Azimuthal Instabilities Arising in an Annular Combustor Equipped With Pure Hydrogen Injection Units

At a stage where hydrogen is being considered as a possible fuel for aeroengines or industrial gas turbines, it is important to examine operability issues and in particular those induced by the coupling between acoustic modes of the system and combustion. The aim of this investigation is to analyze instabilities coupled by azimuthal modes in annular configurations. The present study is carried out in a laboratory scale annular combustor equipped with 16 burners featuring cross-flow hydrogen injection in a swirled air flow. A mapping of the operational domain reveals many different regimes of instability, some of them arising at high frequencies. An examination of the pressure signals detected by the eight microphones reveals high-amplitude oscillations. It is found that instabilities are most often coupled by standing modes with well defined nodal lines. The pressure signals are used to infer the nature of the modes and determine their azimuthal structure. Various types of instabilities are identified involving the 1A1L, 2A1L, 2A2L, 3A1L and 4A1L modes at frequencies ranging from 800 to 2800 Hz in agreement with theoretical estimates of the resonant frequencies. The pressure and OH* light intensity data are used in a second stage to characterize the growth rates of the various unstable regimes and deduce the damping rates pertaining to the various modes. It is shown that flames established by injection of pure hydrogen are highly sensitive to disturbances and that they easily couple with azimuthal modes giving rise to higher order modes at elevated frequencies.