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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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The objective is to study the effects of acoustics on a flame in microgravity. This research is meant to provide a new approach to reducing and extinguishing a combustion reaction in space (where a conventional fire extinguisher is hazardous). The group proposes to test the following hypotheses: (1) as sound intensity increases, so does the magnitude of the effect on the flame; (2) there is one optimal frequency for maximizing the effects of sound waves on a flame; (3) homogeneous flames (found only in microgravity) can be affected in a single area separate from others; (4) a sustained pulse of sound, rather than a single, brief pulse, can be used to extinguish a flame. The group will describe the experimental apparatus in detail, which was flown aboard a NASA C-9B Aircraft through their Reduced Gravity Student Flight Opportunities Program (RGSFOP), and present the findings and compare data obtained in a 1-g environment with data collected in a microgravity environment. [Thanks to the following organizations for their support: Kicker Audio, Georgia Space Grant Consortium, Siemens, FLIR Systems, PCB Piezotronics, UWG Honors College, and NASAs Reduced Gravity Student Flight Opportunities Program.]

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.