There has been increased demand in recent years for gas turbines that opera
te in a lean, premixed (LP) mode of combustion in an effort to meet stringe
nt emissions goals. Unfortunately detrimental combustion instabilities are
often excited within the combustor when it operates under lean conditions,
degrading performance and reducing combustor life. To eliminate the onset o
f these instabilities and develop effective approaches for their control, t
he mechanisms responsible for their occurrence must be understood. This pap
er describes the results of an investigation of the mechanisms responsible
for these instabilities. These studies found that combustors operating in a
LP mode of combustion are highly sensitive to variations in the equivalenc
e ratio (phi) of the mixture that enters the combustor. Furthermore, it was
found that such phi variations can be induced by interactions of the press
ure and flow oscillations with the reactant supply rates. The phi perturbat
ions formed in the inlet duct (near the fuel injector) are convected by the
mean flow to the combustor where they produce large amplitude heat release
oscillations that drive combustor pressure oscillations. It is shown that
the dominant characteristic time associated with this mechanism is the conv
ective time from the point of formation of the reactive mixture at the fuel
injector to the point where it is consumed at the flame. Instabilities occ
ur when the ratio of this convective time and the period of the oscillation
s equals a specific constant, whose magnitude depends upon the combustor de
sign. Significantly, these predictions are in good agreement with available
experimental data, strongly suggesting that the proposed mechanism properl
y accounts for the essential physics of the problem. The predictions of thi
s study also indicate, however, that simple design changes (i.e., passive c
ontrol approaches) may not, in general, provide a viable means for controll
ing these instabilities, due to the multiple number of modes that may be ex
cited by the combustion process.