Forced Low-Frequency Spray Characteristics of a Generic Airblast Swirl Diffusion Burner

Abstract

The low frequency response of the spray from a generic air-blast diffusion burner with a design typical of an engine system has been investigated as part of an experimental study to describe the combustion oscillations of aero engine combustors called rumble. The atomization process was separated from the complex instability mechanism of rumble by using sinusoidal forcing of the air mass flow rate without combustion. Pressure drop across the burner and the velocity on the burner exit were found to follow the steady Bernoulli equation. Phase-locked PIV measurements of the forced velocity field of the burner show quasi-steady behavior of the air flow field. The phase-locked spray characteristics were measured for different fuel flow rates. Here again quasi-steady behavior of the atomization process was observed. With combustion, the phase-locked Mie-scattering intensity of the spray cone was found to follow the spray behavior measured in the non-combusting tests. These findings lead to the conclusion that the unsteady droplet SMD mean and amplitude of the air-blast atomizer can be calculated using the steady state atomization correlations with the unsteady burner air velocity.

Time-Response of Recent Prefilming Airblast Atomization Models in an Oscillating Air Flow Field

“…Two fluctuation frequencies ( f = 62 and 500 Hz) are selected and will be discussed in this study. The former frequency corresponds to the lower bound of a resonance frequently observed in combustion chambers [2] and referred to as rumble, while the latter frequency is representative of the first longitudinal mode of usual combustion chambers [5].…”

“…The operating principle of prefilming airblast atomization is the momentum transfer from a high speed gas flow to the liquid phase of a thin film. This type of injector was previously investigated under fluctuating gas velocity conditions [2][3][4][5] and was found to be sensitive to flow fluctuations [4,5]. Consequently, typical spray characteristics such as the droplet flux and the Sauter Mean Diameter might also be influenced by thermoacoustic instabilities.…”

Towards the optimization of a pulsatile three-stream coaxial airblast injector

“…They further speculated that, with knowledge of said orifice contributions, atomization systems could be optimized via harmonic amplification of instabilities. The concept of jet shedding tuning with combustion was explored by Ibrahim et al (1998) and Eckstein et al (2003). They propose that jet shedding frequencies may couple with combustor dynamics to cause combustion instability.…”

Numerical analysis of the dynamic flame response of a spray flame for aero-engine applications

“…Eckstein et al proved that for low-frequency combustion oscillations a quasi-steady description of the airblast atomizer is appropriate and droplet diameter fluctuations can be directly related to air velocity fluctuations at the injection plane. 7 They in fact observed that positive air velocity fluctuation leads to a positive oscillation of the heat release rate because a higher velocity causes a reduction of droplet mean diameter and therefore a greater heat release rate.…”

“…The interaction of the acoustic field with the fuel spray can produce periodic variation of the spray shape, droplet size distribution and, in turn, a variation in the evaporative and mixing processes. [5][6][7] These periodic variations in fuel supply rate and/or periodic variations of the equivalence ratio at the flame location produce heat release oscillations that drive the acoustic field. [8][9][10] The physical processes involved are very complex and not completely understood with very few investigations available in literature especially for the interaction between acoustics and droplet primary breakup.…”