The Alteration of Turbulent Jet Flame Structure and Combustion by Axisymmetric Jet Pulsing.

Flow structure plays an important role in the mixing and chemical reaction processes of turbulent jet diffusion flames. Enhancement or control of jet flame structure may allow for control of the combustion characteristics of such flames. An experimental study has been conducted to investigate the effects of strong axisymmetric pulsing on a free, vertical, turbulent, propane, jet diffusion flame having an exit Reynolds number of 10,000. The jet flame exit velocity was modulated over a frequency range of 2 to 1300 Hz with pulse amplitudes ranging from 13 to 89% of the centerline exit velocity. Conditionally averaged velocity and temperature measurements, post-flame emission measurements, and 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 produces significant alterations in the structure of a turbulent jet diffusion flame. The effects are frequency dependent and require relatively large pulse amplitudes. Pulsing produces a local interaction with the jet flame structure that is described by a preferred-mode coupling which scales with the local large-scale jet variables. The enhanced structure increases the local mixing and combustion which decreases the overall length of the jet flame. Pulsing produces a periodic alteration of the mixing processes of the jet and generates a periodic, quasi-steady distribution of turbulence in the jet flame. Forcing with a low frequency near 10 Hz produces an isolated large-scale region of combustion and volumetric expansion which widens the jet flame and couples with the buoyancy in the flame far downstream. Pulsing at high frequencies intensifies the turbulence and mixing near the jet exit; the increased mixing increases the local fuel consumption rate which can significantly alter the overall flame length. High frequency forcing is limited by the rapid breakdown and dissipation of the pulse wave. Post-flame emission measurements show that the overall production of NO_{rm x} is not significantly affected by pulsing, indicating that the large-scale turbulence is not important. Measurements in the unpulsed jet flame show that the emission index increases with increasing jet velocity.