Closed-Loop Control of a Flame and its Application in a Dump Combustor

A closed-loop control system was constructed to control the lift-off height of a premixed flame at the lean flammability limit. Control authority was obtained by forcing the initial shear layer of the fuel/air mixture, thus generating small scale vortices which improved the flame stability. The optimal forcing was determined in open-loop control tests, which defined the effective range and relate it to the Kelvin-Helmholtz instability of the shear layer. An amplitude modulation (AM) controller was developed using the open-loop frequency as a carrier signal. Bode and Nyquist analyses showed that the flame is stable only in a limited range due to convective time lag, related to the convection time of the vortices. In spite of the high noise level of this turbulent combustion system, it was possible to use linear control theory to analyze the time-averaged transfer functions. The application of lead and lag compensations was studied to extend the stability margins. The control system response to changes in operating conditions such as set-point and flow rates were compared to the open-loop control operation and found to be superior, leading to a small extension of the lean flammability limit relative to an open-loop controller. The same approaches investigated in the flame experiments are used for active combustion control in the dump combustor. Shear-layer excitation was investigated using acoustic drivers. Various parameters affecting the efficiency of the methods for active combustion control were studied, including frequency, amplitude, and phase information.