Experimental determinations of Flame Describing Functions of swirling spray flames

International audienceStudies of the dynamics of spray swirling flames are of importance from a fundamental point of view and also because these flames are used in many practical systems like aero-engine combustors. Under certain conditions these flames give rise to various types of undesirable combustion instabilities. This article focuses on the measurement of the combustion dynamics of a system comprising a single injector designated as SICCA-Spray and reports novel Flame Describing Function (FDF) data on spray swirling flames. The test configuration comprises a tubular plenum fed with air by a swirling injector, a simplex atomizer that injects the liquid fuel (n-heptane) within a hollow cone spray and a cylindrical combustion chamber where the air and the fuel spray mix and burn. The air flow can be excited with two driver units plugged on the plenum, creating a flow perturbation which acts on the flame. The amplitude of fluctuation u is controlled with a hot wire in the plenum. The flame response to incoming perturbations is then examined by recording the OH* signal with a photomultiplier equipped with an interferometric filter. It is assumed that fluctuations of the light emission from this excited radical is proportional to heat release rate perturbationsQ. The acoustic pressure p at the backplane of the chamber is also recorded with a cooled waveguide microphone. Two different FDFs are defined and compared:Q /Q = f (p /p) andQ /Q = g (u /ū). The data are then used to analyze self-excited instabilities observed in the SICCA-Spray burner and to interpret some recent observations of strong self-sustained azimuthal instabilities in the MICCA-Spray annular combustor, equipped with sixteen injectors identical to that used in the SICCA-Spray setup