Methane Ignition in a Shock Tube with High Levels of CO2 Dilution: Consideration of the Reflected-Shock Bifurcation

Experiments were performed in a shock-tube facility to examine experimentally the kinetic effect, if any, of excess amounts of CO2 as part of natural-gas-based fueloxidizer mixtures. An important aspect of these experiments was to also observe the role excess amounts of CO2 play in causing nonidealities, particularly shock bifurcation, in shock-tube experiments using real (nondilute) fuelair mixtures. Mixtures were composed of methane fuel at an equivalence ratio of 0.5 to represent a typical natural gas in a modified air mixture designed to study the effect of large levels of CO2 dilution. These oxidizer compositions maintained constant levels of O-2 while exchanging N-2 for CO2 in stages to give oxidizer mixture concentrations ranging from (0.21O(2) + 0.79N(2)) to (0.21O(2) + 0.79CO(2)). Low-pressure and high-pressure (near 1 and 10 atm, respectively) experiments were conducted over an approximate temperature range of 1450 to 1900 K. Results showed that the observed effect of CO2 relating to reflected-shock bifurcation was quite significant, giving stronger bifurcation as amounts of CO2 increased, as determined by a sidewall pressure transducer. Despite the presence of significant reflected-shock bifurcation in the mixtures containing high levels of CO2, the resulting ignition delay times were commensurate with the results expected if one were to assume the test conditions were at the inferred temperature and pressure immediately behind the reflected shock wave. That is, the main ignition events occurred in the gas closest to the endwall, where the effects of the shock bifurcation were minimal for the ignition delay time range of the present study. When the ignition delay times for mixtures with and without CO2 dilution were compared, the effect of the CO2 was minimal and within the uncertainty of the data, particularly for the experiments near 1 atm. A small effect of CO2 addition was seen for the higher pressure near 10 atm, with a general increase in ignition delay time for the largest levels of CO2 dilution. Predictions from a modern chemical kinetics model also showed a minimal effect of CO2 addition on the methane ignition delay times, in agreement with the shock-tube data.