Non-linear Response of Turbulent Premixed Flames to Imposed Inlet Velocity Oscillations of Two Frequencies

Abstract

This paper describes an experimental study investigating the non-linear response of lean premixed air/ethylene flames to strong inlet velocity perturbations of two frequencies. The combustor has a centrally-placed bluff body and a short quartz section. The annulus between the bluff body and the flow tube, which also housed the acoustic pressure transducers, allowed the reactants into the combustor. The inlet flow was perturbed using loudspeakers. High speed laser tomography, OH* chemiluminescence and OH Planar Laser Induced Fluorescence (PLIF) have been used for flow visualization, heat release and flame surface density (FSD) measurements respectively. The heat release fluctuations increased initially linearly with inlet velocity amplitude for a single frequency forcing, with saturation occurring after forcing amplitudes of around 15% of the bulk velocity, which was found to occur due to vortex roll up and subsequent flame annihilation. The introduction of energy at the second frequency (i.e, the harmonic) was found to change the vortex formation and shedding frequency, depending on the level of forcing. This resulted in a non-linear flame response transfer function (defined as the amplitude of unsteady heat release divided by the amplitude of velocity perturbation at the fundamental) whose amplitude depended greatly on the amount of harmonic content present in the perturbations. The introduction of higher harmonics reduced the flame 456 Flow Turbulence Combust (2008) 80:455-487 annihilation events, which are responsible for saturation, thus reducing non-linearity in the amplitude dependence of the flame response. These results were further verified using sequential time-resolved OH PLIF measurements. The findings from this study suggest that the acoustic response of the flame was mostly due to flame area variation effected by modulation of the annular jet and evolution of the shear layers.

Proper Orthogonal Decomposition Analysis of a Turbulent Swirling Self-Excited Premixed Flame

“…In addition, in the case of L duct =510mm at t=2ms, two sites of local flame annihilation are noted. This phenomenon occurs as the inner and outer flame fronts are brought together and it ultimately leads to an overall decrease in flame surface area and hence local heat release rate, as it has been reported before in externally forced flames 7,9 . In these works, the magnitude of flame annihilation was found to be dependent on the amplitude of the induced forcing.…”

“…Planar laser-induced fluorescence (PLIF) on the other hand, gives more reliable information about the fuel-air mixing and the flame structure, advantages that qualify it as a suitable technique for the investigation of turbulent flames. 1 PhD student, Email: amk74@cam.ac.uk 2 Professor of Mechanical Engineering, Fellow AIAA, Email: apd1@cam.ac.uk 3 Professor of Energy Technologies, Associate Fellow AIAA, Email: em257@eng.cam.ac.uk 4 Lecturer in Systems Power and Energy, Email: Nader.Karimi@glasgow.ac.uk C Previous works from our group applied simultaneous OH* chemiluminescence and OH and CH 2 O PLIF imaging, to provide a more accurate measurement of heat release rate in turbulent premixed flames [7][8][9] . As for most of such research until now, the imaging was done at a low sampling rate .…”

Non-Linear Response to Periodic Forcing of Methane-Air Global and Detailed Kinetics in Continuous Stirred Tank Reactors Close to Extinction Conditions

“…The shift from the diffusion to the LPP combustion regime is not straightforward: the possibility of occurrence of combustion instability has been widely reported in the literature [2] and promoted many studies to understand the mechanism of their generation. Both theoretical and experimental studies highlighted the role of different mechanisms, like unmixedness of reactants [3], flame dynamics response to perturbations [4][5][6][7][8][9][10] and thermo-acoustic instabilities [2]. The chemical kinetics response to imposed oscillations has been clearly recognized for the proper estimation of pollutant emissions [11].…”

Measurements of ensemble averaged flame dynamics using spatially resolved analysis

“…Obtaining knowledge of the flame dynamics remains a significant area of research and many studies, both experimental [10][11][12][13][14][28][29][30][31][32] and theoretical [2,3,9,[15][16][17][18], have been aimed at achieving greater understanding of the flame response to perturbations. The flame is known to behave as a low pass filter with respect to excitations, exhibiting a response at low frequencies which rolls off at higher frequencies.…”