Ignition of counterflowing methane versus heated air under reduced and elevated pressures

This article presents experimental and computational results on ignition of nonpremixed, counterflowing jets of nitrogen-diluted methane versus heated air within a wide range of pressures, fuel concentrations, and flow strain rates. The system was brought to ignition by increasing gradually the temperature of the air stream. Each steady-state situation just prior to ignition was experimentally characterized by measuring detailed centerline axial flow velocity and temperature distributions, for ambient pressures between 0.5-8.0 atm, fuel concentrations in the range of 6%-100% methane in nitrogen, and pressure-weighted strain rates between 100-700 s(-1). In addition, each situation was modeled numerically, using detailed transport properties and full chemical kinetics based on the GRI (Gas Research Institute) Mech v1.2 mechanism. As in our previous work with hydrogen/air ignition, we have identified computationally the existence of a localized ignition kernel of maximum reactivity and heat release. In contrast to the hydrogen case, however, we have shown that heat release and the thermal feedback are indispensable at ignition in the methane/air system. The ignition temperature, defined as the boundary temperature of the air jet just prior to ignition, was found to increase with increasing flow strain rate at all pressures. This has been shown numerically to be an effect of heat and radical loss out of the ignition kernel by convective-diffusive transport. The ignition temperature decreased abruptly with increasing fuel concentration, for dilute conditions. For CH4 concentrations in excess of 20%-30%, however, the ignition temperature became insensitive to further increase in the fuel concentration. Ignition temperatures at constant pressure-weighted strain rates decreased monotonically with increasing system pressure, similar to the homogeneous explosion limits. Over this range of pressures the numerical simulation indicated that the dominant chemical pathways at ignition do not change significantly. Flux, sensitivity, and the Computational Singular Perturbation (CSP) method were used to identify the ignition chemistry and provide several simplified kinetic mechanisms. The results obtained using a skeletal mechanism M4, with 22 species and 64 irreversible reactions, were found to agree closely with those obtained using the full chemistry. The experimental data were compared with computations using several kinetic mechanisms. Copyright (C) 1997 by The Combustion Institute.