Flame and flow characteristics of lean premixed turbulent NH3/H2/N2 - air flames with increasing Karlovitz numbers

Premixed flames of partially cracked ammonia (NH3) hold significant promise for the decarbonization of internal combustion engines and gas turbines, since they can burn at a similar laminar flame speed to methane but have notably high blow-out resistance. Understanding turbulent premixed flames with partially cracked NH3 is highly relevant from both academic and application perspectives. This study aims to enhance our understanding of such premixed NH3/H2/N2-air flames subjected to increasing turbulence. For this purpose, a specific fuel mixture, consisting of 40vol% NH3, 45vol% H2, and 15vol% N2, is selected to match the laminar flame characteristics of methane at the same equivalence ratio. Turbulent jet flames are stabilized in a piloted burner with increasing bulk velocities from 30 to 180 m/s and Karlovitz numbers from approximately 75 to 2,140. One-dimensional (1D) simulations of freely propagating flames and strained counter-flow flames are performed, emphasizing temperature and species axial profiles and flame response to strain rate. Further, turbulent flow and flame structures are characterized using simultaneous particle image velocimetry (PIV) and laser-induced fluorescence of OH radicals (OH-LIF) measurements. Flame surface density and curvature distributions are evaluated, revealing the dominant role of turbulence over differential diffusion in shaping the flame surface topology. It is also found that the OH intensity gradient serves as a marker for local reactivity and thermo-diffusive instabilities, being higher at positive curvatures than at negative ones. Flat flames dominate the surface topology but show significant discrepancies in as they appear both upstream and downstream of leading edges. The thickness of the OH layer is not broadened by turbulence, even at Ka = 2,140, suggesting that eddies cannot penetrate into the main reaction zone marked by OH radicals, which are formed at higher temperatures than the preheat layer.