Abstract
Experiments are carried out on the laboratory-scale MICCA-Spray annular combustor to examine the effects of swirlers on combustion instabilities. This system comprises 16 spray-swirl injectors and gives rise to instabilities coupled by azimuthal modes. Five types of swirlers producing clockwise rotation and varying in swirl numbers and pressure drops are considered. These swirlers can be broadly categorized into two groups: lower-swirl and higher-swirl groups, based on their swirl numbers. An arrangement where clockwise and counterclockwise swirlers alternate is also studied. Experiments are performed systematically with liquid heptane at five levels of thermal power and six equivalence ratios. Results reveal that none of the swirlers in the lower-swirl category exhibit instability in the operating region considered, whereas the higher-swirl units feature strong azimuthal instabilities that trace an overall limit cycle envelope with a few short and random bursts. Among the higher-swirl group, a higher pressure drop swirler is associated with a broader instability map. This shows that the transition to instability mainly depends on the swirl number through its effect on the flame structure and that the pressure drop adds to further variations in amplitude and frequency of oscillation. The spin ratio time series indicate that the modes are of mixed type and that their distribution depends on the operating condition. On specifically comparing the spin ratio distribution between a full set of clockwise rotating (CR) swirlers and a configuration where clockwise and counterclockwise rotating swirlers (CCR) are alternatively placed, it is found that there is no definite statistical preference for spin ratio linked to the effect of bulk swirl. In some cases, however, the CCR configuration promotes a broader distribution of spin ratios centered around the standing mode (sr = 0) while the CR setup favors azimuthal modes spinning in the counterclockwise direction. An attempt is made to interpret the occurrence of instabilities by making use of flame describing functions (FDFs) measured in a single-injector combustor. It is found that the FDFs corresponding to the two swirler categories (lower-swirl and higher-swirl) are relatively distinct. The observed behavior is tentatively interpreted using an instability analysis in which the injector and upstream plenum are represented by an impedance that shifts the band of instability. The unstable behavior is then linked to the relative position of the FDF phase with respect to the instability band in the frequency range corresponding to the expected azimuthal mode frequency. The phase and gain of the FDF notably depend on the swirl number, and it is possible to distinguish, for the present configuration, a category of low swirl number injectors inducing stable operation and another category of high swirl number units leading to oscillations.
