Experimental Validation of Linear Stability Analysis in a Premixed Combustor by Actively Tuning Its Acoustic Boundary Conditions

Linear stability analysis by means of low-order network models is widely spread in industry and academia to predict the thermoacoustic characteristics of combustion systems. Even though a vast amount of publications on this topic exists, much less is reported on the predictive capabilities of such stability analyses with respect to real system behaviour. In this sense, little effort has been made on investigating if predicted critical parameter values, for which the combustion system switches from stability to instability, agree with experimental observations. Here, this lack of a comprehensive experimental validation is addressed by using a model-based control scheme. This scheme is able to actively manipulate the acoustic field of a combustion test rig by imposing quasi arbitrary reflection coefficients. It is employed to continuously vary the downstream reflection coefficient of an atmospheric swirl-stabilized combustion test rig from fully reflecting to anechoic. By doing so, the transient behaviour of the system can be studied. In addition to that, an extension of the common procedure, where the stability of an operating point is classified solely based on the presence of high amplitude pressure pulsations and their frequency, is given. Generally, the predicted growth rates are only compared to measurements with respect to their sign, which obviously lacks a quantitative component. In contrast to that, in this paper, validation of linear stability analysis is conducted by comparing calculated and experimentally determined linear growth rates of unstable modes. Besides this, experimental results and model predictions are also compared in terms of frequency of the least stable mode. Excellent agreement between computations from the model and experiments is found. The concept is also used for active control of combustion instabilities. By tuning the downstream reflectivity of the combustion test rig, thermoacoustic instabilities can be suppressed. the underlying mechanism is an increase of the acoustic energy losses across the system boundary.