Phd in AERONAUTICS AND SPACE ENGINEERING

Thesis title: Intrinsic flame instabilities in hydrogen enriched Ammonia/Air premixed flames.

The growing demand for sustainable energy solutions has intensified interest in alternative fuels such as hydrogen and ammonia, which offer carbon-free combustion potential. However, their practical implementation is challenged by inherent instabilities that affect flame propagation, pollutant emissions, and overall combustion performance. This thesis investigates intrinsic flame instabilities in hydrogen-enriched ammonia/air premixed flames through high-fidelity numerical simulations, focusing on their impact on flame dynamics and nitrogen oxide (NO) formation. To achieve this, a Low-Mach number chemically reacting flow framework is developed and implemented within a massively parallel spectral element code, enabling direct numerical simulations (DNS) of laminar premixed flames. This computational approach captures the effects of pressure, differential diffusion, and second-order transport phenomena such as the Soret effect. Additionally, the computational efficiency and scalability of the reactive flows solver are assessed, ensuring its viability for large-scale simulations of alternative fuel combustion. The study systematically investigates the influence of thermo-diffusive instabilities on hydrogen-enriched ammonia combustion, focusing on how increasing pressure, particularly at atmospheric and elevated conditions (up to 10 atm), affects flame stability and pollutant formation. A numerical linear stability analysis is performed to establish theoretical stability limits and evaluate dispersion relations measuring the growth rates of flame perturbations under various conditions. This analysis is extended into the non-linear regime through direct numerical simulations, enabling a detailed examination of how intrinsic flame instabilities evolve over time and their impact on NO formation. The findings uncover key mechanisms that link instability-driven flame wrinkling to pollutant formation, offering deeper insights into the interplay between combustion chemistry and hydrodynamics. Beyond the fundamental analysis of flame instabilities, the study also explores the integration of these effects into reduced-order modelling frameworks. The dimensionality of the low-dimensional manifold (LDM) required for accurately predicting NO reaction rates and key flame characteristics is identified, laying the groundwork for data-driven models that bridge high-fidelity simulations with engineering-scale applications. Finally, an attempt is made to model the interaction between turbulence and intrinsic flame instability within an industrially relevant framework. The proposed model is evaluated against experimental results from the NTNU hydrogen-air premixed burner, allowing for a comparative analysis that assesses its predictive accuracy and practical applicability. Overall, the findings of this research offer valuable insights into the behaviour of hydrogen-enriched ammonia flames, advancing the understanding of their stability and pollutant emissions.