Impact of turbulence on the response of flames to external coherent excitation

Large-scale coherent oscillations in flow fields, which are often a consequence of hydrodynamic instabilities, can significantly modulate the response of flames by creating oscillations in flame surface area and the unsteady rate of heat release. Such oscillations in the heat release rate have been shown to be the drivers of thermo-acoustic instabilities in some combustor systems, and occurrence of thermo-acoustic instabilities can be detrimental to combustor performance and operability. In this study, we examine the impact of varying in-flow turbulence on the response of rod-stabilized flames to external coherent excitation. We systematically vary the amplitude of the longitudinal coherent excitation to simulate coherent perturbations created by hydrodynamic flow instabilities at varying strengths and the frequency of forcing is set to match the natural frequency of vortex-shedding from the cylindrical bluff body. The in-flow turbulence is varied using turbulence-generating screens upstream of the flame. High speed, stereoscopic, particle image velocimetry is used to obtain three-component velocity fields to characterize the flow response and Mie scattering scattering images are analyzed to obtain the flame location at each condition. Our results show that, in conditions with low levels of in-flow turbulence, high amplitudes of coherent excitation can modulate not only the flame response, but also the time-averaged base flow. By contrast, the conditions with high levels of in-flow turbulence exhibit a significantly damped flame response and no modulation of the base flow can be seen. The results indicate that turbulence intensity can not only impact the response of flames to flow perturbations, but can also impact the underlying flow field.