Thermoacoustic instabilities often occur in a combustion chamber of a jet engine when pressure fluctuations from the released heat create a closed feedback loop with the fluctuations of the inlet air velocity, causing further combustion instabilities that enhance the cycle back again. Since most jet engine combustion chambers use liquid fuel dispersed into droplets by an airblast atomizer, any fluctuations occurring in the air entering the airblast nozzle could affect the atomization process of the liquid fuel and the air to fuel ratio distribution inside the combustor. The present experimental study focuses on the influence of the pulsating airflow on the spray characteristics during the airblast atomization process. The experiments were carried out with a two-dimensional prefilmer in which a water film flow was generated on one surface. A siren excited the airflow at approximately 120 Hz. The oscillation of the airflow was characterized by a Constant Temperature Anemometer, while the developed spray was examined using Phase Doppler Anemometry where droplet velocities and the droplet size distribution were measured. By processing the droplet characteristics, an oscillation of the mean diameter, Sauter Mean Diameter and droplet velocity is observed. In comparison to a non-forced flow case as well as theoretical predictions of the SMD based on the air velocity, these results may indicate that the oscillating air velocity causes a timedependent separation of the droplets which results in those fluctuating spray characteristics.

No abstract

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.

An appealing concept for jet engine combustors is the Lean Premixed Prevaporized (LPP) combustor, which operates at high pressures. The low NOx emissions achieved by lean combustion are one of the targets for modern aircraft engines. However, these types of combustors can introduce thermoacoustic instabilities that can potentially damage the engine and reduce its lifespan. Since the potential instabilities on the fuel spray characteristics, i.e. the spray mass flux, can affect the flame stability, the need arises to investigate the spray response under an unsteady airflow. For this study, a model prefilmer was experimentally investigated to produce a two-dimensional droplet flow without swirl flow. An acoustic forcing in the range of 100–500 Hz was introduced into the airflow, characterized by a hot wire Constant Temperature Anemometry (CTA) setup. Droplet characteristics, namely the droplet diameter distribution and velocity, were determined using a Phase Doppler Anemometry (PDA) setup, while the acquired data were phase-averaged in one period of the airflow oscillation. The influence of the excitation frequency and the air-to-liquid ratio (ALR) on the spray was studied: the spray responded to the acoustic excitation and therefore critical performance parameters, such as the spray mass flux, oscillated indicating potential problems regarding the flame stability.