Scale-Up Experiments on Liquid-Fueled Active Combustion Control

Liquid-fueled active instability suppression and the associated scaling issues were investigated in a model ramjet dump combustor that was operated between 70 kW and 1.3 MW output conditions. When the secondary fuel injection was pulsed and its timing was controlled, pressure oscillations were suppressed. The controller performance was adversely affected by reducing the flow residence time, but the correlation was modest. The amount of pulsed fuel, on the other hand, had a significant effect on the controller performance, which suggested the existence of a critical fuel flux. To maintain control authority for instability suppression, a fuel amount in excess of the critical fuel flux was required for the secondary injection. In the scale-up experiments, when the Sauter mean diameter of fuel droplets was decreased from 40 to 10 μm, the critical fuel flux was lowered from 8% of the total fuel flux to about 2%, which indicated that it was a strong function of fuel droplet size. Last, a novel control approach was demonstrated using pulsed fuel injection that was closed-loop controlled at the second harmonic of the instability. This showed that instabilities could be suppressed with closed-loop control even when the frequencies are outside the actuator's repetition capability.