Extinction criterion for unsteady, opposing-jet diffusion flames

Dynamic flames are known to survive at strain rates that are much higher than those associated with steady-state flames. A numerical and experimental investigation is performed to aid the understanding of the extinction process associated with unsteady flames. Spatially locked unsteady flames in an opposing-jet-flow burner are established and stretched by simultaneously driving one vortex from the air side and another from the fuel side. Changes in the structure of the flame during its interaction with the incoming vortices and with the instability-generated secondary vortices are investigated using a time-dependent computational-fluid-dynamics-with-chemistry (CFDC) code known as UNICORN (UNsteady Ignition and Combustion with ReactioNs). The combustion process is simulated using a detailed-chemical-kinetics model that incorporates 13 species and 74 reactions. Slow-moving vortices produce a wrinkled but continuous flame, while fast-moving vortices create a locally quenched flame with its edge wrapped around the merged vortical structures. In an attempt to characterize the observed quenching process, five variables-namely, air-side, fuel-side, and stoichiometric strain rates and maximum and stoichiometric scalar dissipation rates-are investigated. It is found that these characteristic parameters cannot be used to describe the quenching process associated with unsteady flames. The flow and chemical nonequilibrium states associated with the unsteady flames are responsible for changes in the extinction values of these traditional characteristic variables. However, even though the quenching values of the scalar dissipation rates increase with the velocity of the incoming vortices, the variations are much smaller than those observed in the strain rates. It is proposed that a variable that is proportional to the air-side strain rate and inversely proportional to the rate of change in the flame temperature can be used to characterize the unsteady quenching process uniquely. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.