Spray Response to Acoustic Forcing of a Multi-Passage Lean-Burn Aero-Engine Fuel Injector
Authors: Su Jialin , Barker Ashley , Garmory Andrew , Carrotte Jon 
Published: August 2018

An air-blast fuel injector in an aero-engine plays an important role in the thermoacoustic behaviour of the combustion system. Previous studies have looked at the response of the gas phase to acoustic forcing of fuel injectors but less attention has been paid to the liquid phase. Even if the fuel flow rate is assumed to remain constant, the atomisation process is driven by the relative velocity of the air and fuel. Hence air velocity fluctuations will lead to changes in atomisation which will significantly affect the flame. In this work, we use both experiment and computational fluid dynamics (CFD) to investigate the response of the spray produced by a multi-passage lean-burn aero-engine fuel injector to acoustic forcing. A Phase Doppler anemometry (PDA) system was used to measure the spray size and velocity statistics at points downstream of a fuel injector operating under atmospheric conditions. The plenum-fed injector was subject to acoustic forcing from downstream at a range of frequencies from 50 to 450 Hz. The data were sorted according to phase bins to observe how the spray statistics change during the acoustic forcing cycle. To further understand these results and assess numerical prediction strategies, CFD simulations were performed. A compressible unsteady Reynolds-averaged Navier-Stokes (URANS) method was used to predict the acoustically forced air-flow through the injector. Lagrangian spray particles were introduced into the flow, close to the prefilmer lip, with a time varying size distribution determined using existing empirical breakup correlations based on the instantaneous air velocity field. Constants needed for the breakup correlations were calibrated using the data from the unforced and lowest frequency results. Results were then sampled at the same downstream location as in the experiment. With this approach, better agreement was observed between experiment and CFD when the instantaneous air mass flow rates and velocities are based on the local flow field, close to the fuel-prefilmer lip, rather than the whole passage. This shows the importance of including the details of the response of the separate passages, including phase differences, for accurate predictions of unsteady fuel injector flows. Experimental results also show a phase difference between the peak air velocity and the Sauter Mean Diameter (SMD) of the particles at the measurement plane. At lower frequencies, the phase shift is captured by the CFD results based on a quasi-steady breakup correlation. This indicates that the phase shift at low frequencies can be explained mainly by the particles travelling to the measurement plane at a slower speed than the air. However, as frequency increases, CFD based on the quasi-steady assumption alone is seen to no longer capture the phase shift, indicating that the phase shift in the experiment is not just due to the time of flight of the particles. Introducing a time delay into the breakup model allows the experimental phase shift to be matched. However, if running averaging is used, the variation in the SMD at a high frequency is over attenuated compared to the experimental measurements. More research is needed to fully understand the unsteady atomisation behaviour of this type of fuel injector, particularly its dependency on the time history of the unsteady air flow.