Publications

The role of cool-flame dynamics in high-pressure spray ignition

Dahms, Rainer N.; Paczko, Günter A.; Skeen, Scott A.; Pickett, Lyle M.

The mechanism of high-pressure spray flame ignition is still poorly understood. However, recent high-speed laser imaging has provided important insights into the spatial and temporal progression of the formation and consumption of formaldehyde which indicates the presence of cool-flame chemistry and its interplay with high-temperature ignition. A corroborating theoretical-numerical analysis, based on the Lagrangian flamelet equations, is presented. The validity of Peters' two-scale asymptotic for cool-flame dynamics is established by a coupled chemical explosive mode and molecular diffusion time scale analysis. Contrary to conventional wisdom, this analysis reveals that the flamelet derivation from the reactive Navier-Stokes equations applies during the entire turbulent two-stage ignition process. In combination with high-fidelity LLNL reference kinetics, the simulation then establishes the presence of a turbulent cool flame wave. It significantly decreases the low-temperature ignition delay in lower temperature regions in comparison to their homogeneous reactor reference. These waves facilitate high-temperature turbulent ignition in preferably rich mixture regions. It is shown that the ignition process follows a distinct pattern which can be characterized by a set of measurable time scales.