Shear-layer effects on the dynamics of unsteady premixed laminar counterflow flames

The influence of flow non-uniformity and unsteadiness on premixed flames is of considerable interest due to its relevance to practical combustion systems. The steady counterflow flame has long served as a canonical configuration for investigating flame dynamics under controlled, spatially non-uniform conditions. A commonly studied variation, referred to as the unsteady counterflow, introduces a temporal perturbation to the otherwise steady flow from the nozzles, thereby enabling the systematic examination of the coupled effects of unsteadiness and non-uniformity. Prior investigations have focused on flame dynamics along the line of symmetry, where the reduced dimensionality of the problem facilitates analysis. In the present study, we extend this perspective by experimentally examining flame behavior at off-center locations, where multi-dimensional effects of non-uniformity and unsteadiness are more pronounced. Results reveal markedly different dynamics away from the centerline, characterized by a dominant contribution from higher harmonic responses. Further analysis of the associated vortex dynamics in the shear layer demonstrates that the radial variations in the intensity of these vortical structures directly govern the variations in the strength of the observed higher harmonics, and thereby the altered flame behavior. Novelty and significance statement While the counterflow configuration is a widely used canonical model for studying flames subjected to unsteady strain rates, prior investigations have primarily focused on centerline or symmetry-plane behavior. This study expands that framework by systematically examining both centerline and off-center flame dynamics, revealing pronounced spatial variations in the spectral response. In particular, the results uncover distinct spectral signatures associated with the coupling between imposed unsteadiness and vortex shedding in off-center regions, which are not observable from centerline analyses alone. These off-center perspectives extend the relevance of counterflow studies to other canonical flame configurations, such as bluff-body-stabilized and jet flames, where flame-vortex interactions play a central role in stabilization. The explored unsteady dynamics of off-center locations are also relevant for practical combustors, where flames are often asymmetric and highly unsteady.