Characterisation of three-port burner acoustics using a reduced-order model

Combustion of steam-diluted fuels offers low emissions and fuel flexibility, but thermoacoustic instabilities remain a challenge. Burner acoustics play a crucial role in the prediction and control of such instabilities. This study reports an experimental investigation of the acoustic characteristics of a dual-swirl burner integrated in a humid combustor. In contrast to conventional two-port burners, this burner features three ports of acoustic flux: Two non-stiff inlets corresponding to air and steam-fuel mixture, and one outlet. A full characterisation of such a three-port is demanding for practical burners. Based on the assumption of acoustic compactness, a Reduced-order Model is proposed for the three-port Burner Transfer Matrix (BTM). The model treats the three-port as a combination of two compact two-ports with no interaction between the two inlets. It allows obtaining the reduced three-port BTM with only two linearly independent acoustic states, as is done for a two-port, but with additional microphones for the third port. First, the two-port characteristics of the burner were studied by blocking one of the two inlets at a time. The results substantiate the model assumptions for the three-port. However, the simplifications are valid for frequencies up to 400 Hz, beyond which some deviations are observed. The three-port shows higher resistive and reactive characteristics between the fuel inlet and the outlet ports, compared to the two-port case with the air inlet blocked. The ratio of flow momentum between the two inlets has only a mild effect on the BTM, whereas the speed of sound ratio exhibits a stronger influence.