Non-Linear Effects in a Lean Partially Premixed Combustor During Limit Cycle Operation

In a gas turbine combustor limit cycles of pressure oscillations may occur due to a coupling of combustion dynamics to the acoustic field inside the system. In this case, the engine is subjected to high vibrations and the possibility of structural damage. Experimental research in this subject was carried out in a laboratory combustor operating in a lean, partially premixed methane/air flame, where the flame stabilizes on a triangular bluff body inside a rectangular combustor duct. Depending on the operating point, the flame shows a stable or unstable behavior. In this last case, amplitudes up to 155 dB (ref 20 μPa) have been recorded. The variation of behavior of the instability with operating conditions is well known. The stable combustion presents a low amplitude broadband noise. The unstable regime is more interesting. It has a main peak with high amplitude and fixed frequency and several secondary peaks at multiple times the frequency of the fundamental one. This peaks can be seen in the pressure and heat release spectrum. The secondary peaks of the pressure spectrum are due to non-linear effects. Odd numbered peaks came from a change in the acoustic boundary conditions in the burner. The even peaks are the result of frequency doubling of the odd frequencies. The frequency doubling comes from a second order source term of the Ligthill’s analogy.