Direct numerical simulation of n-heptane/air auto-ignition with thermal and charge stratifications under partially-premixed charge compression ignition (PCCI) engine related conditions

Two-dimensional direct numerical simulations (2-D DNS) are used to study auto-ignition of n-heptane/air mixture with charge and temperature stratifications under elevated pressure and temperature conditions relevant to PCCI (Partially-premixed Charge Compression Ignition) engines. Detailed species transport and a skeletal n-heptane chemical kinetics mechanism are employed; the effect of compression/expansion due to piston motion is also taking into account. The result shows that charge and temperature stratifications have an important impact on the ignition delay and the spatial structure of the reaction zones regardless the initial mean temperature (T0) is within or outside the negative temperature coefficient (NTC) regime. The second-stage ignition delay time decreases as charge stratification increases but it increases with increasing temperature stratification. Larger charge stratification can spread out heat release due to the high sensitivity of ignition delay time to the equivalence ratio (φ) and the temperature gradient. Furthermore, stronger turbulence can homogenize mixture fluctuations, but the ignition delay time is similar as that in weak turbulence case because the species diffusion rate is rather weak compared with chemical reaction rate. A series of 1-D simulations are carried out under different T0 and T-φ correlations to investigate their effect on the combustion process.