PERFORMANCE OF PREFILMING AIRBLAST ATOMIZERS IN UNSTEADY FLOW CONDITIONS

摘要

Within the context of lean premixed prevaporized combustion (LPP) which is considered as most promising technology for the next generation of low emission combustors for aero engines, combustion instabilities are a major issue. These combustion instabilities may compromise the pollutant emissions and even cause damage to the combustion chamber structure. In the literature, numerous phenomenological studies on combustion oscillation are available, but a comprehensive theory is still missing. One potential excitation mechanism is the interaction of strong air velocity fluctuations and pressure oscillations with the airblast atomizer leading to temporal fluctuations of the spray characteristics. This phenomenon was investigated experimentally at the Institute of Thermal Turbomachinery (ITS) within a parametric study. A duct with a prefilming surface was set up as an abstraction of a prefilming airblast atomizer. A mean air velocity up to 65 m/s can be reached, and periodic oscillations can be superimposed by means of a siren with a frequency up to 570 Hz. The disintegration process of the liquid fuel was studied downstream the atomizing edge of a plain airblast nozzle. Several optical diagnostics like phase resolved LDV (Laser Doppler Velocimetry) and an improved PTV technique (Particle Tracking Velocimetry) were used. The mean air velocity, the film load, the kinematic viscosity and the surface tension of the fluid as well as the pulsation frequency and amplitude of the siren were varied, and their effect on the temporal evolution of the droplet size and droplet rate was studied. It was found that the amplitude of fluctuations of the droplet size and the droplet rate is almost proportional to the air velocity fluctuations at low frequencies. At higher frequencies, however, both are nearly unaffected. In addition, the fluctuations of droplet diameter and rate increase strongly if the mean air velocity is increased. The phase shift between particle diameter, particle rate and air velocity fluctuations was found to increase at higher excitation frequencies.