CONTROL OF COMBUSTION OSCILLATIONS BY FORCED OSCILLATION OF PART OF THE FUEL SUPPLY

Naturally occurring oscillations in ducted premixed flames have been c ontrolled by imposed oscillation of the fuel. The Barnes were stabilis ed behind a bluff-body and a step with methane as the main fuel and ke rosene as the oscillated secondary fuel and with heat release rates of up to 150 kW. Some experiments with oscillation of gaseous fuel were performed to supplement and to complement earlier results. Two types o f actuator were used to oscillate gaseous fuels, one involving the osc illation of the mean fuel composition and the other the pulsed ignitio n of a fuel-air mixture. Liquid fuel flow was oscillated with a pintle -type injector with a jet of air to further atomise the fuel spray. Th e amplitude of the oscillated input is quantified for the different ac tuators in terms of the flow variables and frequency of oscillation. C ombustion oscillations were ameliorated by oscillating fuel at selecte d frequencies and more effectively by the controlled oscillation of fu el out of phase with the combustion instability. Pressure fluctuations of 7 kPa rms were reduced by 16 dB to 1.2 kPa with flame stabilisatio n on the rim of the pulsation device, but control was less effective i n the presence of an additional main flame holder due to uncertainty i n the dominant frequency. Control by the oscillation of liquid fuel wa s sensitive to atomisation and to the location of fuel injection and w as equally effective with the oscillation of fuel at the frequency of the instability and at its first subharmonic with rms pressure fluctua tions of 10 kPa reduced by 12 dB to 2.3 kPa despite limitations in mod ulating the amplitude of the input. Sequential injection at circumfere ntially distributed locations is also shown to provide a satisfactory means of control.