The Structure of Pulsed Turbulent Nonpremixed Jet Flames

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Abstract
The enhanced mixing observed in excited jets may be used to alter jet flame characteristics. An experimental study was conducted to investigate the effects of strong axisymmetric pulsing on a free, vertical, nonpremixed, turbulent jet flame. The majority of measurements were made with a propane jet flame having an exit Reynolds number of 10,000. The jet flame exit velocity was modulated over a frequency range of 2 to 1340 Hz with pulse amplitudes ranging from 13 to 89% of the centerline exit velocity. Phase-averaged velocity and temperature measurements, and postflame emissions measurements, as well as natural light, schlieren, and laser-plane flow visualizations were obtained to characterize the effects of pulsing on the jet flame. The results show that axisymmetric forcing of the fuel jet significantly alters the structure of a nonpremixed turbulent jet flame. The effects of forcing are frequency dependent, producing a local interaction with the jet flame structure that is described by a preferred-mode coupling characterized by the large-scale jet variables. Forcing with a low frequency near 10 Hz produces a strong coupling with the buoyant structure of the jet flame in the far-field. Pulsing at high frequencies intensifies the local turbulent mixing and fuel consumption near the jet exit. The appearance of enhanced mixing and combustion at these two forcing-frequency regimes reduces the overall length of the jet flame but had no effect on the overall mass emission of oxides of nitrogen.

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