Coordinator: Dr. Wolfgang Meier
Gas turbine combustors are affected by a variety of unsteady combustion phenomena including ignition (auto ignition or external ignition) and extinction, flame stabilization, thermo-acoustic pulsation and fluid-dynamic instabilities. These processes are characterized by a strong interaction between the flow field and the chemical reactions. Due to their strong dynamics, unsteady combustion processes present a major challenge for both numerical simulations and experimental investigations.
Gas turbine flames are operated in highly turbulent flow fields in order to achieve a large energy conversion in a small volume. Excessive turbulence, however, can result in blow-out, making flame stabilization in these combustors a competition between turbulence and chemistry. To avoid blow-out at these high turbulence conditions, gas turbine combustors frequently employ exhaust gas recirculation to increase the temperature and reactivity of the incoming fuel-air mixtures. This is achieved by generating recirculation zones in the combustor through aerodynamic effects such as swirl or jet-stabilized mixing. The combustion reactions are sustained by two different mechanisms; by the propagation of flame fronts or through auto-ignition.
The auto-ignition of a fuel/air mixture is a spontaneous process that is highly sensitive to temperature, pressure, gas composition and local turbulence characteristics. The Institute performs research on auto-ignition in realistic gas turbine combustors as well as on fundamentals of auto-ignition [1,2]. For example, the auto-ignition of fuel during its injection into hot, oxygen-containing exhaust gas is studied by experimental and numerical methods. The figure below shows three images from a series of high-speed measurements acquired in collaboration with the Ohio State University. The temperature distributions illustrate how the cold, turbulent fuel jet mixes with the ambient hot gas. 2.4 ms after the start of the injection an auto-ignition kernel (marked by the white outline) appears in the image then grows rapidly and engulfs the entire fuel jet.
Coherent vortex structures
Precessing helical vortices (Precessing Vortex Core, PVC) are often observed in swirling flows and can have a strong influence on the flame structure and stabilization [3-6]. They can intensify mixing, roll up flame fronts and lead to locally increased heat release. The precession frequency of these tube-like structures depends, among other factors, on the flow velocity in the nozzle and can be higher than 1000 Hz. For the gas turbine burner shown in the figure below, experimental and numerical investigations have been performed to reveal the effects of the PVC on the combustion process. Also shown is the result of a calculation performed using Large Eddy Simulation (LES). The sectional image displays instantaneous distributions of the flow field and temperature. The vortex pattern that can be seen in the cold inflow belongs to the three-dimensional structure of a PVC.
 J. Fleck, P. Griebel, A.M. Steinberg, M. Stöhr, M. Aigner, A. Ciani, J. Eng. Gas Turbines Power 134, 041502 (2012)
 C.M. Arndt, J.D. Gounder, W. Meier, M. Aigner, Appl. Phys. B 108, 407-417 (2012)
 A. Widenhorn, B. Noll, M. Aigner, Proc. ASME Turbo Expo 2009, GT2009-59038
 I. Boxx, M. Stöhr, C. Carter, W. Meier, Combust. Flame 157, 1510-1525 (2010)
 M. Stöhr, C. M. Arndt, W. Meier, Proc. Combust. Inst 34, 3107-3115 (2013)
 M. Stöhr, C. M. Arndt, W. Meier, Proc. Combust. Inst 35, 3327-3335 (2015)