Experiments and modeling of forced ignition in methane/air mixtures with added NO

Abstract

Ignition of fuel/oxidizer mixtures containing combustion products is important to the operation of advanced combustors. Previous work indicates that the presence of NO can reduce ignition delay times, yet the influence of NO on forced ignition is not clear. This work examines ignition kernels generated by spark discharges in CH4/air flows diluted with a mixture of NO and N2. Differences in chemistry initiated by the presence of NO are examined by comparing mixtures diluted with only N2 to mixtures diluted with both NO and N2. Ignition probability and kernel growth rates are determined using measurements of infrared radiation emissions from the developing ignition kernels. The presence of NO in the unburned mixture does not affect ignition probability. However, the addition of small quantities of NO on the order of 300–1200 parts per million increases the size of ignition kernels relative to those generated in mixtures diluted with pure N2 by up to 20%. Numerical modeling is performed to examine the role of low temperature chemistry on ignition behavior. The sensitivity of ignition kernel growth to the presence of NO is attributed to increased CH4 oxidation rates caused by low-temperature chemistry activated in the early stages of kernel development.