Self-Excited Oscillations in Combustors With Spray Atomisers

Abstract

Combustors with fuel-spray atomisers are susceptible to a low-frequency oscillation, particularly at idle and sub-idle conditions. For aero-engine combustors, the frequency of this oscillation is typically in the range 50–120Hz and is commonly caused ‘rumble’. In the current work, Computational Fluid Dynamics (CFD) is used to simulate this self-excited oscillation. The combustion model uses Monte Carlo techniques to give simultaneous solutions of the Williams’ spray equation together with the equations of turbulent reactive flow. The unsteady combustion is calculated by the laminar flamelet presumed pdf method. A quasi-steady description of fuel atomiser behaviour is used to couple the inlet flow in the combustor. A choking condition is employed at turbine inlet. The effects of the atomiser and the combustor geometry on the unsteady combustion are studied. The results show that, for some atomisers, with a strong dependence of mean droplet size on air velocity, the coupled system undergoes low frequency oscillations. The numerical results are analysed to provide insight into the rumble phenomena. Basically, pressure variations in the combustor alter the inlet air and fuel spray characteristics, thereby changing the rate of combustion. This in turn leads to local ‘hot spots’, which generate pressure fluctuations as they convect through the downstream nozzle.