Strong Azimuthal Combustion Instabilities in a Spray Annular Chamber With Intermittent Partial Blow-Off

The present article reports original experiments carried out in the MICCA-Spray combustor developed at EM2C, CNRS and CentraleSupélec. This system comprises 16 swirl spray injectors. Liquid n-heptane is injected by hollow cone simplex atomizers. The combustion chamber is formed by two cylindrical quartz tubes allowing full optical access to the flame region and it is equipped with eight pressure sensors recording signals in the plenum and chamber. A high speed camera provides images of the flames and photomultipliers record the light intensity from different flames. For certain operating conditions, the system exhibits well defined instabilities coupled by the first azimuthal mode of the chamber at a frequency of about 750 Hz. These instabilities occur in the form of bursts with a moderate level of growth. Examination of the pressure and the light intensity signals gives access to the acoustic energy source term. Analysis of the phase between the two signals during the instability bursts (growth, limit cycle, decay) is carried out using cross-spectral analysis. At limit cycle, large amplitude of pressure oscillations are reached with peak values around 5000 Pa (or 5% of the mean pressure in the chamber), and these levels persist over a finite period of time. Detailed analysis of the signals using the spin ratio indicates that the standing mode is predominant. The chamber can exhibit a spinning mode but with a lower amplitude of acoustic fluctuation. Analysis of the flame dynamics at the pressure anti-nodal line reveals a strong longitudinal pulsation with heat release rate oscillations in phase and increasing linearly with the acoustic pressure even at the highest oscillation levels. At the pressure nodal line, the flames are subjected to large transverse velocity fluctuations leading to a transverse motion of the flames and partial blow-off. Scenarios and modeling elements are developed to interpret these features. To the best of our knowledge, this is the first time that azimuthal instabilities are characterized in a well-controlled annular combustor with swirled spray flames.