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

Abstract

Experiments were performed in a shock-tube facility to examine experimentally the kinetic effect, if any, of excess amounts of CO 2 as part of natural-gas-based fuel−oxidizer mixtures. An important aspect of these experiments was to also observe the role excess amounts of CO 2 play in causing nonidealities, particularly shock bifurcation, in shock-tube experiments using real (nondilute) fuel−air 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 CO 2 dilution. These oxidizer compositions maintained constant levels of O 2 while exchanging N 2 for CO 2 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 CO 2 relating to reflected-shock bifurcation was quite significant, giving stronger bifurcation as amounts of CO 2 increased, as determined by a sidewall pressure transducer. Despite the presence of significant reflected-shock bifurcation in the mixtures containing high levels of CO 2 , 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 CO 2 dilution were compared, the effect of the CO 2 was minimal and within the uncertainty of the data, particularly for the experiments near 1 atm. A small effect of CO 2 addition was seen for the higher pressure near 10 atm, with a general increase in ignition delay time for the largest levels of CO 2 dilution. Predictions from a modern chemical kinetics model also showed a minimal effect of CO 2 addition on the methane ignition delay times, in agreement with the shock-tube data.

Effects of High Fuel Loading and CO2 Dilution on Oxy-Methane Ignition Inside a Shock Tube at High Pressure

“…For example, Hargis et al has investigated oxy-fuel combustion within a high pressure shock tube at pressures from 1 to 10 atm and 1450 to 1900 K; it was observed that increasing the level of CO2led to increased bifurcation. Despite the presence of significant reflected-shock bifurcation in mixtures containing high levels of CO2, the resulting ignition delay times were commensurate with calculated temperature and pressure behind reflected shock waves [20]. Other studies of oxymethane mixtures include different levels of CO2 dilution, but with a final conclusion finding that as the amount of CO2 is increased, there is an insignificant, gradual increase in ignition delay time [14,21,22], though as Hargis et al [20] explains, this becomes more apparent at higher pressures.…”

“…Despite the presence of significant reflected-shock bifurcation in mixtures containing high levels of CO2, the resulting ignition delay times were commensurate with calculated temperature and pressure behind reflected shock waves [20]. Other studies of oxymethane mixtures include different levels of CO2 dilution, but with a final conclusion finding that as the amount of CO2 is increased, there is an insignificant, gradual increase in ignition delay time [14,21,22], though as Hargis et al [20] explains, this becomes more apparent at higher pressures. In an effort to extend understanding of highly CO2dilute mixtures with high fuel loading, steps here are being made firstly at a moderate pressure with future work aimed at elevated pressures near 100 atm.…”

“…Because of this, the time of DDT was interpreted on a case-by-case basis, as was done previously by Hargis et al [20]. The associated uncertainty in τDDT ¬was larger than any uncertainty in T5 or P5, with the most impacted experiment revealing a 48% uncertainty in this ignition definition.…”

“…Bifurcation can have multiple effects on the system, including a highly nonuniform velocity field, induced vorticity, and turbulence aiding in other nonuniformities and inhomogeneities [34][35][36]. The fluid flow issues imposed by bifurcation immensely affect pressure measurements as well, resulting in observed fluctuations or oscillations that increase with increasing amounts of CO2, which ultimately add greater uncertainty [20]. Typically, end wall located diagnostics (pressure/emission) are preferred to understand ignition of non-dilute fuel mixtures primarily due to the tendency of sidewall mounted diagnostics to observe earlier ignition delay times [37].…”

Numerical study on the effects of CO2/H2O dilution on the ignition delay time of methane

“…Since the main purpose of this research focuses on the methane IDT under different mixtures (CO 2 /H 2 O) diluted conditions, the results obtained TA B L E 2 Data obtained from Refs. 19,21 Mixture…”