EXPERIMENTAL INVESTIGATION OF UNSTEADY FLOW PHENOMENA IN HIGH INTENSE COMBUSTION SYSTEMS

摘要

The advantages of modern gas turbine burners operating with premixed combustion are a low NO_x-emission, but the drawback of this technology is that it is prone to combustion instabilities. One of the mechanisms leading to instabilities may be linked to coherent flow structures. In this study a full-scaled stationary gas turbine burner has been investigated with special emphasis on flow insta- tionarities that may trigger combustion instabilities. The test rig allows detailed investigations of the flow inside the combustion chamber at atmospheric pressure. Different flame settings (pilot, non-premixed and premixed operation) were analysed at different air flows by microphone probes and Laser Doppler Anemometry (LDA). Previous investigations of the cold flow (Schildmacher et al., 2000) indicated that vortex shedding is present at the burner mouth with a pronounced frequency peak in the range of 250 Hz. The frequency was found to be proportional to the air flow rate, i.e. the Strouhal number corresponding to the vortex shedding mechanism is constant. In the present work premixed operation of the burner with an additional pilot flame has been investigated with respect to the influences of the vortex shedding at the burner mouth on combustion instabilities. Under reacting conditions in premixing operation, the pronounced frequency peak and thus the Strouhal number are nearly the same as for cold flow conditions indicating that vortex shedding at the burner mouth is also the driving mechanism at reacting flow conditions. Laser Induced Fluorescence (LIF) measurement revealed alternating pattern of fuel lean and rich pockets which may also be attributed to vortex shedding. In addition, a modified burner mouth design was studied under non-reacting conditions by which the amplitude of the cold flow instabilities could be reduced.