Pressure effects on pure and hydrogen-enriched ammonia flames relative to methane, with LES validation for 30% H2

The flame propagation behaviour of NH 3 /H 2 mixtures at intermediate pressures is still not well understood, despite the growing interest in ammonia (NH 3 ) as a carbon-free energy carrier. In this work premixed NH 3 /H 2 combustion at 0.1–0.5 MPa and stoichiometric conditions (φ = 1.0) is investigated and compared to CH 4 experimentally, with LES-ECFM comparison restricted to the NH 3 /H 2 blend containing 30% H 2 in the fuel. A constant volume combustion chamber (CVCC) is used, while high-speed natural-light front-view imaging techniques are employed to analyse the combustion of 100% CH 4 , 100% NH 3 , and NH 3 /H 2 blends (30% and 50% H 2 on a fuel-only molar basis). Apparent flame propagation speeds and the applicability of stretch extrapolation are examined. Pure NH 3 did not sustain a propagating flame front at 0.1 MPa with the wall-mounted ignition arrangement. Hydrogen enrichment markedly accelerated ammonia flame propagation. However, the 30% H 2 blend still propagated more slowly than CH 4 , whereas the 50% H 2 blend exhibited a greater enhancement and the fastest flame growth among the ammonia-based cases. Unstretched flame propagation speeds and burned-gas Markstein lengths were extracted from the measured stretched flame speeds using linear and nonlinear (Kelley–Law) extrapolation. Across the investigated mixtures, the derived Markstein lengths showed no consistent trend with pressure. Stretch-parameter estimates were inconclusive for pure NH 3 and the 50% H 2 blend at 0.3 and 0.5 MPa. Large eddy simulations (LES) of the 30% H 2 blend reproduced the measured flame arrival times at common flame diameters more closely at 0.3 and 0.5 MPa than at 0.1 MPa.