On the Z-shaped explosion limits of acetylene-oxygen mixtures

The explosion limits of the acetylene-oxygen (CH-O) mixture have been investigated and it is demonstrated, for the first time, that the CH-O explosion exhibits a Z-shaped response in the pressure-temperature regime with three distinct limits from low to high pressures, which resembles the explosion limits of hydrogen-oxygen (H-O) mixtures. Kinetic analysis shows that the is mainly controlled by the reaction of CH (+ M) = HCC (+ M) and the subsequent pathways of HCCHCOH. The second and third limits are controlled by the H-addition reaction of CH to form the CH radical. The CH radical is less reactive at intermediate pressures, and its pressure dependent formation, CH + H (+ M) = CH (+ M), results in the . However, CH radical is reactivated at high pressures through CH + HO = CH + O, from which the HO radical formed activates the HOHOOH chain branching channel, leading to the . Further, we explored the explosion limits of the H-CH-O mixtures, which shows that with increasing H concentrations, the explosion limit curve rotates clockwise around a . Such rotation only occurs in the second and third limits, while the first limit basically coincides with that of the CH-O system. The kinetic reasons responsible for the observed behavior are identified through the competing pathways of H and CH. Furthermore, the effects of changing of equivalent ratio, dilutions of nitrogen and water, with and without surface reactions on the explosion limits are also discussed. The results of the present work are useful to better understand the explosion limits of CH-O and H-CH-Omixtures and further study of more complex hydrocarbon fuels.