Nonpremixed flames occur at the stoichiometric interface between fuel and oxidizer streams. The rate of combustion in these flames is limited by the rate of diffusive mixing of the streams. Turbulence acts to increase flame surface area and scalar gradients, hence increasing mixing. As mixing rates increase, finite rate chemical kinetic effects become important, and flame extinction may occur if rates of diffusive heat loss exceed the heat release rates of combustion. Local flame extinction results in flame holes through which unburned fuel may escape, reducing combustion efficiency and allowing fuel emission. High rates of flame extinction may result in unstable combustion, flame liftoff, and if excessive, global flame blowout, posing operational and safety hazards.
Flame extinction and reignition are notoriously difficult processes to model due to the finite rate chemical kinetic effects, and complex reignition mechanisms that depend on the turbulent environment and flame structure. The degree of mixing in extinguished regions prior to reignition may affect the mode of reignition. At right are a series of three parametric simulations of planar nonpremixed jets with increasing levels of extinction. The simulations allow detailed investigation into reignition modes, and flame structures, providing high fidelity data for model validation, and fundamental insight into turbulent flame structure.