Laminar Flame Speed Measurements of a NH3/H2/N2 Natural Gas Surrogate at Elevated Pressures

Abstract Ammonia is a promising carbon-free fuel alternative, offering higher energy density and easier storage compared to liquid hydrogen. This study presents experimental measurements of the laminar flame speed for an NH3:H2:N2 mixture with a 40:45:15 volume ratio at elevated pressures, contributing to the understanding of ammonia's potential for clean energy applications. Experiments were conducted using the Turbulent Flame Speed Vessel (TFSV) at Texas A&M University, employing high-speed schlieren photography to capture spherical flame propagation. Flame stretch effects were accounted for to determine unstretched laminar burning velocities over equivalence ratios ranging from 0.7 to 1.4 at pressures of 1 – 4 atm and an initial temperature of 293 K. At 1 and 2 atm, with air as the oxidizer, peak laminar flame speeds were 42.5 and 34.1 cm/s at ϕ = 1.1. Leaner mixtures at these conditions exhibited cellular instabilities. To mitigate these instabilities at 3 and 4 atm, the oxidizer was replaced with a 1:6 O2:He mixture, resulting in increased peak laminar flame speeds of 54.6 and 48.7 cm/s at ϕ = 1.1, respectively. Comparative experiments using a Bunsen flame at 1 atm with planar laser-induced fluorescence (PLIF) imaging of OH revealed flame speeds approximately 10 cm/s slower than those of spherically propagating flames. Experimental results were compared with predictions from four chemical kinetics models, identifying discrepancies and emphasizing areas for refinement. These findings offer valuable insights for optimizing ammonia-hydrogen combustion systems under high-pressure conditions, supporting the advancement of ammonia as a viable, carbon-free fuel option in future energy systems.