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 twophase 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 onedimensional 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, selfexcited highamplitude acoustic oscillations are observed in the combustor. We observe the transition of the system behavior from lowamplitude aperiodic oscillations to large amplitude limit cycle oscillations occurring through intermittency. This intermittent state mainly consists of a sequence of highamplitude bursts of periodic oscillations separated by lowamplitude aperiodic regions. Moreover, the experimental results highlight that during intermittency, the maximum amplitude of bursts, near to the onset of intermittency, is as much as three times higher than the maximum amplitude of the limit cycle oscillations. These highamplitude intermittent loads can have stronger adverse effects on the structural properties of the engine than the lowamplitude 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) is studied in threedimensional phase space by using a phase space reconstruction tool from the dynamical system theory. We report the first experimental observation of typeII 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 (RP) techniques are employed to confirm the type of intermittency.