Effects of multicomponent diffusion on predicted ignition characteristics of an n-heptane diffusion flame

Flamelet models for turbulent combustion typically employ the assumption of unity Lewis number, i.e., equal thermal and species diffusivities. These models have been employed to predict ignition delay times and ignition location in combusting sprays. However, there is the interesting question: what would be the effects of including multicomponent species diffusion on the ignition predictions? In this work, a one-dimensional n-heptane-air diffusion flame is chosen to study the effects of multicomponent diffusion on predicted ignition characteristics. The ambient conditions selected include typical in-cylinder conditions of a medium-duty diesel engine: pressure 10-40 bar and air temperature 850-1000 K. The ignition and oxidation of n-heptane are predicted using a reaction mechanism consisting of 34 species and 56 steps. The mixture fraction is computed separately as a passive species, the diffusion coefficient, of which is equal to the local thermal diffusion coefficient. From these computations, the transient structure of the flamelet, including ignition, is obtained. The results are compared with those obtained with the unity Lewis number assumption. The implications of the unity Lewis number assumption on the predicted ignition characteristics are discussed.