Experimental and Numerical Investigation of the Mechanisms of Local Extinction Using Flame Kernel-Vortex Interactions

North Carolina State University, Raleigh NC

Investigators

Abstract

Project Summary The objective of this research is to investigate, both experimentally and computationally, the local extinction processes occurring in premixed flames using a novel flame kernel-vortex configuration. The kernel-vortex interaction provides a rich configuration to study at least two important mechanisms responsible for extinction: flame-flow field topologies (curvature and strain); and flame-flame interactions (or mutual flame annihilation processes). Conditions of positive and negative unsteady strain rate and curvature may be generated simultaneously within the same interaction, providing the ideal condition for comparing the flame's response to different types of flame stretch. Moreover, the relative size of the expanding kernel to the vortex results in an evolving contribution of the vortex to flame wrinkling and its advection during the kernel evolution. The two complementary (experimental and computational) approaches are coordinated to provide improved understanding of the role of flame and flow field topologies (mainly unsteady strain and curvature effects) and mutual flame annihilations on local flame extinction, and to identify experimental observables that measure the local burning intensity in premixed hydrocarbon flames. Three important elements represent the main thrusts of the work and its technical merit: (1) An experimental study of extinction processes during flame kernel-vortex interactions. The study relies on a broad range of experimental conditions that span different flame and vortex strengths. Simultaneous measurements of intermediate species, temperature, and velocity provide a more robust link between flame and flow-field topologies and the local flame response; (2) Numerical studies of the general structure as well as the structure of two important regions in the flame kernels that are subject to quenching by unsteady strain or mutual flame annihilation. The numerical studies are used to identify relevant experimental conditions and explore, using detailed chemistry, the chemical mechanisms involved during extinction. Initial conditions for the detailed chemistry simulations are provided by the selected experimental conditions; and (3) The combined approaches provide complementary information to identify conditions and mechanisms for local extinction. An equally important endeavor is to use the simulations to identify candidate experimental observables that may be used to quantify the burning intensity of hydrocarbon premixed flames for unsteady and extinction conditions. Broader impact The kernel-vortex offers a realistic and advantageous configuration to develop models for flame extinction that will be of value to our understanding of turbulent flames in general and turbulent kernels specifically, such as those observed in spark-ignited internal combustion engines and other practical combustion devices. The methodology proposed can have a broad impact on similar problems in combustion science and related areas of research where neither computational nor experimental approaches alone can adequately explain observations. The paradigm of coordinated experimental-computational approaches provides unique opportunities for the education of graduate students in both approaches as the project requires.