Modeling ethylene combustion from low to high pressure

With the same reaction set, data have been modeled successfully from two ethylene-oxygen combustion systems at greatly different pressures. New data are from the Princeton flow reactor, a lower temperature (850-950 K) and high-pressure (5-10 atm) data set (Phi = 2.5). The second set is for a high-temperature (>2000 K) and low-pressure (20 torr), premixed laminar fuel-rich flame (Phi = 1.9) studied by Bhargava and Westmoreland. Several reaction sets were tested, but only the present set demonstrated good agreement in both cases. A key difference between this set and previous ones lies in the modeling of the complex C2H3 + O-2 reaction. New rate constants were calculated based on recent findings regarding the potential energy surface of the C2H3 + O-2 system. In the 850-1600 K range that is crucial in these experiments, the product set CH2CHO + O was found to contribute much less than reported in earlier studies, and the HCO + CH2O channel dominated. The present reaction set predicted the species profiles in both cases with reasonable accuracy, allowing us to interpret and compare the reaction pathways over a wide range of conditions. In the low-pressure flame, C2H4 is mainly consumed by abstraction, while in the high-pressure system, abstraction (mainly by OH instead of H) competes with H addition that forms C2H5. In both cases, abstraction forms C2H3 that reacts with O-2 to make HCO and CH2O and eventually CO and CO2 However, higher levels of C2H5 and HO2 at the high-pressure, lower temperature flow reactor condition drive distinct pathway differences. The key role of HO2 chemistry is particularly emphasized through the reaction CH2O + HO2. Model comparisons support a lower value of the rate constant for this reaction, consistent with that recommended by Hochgreb and Dryer.