Self-excited oscillations in combustors with spray atomizers

Combustors with fuel-spray atomizers are susceptible to a low-frequency osc illation, particularly at idle and sub-idle conditions. For aero-engine com bustors, the frequency of this oscillation is typically in the range 50-120 Hz rind is commonly called "rumble." In the current work, compatational flu id dynamics (CFD) is used to simulate this self-excited oscillation. The co mbustion model uses Monte Carlo techniques to give simultaneous solutions o f the Williams' spray equation together with the equations of turbulent rea ctive flow. The unsteady combustion is calculated by the laminar flamelet p resumed pdf method. A quasi-stead, description of fuel atomizer behavior is used to couple the inlet flow in the combustor. A choking condition is emp loyed at turbine inlet. The effects of the atomizer and the combustor geome try on the unsteady combustion are studied. The results show that, for some atomizers. with a strong dependence of mean droplet size on air velocity, the coupled system undergoes low-frequency oscillations. The numerical resu lts are analyzed to provide insight into the rumble phenomena. Basically, p ressure variations in the combustor alter the inlet air and fuel spray char acteristics, thereby changing the rate of combustion. This in turn leads to local "hot spots," which generate pressure fluctuations as they convect th rough the downstream nozzle.