Investigation on flame propagation and end-gas auto-ignition of ammonia/hydrogen in a full-field-visualized rapid compression machine

Abstract

Ammonia-hydrogen engine has attracted much attention due to its carbon-free nature. Given that ammonia has high knock resistance (indicated by its high research octane number of nearly 130), researchers are trying to increase the compression ratio (CR) to improve engines’ thermal efficiency. However, knocking cycles can still be detected in ammonia-hydrogen engines under elevated thermodynamic conditions. To further explore the ammonia-hydrogen knocking process, this study conducted a series of spark-ignition combustion experiments focusing on flame propagation and end-gas auto-ignition in a full-field-visualized rapid compression machine. Four ammonia-hydrogen blended fuels with hydrogen energy fractions of 0 % (H0), 10 % (H10), 20 % (H20), and 100 % (H100) were comparatively tested under the thermodynamic conditions of 30 bar and 750–985 K. The experimental results showed that the flame speed of H0, H10, and H20 is around 3–6 m/s under test conditions, much lower than that of H100, which exceeds 37 m/s at 30 bar/750 K. Strong end-gas auto-ignitions with maximum pressure amplitudes of 77 bar and 101 bar were recorded for H0 at 30 bar/985 K and H10 at 30 bar/915 K, respectively. The two auto-ignition events both exhibited detonation characteristics by clear wavefronts with speeds reaching up to 1620 m/s (H0) and 1812 m/s (H10). Based on the experimental results, simulations were carried out to analyze the chemical process during the end-gas auto-ignition. The simulated results showed that for H0 at 30 bar/985 K, the NO-, NO-, and HNO-related reactions contributed most to heat production, which improved the end-gas reactivity and promoted the occurrence of auto-ignition. For H10 at 30 bar/915 K, the H + O (+M) = HO (+M) became the most exothermic reaction, indicating the hydrogen addition significantly promoted the ammonia's end-gas auto-ignition. Additionally, the detonation events observed in this study can be well classified in the -diagram.