Visualization of Shear Layer Dynamics in a Transversely Excited, Annular Premixing Nozzle

Abstract

In this study, we investigate the response of a swirling annular jet flow and flame to transverse acoustic excitation. Characterizing this response is a necessary step towards understanding velocity-coupled transverse combustion instabilities in lean, premixed flames. These effects are investigated using smoke visualization, particle image velocimetry (PIV), and high-speed flame imaging. This study particularly focuses on the effects of excitation on unsteady vortex development in the shear layers, as well as the effects of high-amplitude acoustics on the time-averaged flow field. First, the time-averaged characteristics of the flow are discussed under both nominal and forced conditions. The shape of the flow changes significantly at high forcing amplitudes as a result of changes in the vortex breakdown structure. Next, the shear layer dynamics with and without acoustic forcing are considered. The shear layers are visualized using smoke visualization and PIV, and the result of vortex rollup on the flame is imaged using high-speed imaging of the flame. We hypothesize that the convectively unstable shear layers and absolutely unstable vortex breakdown bubble play different dynamical roles in controlling the flame response to excitation. The unsteady vortex breakdown bubble is primarily important through its impact on the time-averaged flow field upon which perturbations evolve. It really only changes character at high acoustic forcing amplitudes, resulting in significant variations to the time-averaged flame and flowfield. The shear layer rollup, responding at the frequency of acoustic forcing, creates large-scale wrinkles on the flame and is the main driver of flame response.