58862 - Influence of Acoustically Excited Airflows on a Planar Airblast Prefilmer 

In order to meet the higher requirements for clean combustion technology in aircraft engine applications and thus reduce harmful emissions, especially nitrogen oxides emissions, the major jet engine manufacturers are developing lean premixed prevaporized (LPP) combustors that operate at very high pressure. In this context, thermoacoustic instabilities may occur within the combustion chamber. The unsteady heat released by the flame generates pressure waves, which are coupled to the inlet air velocity by a feedback loop. This loop amplifies the instabilities of the inlet air velocity, which in turn influences the atomization process.

Since the atomization process at the airblast atomizers of most jet engine combustors determines critical operating characteristics such as air-to-fuel ratio (AFR), flame stability, or NO_x emissions, predicting the performance of this process under unsteady conditions has a significant value.

The present experimental study focuses on the influence of oscillating airflows on the spray characteristics at the airblast atomization process. The experimental setup was based on a two-dimensional prefilmer where a water film flow was introduced on one surface. The airflow was excited by a siren, whereby excitation frequencies between 90 and 150 Hz were investigated. The airflow oscillation had been characterized in preliminary investigations using a Constant Temperature Anemometer (CTA), while the generated spray was investigated with a Phase Doppler Anemometry (PDA) system.

The performance of the prefilmer was investigated for non-forced flow conditions in different positions and operating conditions. The characterization of the spray via PDA includes a two-component droplet velocity detection and diameter measurement, and the calculation of the spray mass flux for each measured position. When the acoustic forcing was introduced into the airflow, the same spray characteristics were analyzed by phase averaging the measured data via an in-house developed processing algorithm. The excitation frequency strongly influenced all spray characteristics, namely the mean diameter, the Sauter Mean Diameter (SMD), the droplet velocities, the number of measured droplets as well as the mass flux. Depending on the region of the spray and the excitation frequency, the size distribution of the spray changed, indicating a different behavior on the trajectory of the small droplets.