Transition from noise to instability in a laboratory-scale swirl combustor

Experiments performed in a laboratory scale gas turbine combustor with a vane swirler to investigate its combustion instability characteristics are reported in the present paper. Both premixed and partially premixed flames are studied. The fuel is liquefied petroleum gas (LPG) and its flow rate is fixed at different levels while the air flow rate is varied. Blow-off limits are obtained by increasing the air flow rate with corresponding increase in flow Reynolds number (Re). Piezoelectric transducers are used to acquire static pressure from the walls of the combustor. Acoustic characterization of the system is performed as the combustor moves from stable conditions to unstable conditions. Evidence of acoustically induced blow-off of the flame for long ducts where flow-acoustic lock-on occurs and high-amplitude discrete tones are excited is found. During flow-acoustic lock-on, the acoustic pressure amplitude increases significantly, reaching a maximum value of up to 6 kPa, in the frequency range of 150-200 Hz. Considerable variation in acoustic pressure amplitude is observed during blow-off when the mixture is lean. Flow-acoustic lock-on is observed during Re variation and the phenomenon is explained by plotting corresponding variations in Helmholtz and Strouhal numbers. The length of the combustor downstream of the swirler is observed to play a dominant role in determining the lock-on, amplitude levels, and the blow-off limits. The present experimental results can be used for further studies involving active control of combustion instability. Copyright © (2011) by the International Institute of Acoustics & Vibration.