UC San Diego

Thermoacoustic instability which stems from the non-linear interactions between heat release and pressure waves in a confined combustion chamber, can create large amplitude pressure oscillations. These oscillations may cause significant damage to the combustor hardware, and even surroundings. Accurate characterization and prediction of such instability is challenging due to its inherent complexities arising from the non-linear nature of the problem.In this thesis, we design a Rijke tube, a device in which self-excited thermoacoustic instabilities can be studied for relatively simple flame geometries. Subsequently, we used the Rijke tube to investigate dynamics of thermoacoustic instability in premixed laminar flame. We focused on the role of burner design on the ensuing instability and flame dynamics by investigating pressure oscillations observed in four different single- and multi-flame burners. We observed that %a single- and seven-flame burner tip using similar flame conditions were experimentally compared, by recording pressure data. The the single-flame burner did not display transition to an unstable state, when the burner was traversed along the length of the Rijke tube, while a seven-flame burner showed various states of instability and multiple bifurcation points. This observation confirms that the increased degree of freedom, arising from the interaction between individual flames in a multi-fame burner, is responsible for the wider dynamics observed.

Subsequently, to quantify the dynamics of individual flames and their roles in the ensuing pressure dynamics, a three flame burner, where the flames are located in-line, were studied in details. The Phase Lock Value, Root Mean Frequency, and Power Spectral Density were analyzed for the heat release rates from individual flames and the acoustic pressure. We observed that the center flame possessed stronger synchronization with the pressure. But, the left and right flames shared a few modes (frequencies) of oscillations with the pressure, that were not present in the center flame. This suggests that the dynamics that effect of the individual flames pose varying degrees of influence on the pressure dynamics.

An additional experiment was also conducted using an asymmetric three-flame burner, where the flames are located at the tip of an isosceles triangle, using the same flame conditions. This flame showed stronger pressure oscillations, perhaps due to enhanced degrees of interactions between three flames with the symmetric design, and eventually suffered blow-off. These systematic investigations highlights the strong influence of local flame interactions on the observed pressure dynamics, which is relevant for design of multi-nozzle combustors.