Ammonia/methane ( \(\textrm{NH}_3\) / \(\textrm{CH}_4\) ) co-combustion offers a promising route toward high-efficiency, low-carbon, and low-NOx combustion, yet experimental insights into local displacement speeds in ammonia-containing flames remain scarce. Here, 2D particle image velocimetry (2D-PIV) was applied to turbulent premixed Bunsen flames at atmospheric pressure, covering ammonia fractions up to 0.6 and equivalence ratios between 0.7 and 1.0. Streamline-based decomposition of the progress-variable transport equation showed that the measured flame propagation speed sT is broadly consistent with the closure relation sR + sF at low turbulence intensities, supporting the applicability of the Bray-Moss-Libby (BML) thin-flame concept to \(\textrm{NH}_3\) / \(\textrm{CH}_4\) flames. The reaction and convection terms dominated the balance, while turbulent fluxes, though weaker, remained non-negligible. Closure errors increased with higher ammonia content or leaner mixtures but were greatly reduced by introducing Markstein corrections and approximating 2D flame surface density as 3D. Analysis of reaction rates further revealed that the near-axis region contributed most, while ammonia addition lowered local reactivity, increased flame height, and promoted local extinction at the root. The findings provide experimental guidance for source-term estimation and turbulence–flame interaction modeling.
