THE STRUCTURE OF PULSED TURBULENT NONPREMIXED JET FLAMES

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 shaw 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 law 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.