A technique has been devised that can provide information about the local dynamic response of a flame to an acoustic field. In the experiments, a flame in an acoustic chamber is subjected to pressure oscillations by a pair of lowfrequency drivers. The response of the flame is visualized by planar laserinduced fluorescence of the hydroxyl radical, which is used as a qualitative indicator for heat release in the flame. The resulting images are phase-resolved and averaged to yield a qualitative picture of the fluctuation of the heat release. This is correlated with the output of a pressure transducer near the flame. The tendency to cause instability can then be evaluated using Rayleigh's criterion. Results indicate that the forcing frequency and burner configuration have pronounced effects on the response of the flame. Forcing frequencies ranging from 22 to 55 Hz are applied to the jet-mixed burner supplied with a premixed 50=50 mixture of methane and carbon dioxide at a Reynolds number of 20,000. The burner is operated in two configurations: with an aerodynamically stabilized flame, and with a flame stabilized by two protruding bluff-bodies. Results indicate that, in general, the bluff-body stabilized flame is less sensitive to chamber acoustic excitation. The authors are especially indebted to Prof. Chris Cadou (University of Maryland) for his very considerable advice and help during the initial stages of designing the PLIF apparatus, when he was a postdoctoral scholar at Caltech. We also thank Mr. Konstantin Matveev for his help in completing this article, and Cerwin-Vega for their generous donation of the acoustic drivers.

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-A technique has been devised which can provide insight into the local dynamic response of a flame to an acoustic field. In the experiments, a test chamber is acoustically excited by a pair of low frequency drivers. The response of the flame is visualized by planar laser-induced fluorescence (PLIF) of the hydroxyl (OH) radical, which is a good indicator for heat release in the flame. The resulting images are phase-locked and averaged to yield a qualitative picture of the fluctuation of the heat release. This is correlated with a pressure transducer near the flame, which allows stability to be evaluated using Rayleigh's criterion. Results indicate that the drive frequency and burner configuration have a pronounced effect on the response of the flame. Drive frequencies ranging from 22 Hz to 55 Hz are applied to the jet mixed burner, supplied with a premixed 50/50 mixture of methane and carbon dioxide at a Reynolds number of 20,000. The burner is operated in two configurations; with an aerodynamically stabilized flame, and with a flame stabilized by two protruding bluff-bodies. Results indicate that in general, the bluff-body stabilized flame is less sensitive to chamber acoustic excitation

-The California Institute of Technology's Combustion Acoustics Facility is used to measure the changes in the creation of NO in a partially premixed jet flame due to acoustic forcing at frequencies ranging from 22 to 55 Hz. The facility generates a quarter-wave mode so that the test flame is in a region where the acoustic velocity is nearly zero. This facility and a similar burner have been previously used to measure the phase-resolved response of the OH field. In this experiment, phase-resolved NO planar laserinduced fluorescence (PLIF) measurements are recorded. The location and phase coupling of the NO field are analyzed, and the production and transport of NO are compared with previously reported OH field measurements. The NO levels increase for frequencies that exhibit stronger acoustic coupling to the flame. The NO concentration field variations lead (in phase space) the OH field variations. This is probably a result of the greater NO sensitivity to temperature (which itself is closely coupled to the chamber pressure).

-The California Institute of Technology's Combustion Acoustics Facility is used to measure the response of a partially premixed jet flame to acoustic forcing at frequencies ranging from 22 Hz to 55 Hz. The facility generates bulk acoustic modes that simulate unstable combustor conditions. This same facility and burner has been previously used to measure the phase-resolved response of the OH PLIF field. In this experiment, phase-resolved chemiluminescence measurements are recorded and analyzed. Flame base oscillations are quantified and compared for two different burner configurations. The chemiluminescence also shows that frequencies that exhibit stronger acoustic coupling to the flame tend to have decreased luminosity in the flame stabilization zone, while frequencies with weaker coupling tend to produce greater luminosity at the base of the flame.