Investigating combustion oscillations with phase-resolved diagnostics
In advanced stationary gas turbines, the concept of lean premix combustion is applied to reduce NOx emissions. Using a homogeneous, lean mixture prevents temperature peaks in the combustion chamber, thereby significantly reducing thermal NO formation. However, lean premix combustion is very susceptible to combustion instabilities, especially to thermoacoustic pulsations. Since the underlying physical and chemical processes of instabilities are inadequately understood, “thermoacoustics” is a current research topic. We apply measurement techniques in a phase-correlated manner, i.e. the lasers and detectors are triggered with a defined phase relationship to the pressure fluctuation in the combustion chamber, in order to record the dynamic behaviour of key measurement quantities. By applying Raman, LIF and PIV measurement techniques, it has been possible to calculate the phase-dependent variations of mass flows in the combustion chamber and to show how they are related to the heat release. The sets of measurement data thus obtained show quantitatively, for the first time, how the combustion pulsations are sustained by the interaction between heat release, flow and acoustics.
Thermoacoustic oscillations have been studied by us in various swirling flames, for example, from laboratory scale to technically relevant high-pressure burners, e.g. the DLR double-swirl burner (gas film nozzle), the TM premix burner of the EU PRECCINSTA project, the double-swirl burner of the Collaborative Research Center 606 or scaled industrial gas turbine burners.