Control of self-excited thermoacoustic oscillations using transient forcing, hysteresis and mode switching

Abstract

In many combustion devices, strong self-excited flow oscillations can arise from feedback between unsteady heat release and acoustics, resulting in increased vibration and pollutant emissions. Open-loop acoustic forcing has been shown to be effective in weakening such thermoacoustic oscillations, but current implementations of this control strategy require the forcing to be continuously applied. In this proof-of-concept study, we experimentally demonstrate an alternative method of weakening thermoacoustic oscillations in a self-excited combustion system – a laminar premixed flame in a double open-ended tube. Unlike existing methods, the proposed method combines the use of transient forcing with hysteresis and mode switching, thus avoiding the need to continuously supply energy to the control system. Control is achieved by exploiting the fact that most combustors have a multitude of natural thermoacoustic modes, some of which are linearly unstable but some are nonlinearly unstable. By applying open-loop acoustic forcing at an off-resonance frequency and at an amplitude higher than that required for synchronization, we find that the combustor can switch to one of the nonlinearly unstable natural modes (f2) and remain there, even after the forcing is removed. Dynamic mode decomposition of high-speed chemiluminescence videos shows that this mode switching occurs because the flame structure at f2 is more robust than that at the original linearly unstable natural mode. The final unforced state has a thermoacoustic amplitude of just half that of the initial unforced state, even though the Rayleigh index of the former is higher than that of the latter. Although this 50% reduction in thermoacoustic amplitude is not as large as the 95% reduction achieved with asynchronous quenching, it is achieved without the use of continuous forcing. This is a distinct advantage over existing control strategies as it allows the complexity and power requirements of the control system to be reduced. With further development and testing, particularly on turbulent swirling combustors, the proposed control strategy could pave the way for a new class of open-loop control techniques based on transient forcing rather than continuous forcing.