The effects of long-arc plasma ignition on flame propagation and buoyancy-induced upward flame displacement of ammonia/air mixtures at elevated pressures

Ammonia is a promising zero-carbon fuel, but its combustion is hindered by difficult ignition, low flame propagation speed, and buoyancy-induced upward flame displacement, particularly at elevated pressures. This study develops a long-arc plasma ignition strategy integrating bipolar nanosecond pulsed excitation with a specially designed electrode for spatially extended discharge. Comparative experiments were conducted in a constant-volume chamber at initial pressures of 1–7 bar with equivalence ratios Φ of 0.7–1.4. The long-arc electrode and conventional spark plug were operated under identical pulse repetition frequency, pulse number, and deposited energy, enabling direct assessment of the effect of spatial energy deposition. It was demonstrated that the long-arc strategy generated a larger initial flame kernel that could significantly enhance flame propagation compared with conventional spark ignition, thereby suppressing buoyancy-induced upward flame displacement, as quantitatively reflected by consistently higher Froude numbers. When pressure raised up to 7 bar, flame-wrinkling analysis showed that long-arc discharge promoted flame wrinkling over Φ = 0.9–1.3. Stronger buoyancy at Φ = 0.9 and 1.3 retained more wrinkles, resulting in a greater enhancement in stretched flame speed. Heat-release analysis further indicated that long-arc ignition shortened the ignition delay, weakened the double-peak heat-release-rate profile, and reduced the overall heat-release duration. It is worthy noted that the spatial distribution of plasma energy deposition plays an important role in elevated-pressure ammonia ignition and combustion, highlighting the potential of long-arc plasma ignition to address the combustion bottleneck problem in advanced ammonia-fueled internal combustion engines.