IGNITION OF COUNTERFLOWING METHANE VERSUS HEATED AIR UNDER REDUCED AND ELEVATED PRESSURES

This article presents experimental and computational results on igniti on of nonpremixed, counterflowing jets of nitrogen-diluted methane ver sus heated air within a wide range of pressures, fuel concentrations, and flow strain rates. The system was brought to ignition by increasin g gradually the temperature of the air stream. Each steady-state situa tion just prior to ignition was experimentally characterized by measur ing detailed centerline axial flow velocity and temperature distributi ons, for ambient pressures between 0.5-8.0 atm, fuel concentrations in the range of 6%-100% methane in nitrogen, and pressure-weighted strai n rates between 100-700 s(-1). In addition, each situation was modeled numerically, using detailed transport properties and full chemical ki netics 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 maxim um reactivity and heat release. In contrast to the hydrogen case, howe ver, we have shown that heat release and the thermal feedback are indi spensable at ignition in the methane/air system. The ignition temperat ure, defined as the boundary temperature of the air jet just prior to ignition, was found to increase with increasing flow strain rate at al l pressures. This has been shown numerically to be an effect of heat a nd radical loss out of the ignition kernel by convective-diffusive tra nsport. The ignition temperature decreased abruptly with increasing fu el concentration, for dilute conditions. For CH4 concentrations in exc ess of 20%-30%, however, the ignition temperature became insensitive t o 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 limit s. Over this range of pressures the numerical simulation indicated tha t the dominant chemical pathways at ignition do not change significant ly. Flux, sensitivity, and the Computational Singular Perturbation (CS P) method were used to identify the ignition chemistry and provide sev eral simplified kinetic mechanisms. The results obtained using a skele tal mechanism M4, with 22 species and 64 irreversible reactions, were found to agree closely with those obtained using the full chemistry. T he experimental data were compared with computations using several kin etic mechanisms. Copyright (C) 1997 by The Combustion Institute.