Abstract

In industrial applications, such as gas turbine combustion chambers, global shear instabilities can be exploited to generate good mixing. These instabilities lead to large scale vortical structures, secondary instabilities and high turbulent kinetic energy. In this study, we ask whether global shear instabilities can also be used to disrupt global thermo-acoustic instabilities. An internal combustion system consists of one or more burners inside a combustion chamber. Thermo-acoustic instabilities arise when pressure perturbations in the chamber are amplified by the burner and cause the frequency of heat release to be in phase with one of the acoustic eigenmodes of the chamber. It is often said that global shear instabilities at one frequency are insensitive to external forcing at other frequencies, except at very high forcing amplitudes. If this is the case, then it should be possible to weaken the feedback loop that causes combustion instability by designing a burner to have a natural frequency well away from the acoustic eigenmodes of the chamber. In this experimental study, we examine acoustically-forced buoyant jet diffusion flames. In these flames, a global shear instability can be turned on and off by careful adjustment of the composition of the fuel. We find that a flow that has a global shear instability is not, in fact, insensitive to external forcing at other frequencies. At all forcing amplitudes, the natural and forced mode co-exist. The growth rate of the forced mode, however, is lower in the flame that contains a global shear instability. Furthermore, with non-linear development, the frequency of heat release will be determined by the frequency of the mode that rolls up first. There is some evidence from real combustion systems that this process reduces the amplitude of thermo-acoustic instabilities.