Detonation limits in binary fuel blends of methane/hydrogen mixtures

Binary fuel blends of methane and hydrogen have a wide application in the internal combustion engines due to their promising combustion performance, although substantial studies have been carried to investigate the combustion characteristics, very limited study focused on its detonation limits for propagation in tubes or pipes. In this study, near detonation limits behavior, which includes velocity deficit and cellular structure, of binary fuel blends of methane and hydrogen mixtures with different compositions (i.e., CH4–2H2–3O2, CH4–H2–2.5O2 and CH4–4H2–4O2) are experimentally studied, experiments are carried out in a 36 mm inner diameter round tube and annular channels with three gaps (w = 2 mm, 4.5 mm and 7 mm). The results show the maximum detonation velocity deficit is 7% of CJ (Chapman–Jouguet) velocity for three mixtures in the 36 mm inner diameter round tube, and this velocity deficit is universal in the mixtures with different compositions. As detonations transmit into the annular channels, the velocity deficits in CH4–2H2–3O2 and CH4–4H2–4O2 mixtures are very close, i.e., within 10–20% VCJ in the different scale of channels. For CH4–H2–2.5O2 mixtures, velocity deficit varies from 15.0% to 34.1% VCJ as the annular channel gap reduces from 7 mm to 2 mm, which is due to it has a higher degree of instability and hence more robust than other mixtures, a critical value of stability parameter χ is determined as 15–20, below which the instability has no significant effect on the velocity deficit. The cellular pattern from the smoked foils indicates single-headed spinning detonation in CH4–H2–2.5O2 mixture appears at lower initial pressure than other two mixtures, and the detonation cell size for this mixture is larger at the same initial condition, which is verified by the evidence from ZND induction zone length analysis.