Laminar flame speed measurements of ethylene at high preheat temperatures and for diluted oxidizers

Laminar flame speeds of ethylene are measured and the results are compared to predictions based on detailed chemical kinetic mechanisms. As important aspects of jet fuel chemistry involve ethylene, the flame speeds are determined at conditions relevant to jet engine combustors that have not been previously investigated, specifically high preheat temperatures (up to 650 K) and reduced O2 levels (down to 15% mole fraction in the oxidizer). The latter is relevant to staged-combustion and vitiation, where there is partial pre-burning of the oxidizing flow; so flame speeds are also measured with both CO2 and N2 dilution. Flame speed measurements of ethylene-air mixtures at room temperature conditions agree well (within 10%) of measurements with previous results from the literature and the chemical kinetic mechanism predictions. As the preheat temperature is increased to 650 K and with N2 dilution, the chemical mechanism predictions agree reasonably well with the measurements (generally within 10%). With CO2 dilution of the oxidizer, however, larger prediction discrepancies are found. The mechanisms over predict the measured flame speeds, and the prediction error increases with the amount of CO2 in the oxidizer. Analysis of different possible sources of discrepancy point to the third-body efficiency of CO2 as a likely reason; specifically, it may be underestimated for three-body association reactions such as H + O2 (+M) ⇔ HO2 (+M).