Ignition Characteristics in Spatially Zero-, One- and Two-Dimensional Laminar Ethylene Flames

Combustion in next-generation aero-engines may occur in conditions in, or approaching, moderate or intense low oxygen dilution (MILD) combustion. Under these conditions, fresh fuel is injected into a hot, low oxygen environment. Autoignition delays for ethylene, with detailed chemical kinetics, in three different oxidants are systematically analysed in a simulated perfectly-stirred batch reactor (batch PSR), with initial conditions based on the composition and adiabatic mixing temperature of the separate fuel and oxidant streams. The analysis of the same non-premixed streams in a transient, one-dimensional, opposed laminar dffusion flame shows ignition initiating at the same equivalence ratio for a diluted ethylene fuel with a high temperature air oxidant, albeit over-predicting ignition delay. Further analysis through means of a two-dimensional simulation shows good agreement between the batch PSR analysis and the location of the flame base. These analysis are subsequently applied to a MILD flame and a flame bordering the MILD and autoignitive lifted-flame regimes. Whilst the batch PSR analysis is able to predict the ignition equivalence ratio of the transitional flame, agreement with lift-off height is superior using a measure of 10K increase above oxidant conditions. This temperature metric is also applied to a MILD flame to show good agreement between a 10K temperature increase and visible chemiluminescence, however occurring in richer conditions than predicted by the batch PSR. The onset of thermal runaway in a batch PSR shows good predictive value for ignition in hot, vitiated environments, such as those anticipated in next generation aero-engines, despite the signicant differences in flame structure between ignition in the lifted and MILD combustion regimes.