Influence of an Oscillating Airflow on the Prefilming Airblast Atomization Process

Abstract

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.

Spray response on a model prefilmer under unsteady airflows of various frequencies

“…Similar behavior is observed in both Figure 4(a) and Figure 4(b) in terms of the mean velocity trend. As shown in previous studies with similar operating conditions ⁠ 8 , approximately 5 mm away from the centerline the mean droplet velocity is higher compared to the velocity at X = 0 mm; this is observed in Figure 4 as well for all four excitation frequencies examined.…”

“…Previous studies in the test facility of the Engler-Bunte-Institute had analyzed the behavior of the spray in a single excitation frequency near 100 Hz ⁠ 7,8 . In this study, the influence of the excitation frequency and the air-to-liquid ratio (ALR), i.e.…”

Comparison of the flame dynamics of a premixed dual fuel burner for kerosene and natural gas

“…Recent investigations suggest that the average evaporation quality is only marginally affected by acoustic excitation. However, the different inertia of droplets of different size cause a segregation of small droplets at velocity antinodes whereas bigger droplets remain unaffected by the excitation, 6,7 acting as a potential source of instability. 8–10 While the research on liquid combustion has mainly focused on aero engines which operate under rich non-premixed conditions, 9,11 only little research has been conducted on lean premixed liquid fuel combustion so far.…”

Optical Equivalence Ratio Measurement of a Dual Fuel Burner for Natural Gas and Kerosene

“…For liquid fuel combustion, additional sub-processes involving fuel atomization and evaporation may act as sources of instabilities influencing the generation of equivalence ratio waves. Recent studies indicated that even small velocity perturbations significantly affect the droplet size distribution contributing to the formation of equivalence ratio waves [16][17][18].…”