Response of Rod Stabilized Flames to Harmonic Excitation: Shear Layer Rollup and Flame Kinematics

Shanbhogue, Santosh J.; Plaks, Dmitriy; Nowicki, G.; Lieuwen, Tim; Guggenheim, Daniel

doi:10.2514/6.2006-5232

Cited by 6 publications

(6 citation statements)

References 12 publications

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“…Due to the flame propagation normal to itself (kinematic restoration), as well as the dissipation in amplitude of the vorticity field (detailed in this paper), these wrinkles are not present farther downstream, where only smooth crest and cusped troughs of the flame can be seen. The flame dynamics under these circumstances were quantified in a previous work 32,29 , where it was show that the amplitude of flame front position fluctuation, L', at the forcing frequency first increased with downstream distance from the bluff body, reached a maximum, and then decreased to essentially zero (Figure 3, right). It was suggested that this behavior was controlled by flame stabilization in the initial, growth region.…”

Section: Resultsmentioning

confidence: 94%

“…Previous work 29 has shown that acoustic excitation influences the KH instability of the separated shear layer of the bluff body that, in turn, distorts the flame. Figure 3 illustrates a typical image showing this interaction.…”

Section: Resultsmentioning

confidence: 99%

“…The choice of the phrase 'harmonic waves' over 'acoustic waves' was intentional despite the fact that the excitation source used in the experiments (as will be described next) is acoustic. This allows us to account for the possibility of acoustic waves leading to periodic vortical structures that in turn perturb the flame 29 .…”

Section: Effect Of Heat-release On the Flow-fieldmentioning

confidence: 99%

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The K-H Instability of Reacting, Acoustically Excited Bluff Body Shear Layers

Shanbhogue¹,

Plaks²,

Lieuwen³

2007

43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference &Amp;amp; Exhibit

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This paper describes an experimental study of the effect of acoustic excitation on bluffbody stabilized flames. The Kelvin-Helmholtz (KH) instability of the shear layer is excited due to the incident acoustics. In turn, the KH instability imposes a convecting, harmonic excitation on the flame, which leads to spatially periodic flame wrinkling and heat-release oscillations. Understanding the factors influencing these heat release oscillations therefore requires an understanding of the generation, convection, and dissipation of these vortical disturbances. The evolution of these vortical disturbances is strongly influenced by the presence of combustion due to enhanced diffusivity in the hot products, volume dilatation, and baroclinic torque. PIV measurements are reported of the decay of these vortices over a range of conditions, which suggest that the high product diffusivity controls the reduction in vorticity amplitude downstream. Of particular significance is the relative location of the flame and vortex sheet. If the vortex sheet is inside the hot products, it dissipates much more rapidly than if it lies in the reactants. In addition, experiments were performed with two bluff bodies, one with a triangular cross section and another with a circular cross section. The triangle has a well defined separation point, leading to phase locked and transversely symmetric vorticity and flame wrinkling. In contrast, while instantaneous images from the circular bluff body look similar to those of the triangle, overlays from cycle to cycle reveal a substantial amount of phase jitter in the vortex sheet, and therefore flame position. Vorticity fluctuations of comparable magnitudes are generated instantaneously for both bluff body shapes, but the spatial jitter leads to reduced ensemble averaged amplitudes for the circular bluff body. This phase jitter is reduced by increasing the amplitude of acoustic excitation.

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Section: Resultsmentioning

confidence: 94%

Section: Resultsmentioning

confidence: 99%

Section: Effect Of Heat-release On the Flow-fieldmentioning

confidence: 99%

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The K-H Instability of Reacting, Acoustically Excited Bluff Body Shear Layers

Shanbhogue¹,

Plaks²,

Lieuwen³

2007

43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference &Amp;amp; Exhibit

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“…First, future measurements would benefit from laser sheet imaging to characterize the flame edge, as done in our earlier studies [24][25][26]. Second, they show that resolution of the bluff body near field is critical to verify the flame anchoring, which plays such an important role in the flame's near-field response character.…”

Section: Discussionmentioning

confidence: 97%

“…The objective of this study is to characterize this relationship between acoustic, vortical, and flame sheet fluctuations. It extends prior measurements and analysis by the Georgia Tech group from lower velocity and Reynolds number situations to much higher velocity flames [24][25][26]. The next section describes the experimental facility and diagnostics.…”

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confidence: 95%

Dynamics of a Longitudinally Forced, Bluff Body Stabilized Flame

Shin¹,

Plaks²,

Lieuwen³

et al. 2011

Journal of Propulsion and Power

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This paper describes an investigation of the response of bluff body stabilized flames to harmonic oscillations. This problem involves two key elements: the excitation of hydrodynamic flow instabilities by acoustic waves, and the response of the flame to these harmonic flow instabilities. In the present work, data were obtained with inlet temperatures from 297 to 870 K and flow velocities from 38 to 170 m=s. These data show that the flame-front response at the acoustic forcing frequency first increases linearly with downstream distance, then peaks and decays. The corresponding phase decreases linearly with axial distance, showing that wrinkles on the flame propagate with a nearly constant convection velocity. These results are compared with those obtained from a theoretical solution of the G-equation excited by a harmonically oscillating, convecting disturbance. This kinematic model shows that the key processes controlling the response are 1) the anchoring of the flame at the bluff body, 2) the excitation of flame-front wrinkles by the oscillating velocity, 3) interference of wrinkles on the flame front, and 4) flame propagation normal to itself at the local flame speed. The first two processes control the growth of the flame response and the last two processes control the axial decrease observed farther downstream. These predictions are shown to describe many of the key features of the measured flame response characteristics. Nomenclature A = flame area D = burner depth d = bluff body width f = frequency f o = forcing frequency f BvK = Bénard-von Kármán instability frequency f KH = Kelvin-Helmholtz frequency G = isoscalar contour variable describing flame position H = burner height I t = edge detection threshold k = wave number L = flame position L 0 = fluctuation of flame position jL 0 j = gain of Fourier transformed flame position Q = heat release R 2 = goodness-of-fit parameter Re = Reynolds number S = distance along time-averaged flame front S L = laminar flame speed S T = turbulent flame speed t = time u = flow velocity in the x-direction u 0 = unsteady axial flow velocity u 0 = mean flow velocity u c;eff = effective convection velocity of a flame disturbance u c;v = convective velocity of a flow disturbance u 0 n = fluctuating flow velocity normal to the flame u t;0 = mean flow velocity tangential to the flame V = excitation voltage v = flow velocity in the y-direction W = burner width x = position along mean flow direction y = position perpendicular to mean flow direction = parametric flame-front angle " = nondimensionalized disturbance amplitude = time-averaged flame-front angle c = convective wavelength, u o =f 0 f = phase of a flame wrinkle x = phase of velocity perturbations in the x-direction y = phase of velocity perturbations in the y-direction ! = disturbance radial frequency

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