Coupling Between Combustor Pressure and Fuel/Oxidizer Mixing and the Consequences in Flame Behavior

Abstract

The impact on flame behavior of unsteady fuel-air mixing due to acoustic oscillations was investigated by examination of the mixing response to imposed chamber acoustic oscillations (in the range of 22–55 Hz). The distribution of local fuel mixture fraction inside the mixing zone, which evolves into the local equivalence ratio in the flame zone, is tightly coupled to flame instability and oscillatory behavior. A custom made aerodynamically stabilized burner was employed in this study along with acetone seeding into the fuel stream to mark the location and concentration of the primary fuel (methane). Phase-resolved acetone PLIF was used to image the upstream flow field of both reacting and non-reacting flows. Unmixedness was calculated from these measurements to quantify the degree of fluctuations in fuel mixture fraction in the region preceding the flame. The fluctuations were then analyzed to extract the dynamics of fuel-air mixing. It was found that the presence of a flame has a strong effect on the degree and type of pressure-mixing coupling. Also, both the frequency and the phase of the imposed pressure oscillation significantly affect flow coupling, with non-reacting flows experiencing peak coupling at lower frequencies than corresponding reacting cases.

Keywords:

• Acetone PLIF
• Acoustically excited
• Acoustically forced
• Combustion dynamics
• Combustion instabilities
• Fuel/ air mixing
• Oscillating flame
• Phase-resolved
• Rayleigh index
• Unmixedness

Acknowledgments

This work was supported in part by the California Institute of Technology and partly by the Air Force Office of Scientific Research (AFOSR) under Grant No. F49620-03-1-0384 (Dr. Mitat Birkan, Program Manager). We thank Carlos Pinedo for assistance in configuring the experiment.