A NUMERICAL INVESTIGATION OF METHANE AIR PARTIALLY PREMIXED FLAMES AT ELEVATED PRESSURES

Abstract

A numerical investigation of high pressure, counterflow methane-air partially premixed flames (PPFs) is presented to characterize the effect of pressure on flame structure. Four different mechanisms, namely GRI-2.11, GRI-3.0, the San Diego, and the C2 mechanisms have been examined. The mechanisms have been validated by comparing the predicted laminar flame speeds with experimental data. While there is excellent agreement between the measured and predicted laminar flame speeds based on these mechanisms, there are noticeable discrepancies amongst the mechanisms for the prediction of high-pressure PPFs. The GRI 3.0 mechanism is used to examine the detailed PPF structure at elevated pressures. At low pressures, the PPF exhibits a typical double flame structure that consists of rich premixed and nonpremixed reaction zones, and the separation distance between the reaction zones decreases with increasing pressure. However, for pressure above 10 atm, the separation distance becomes nearly independent of pressure. In addition, a critical pressure is observed, above which the PPF structure exhibits anomalous behavior, characterized by the presence of endothermic reactions and the production of CO in the region between the two reaction zones. A rate of production analysis revealed that the anomalous behavior is attributable to the following reactions: H + H2O → OH + H2, CH2CO + M → CH2 + CO + M, and HCO + M → H + CO + M. The critical pressure at which this behavior is first observed increases with the increase in equivalence ratio and/or strain rate. Thermal radiation is found to have a negligible effect on the flame structure for the range of conditions investigated.