Combustion Oscillation Control by Cyclic Fuel Injection

Abstract

A number of recent articles have demonstrated the use of active control to mitigate the effects of combustion instability in afterburner and dump combustor applications. In these applications, cyclic injection of small quantities of control fuel has been proposed to counteract the periodic heat release that contributes to undesired pressure oscillations. This same technique may also be useful to mitigate oscillations in gas turbine combustors, especially in test rig combustors characterized by acoustic modes that do not exist in the final engine configuration. To address this issue, the present paper reports on active control of a subscale, atmospheric pressure nozzle/combustor arrangement. The fuel is natural gas. Cyclic injection of 14 percent control fuel in a premix fuel nozzle is shown to reduce oscillating pressure amplitude by a factor of 0.30 (i.e., −10 dB) at 300 Hz. Measurement of the oscillating heat release is also reported.

When Do Open-Loop Strategies for Combustion Control Work?

“…In particular, we have shown that the same proposition that explained the idea behind stabilization using the switching strategy in [1,2] provides the analytical basis for the results in [4,5] as well. We showed that our observations based on the model and the proposition matches the experimental observations in [4,5].…”

“…It should be noted that the combustor in [1] differs from that in [2] in two respects. First, variations in ¦ t a re present in the rig in [1], contrary to the case in [2] where a decoupler is introduced between the feed system and the burning zone. This implies that the time-delay in [1] may be due to t…”

“…In particular, we will focus on the strategies used in [1,2] which consists of changing the equivalence ratio between two constant values at a frequency that is significantly lower than the resonant frequency. The discussions in the previous section indicate that while the specific dependence of the individual parameters may be different, the general class of models that describe the combustion instability are of the form of …”

“…(ii) model-based control, which consists of developing a physically based model of the combustion dynamics and have been demonstrated to yield an optimal stabilizing response under certain conditions, [14]- [17], (iii) observer-based control [18], which have been shown to result in satisfactory pressure suppression using an on-line determination of an observer and a phase-shift controller based on the observer parameters, (iv) control based on system-identification based modeling [19,20] and (v) open-loop control strategies [1,2], [21]- [5] where the mass-flow rate of a fuel injector (and therefore the equivalence ratio) is switched between two different values at a significantly slower frequency than the resonant frequency. This paper concerns analytical discussions of the last method listed above.…”

“…In [1,2,21] a low frequency pulsing strategy was used to stabilize the pressure oscillations, with no feedback. In [3], a similar low-frequency modulation was used.…”