the mechanism are discussed qualitatively.