DETAILED EXAMINATION OF A MODIFIED TWO-STAGED MICRO GAS TURBINE COMBUSTOR

Jet-stabilized combustion is a promising technology for fuel flexible, reliable, highly efficient combustion systems. The aim of this work is a reduction of NOx emissions of a previously published two-staged MGT combustor [1, 2], where the pilot stage of the combustor was identified as the main contributor to NOx emissions. The geometry optimization was carried out regarding the shape of the pilot dome and the interface between pilot and main stage in order to prevent the formation of high temperature recirculation zones. Both stages have been run separately to allow a detailed understanding of the flame stabilization within the combustor, its range of stable combustion, the interaction between both stages and the influence of the modified geometry. All experiments were conducted at atmospheric pressure and an air preheat temperature of 650 degrees C. The flame was analyzed in terms of shape, length and lift-off height, using OH* chemiluminescence images. Emission measurements for NOR, CO and UHC emissions were carried out. At a global air number of lambda= 2, a fuel split variation was carried out from 0 (only pilot-stage) to 1 (only main stage). The modification of the geometry lead to a decrease in NO and CO emissions throughout the fuel split variation in comparison with the previous design. Regarding CO emissions, the pilot stage operations is beneficial for a fuel split above 0.8. The local maximum in. NOx emissions observed for the previous combustor design at a fuel split of 0.78 was not apparent for the modified design. NOx emissions were increasing, when the local air number of the pilot stage was below the global air number.- In order to evaluate the influence of the modified design on the flow field and identify the origin of the emission reduction compared to the previous design, unsteady RANS simulations were carried out for both geometries at fuel splits of 0.93 and 0.78, respectively, using the DLR in-house code THETA with the k-w SST turbulence model and the DRM22 [3] detailed reaction mechanism. The numerical results showed a strong influence of the recirculation zones on the pilot stage reaction zone.