Intermittency Route to Combustion Instability in a Laboratory Spray Combustor

Abstract

In the present study, we investigate the phenomenon of transition of a thermoacoustic system involving two-phase flow, from aperiodic oscillations to limit cycle oscillations. Experiments were performed in a laboratory scale model of a spray combustor. A needle spray injector is used to generate a droplet spray having one dimensional velocity field. This simplified design of the injector helps in keeping away the geometric complexities involved in the real spray atomizers. We investigate the stability of the spray combustor in response to the variation of the flame location inside the combustor. Equivalence ratio is maintained constant throughout the experiment. The dynamics of the system is captured by measuring the unsteady pressure fluctuations present in the system. As the flame location is gradually varied, self-excited high amplitude acoustic oscillations are observed in the combustor. We observe the transition of the system behaviour from low amplitude aperiodic oscillations to large amplitude limit cycle oscillations occurring through intermittency. This intermittent state mainly consists of a sequence of high-amplitude periodic bursts separated by low amplitude aperiodic regions. Moreover, the experimental results highlight that during intermittency, the maximum amplitude of bursts oscillations, near to the onset of intermittency, is as much as three times higher than the maximum amplitude of the limit cycle oscillations. These high amplitude intermittent loads can have stronger adverse effects on the structural properties of the engine than the low amplitude cyclic loading caused by the sustained limit cycle oscillations. Evolution of the three different dynamical states of the spray combustion system (viz. stable, intermittency and limit cycle) are studied in three-dimensional phase space by using a phase space reconstruction tool from the dynamical system theory. We report the first experimental observation of type-II intermittency in a spray combustion system. The statistical distributions of the length of aperiodic (turbulent) phase with respect to the control parameter, first return map and recurrence plot techniques are employed to confirm the type of intermittency.

Sensitivity and Nonlinearity of Thermoacoustic Oscillations

“…These intermittent bursts last longer as the point of onset is approached. These intermittent bursts are also reported in other studies (Clavin et al, 1994;Gotoda et al, 2014;Kabiraj et al, 2015a;Huang, 2015;Pawar et al, 2016;Wilhite et al, 2016). Intermittency is often analysed using recurrence plots (figure 11 and supplementary materials), which represent time series data graphically.…”

NOx emission reduction reaction of ammonia-hydrogen with self-sustained pulsating oscillations

“…11(d) indicates stronger nonlinearity. Such large-amplitude intermittency oscillations are unwanted in propulsion systems, since it may lead to fracture of the brittle material or may cause deformation of the ductile material [48,49]. In general, this study confirms the strong impact of hydrogen on producing and the characteristics of the NH 3 -H 2 -combustion-driven pulsating oscillations.…”

“…15. This is based on the well-known Rayleigh criteria [28,30,49]. It states that limit cycle oscillations may be produced when acoustic pressure and unsteady heat release are in phase.…”

Common features in the thermoacoustics of flames and engines

“…lifting the flame) and the subsequent heating or cooling of the burner mouth changing the thermal radiation in such a way that the flame reverts to its original position. The position of the flame was artificially controlled by movement of a mesh in a spray combustor by Pawar et al 134 and they observed a progression from stable operation, through periodic bursts of high intensity at 10 s intervals, to shorter intervals, and finally to limit cycling, as the mesh was moved down the tube.…”