DETAILED EXAMINATION OF TWO-STAGED MICRO GAS TURBINE COMBUSTOR

Jet-stabilized combustion is a promising technology for fuel flexible, reliable, highly efficient combustion systems. In this work, experiments have been carried out on a two-staged combustor, with a jet-stabilized main stage and a swirl-stabilized pilot stage. Both stages have been run separately to allow a more detailed understanding of the flame stabilization within the combustor and its range of stable operation. All experiments were conducted at atmospheric pressure and preheating temperatures of 650 degrees C. The air was fed to both stages of the combustor for all experiments. The flame was analyzed in terms of shape, length and lift-off height, using the OH* chemiluminescence signal detected by an ICED-camera. Emission measurements for NOR, CO and UHC emissions were carried out. The pilot stage was examined at a local air number between 0.14 and 1.43, which corresponds to a global air number of 2.0 to 20.7. For lowest air numbers, the combustor works with the RQL principle with lowest emissions in pilot stage only operation. This is because the remaining fuel fed to the pilot stage mixes rapidly with the air from the main stage and reacts under lean conditions. The optimum operating range of the main stage is at global air numbers between 3 and 3.2 with a blow-off limit beyond lambda(g) = 4.0. At a global air number of lambda(g) = 2, a fuel split variation was carried out from 0 (only pilot stage) to 1 (only main stage). In combined operation and at higher fitel splits, the NOx emissions are reduced compared to the main stage only operation, while the opposing effect on NOx emissions was observed for lower fuel splits. CFD simulations of the combustor test rig showed higher residence times in the pilot stage compared to the main stage which facilitates higher NOx formation rates in the pilot stage. This could be improved by a geometry optimization. The operation of the pilot stage was beneficial at fuel splits above 90 %, especially concerning an extended operating range to higher global air numbers. In addition, the capability of the combustor to operate at higher thermal power inputs was investigated. Originally designed for the Turbec T100 micro gas turbine, the combustor was operated at 160 % of the original design point. At a constant air number, this led to a decrease in NOx and to an increase in CO emissions, caused by shorter residence times in. the combustion chamber at higher power input. An operation strategy of constant pilot air number increases the envelope of a stable operation regime fitrther.