Direct numerical simulations of n-heptane spray autoignition in methane-air mixtures relevant to dual-fuel engines

The ignition behaviour of n-heptane droplets in hot air mixed with methane is studied by means of Direct Numerical Simulations with complex chemistry. The configuration studied is relevant to dual-fuel engines, where methane-air premixed flames are ignited by a pilot injection of liquid fuel, and to multi-fuel turbulent combustion in general. It was found that the presence of methane delayed the autoignition of n-heptane. Because autoignition happens at rich mixture fractions, oxygen is consumed leaving behind unburnt methane that, in practice, could produce unwanted emissions. The methane-air premixed flame is eventually initiated when the reaction zone spreads from the various autoignition sites.