Visualization of Shear Layer Dynamics in a Transversely Forced Flow and Flame

Abstract

This study addresses the response of a swirling annular jet flow and flame to transverse acoustic excitation in order to better describe key velocity-coupled processes during transverse combustion instabilities in lean, premixed flames. In particular, visualization and velocimetry techniques provide information about the effects of acoustic excitation on unsteady vortex development in the shear layers. Without acoustic forcing, the shear layers roll up into small vortices, driven by the Kelvin-Helmholtz instability, that convect downstream and grow. In the presence of high-amplitude acoustic forcing, as would be present during a combustion instability, the acoustic oscillations drive shear layer vortices to undergo a strong rollup event. Smoke visualization provides visual evidence of the rollup, while particle image velocimetry measurements show the development and trajectory of this large structure. Finally, planar laserinduced fluorescence of OH shows how the large coherent structure causes flame wrinkling at the base of the flame. The vortex rollup, responding at the frequency of acoustic forcing, creates large-scale wrinkles on the flame and can be the main driver of flame response during combustion instability. Nomenclature D = outer diameter of nozzle F = flame describing function f o = forcing frequency f r = natural shedding frequency h = centerbody length Q = global heat release rate Re D = Reynolds number based on nozzle diameter, D r = radial coordinate r 1 = inner radius of annular jet r 2 = outer radius of annular jet t = time U o = bulk velocity u = velocity vector u 0 = axial velocity perturbation v 0 = transverse velocity perturbation x = axial coordinate θ = momentum thickness Subscripts a = acoustic inc = incident L = longitudinal T = transverse ω = vortical

Symmetry breaking of azimuthal waves: Slow-flow dynamics on the Bloch sphere

“…The pure spinning modes in both directions are stable solutions. This result and the amplitudes predicted for the spinning and the standing modes reconstructed with the real expression of the pressure given in (31) are in agreement with the results of [15]. Using the Bloch-Poincaré spherical representation introduced in the section D, Fig.…”

“…In that regard, one can also refer to the interesting experimental studies dealing with the response of flames to transverse acoustic forcing [30,31,32,33,34]. Most of the theoretical findings from 1D models that are based on projection of the acoustic field onto pairs of orthogonal standing or spinning modes have not yet been validated with well-controlled experiments and these two modelling approaches have shortcomings.…”

Comprehensive Combustion Stability Analysis Using Dynamic Mode Decomposition

“…The same flame with acoustic perturbation at 50 Hz is shown in the bottom row of Figure . Strong oscillatory behavior near the recirculation zone and the shear layer is observed at the resonant frequency. DMD analysis should not only provide similar findings but also quantify these dynamics to serve as an effective combustion instability analysis technique.…”

Cluster-based Reduced-Order Modeling to Capture Intermittent Dynamics of Interacting Wakes

“…Interestingly, the region where the disturbances gain or lose coherence is similar in the right wake of the w/D = 1.94 case and the central wake of the w/D = 2.99 case. This distance is approximately twice the vortex development length [31], and given its proximity to the end of the recirculation zone, may also be a highly sensitive region of the flow to external perturbations. Further adjoint analysis of this flowfield and its stability is recommended to understand the structural sensitivity of the flow and further understand why these switching points in the oscillation dynamics occur [11].…”