Active control : an investigation method for combustion instabilities

Abstract

Closed-loop active control methods and their application to combustion instabilities are discussed. In these methods the instability development is impeded with a feedback control loop : the signal provided by a sensor monitoring the flame or pressure oscillations is processed and sent back to actuators mounted on the combustor or on the feeding system. Different active control systems tested on a non-premixed multiple-flame turbulent combustor are described. These systems can suppress all unstable plane modes of oscillation (i.e. low frequency modes). The active instability control (AIC) also constitutes an original and powerful technique for studies of mechanisms leading to instability or resulting from the instability. Two basic applications of this kind are described. In the first case the flame is initially controlled with AIC, the feedback loop is then switched off and the growth of the instability is analysed through high speed Schlieren cinematography and simultaneous sound pressure and reaction rate measurements. Three phases are identified during th growth of the oscillations : (1) a linear phase where acoustic waves induce a flapping motion of the flame sheets without interaction between sheets, (2) a modulation phase, where flame sheets interact randomly and (3) a nonlinear phase where the flame sheets are broken and a limit cycle is reached. In the second case we investigate different types of flame extinctions associated with combustion instability. It is shown that pressure oscillations may lead to partial or total extinctions. Extinctions occur in various forms but usually follow a rapid growth of pressure oscillations. The flame is extinguished during the modulation phase observed in the initiation experiments. In these studies devoted to transient instability phenomena, the control system constitutes a unique investigation tool because it is difficult to obtain the same information by other means. Implications for modelling and prediction of combustion instabilities are discussed

Saturation of a turbulent mixing layer over a cavity: response to harmonic forcing around mean flows

“…The effect of an acoustic forcing imposed at the cavity end is investigated by means of three- Poinsot et al, 1992]). In this study, the forcing frequency is set to ω 1 /2π = 750 Hz, close to the frequency of a marginally stable eigenmode and of the largest harmonic response in the unforced flow (see § 3.3).…”

Dynamics and stability of lean-premixed swirl-stabilized combustion

“…The latter can be realized by modulation of the primary or secondary fuel flow, or indirectly by excitation of shear layers, which in turn affect fuel/oxidizer mixing and flame structures. Acoustic devices have also been used for fuel flow modulation, as well as excitation of coherent structures in shear layers [294,[333][334][335][336][337][338][339][340]. It should be noted that, although acoustic devices have good high-frequency responses, they may not be robust and powerful enough for use in large-scale combustion systems.…”

Large eddy simulation of piloting effects on turbulent swirling flames

“…Furthermore, stabilisation is a difficult task because its basic mechanisms in a piloted swirled zone are not well understood. Proof of the importance of fuel injection are observed in active control examples in which a small modulation of flow rate in the fuel lines feeding the pilot flame can be sufficient to alter the stability of the combustor [13][14][15].…”