Combustion dynamics of swirl-stabilized lean premixed flames in an acoustically-driven environment

Abstract

Combustion instability is a process which involves unsteady chemical kinetic, fluid mechanic, and acoustic processes. It can lead to unstable behavior and be detrimental in ways ranging from faster part fatigue to catastrophic system failure. In terms of combustion methodology, combustion instability has been a key issue for lean premixed combustion. The primary objective of this work is to improve understanding of combustion dynamics through an experimental study of lean premixed combustion using a low swirl combustor. This special burner was developed at the Lawrence Berkeley National Laboratory and has recently received significant interest from the gas turbine industry.In these experiments, acoustic perturbations (chamber modes) are imposed on a low swirl stabilized methane-air flame using loudspeakers. The flame response is examined and quantified with OH planar laser induced fluorescence. Rayleigh index maps of the flame are computed for each frequency and operating condition. Examining the structures in the Rayleigh maps, it is evident that, while the flame shows no significant response in some cases, acoustic forcing in the 70-150 Hz frequency range induces vortex shedding in the flame shear layer. These vortices distort the flame front and generate locally compact and sparse flame areas. This information about the flow field shows that, besides illuminating the combustion dynamics, the Rayleigh index is a useful way to reveal interesting aspects of the underlying flow.The experiments also revealed other interesting aspects of this flame system. It was found that the flame becomes unstable when the perturbation amplitude reaches 0.7% of the mean pressure. Decreasing the swirl number makes the flame shape more jetlike, but does little to alter the shear-layer coupling. In a similar fashion, increasing the pressure was found to alter the flame shape and flame extent, but the thermo-acoustic 2 coupling and induced large scale structure persisted to 0.34MPa, the highest pressure tested.Abstract Approved: ____________________________________ Thesis Supervisor ____________________________________

Extinction strain rate suppression of the precessing vortex core in a swirl stabilised combustor and consequences for thermoacoustic oscillations

“…The two sets of reconstructions describe, explicitly, the different mechanisms through which the acoustic and the aerodynamic time scales introduce resonance. The f a reconstruction is related to the well described mechanisms [52,58] of thermoacoustic coupling causing the "breathing" motion of the recirculation zone evidenced in Figures 10 and 20. The f i reconstruction is associated with the PVC and the azimuthal asymmetries introduced due to the above described axial advection of the recirculation zone.…”

Combustion instabilities

“…Various fuels and mixtures have been tested utilizing the designed low swirl burner, including methane-and hydrogen-based mixtures [Huang (2008)…”

“…Lastly, the speakers are located downstream from the chamber and, therefore, the flame. This eradicates the risk of acoustic perturbations affecting the fuel supply and mixing process when the acoustics are located Figure 3.1, monitored the chamber pressure oscillations where the pressure wave may be assumed to be nearly uniform over the extent of the flame [Huang (2008)].…”

“…These acoustic modes are natural resonant frequencies based on the burner and the chamber geometry. Huang (2008) reported no natural acoustic resonance, but this was due to the smaller 1" burner and a smaller fuel flow rate. Since the area in the current configuration has increased and is coupled with an increase in the flow rate, the effective increase in power is much larger than that of Huang's acoustic signal.…”

An experimental study of the global and local flame features created by thermoacoustic instability

“…It implies that the elevated pressure can play a critical role in determining the instability trend. Yun [41] flame [42]. The wavelength of OH* is centered at 309 nm, whereas the wavelength of CH* is centered at 431 nm.…”