Effect of differential diffusion on the linear dynamics of premixed flames in swirling jets

Hydrogen's exceptionally low molecular weight causes its ratio of thermal to mass diffusivity (the Lewis number) to be less than one in lean flames. This property leads to a phenomenon known as thermal-diffusive instability, which is expected to have a significant impact on the response of hydrogen flames to flow disturbances. Recently, we have used modal and nonmodal stability analysis to show how such differential diffusion influences the linear dynamics of laminar Bunsen flames (EFMC, Athens, Sept. 2022). We now extend those results to swirling jet flames in order to explore how non-unity Lewis number effects interact with swirl. A low-Mach, single-step reacting flow model is used to study the three-dimensional linear flame dynamics via eigenmode and resolvent analysis, at a variety of swirl and Lewis numbers. The results reveal that swirl has a significant destabilizing influence on the flame behavior regardless of the Lewis number. However, the combination of swirl with differential diffusion leads to far greater nonmodal amplification of flow disturbances at low Lewis numbers compared to unity Lewis number flames. The implications of these findings towards the fundamental behaviors of lean-premixed hydrogen flames and their thermoacoustic stability characteristics is discussed.