Studies of Gas Turbines Combustion Chambers: Use of New Fuels and Development of New Tools

Sammanfattning: The growing world electricity demand and environmental concerns pushes toward new technology for electricity production. Within the thermal processes for electricity production, two axis of research are being developed to meet the needs of the industry, namely using renewable fuels and new generation burners technology. The first direction is to develop new processes able to use renewable fuels. Biomass is believed to have a significant potential for electricity production using advanced technology like gasification and combined cycles. In the present thesis, a pilot combustion chamber is studied numerically and experimentally for gasified biomass operations. Firstly, a numerical tool was used to understand/improve the combustion chamber aerodynamics. I was shown that the airflow repartition within the combustion chamber has a significant influence on the operation and even small variations have important effect on the efficiency/viability of the device. Secondly, experimental pressurised tests were carried out in order to study ammonia to NO conversion. The experiments show that about 40% of the ammonia was converted into NO. In addition the combustor behaviour in a wide range of operating conditions was tested and was found stable from idling to full load conditions. The second direction is to improve the combustion chamber design. Modern Dry Low Emissions burners operate at uniform temperature avoiding NOx production in hot regions. Theses new types of burner operate under lean premixed conditions and combustion stability started to become an important concern. If swirl stabilisation has proven to be an efficient concept, the intrinsic unsteady nature of the breakdown is a risk for the device. In order to investigate and understand the interaction between the flame and the large turbulent structures, time accurate numerical tools are used (i.e. Large Eddy Simulation). The contribution of the present work in the field is to propose an original and consistent model for Large Eddy Simulation of premixed turbulent combustion. The model results of a formulation into a filtered space of the flamelet hypothesis. In addition, the subgrid flame thickness is modelled using physical arguments and it was shown how it affects the flame dynamics. A robust version of the model was used for simulations of an industrial GT burner (Alstom AEV). The numerical tool was used to study the sensitivity to the boundary conditions and in some cases flashback/flame-back was predicted. In addition, the simulations enabled to capture vortex breakdown structures/features helping to understand the flame/breakdown interaction, which is an essential point for flame stabilisation.