The effect of asymmetric can-to-can coupling on thermoacoustic modes in a can-annular laboratory combustor

Can-annular combustors are equipped with a set of nominally identical cans, circumferentially arranged around the shaft. Adjacent cans are coupled acoustically via a small gap at the downstream end, where the circular cross-section transitions into the annular turbine inlet. Recent experimental and theoretical work has shown that the coupling strongly affects thermoacoustic system stability and, hence, may give rise to damaging pressure oscillations that originate from a constructive interference of pressure waves and heat-release rate fluctuations. A laboratory-scale can-annular combustor has been operated with premixed CH4-H2-air mixtures to study these instabilities. The combustor consists of eight identical cans, connected acoustically to their respective neighbours via size-adjustable side branches; the latter allow for a variation of the coupling strength. Experimental results are presented in which five asymmetric sets of coupling strengths are investigated. The sets are chosen such that the symmetry is gradually reduced until a fully asymmetric configuration is realized. The can-to-can coupling strength patterns strongly affect the amplitude and structure of the dominant mode.