Direct Numerical Simulation of turbulent nonpremixed "cool" flames: Applicability of flamelet models

"Cool" flames result from the coupling of low-temperature chemistry with molecular transport. These flames have been experimentally and computationally observed under laminar flow conditions but have not been isolated under turbulent flow conditions. In this work, a skeletal n-heptane chemical mechanism including low-temperature chemistry is used to conduct two-dimensional Direct Numerical Simulations (DNS) of nonpremixed "cool" flames subjected to unsteady, two-dimensional flow initialized from a plane of isotropic turbulence. Like conventional "hot" flames, at high Damköhler numbers, these "cool" flames are found to be adequately described by a steady flamelet model. However, at low Damköhler numbers, both nonpremixed and premixed behavior is observed locally. When the combustion is locally nonpremixed, these "cool" flames can be described by the unsteady flamelet model only if the correct scalar dissipation rate profile and associated boundary conditions are considered.