In this work, an unsteady flamelet/progress variable (UFPV) model is applied in large-eddy simulation of a lifted methane/air flame in a vitiated co-flow. In this burner configuration, the flame is stabilized by autoignition. This ignition mode is of particular relevance to a number of practical applications, including furnaces, internal combustion engines, and flame stabilization in augmentors. Autoignition of a reactant mixture is typically initiated in localized regions of low scalar dissipation rate having a mixture composition that favors short ignition times. Since the prediction of autoignition events, however, is strongly dependent on the structure of the surrounding turbulent reacting flow field, combustion models are required that are able to provide an accurate characterization of the spatio-temporal flow field. Although LES techniques have been shown to provide improved predictions for turbulent mixing processes, these localized ignition kernels are computationally not resolved. Therefore, subgrid-scale closure models are required to characterize effects of unresolved scales and ignition kinetics. Another computational challenge arises from the transient evolution of these localized ignition events. Since such ignition events are only inadequately represented by steady-state flamelet models, it is therefore necessary to utilize an unsteady combustion model. To this end, an unsteady flamelet/progress variable (UFPV) model has been developed [1]. In the following, the mathematical model describing the UFPV formulation and the presumed PDF closure is summarized. The experimental configuration and computational setup are discussed in Sec. 3, and computational results are presented in Sec. 4. The paper finishes with conclusions.
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