Theoretical investigation on intrinsic thermoacoustic instability in long flame combustion chambers

This paper investigates the recently identified phenomenon of combustion instability, termed Intrinsic ThermoAcoustic oscillations (ITA). Through the integration of the low-order acoustic network model and the linear Euler equations, a predictive model for thermoacoustic oscillations in duct and annular combustion chambers with long flames is developed. Additionally, the effect of flame distribution on ITA modes in combustion chambers is examined. The results indicate significant differences between ITA modes in long flame combustion chambers and those in compact flame models. Furthermore, the ITA modes in long flame combustion chambers do not conform to the −π criterion. Moreover, the flame distribution has a substantial impact on the ITA modes in combustion chambers, with the distribution of time delay in the flame transfer function determining the ITA modes. The interaction between the time delay distribution and the coupling strength distribution in the flame transfer function further amplifies this impact. Additionally, time delay distributions determine the effects of various parameters, such as temperature ratio and flame length, on ITA modes in combustion chambers. For higher-frequency ITA modes, the effect of flame distribution becomes more pronounced. Compared to duct combustion chambers with long flames, the influence of flame distribution on ITA modes in annular combustion chambers is relatively smaller, yet the ITA modes are still determined by the time delay distribution. In conclusion, the distribution of ITA modes in long flame combustion chambers is considerably complex and requires further research and exploration.