Visualization of the lean blowout process in a model combustor with a swirl cup

Lean blowout (LBO) limit is an important parameter for aero-engine combustors. There are some LBO correlations used by designers to estimate the LBO fuel/air ratio (FAR). Among these correlations, Lefebvre's model is employed extensively. In his model, the parameters and are the combustor volume and inlet air mass flowrate. However, it is argued that the flame volume near LBO is not the whole combustor volume (VcVAm&c), and the air involved in combustion near LBO is not the entire inlet air mass flowrate. To find out the mechanisms behind LBO for combustors with swirl cups, a visualization window was designed in a single dome rectangular model combustor with a dual-radial swirl cup, and flame images of the LBO process were recorded. In the experiments, three configurations of dual-radial swirl cups were employed, the differences among them were the variations of the flow areas of the primary and secondary swirlers. The air mass flowrate was about 0.6kg/s. The pressure in the combustor was kept 220KPa under each operation condition. Different flame shapes, flame colors, and flame anchoring locations were observed and recorded, while the LBO fuel/air ratios were also measured. It is found that the visualization window causes the combustor LBO limit to occur at a higher FAR by 8% (i.e. less stability). The flame images indicate that the flame only exists in a small space of the primary zone instead of the whole combustor near LBO. Moreover, the flow area of the primary swirler has the great effect on the LBO FAR. It is confirmed that the air involved in combustion mainly comes from the primary swirler while approaching to LBO condition. Furthermore, a correlation between LBO equivalence ratio and flame area is obtained. If the flame area is correlated to other geometrical and conditional parameters, the accuracy of Lefebvre's LBO model will be improved further.