Acoustic Characterization of Combustors with Azimuthally Resolved Acoustic Forcing

Open Access
Author:
Levenson, Shayna Nicole
Area of Honors:
Mechanical Engineering
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Jacqueline Antonia O'connor, Thesis Supervisor
  • Hosam Kadry Fathy, Honors Advisor
Keywords:
  • thermoacoustic instabilities
  • circumferential
  • longitudinal
  • azimuthal
  • Comsol
  • eigenmode
  • eigenfrequency
Abstract:
Thermoacoustic instabilities are a significant operational issue for low-emissions gas turbines in both the power and propulsion industries. Specifically, gas turbines that use lean-burn technology in an effort to reduce NOx emissions are highly susceptible to instabilities. Within the combustor, thermoacoustic instabilities can couple with a number of acoustic modes. Focusing on annular combustors, common in aircraft and power generation engines, the azimuthal mode that dominates the thermoacoustic feedback process excites the flame asymmetrically, which can lead to unique flame response. Therefore, the goal of this research is to create asymmetric acoustic fields to understand the acoustic field and how it’s affecting the flow and the flame. In order to better understand these acoustic fields, Comsol models were generated to mimic both a laboratory combustor and a simplified system in which only the combustion chamber is considered. From this analysis, it is clear that the exit boundary condition of the combustion chamber has a large impact on the acoustic field. It is also evident that if the system is forced at a frequency close to one of its natural frequencies, it will experience a stronger response. However, it has also been shown that higher modes of forcing will produce a weaker acoustic response. Finally, the same acoustic modes were visible in both the combustion chamber and full lab experiment at the same eigenfrequencies when acoustic forcing was applied at lower modes. Experimental validation of the Comsol models mimicked the results from both the simplified geometry and laboratory experimental simulations. Additional resonant frequencies were identified through the use of white-noise excitation in the experiment. The implications of these results are discussed and future work proposed.