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 oxide 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 NOx 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 an excitation frequency near 94 Hz was investigated. The airflow oscillation under this excitation frequency was characterized using a Constant Temperature Anemometer (CTA), while the generated spray was investigated with a Phase Doppler Anemometry (PDA) system. The spray was investigated in a variety of positions along the radial axis, providing spatial information, apart from temporal. The characterization of the spray via PDA includes a two-component droplet velocity detection and diameter measurement, while the spray mass flux for each measured position was also calculated. The acquired data were phase averaged via an in-house developed processing algorithm, while through a statistical analysis the confidence intervals of the calculations were included. The excitation frequency strongly influenced all spray characteristics, namely, the Sauter Mean Diameter (SMD), the droplet velocities, the mass flux, as well as the local air-to-liquid ratio (ALR). Depending on the phase angle, the size distribution of the spray changes, explaining the observed oscillating behavior of the spray characteristics.