Cosway, Benjamin; (2025) Nitric Oxide Formation Characteristics and Dynamics of Ammonia/Hydrogen Flames Using Simultaneous Multi-Species Imaging. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Ammonia/hydrogen fuel blends have recently emerged as a promising solution for the decarbonisation of the energy sector. However, concerns over efficiency and crucially, NOx emissions have impeded their wide-spread use so far. Before effective NOx mitigation strategies can be developed, the fundamental chemical mechanisms involved in NOx production in NH3/H2 flames must be well understood. Key insights into NO formation mechanisms and flame structures for NH3/H2 mixtures are required to develop and improve chemical kinetic models. Combustion instabilities can also greatly affect their performance in gas turbines. While there is a large body of research investigating the thermoacoustic characteristics of hydrocarbon fuels, the dynamic behaviour of NH3/H2 flames is not yet well defined. In the first part of this thesis, experimental 1-D species profiles for NO and key intermediate species, NH*, NH2* and OH*, were determined from laminar premixed NH3/H2 Bunsen flames, using simultaneous PLIF and chemiluminescence imaging. Two experimental campaigns were conducted, investigating the effect of varying equivalence ratio and NH3/H2 volume fraction on flame structure and NO formation. Each set of experimental results were compared to computed 1-D species profiles, using different sets of mechanisms available from literature. Both sets of results indicated that all the mechanisms underestimate the profile widths of the measured species and relative changes in width, due to changes in equivalence ratio and volume fraction. NH* also showed a positive correlation with the computed HRR values, with both varying equivalence ratio and volume fraction. This indicates that it is a promising candidate for direct HRR measurement but warrants further investigation over a wide range of conditions. These comparisons will help inform the optimisation of chemical kinetics mechanisms for NH3/H2 flames. In the second part of this thesis, the behaviour of turbulent NH3/H2 flames and their response to acoustic forcing was studied. The dependence of global heat release on forcing amplitude was quantified and flame describing functions produced. The results showed that the non-linearity in the flame response was achieved at lower forcing amplitudes for higher H2 volume fractions. In the non-linear region, the flame response was observed to decrease with increasing forcing amplitude for some mixtures with higher H2 content. The effect of imposed inlet velocity fluctuations on the flame surface and NO formation was further investigated using phaselocked simultaneous OH/NO PLIF. At higher forcing amplitudes and H2 volume fractions, the OH PLIF images showed the existence of a separate flamelet inside the outer shear layer vortex. This lead to destruction of the flame surface area, re- sulting in the observed reduction in the heat release response. NO PLIF images showed that flame-vortex interactions had a key influence on NO distribution in the flames, resulting in high concentrations along the outer shear layer at high forcing amplitudes.

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