The Response of Fuel Injector Sprays to Acoustic Forcing

Combustion instabilities have impeded the development of lean burn low NOx gas turbine combustors. Acoustic waves in the combustor interact with the fuel/air mixing process to modulate the air-fuel ratio (AFR). The fluctuating AFR causes the heat release rate to fluctuate. If the Rayleigh criterion is obeyed the acoustic wave is amplified. The acoustic intensity may reduce the lifetime of the mechanical parts, due to vibration and increased heat transfer, or cause the operating noise levels to exceed regulatory limits. The fuel injector has an important role in the interaction of the acoustic field and the AFR. In rig tests, injectors of slightly different design can yield different noise levels in the same combustor. We have applied planer LIF and Mie imaging and Laser Sheet Dropsizing (LSD) to study the interaction of the fuel injector spray with an imposed acoustic disturbance in low cost non-combusting tests. The injector is run at pressures up to 7 bar in the Cranfield optical sprays rig. By careful scaling of the injector operating conditions the spray structure found in the rig can be made representative of that in the engine. An acoustic perturbation is imposed on the airflow. The spray field is probed with a dual LIF/Mie laser sheet imaging system. Laser Induced Fluorescence (LIF) images are calibrated to yield maps of AFR. The LIF and Mie images are ratioed and calibrated to produce maps of the Sauter Mean Diameter. Images are taken at several phases in the acoustic perturbation cycle. From these the oscillatory response of the spray to the acoustic perturbation is extracted. A transfer function describing the strength and phase of the response of AFR and SMD to the acoustic perturbation is extracted. Both AFR and SMD exhibit fluctuations which are coherent with the imposed acoustic oscillation. Comparison of the transfer functions to the results of combustion tests suggest that the spray measurements can predict which injectors are likely to encourage strong instability in the combustor.