RANS and LES-Based Analyses of Combustion Technologies for 100% Methane and Hydrogen-Fueled mGTs

The global commitment to reducing greenhouse gas emissions and achieving climate neutrality has accelerated research into sustainable energy conversion technologies. Hydrogen has emerged as a key energy carrier due to its potential for carbon-free combustion and seamless integration with renewable energy sources. Among various power generation solutions, Micro Gas Turbines (mGTs) have gained attention for decentralized Combined Heat and Power (CHP) systems, offering high operational flexibility, compact size, and multi-fuel capability. However, the transition to 100% hydrogen-fueled mGTs presents significant challenges, including flashback, auto-ignition, increased flame temperatures leading to higher NOx emissions, and the need for optimized burner designs to ensure safe and stable operation. This thesis presents a comprehensive numerical investigation of hydrogen and methane combustion in different mGT burner configurations using Computational Fluid Dynamics (CFD) approaches. The study employs Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) methodologies to analyse turbulent combustion in two burners designed for the Ansaldo Green Tech AE-T100 mGT. The first burner, the original AE-T100 combustor, was investigated in its standard 100% methane-fueled configuration to characterize the computational domain and identify key parameters influencing flame morphology and stability. This preliminary study provided a fundamental understanding of the dominant physical and chemical mechanisms affecting combustion performance. The second burner, the F400S.3, developed by the German Aerospace Center (DLR), was specifically designed for stable 100% hydrogen combustion. Insights gained from the AE-T100 methane simulations were leveraged to refine predictive modelling strategies and assess the feasibility of hydrogen as a primary fuel. A major focus of this research is the investigation of non-adiabatic boundary conditions and heat transfer effects on flame behavior. A key contribution of this study is the development, implementation, and validation of an extended Flamelet Generated Manifold (FGM) model that explicitly incorporates the effects of flame stretch, heat loss, and heat gain. Notably, the implementation of heat gain has not been previously evaluated in the literature. Its inclusion is particularly important for micro gas turbines, where common geometric features often lead to significant preheating of the mixture before it enters the combustion chamber. This preheating can substantially influence flame stabilization and overall flame topology. The predictive capabilities of the proposed model were assessed using the NTNU burner, a well, established academic test case, demonstrating its potential to significantly enhance the accuracy of numerical flame simulations. While the extended FGM approach provides a computationally efficient alternative for capturing key combustion phenomena, comparisons with species transport-based combustion models, such as the Partially Stirred Reactor (PaSR) model and the Thickened Flame Model (TFM), reveal certain limitations when applied to hydrogen-fueled flames. Specifically, the FGM model underestimates critical flame characteristics, resulting in a less accurate representation of flame structure and dynamics compared to species transport-based approaches. As said, the critical role of heat transfer effects in accurately predicting flame behaviour, particularly in reverse-flow burner configurations, was assessed. Neglecting these effects leads to a significant underestimation of flame morphology and reactivity for both methane and hydrogen combustion, highlighting the necessity of detailed heat transfer modelling in combustion simulations. Furthermore, LES-based simulations demonstrate a clear advantage over RANS by offering superior resolution of turbulence-chemistry interactions, making them the preferred approach for accurately capturing the complex physics of hydrogen and methane combustion in mGT burners. The insights gained from this research contribute to the ongoing development of optimized hydrogen-compatible mGTs burner designs, supporting the transition toward low-emission, high-efficiency hydrogen combustion technologies for future sustainable energy systems.