Experimental studies were performed to understand the combustion instability mechanisms of turbulent lean premixed flames in various model gas turbine combustors. Premixed gases composed of different hydrocarbon fuels (C2H4, C2H6, C3H8, and CH4) and air were burned under different geometric and flow conditions. Dynamic pressure transducers, a photomultiplier tube, and a hot-wire anemometer were installed to detect the dynamic combustion behaviors. An ICCD camera and a high-speed camera were used to capture the time-averaged and instantaneous flame structure, respectively.
The mixed convective-acoustic mode instability characteristics were investigated in dump combustors experimentally. The results showed that dominant frequencies and their maximum power spectral densities of filtered pressure fluctuations had different trends for each fuel at same equivalence ratios. However, the combustion instability characteristics represented consistent results under similar characteristics chemistry times regardless of the hydrocarbon fuels. It was found that when the characteristic chemistry time and convection time increased, the mixed convective-acoustic instability could be prevented.
Open-loop control effectiveness of the combustion instability was studied in a dump combustor with an exhaust nozzle using an acoustic driver. Hot-firing tests were performed by changing acoustic forcing frequencies and amplitudes. The results indicated that the similar level of suppression was obtained regardless of the used fuels for the case of the similar characteristic chemistry time.
An experimental investigation was carried out to study the combustion instability characteristics in open-type swirl-stabilized combustors. The objective of this study was to understand the relationship between dynamic combustion behavior and the Rayleigh criterion/the flame structure. It was found that even if the Rayleigh criterion was satisfied, stable combustion was possible. The RMS value of the heat release fluctuations and magnitude-squared coherence between pressure oscillation and heat release fluctuation were good indicators to predict the thermo-acoustic instability region. The bifurcation phenomenon of the flame structure was a necessary condition in this combustor.
Two different combustion instability modes in the closed-type swirl-stabilized combustors were observed. The first was half-wave eigenmode of the combustion chamber and the second was eigenfrequency corresponding to the quarter-wave of the inlet mixing section. The combustion instability modes were dependent on the swirl number. For the case of low swirl number, the combustion system was mainly associated with the quarter-wave eigenmode of the inlet section. When the swirl number was high, however, instability mode was indifferent to the inlet acoustics. This phenomenon was analyzed by the phase difference, flame structure. and change in fuel composition. The results provided the importance of the eigenmode of the inlet mixing section in analyzing combustion instability of gas turbine engines.