NH_<b>3</b>/CH_<b>4</b>/Air Partial Premixed Flames: Flame Stability, Emissions, and Thermal Structure

Partially premixed combustion has proven effective in enhancing flame stabilization and reducing emissions in hydrocarbon systems. However, the widespread adoption of ammonia (NH3) as a carbon-free fuel remains limited due to weak flame stabilization and high NO emissions. Therefore, this study investigates flame stability, NO emissions, flame appearance, and thermal structure in NH3/CH4/air flames under varying degrees of mixture inhomogeneity. Flames were stabilized using a conical nozzle fitted at the outlet of two concentric tubes: an NH3/air mixture was introduced through the inner tube, while a CH4/air mixture was flowed through the outer annular passage. The equivalence ratios and velocities of the inner (Φin, Vmi) and outer (Φout= 0.9, 1.1, 1.3; Vmo) streams were systematically varied. The mixture inhomogeneity of NH3/air and CH4/air streams was controlled by adjusting the axial recession distance (L) between the two tube exits, which was expressed as L/D (0–10), where D is the outer tube’s inner diameter. Flame stability was found to depend strongly on the interaction between L/D, Φout, and NH3 content. At lean Φout = 0.9, increasing L/D (i.e., enhancing premixing) reduced flame stability, while at Φout = 1.3, optimum stability occurred at L/D ≈5, with stability deteriorating at both lower and higher values. Partially premixed NH3/CH4 flames exhibited improved NO emission performance and combustion efficiency relative to fully premixed NH3/CH4/air and pure NH3/air flames. Co-firing NH3 in the inner stream with CH4 in the outer stream enabled the use of richer NH3 mixtures while limiting NH3 slip. Although NO emissions generally increased with L/D, enrichment of Φout or reduction of the velocity ratio of the inner to the outer stream (VR) mitigated this effect. Under lean Φout, increasing L/D or VR enhanced the mixing degree, shifting the overall mixture toward leaner local conditions and promoting NO formation via enhanced radical activity. Conversely, under rich Φout, these changes reduced flame temperature relative to those seen at lean Φout and suppressed NO pathways by limiting radical production. These findings demonstrate that partially premixed NH3/CH4 combustion─when optimized through control of L/D, Φin, Φout, and VR─can achieve stable, efficient, and ultralow NO operation, supporting its viability in future hydrogen-based energy systems.