Turbulence-chemistry interactions in piloted partially premixed cracked ammonia-air flames with high Reynolds numbers

This study investigates turbulence-chemistry interactions in piloted NH₃/H₂/N₂-air flames at Reynolds numbers of 24,000, 32,000, and 36,000, referred to as Flames D, E, and F, respectively. Raman/Rayleigh scattering and NH₂/OH-LIF measurements are used to analyze flame structure in mixture fraction and temperature space as well as physical space. Probability density functions (PDFs) provide insights on local extinction behavior, while conditional means of the NH₃/H₂ ratio yield insights on differential diffusion. With increasing Re, the flames exhibit stronger entrainment, leading to higher fluctuations in the outer shear layers between the piloted products and coflow air in the near-field (Z/D = 1-2). At Z/D > 15, enhanced turbulent mixing at higher Re results in lower NH₃, H₂, mixture fraction, OH, and NH₂ downstream. The local extinction probability increases with Re, with significant extinction observed in Flames E and F. Three distinct reaction zones are identified, corresponding to peak OH, peak temperature, and peak NH₂. Extinction initially occurs in the fuel-lean side, followed by the fuel-rich side. Reignition occurs earlier in Flame E (by Z/D = 10), whereas in Flame F, it is delayed until Z/D = 20. The flame structure reveals a balance between differential diffusion effects and turbulent mixing in the fuel-rich regions for all three flames. Further downstream, differential diffusion effects are more pronounced in Flame D, resulting in a higher NH₃/H₂ ratio, while in Flames E and F, the influence of differential diffusion diminishes due to the higher Re. This series of flames (D, E, and F) provides a valuable dataset for validating ammonia combustion models, particularly in the context of differential diffusion, local extinction, and turbulence-chemistry interactions in high-Reynolds-number flows.