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Comparison of Flame Describing Functions Measured in Single and Multiple Injector Configurations


 Recent investigations of combustion instabilities in annular systems indicate that considerable insights may be gained by using information gathered in single-sector experiments. Such experiments are, for example, employed to measure flame describing functions (FDFs). These data may be used in combination with low-order models to interpret instabilities in multiple injector annular systems. It is known, however, that the structure and dynamical behavior of an isolated flame do not necessarily coincide with those of a flame placed in an annular environment with neighboring side flames. It is then worth analyzing effects that may be induced by the difference in lateral boundary conditions and specifically examining the extent to which FDF data from single-sector experiments portrays the dynamical response of the flame in the annular environment. These issues are investigated with a new setup that comprises a linear arrangement of three injectors. The central flame is surrounded by two identical side flames in a rectangular geometry with key dimensions identical to those of the annular system. The describing function of the central flame is determined with techniques recently developed in single sector experiments. Differences between FDFs determined in the two test facilities, sometimes modest and in other cases less negligible, are found to depend on the flames' spatial extension and interactions. The general inference is that the FDFs measured in a single-injector combustor are better suited if the flame-wall interaction is weak. Nonetheless, a multi-injector system would be more appropriate for a more precise FDF determination.