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

Abstract

fuel flow 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 flow rate, about 60 Hz below the acoustic resonant frequencies. The invariance in the flametransferfunctionstothefuelmodulationamplitudesuggeststhatthederived flametransferfunctionsarelinear 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 byincreasingtheamountoffuelmodulations.Applicationsofthe flametransferfunctionstomodelingandcontrolof combustion instability and lean blowout for both liquid-fueled and gas-fueled combustion are discussed.