Experimental studies were performed to understand the combustion instability in model gas turbine combustors and swirl-stabilized combustor. Premixed gas composed of air and fuel were burned under different geometric and flow conditions. Combustion instability characteristics, which vary by various swirlers and inlet lengths, were identified through various measuring equipment and analysis programs. Dynamic pressure transducers , photomultiplier tube and hot-wire anemometer were installed to detect the dynamic combustion behaviors. A high-speed camera was used to capture the instantaneous flame structure, respectively. The dominant frequency of the acoustic mode that can be formed in the combustor was determined through the longitudinal mode formula and the OSCILOS program.
Two different combustion instability modes in the model gas turbine combustor were observed. The first was inlet mixing section resonance frequency and the second was combustion chamber resonance frequency. The combustion instability modes were dependent on the swirler geometry. When the swirl number was high, instability mode was indifferent to the inlet acoustics. This phenomenon was analyzed by the phase difference and flame structure.
As the swirler hub length increased in the same combustor, most of the combustion instability corresponding to the mixture resonant frequency shifted to the combustion chamber resonant frequency. In addition, it was observed that the amplitude of the pressure oscillation of the combustion chamber did not increase in the long hub length even when the length of the combustion chamber was increased. Through this, it was found that the mixture and combustion chamber were acoustically separated.
Heat release fluctuations, pressure oscillations and inlet velocity perturbations were observed while varying the inlet length in a swirl-stabilized combustor. Through this, it was observed that the inlet velocity fluctuation decreased as the inlet length increased. In addition, it was found that the pressure oscillation peak appearing at the dominant frequency of the inlet mixing section shifted to a higher order harmonic mode under the condition of a long inlet length. In addition, more combustion instability conditions were observed under the conditions where these mode-shifting phenomena occurred.