Model-based active control of thermoacoustic instability in continuous combustion processes

Abstract

Thermoacoustic instability is frequently found in continuous combustion processes in propulsion, power generation, and heating. Active control has been increasingly pursued in recent years to suppress the pressure oscillations which result from this instability, while maintaining performance objectives such as low NOx emission and high efficiency. This thesis considers the physics behind the thermoacoustic instability and utilizes a model based on the physics to understand the problem and design an active controller to suppress the instability. A one-dimensional, laminar combustor is modeled and a 1 kW bench-top combustor rig constructed for experimental validation of simulation results. The model considers the linear acoustic and flame dynamics, acoustic mode coupling, and actuator dynamics. Several model-based control designs including proportional, phase-lead, and LQG are presented and tested on the bench-top combustor using a 0.2 W loudspeaker as an actuator. Results show that the model-based controllers are effective in suppressing the instability, and that the simulation results accurately predict the response of the real system. Using the LQG controller, a settling time of as low as 23 milliseconds was obtained, significantly faster than those reported on similar setups. The nonlinear dynamics which leads to the limit-cycle behavior in real systems are investigated by looking at several "blackbox" type models of nonlinear behavior. The performance of the linear controllers on the nonlinear models is investigated and an explanation for their success given.