Combustion instabilities in gas turbine engines often give rise to acoustic resonances. These resonances occur as manifestations of different acoustic modes, of which a single or multiple modes may be present. In this work, the acoustic behavior of a gas turbine model combustor, developed at DLR Stuttgart by W. Meier et al., was investigated using dimethyl ether (DME). The equivalence ratio and air mass flow rate were systematically varied. The results did not correspond to any one instability mechanism. It is concluded that, in the current burner configuration, integrated-acoustics occur that involve a combination of mechanisms, including a Helmholtz-type resonance from the plenum and convective-acoustic effects. To understand the instability, accurate measurements are needed of the correlation between heat release rate fluctuations and pressure fluctuations. Thus heat release rate must be recorded as a function of time and space. However conventional chemiluminescence offers only a line-of-sight measurement. High-speed formaldehyde planar laser-induced fluorescence was applied to study the motion of flame surfaces in response to the pressure oscillations of the instability. Flame shape has been correlated with instability strength and presence. The flame surface density and surface area fluctuated at the acoustic frequency and displayed motions correlated with the precessing vortex core (PVC) rotation. In non-resonating flames, the behavior of the formaldehyde structure and marked flame surfaces were dominated by the PVC motion, but the degree of surface area fluctuations was reduced compared to unstable flames. Results show that the frequency of the combustion instability varies with several operational conditions, including gas velocity, equivalence ratio, and convective time delays.
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