Swirler effects on combustion instabilities analyzed with measured FDFs, injector impedances and damping rates
Résumé
The influence of the injection system on combustion instabilities is investigated by using systematic experiments with three swirlers having similar geometries but different pressure losses and swirl numbers. These experiments are carried out in a laboratory-scale single injector combustor fed with liquid heptane delivered as a spray by a pressure atomizer. Self-sustained oscillations (SSOs) examined by varying the chamber length for the different swirlers feature differences in oscillation frequency and in oscillation level. An attempt is made to interpret these results using a low-order model that provides growth rates and frequencies of oscillation. This model uses an experimentally determined flame describing function (FDF) and specific impedance at the injector outlet. For the FDF measurements, it is shown that the reference position for the measurement of incident velocity fluctuations is critical for swirling injectors, and the optimal position needs to be carefully defined. Results from the model indicate that the injector impedance (that depends on the swirler characteristics) shifts the bands of instability and modifies the growth rate magnitude. This has a profound effect on the stability of the system. By combining the injector impedance with FDF data in the model, the stability of the system can be rated along with a prediction for growth rate and frequency of oscillation. This analysis is performed for the three swirlers and the instability predictions from the model generally agree with experimental observations with some limitations. However, it is seen that many of the unstable points would not be identified without considering simultaneously a representation of the injector impedance, suitable FDF data and damping rate estimates.
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