Reynolds number effects in turbulent premixed precracked ammonia-hydrogen flames

Precracked ammonia–hydrogen ( NH X 3 / H X 2 ) mixtures, particularly under fuel-rich conditions, are highly attractive for applications as they enable low- NOx combustion while producing hydrogen in situ, offering a controllable, carbon-free fuel pathway compatible with existing high-temperature energy and propulsion systems. However, their fundamental combustion behavior is still insufficiently understood, especially regarding turbulence-chemistry interaction and pollutant formation mechanisms. Therefore, this study aims to characterize the influence of turbulence on the internal flame structure, thermochemical state, and NO formation mechanisms in piloted, fuel-rich, turbulent premixed precracked NH X 3 / H X 2 / N X 2 jet flames by combining simultaneous 1D Raman/Rayleigh spectroscopy with Direct Numerical Simulations (DNS) using detailed chemical kinetics. Quantitative scalar measurements of instantaneous flame structure and thermochemical states were used to assess NH X 3 / H X 2 interactions, differential diffusion, and NO formation. Increasing the Reynolds number from Re = 5900 to 17,700 results in an increase in global turbulent flame speed by a factor of 1.9 and a corresponding rise in mean NO concentrations. At low and intermediate Reynolds numbers, the increase in turbulent flame speed is primarily governed by flame surface area, whereas at the highest Reynolds number an additional enhancement of mean reactivity is observed. This is reflected by an increase in the observed heat release rate conditioned on reaction progress, indicating increasingly strong differential diffusion effects at elevated turbulence intensities. While no NH X 3 is detected downstream of the main reaction zone, residual H X 2 levels are observed in the products. Due to residual O X 2 in the pilot exhaust gas, a secondary reaction zone forms downstream where H X 2 undergoes further oxidation under near-stoichiometric conditions. At high Reynolds numbers, regions of negative flame curvature exhibit increased NO production and up to a factor of 4 higher local heat release rates compared to one-dimensional laminar flames, highlighting the strong coupling between flame topology, diffusive transport, and pollutant formation. Based on the combined experimental and numerical findings, reactant-to-product counterflow flames are identified as a suitable canonical configuration for future flamelet manifold modeling of fuel-rich NH X 3 / H X 2 combustion.