Forced Flame Response of Turbulent Liquid-Fueled Lean-Direct-Injection Combustion to Fuel Modulations

Reported are the forced flame responses of a turbulent, liquid-fueled, swirl-stabilized, lean-direct-injection combustor to fuel modulations. Fuel modulations are achieved using a motor-driven, high-frequency, rotary fuel valve specially designed for this experiment. This valve is capable of fuel modulations up to 1 kHz. The instantaneous fuel How rate out of a fuel injector is accurately determined from pressure measurements at one or two locations upstream of the fuel nozzle. Linear flame responses are obtained with small-amplitude fuel modulations, typically below 2.0% of the mean fuel How rate, about 60 Hz below the acoustic resonant frequencies. The invariance in the flame transfer functions to the fuel modulation amplitude suggests that the derived flame transfer functions are linear and that the induced heat release rate oscillations mainly respond to variations in the instantaneous fuel flow rate rather than in the droplet size and distribution. Flame transfer functions at different air flow rates, equivalence ratios, and preheat temperature are examined. With fuel modulations around the acoustic resonant frequencies, forcing-induced acoustic feedback on heat release responses cannot be ignored, and the measured flame transfer functions are no longer open-loop and linear. The nonlinearity in the forced flame responses is significantly affected by acoustic damping. Extinction and entrainment of self-excited combustion oscillations by fuel modulations at the approach of the acoustic resonant frequencies are observed. Self-excited combustion oscillations can be diminished by increasing the amount of fuel modulations. Applications of the flame transfer functions to modeling and control of combustion instability and lean blowout for both liquid-fueled and gas-fueled combustion are discussed.