Combustion instabilities of bluff-body stabilized liquid spray flames coupled with pressure oscillations in the fuel line

In this paper, combustion instabilities in a bluff-body combustor are investigated. The study delves into the features of pressure oscillations occurring in both the combustion chamber and the fuel line under various operational conditions. It is observed that at low airflow rates, only a low-frequency longitudinal oscillation mode occurs. However, at elevated airflow rates, both low-frequency longitudinal and high-frequency transverse oscillation modes are observed. Additionally, it is found that the fuel line exhibits significant pressure oscillations with frequencies identical to that of the combustion chamber. When the low-frequency oscillation mode appears in the combustion chamber, the low-frequency pressure oscillation amplitudes in the fuel line are of the same order of magnitude as that of the combustion chamber. In contrast, for high-frequency oscillation modes, the fuel line's high-frequency oscillations are even one to two orders of magnitude higher than that of the combustion chamber. We established a low-order model capable of effectively analyzing the thermoacoustic frequencies and modes of the combustion chamber coupled with the fuel line. The model allows us to predict the ratio of the pressure oscillation amplitude in the fuel line to that in the combustion chamber. Through the analysis of dual-frequency scenarios, it is discovered that over a certain range of fuel line length, the pressure oscillation in the fuel line is one order of magnitude higher than that in the combustion chamber when the high-frequency oscillation mode occurs, agreeing with the experiments. Novelty and significance The innovation of this research lies in the experimental and theoretical modeling study of the longitudinal low-frequency and transverse high-frequency combustion instability characteristics in a bluff-body liquid spray combustion chamber, as well as the coupling issues between fuel line pressure oscillation and combustion chamber pressure oscillation caused by combustion instabilities. The research reveals that substantial pressure oscillations in the fuel line are identified during multimode combustion instabilities in a bluff body combustor. Furthermore, a theoretical model is developed to systematically analyze the thermoacoustic frequencies and modes within the combustion chamber, considering its coupling with the fuel line. A clear relationship is established between the pressure oscillations observed in the fuel line and those occurring in the combustion chamber.