Research on Characteristics of Thermoacoustic Instabilities in Air-Methane-Ammonia Premixed Swirl-Stabilized Combustors

Abstract

As an excellent hydrogen carrier, ammonia is a very good clean energy source. However, due to the poor combustion performance of ammonia, other fuels can be blended to improve the combustion performance. In this paper, the combustion characteristics of ammonia/methane composite fuel and analyzed, and the ability of ammonia/methane ratio to alleviate the thermoacoustic instabilities of burners is discussed, which is of great significance for the development of low-emission and high-reliability combustors. In this study, the pressure and heat release rate oscillation characteristics in the combustion chamber, and the evolution of the flame surface driven by velocity perturbation are investigated. The flow shape and instantaneous OH profile are measured with high-speed camera and OH-PLIF technique, respectively. Analysis shows that its vibration pattern and frequency are similar to the axial first-order acoustic mode; both the heat release rate and the pressure fluctuate at the same frequency. The inlet velocity perturbation leads to the reciprocating retraction and expansion of the flame front as well as the further change of the flame wrinkle. The spatiotemporal varying in the flame surface area are the key mechanism for driving the fluctuation of the heat release rate. When the swirling flame interacts with the combustor wall, the flame surface will be broken out, and the flame surface area will be extended to the maximum. In addition, the stoichiometric ratio and ammonia/methane mixing ratio have a great influence on the thermoacoustic instability of premixed swirl combustion. As the equivalence ratio changes from the lean to the rich, the burner undergoes a transition process: from the stable combustion, quasiperiodic oscillation, limit cycle oscillation, quasi-periodical oscillation, until the stable combustion. Blending ratio mainly change the flame heat release power, flame shape, flame propagation velocity and the interaction between the flame and the combustor wall. As the ratio of ammonia gas increases, the burner changes from limit cycle oscillation to the stable combustion, and the flame heat release power also decreases. Therefore, blending ammonia in methane is a good attempt to alleviate thermoacoustic coupling, and optimizing the overall airflow velocity, equivalence ratio and blending ratio is the key to get the best performance.