Investigation of Dimethyl Ether Combustion Instabilities in a Partially - Premixed Gas Turbine Model Combustor Using High-Speed Laser Diagnostics

Abstract

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 nonresonating 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.

Experimental Studies and Modeling of Acoustic Instabilities in a Gas Turbine Model Combustor

“…Figure 6 compares the power spectral density of pressure oscillation and flame surface area oscillation. Combustion chamber pressure Similar to what has been observed previously, 8,19 we can see that the flame surface area, and in this way heat release rate, is fluctuating almost at the the same frequency as the combustion chamber pressure. The peak of flame surface area spectrum in figure 6 is at 313 Hz and the peak of pressure spectrum is at 314 Hz.…”

“…Figure 7 shows the calculated flame surface density of the current case with a bin size of 10 pixels × 10 pixels (394 μm × 394 μm). Compared to previous studies, 19 this result provides much higher spacial resolution and gradient resolution, while preserving most of the fundamental information. We can see that in this flame, the highest flame surface density, and hence the region where the flame is statistically most intense, is right above the surface of the flame, with a lift-off height of about 5 mm.…”

“…Furthermore, they mentioned that pressure in the plenum is fluctuating at the same frequency, but with a phase lag of 60 to 80 degrees. Based on the trend of changes in instability frequencies, Allison et al 19 stated that it is more likely that the GTMC exhibits a combination of multiple modes. Since it is crucial in the process of understanding the combustion instability in the GTMC, the characterization of the instability mode in the combustion chamber had to precede all other measurements.…”

“…19,22 It is considered to be an improvement over the conventional chemiluminescence detection, because the latter one is a line of sight measurement. The PLIF setup used in present study is based on the aforementioned study, but with improvements.…”

Distributed Low Temperature Combustion: Fundamental Understanding of Combustion Regime Transitions

“…Numerous investigations are found on the utilisation of DME in engines, e.g. [14,15], yet fundamental flame studies are relatively rare. Existing studies have investigated properties such as laminar burning velocities [16,17,18] and extinction strain rates [18].…”