Combustion instabilities in a bifurcating tube: Open- and closed-loop measurements

Combustion instabilities characterized by large-amplitude self-sustained oscillations are detrimental to gas-turbine and aeroengine systems. To mitigate these unwanted oscillations, a dynamic actuator is generally implemented. In this work, combustion instability in a bifurcating tube with a loudspeaker implemented is investigated. The bifurcating system is different from conventional Rijke-type and T-shaped ones. It can produce nonharmonic "hot" and "cold" oscillating flows, which provides a useful platform to aid experimental research and teaching on combustion instability. To monitor the flow and acoustic fields in the bifurcating branches, two arrays of pressure sensors and an infrared thermal-imaging camera are implemented. When the loudspeaker attached near the open end of the bottom stem is not actuated, the tube is corresponding to an open-loop thermoacoustic system. To gain insights on the distinguishing characteristics of the flow and acoustic fields, two-dimensional numerical simulations and experimental measurement of the open-loop system are conducted first. Comparison is then made between the numerical and experimental results. Good agreement is obtained in terms of mode frequencies, mode shapes, and sound pressure level. Describing function analysis is then conducted to predict the limit-cycle frequency and amplitude. Finally, the loudspeaker is actuated via a multi-input/single-output tuning strategy to minimize the combustion oscillations. It is shown that actuating the loudspeaker results in the combustion system being stabilized via reducing the sound pressure level by approximately 45 dB, even when the fuel flow rate is slightly changed.