CLOSED-LOOP AMPLITUDE-MODULATION CONTROL OF REACTING PREMIXED TURBULENT JET

A closed-loop control system was constructed to control the initiation of reaction in a turbulent fuel/air jet 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 frequency was determined in open-loop control tests, which defined the effective range and related 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 reaction is stable only in a limited range due to 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.