Laboratory for Flow Instabilities and Dynamics

Active flow control of hydrodynamic instabilities in gas turbine combustors


Pre-mixed, swirl-stabilized combustion is characterized by low pollutant emissions, which is particularly crucial in terms of compliance with environmental regulations.  This type of combustion can be very efficient, as an excess of air in the ignitable fuel-air mixture can ensure a stable combustion process. However, this type of lean, premixed swirl flame can become thermoacoustically unstable if the heat release from the flame oscillates in phase with the acoustic pressure field in the combustion chamber. The resulting thermoacoustic modes can reach very high amplitudes in the combustor, which jeopardize safe and stable operation of the gas turbine.

The thermoacoustic modes cause axially symmetric coherent vortex structures in the flow field of the combustor. These coherent structures likewise influence the dynamics of the flame and can also drive up the emission of pollutants. Within this complex, reacting flow field, another coherent structure known as the Precessing Vortex Core (PVC) can also occur, exhibiting a helical structure. The PVC occurs as a global hydrodynamic instability that can be described by an azimuthal wavenumber of one. The helical coherent structure of the PVC leads to the formation of downstream meandering vortices in the shear layers, which influence the shape and dynamics of the flame (see Figure 1).


We are engaged in this project to investigate the direct influence of the Precessing Vortex Core (PVC) on the dynamics, thermoacoustic instabilities and emissions of swirl-stabilized flames using active flow control.

We are interested in exploring the effects of the PVC on combustion and flow characteristics in more depth for beneficial use. For this purpose, the possibility was created to control the PVC in experiments specifically and directly at the point of its formation. The actuator used for active flow control (see above) enables azimuthal excitation of the PVC in open and closed loop. This allows the PVC to be selectively damped and amplified.

To characterize the PVC and its influence on flow and combustion, modern optical and laser-based measurement techniques are used in addition to sensor-based pressure measurements. These include chemiluminescence measurements, Particle Image Velocimetry (PIV), Quantitative Light Sheet (QLS) methods and various Planar Laser Induced Fluorescence (PLIF) methods. Data collection is time-resolved (with high sampling rates), which allows the application of advanced data-driven analysis methods, such as Spectral Proper Orthogonal Decomposition (SPOD).

The thermoacoustic behavior of the flame is typically described using a so-called flame transfer function (FTF). The FTF is determined using the Multi Microphone Method (MMM) via measurements of the acoustic field in the combustion chamber. For the measurement of pollutant emissions such as nitrogen oxides (NOx) and carbon monoxide (CO), a modern emission measurement system is available that continuously monitors the composition of the exhaust gas.


With the help of dynamic pressure measurements and OH* chemiluminescence measurements, it was shown that the PVC can change the mean flame shape in such a way that the amplitude of self-excited thermoacoustic fluctuations can be significantly reduced (Figure 2, left).  Two different mechanisms could be derived based on experimental data to explain this phenomenon (Figure 2, right).

First, the PVC alters the mean flame such that most of the heat release occurs near the burner outlet, where the PVC causes increased turbulent fluctuations. As a result, fewer heat release fluctuations occur further downstream, at the flame tips, which are particularly sensitive to thermoacoustic fluctuations. Accordingly, the gain of the FTF decreases with increasing PVC amplitude. Since the position of the thermoacoustic-induced oscillations that occur does not change, the phase of the FTF remains constant.

The second mechanism relates to changes in the averaged flow field induced by the PVC. Using linear hydrodynamic stability analysis (LSA), it can be shown that the PVC reduces the gain of the Kelvin-Helmholtz instability. The Kelvin-Helmholtz instability is responsible for the formation of the axisymmetric vortex structures by the thermoacoustic modes. Therefore, this gain reduction equally leads to a reduction of the FTF gain.

In the investigated flame configuration, it was shown that the vortex structures induced by the PVC slightly increase the NOx emissions. The reasons for this may be manifold and require further investigation. These include investigation of the influence of the PVC on the mixing of fuel and air and associated effects on the formation of pollutants.


The project is funded by the DFG with grant number 247226395.