A Numerical Study of the Effect of EGR on Flame Lift-off in n-Heptane Sprays Using a Novel PaSR Model Implemented in OpenFOAM 2012-01-0153

The effect of exhaust gas recirculation (EGR) on flame lift-off in non-stationary n-heptane sprays was studied under Diesel engine-like conditions using numerical simulation involving complex chemistry and a novel partially stirred reactor (PaSR) model of subgrid turbulence-chemistry interaction.

The flame-stabilization mechanism is a result of complex physical and chemical interactions and cannot be described by a simplified theory. To leading order it is determined by the chemical reaction time at the leading edge, the turbulent diffusivity, and the flow velocity; so that there exists a balance between the local convection velocity and the triple-flame propagation speed. In this study of ignition and flame formation and stabilization processes, the VSB2 stochastic blob-and-bubble spray model was used in combination with the volume reactor fraction model (VRFM) implemented in OpenFOAM. The reacting volume fraction in the VRFM was determined by solving for mixture fraction, progress variable, and their variances in order to estimate the non-uniformities of the fluid cell; rather than simply taking the ratio of the mixing and chemistry time-scales. The chemistry is described by a reduced n-heptane mechanism with 36 species involved in 81 reactions.

The simulated lift-off trends are compared to available experimental data from the Engine Combustion Network, Sandia National Laboratories [1].