Combustion instability is a serious obstacle for the lean premixed combustion of gas turbines, and can even
cause fatal damage to the combustor and the entire system. Thus, improved understanding of the mechanisms of
combustion instability is necessary for designing and operating gas turbine combustors. In this study, in order to
understand the instability phenomena, an experimental study was conducted under the moderate pressure and
ambient temperature. We investigated the cause of combustion instability in a rearward-step dump combustor
with respect to the modulation of the fuel flow—that is, the choked fuel flow and the unchoked fuel flow. The
fluctuation of pressure, heat release and equivalence ratio were measured by piezoelectric pressure sensor, high
speed Intensified Charge Coupled Device (ICCD) camera and gas chromatography respectively. Various types
of combustion instabilities occurred in relation to changes of the equivalence ratio and the fuel flow conditions.
Two representative instability modes were self-excited instabilities at the resonance of combustion chamber
(200 Hz) and instabilities related to the fuel flow modulation (10 Hz). In this experiment, combustion instability
results mainly from thermoacoustic instability, and the modulation of the fuel flow affects the instability mode.
It is founded that the causes of heat release rate fluctuation are not only equivalence ratio fluctuation due to the
fuel and air flow modulation but also large vortical motion of the mixture flow and the unmixedness of the fuel