Forced low-frequency spray characteristics of a generic airblast swirl diffusion burner

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.