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

Abstract

Recent investigations of combustion instabilities in annular systems indicate that considerable insight may be gained by using information gathered in single-sector experiments. Such experiments are, for example, employed to measure flame describing functions (FDFs), which represent the flame response to incident perturbations. 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 the FDF data from single-segment experiments portrays the dynamical response of the flame in the annular environment. These issues are investigated with a new setup, named TICCA-Spray, 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, side-wall separation, and spacing between injectors identical to those of the annular system MICCA-Spray. The describing function of the central flame is determined with techniques recently developed in single sector experiments (SICCA-Spray). The FDFs obtained in the two configurations are compared for two swirler types having different swirl numbers and pressure drops. The effect of the swirl direction of the neighboring injectors is also explored by operating with co- and counter-swirl combinations. 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, and provided that the area is equivalent to that of a single sector of an annular combustor. Nonetheless, using a multi-injector system would be more appropriate for a more precise FDF determination.

Azimuthal Instabilities of an Annular Combustor With Different Swirling Injectors

“…The rig comprises a cylindrical quartz chamber with an inner diameter of 69 mm, allowing optical access to the flame region, and is operated with the same injectors as MICCA-Spray. Although the wall-bounded flames in SICCA-Spray do not exactly represent the flames of MICCA-Spray, where each flame is surrounded by other flames, it is concluded in a later investigation that they approximately represent the flame dynamics in MICCA-Spray to a reasonable degree [34]. This conclusion relies on a comparison between the single injector configuration and a newly developed threeinjector linear test rig TICCA-Spray, where the central flame is surrounded by two side flames and more closely resembles the MICCA-Spray environment.…”