Owing to their reliability and efficiency, gas turbines are widely used for power generation and propulsion. However, they emit harmful nitrogen oxides (NO_x), threatening public health and environment. To reduce NO_x emissions, gas turbine manufacturers are switching to lean premixed combustion, but this tends to provoke thermoacoustic oscillations in the combustor. Such oscillations can increase pollutant emissions, exacerbate mechanical stresses, and even lead to catastrophic structural damage.

Currently, both passive and active methods are available to control thermoacoustic oscillations, but most assume a priori that the oscillations are periodic with a single dominant frequency and a fixed amplitude. Recent studies, however, have shown that in both simple and industrial the...[ Read more ]

Owing to their reliability and efficiency, gas turbines are widely used for power generation and propulsion. However, they emit harmful nitrogen oxides (NO_x), threatening public health and environment. To reduce NO_x emissions, gas turbine manufacturers are switching to lean premixed combustion, but this tends to provoke thermoacoustic oscillations in the combustor. Such oscillations can increase pollutant emissions, exacerbate mechanical stresses, and even lead to catastrophic structural damage.

Currently, both passive and active methods are available to control thermoacoustic oscillations, but most assume a priori that the oscillations are periodic with a single dominant frequency and a fixed amplitude. Recent studies, however, have shown that in both simple and industrial thermoacoustic systems, the oscillations are not necessarily periodic but can be quasiperiodic or chaotic. It is therefore important to have effective methods of controlling such aperiodic thermoacoustic oscillations.

This thesis presents an experimental investigation into open-loop control of a simple thermoacoustic system – a laminar premixed flame in a tube combustor – undergoing periodic, quasiperiodic and chaotic oscillations. Its aim is threefold: (i) to test whether periodic acoustic forcing can be an effective means of controlling periodic and aperiodic thermoacoustic oscillations; (ii) to explore the synchronization dynamics of a forced self-excited thermoacoustic system en route to and after lock-in; and (iii) to phenomenologically model the synchronization dynamics, particularly the different routes to lock-in using a universal low-order oscillator.

This thesis shows that external acoustic forcing can be an effective means of weakening both periodic and aperiodic thermoacoustic oscillations. This implies that different types of nonlinear oscillations can be controlled with the same control system, thus saving time and resources. The control process can be phenomenologically modelled with low-order oscillators within a synchronization framework, which is a computational efficient way to understand and predict the forced dynamics of self-excited thermoacoustic systems.