Unsteady flame and flow field interaction of a premixed model gas turbine burner

Abstract

In recent years, the NOx emissions of heavy duty gas turbines have been significantly reduced by introducing lean premixed combustion. However, these flames are known to be prone to combustion instabilities, which became the key issue of modern gas turbine combustion research. In this paper, investigations of a single model gas turbine burner are presented with special focus on thermo-acoustic eigenmodes of the combustor and the resulting interaction between periodic flow field oscillations and flame front fluctuations. A numerical analysis of the eigenmodes of the combustor rig was preformed and compared to pressure probe measurements. By the numerical analysis Helmholtz mode pressure oscillations were predicted to be present in the air plenum upstream the burner as well as in the combustion chamber. These oscillation modes could well be confirmed by the analysis of the pressure signals taken at different position within the combustor rig. By these pressure oscillations thermo-acoustic flame instabilities are triggered. Due to a phase lag between the pressure oscillations in the air plenum and those in the combustion chamber, fluctuations of air flow through the burner are induced, which in turn cause fluctuations of the equivalence ratio at the burner exit plane. As result, fluctuations of the heat release rate within the flame are established, which interfere with the acoustics of the rig and, thus, unstable combustion is maintained. The dynamics of the flow field and of the flame was studied experimentally by phase-locked LDA and OH-LIF diagnostics. The experimental results give an excellent insight into the chain of processes that are responsible for the thermo-acoustic instabilities. Interestingly, two different oscillation modes could be identified depending on the amplitude of the oscillations. At low oscillation levels only weak and locally confined velocity fluctuations in the main reaction zone were observed which do not have any significant impact on the flame. By increasing the equivalence ratio, the transition from low to high oscillation levels can be triggered. Under high oscillation levels, a pumping motion of the flow field in axial direction could be identified which causes strong periodic disturbances of the flame.

LES and experimental studies of cold and reacting flow in a swirled partially premixed burner with and without fuel modulation

“…Eigenfrequencies coupling the plenum and the chamber, thereby involving the full setup, are marked "S." For modes "P" and "C," this partial decoupling is possible because the inlet section of the chamber acts essentially like a velocity node. These results show that the two strongest modes (433 and 820 Hz) are acoustic modes of the chamber itself, but this is not a general result: many unstable combustors oscillate in modes involving the whole system or even only the plenum [63].…”

Large-Eddy Simulation of combustion instabilities in a variable-length combustor

“…Therefore, the prediction of combustion instabilities through numerical simulation is a major issue. For gas turbine or aeronautic propellers, several studies have already been conducted [6][7][8][9][10][11][12] but for rocket engines the number of unsteady simulations is smaller [13][14][15][16][17][18]. This could be explained by the extreme thermodynamic conditions in rocket engines and the wide range of length scales: the characteristic length of the injector is a fraction of a millimeter while the flame length can be several dozens of centimeters.…”

Numerical Characterization of Entropy Noise With a Density Based Solver

“…For modern aero-engines, one of the dominant noise source and the least studied is combustion noise. Besides, in order to reduce NOx emissions (Schildmacher et al, 2007), lean pre-mixed combustion has been introduced in modern aero-engines. However, lean combustion generates low frequency entropy perturbations which are convected dowstream, in the combustion chamber.…”