Research Projects

Systems with confined combustion (engines, rockets) often exhibit ruinously large pressure oscillations known as thermoacoustic instability, resulting from a positive feedback between unsteady heat release rate and acoustic perturbations of the confinement. Such instabilities have emerged as a concern in gas turbine engines that operate in lean premixed conditions to avoid the generation of harmful NOx emissions. While there have been passive methods of suppression of thermoacoustic instability (baffles, acoustic dampers) that work for certain operational ranges of combustors, new-age fuel-flexible combustors that are expected to operate at a wider range of power levels need a tailorable control solution for suppression of thermoacoustic instability at any desired operational condition. The primary focus of this proposal is on developing an optimized active control mechanism for the suppression of thermoacoustic instability and experimentally validating the proposed methodology. Such an active control mechanism is expected to suppress the onset of multiple modes of instability that can arise at different operational conditions of a combustor. The onset of thermoacoustic instability is the result of nonlinear interactions between the reaction field and the acoustic field of the combustor. Since both these subsystems are spatially extended, their interaction happens through the interplay of spatiotemporal patterns of heat release rate and the pattern of the acoustic field. As a result, certain spatial locations within the combustor emerge as the most critical regions in ensuring the growth of thermoacoustic instability, and a control strategy targeted at these critical regions in the combustor is most effective in suppressing thermoacoustic instability (Krishnan et al., JFM, 916, A20). While the existence of critical regions for control of thermoacoustic instability is established, the optimal methods of introducing control action at such locations are not yet understood. The critical regions are generally deep within the reaction field, away from the peripheral boundaries of the combustor. Hence, the direct introduction of control action (e.g. microjet or plasma injection) at critical locations without significantly affecting the rest of the flow field is almost impossible. Furthermore, the onset of thermoacoustic instability happens through a dynamical state of intermittency characterized by bursts of high-amplitude periodic oscillations interspaced with low-amplitude aperiodic fluctuations. Suppressing the onset of thermoacoustic instability would involve ensuring that the system remains in the regime of low-amplitude aperiodic pressure fluctuations for prolonged periods. This implies that an optimal active control strategy should be targeted not only at the critical region but also should be appropriately timed such that the system can be confined to the dynamical state of low-amplitude aperiodic oscillations. We propose to develop an active control strategy that is based on the introduction of appropriately timed Nanosecond Repetitively Pulsed Plasma Discharges (NRPD) that perturb the reactive field at the appropriate location in the periphery. These perturbations travel downstream and affect the critical region and hence significantly modify the coupling between the acoustic field and the reaction field. We will develop low-order models (synchronization and kicked oscillator models) to help optimize the control strategy (i.e., find both optimal timing and location of plasma injection) using tools from nonlinear dynamics and complex systems theory. We will also perform experiments (simultaneous pressure & heat release rate measurements and high-speed flame images, both with and without plasma injection-based control) to validate the optimization strategy. The results from this study will pave the way for increased operational range for gas turbine engines and better stability for systems involving confined combustion.