Flame propagation mode transition of premixed syngas-air mixtures in a closed duct

Abstract

Knowledge regarding the transformation of the flame propagation dynamics is of importance to predict the safety issues of explosion disasters. The current work experimentally investigated the premixed syngas-air flame propagating in a 2 m duct with a set of obstacles, focusing on the flame propagation mode transition with the change of hydrogen volume fraction (φ). Three modes were identified as the φ increases: steady flame propagation, oscillating flame propagation, and end-gas autoignition. The steady flame propagation mode occurred when φ = 0, where the slow oxidation of pure CO determines its propagation feature. The flame became unsteady and oscillated violently with a slight addition of hydrogen. Flame oscillation resulted from its interaction with pressure wave, and the strength of the pressure wave directly impacted the severity of oscillation. The autoignition appeared when φ ≥ 0.4, resulting from the secondary reflected pressure wave interacted with the region that the thermodynamic state was highly disturbed. What is more, the autoignition kernel migrated from the region ahead of the flame front to the end-wall region when φ ≥ 0.6. The reason is that syngas with higher φ has a lower temperature threshold for explosion and a shorter ignition delay.