Numerical investigation of kinetic energy dynamics during autoignition of N-heptane/air mixture

This paper presents a direct numerical simulation of decaying homogeneous isotropic turbulence interacting with an autoigniting n-heptane/air mixture. The simulation parameters are selected to represent, to some extent, the ignition characteristics of homogeneous charge compression-ignition (HCCI) engines. The Turbulent Kinetic Energy (TKE) balance is analyzed in order to identify important physics associated with impacts of reaction-induced density, viscosity, and pressure variations on turbulence. A mathematical framework is derived to obtain a concurrent view of the TKE evolution in physical and spectral space. Results indicate that the mean pressure work term dominates the evolution of TKE during the period where strong heat release rate takes place. Further, the dominance is attributed to the presence of flame structures in physical space associated with strong local density variations, and thus high volumetric dilatation. In addition, the analysis in spectral space shows that the pressure work not only dominates the evolution of TKE in the mean, but also its dominance occurs over a broad range of scales. The scale distribution of the pressure work spectrum has a power-law behavior as a direct consequence of localized flame structures in physical space.