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Abstract
Naturally occurring oscillations in ducted premixed flames have been controlled by imposed oscillation of the fuel. The flames were stabilised behind a bluff-body and a step with methane as the main fuel and kerosene 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 of actuator were used to oscillate gaseous fuels, one involving the oscillation of the mean fuel composition and the other the pulsed ignition 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. The amplitude of the oscillated input is quantified for the different actuators in terms of the flow variables and frequency of oscillation
Combustion oscillations were ameliorated by oscillating fuel at selected frequencies and more effectively by the controlled oscillation of fuel out of phase with the combustion instability. Pressure fluctuations of 7 kPa rms were reduced by 16 dB to 1.2 kPa with flame stabilisation on the rim of the pulsation device, but control was less effective in the presence of an additional main flame holder due to uncertainty in the dominant frequency. Control by the oscillation of liquid fuel was sensitive to atomisation and to the location of fuel injection and was equally effective with the oscillation of fuel at the frequency of the instability and at its first subharmonic with rms pressure fluctuations of 10 k Pa reduced by 12 dB to 2.3 kPa despite limitations in modulating the amplitude of the input. Sequential injection at circumferentially distributed locations is also shown to provide a satisfactory means of control.