Near-limit detonations of methane-oxygen mixtures in long narrow tubes

The near-limit gaseous detonation behavior of three different methane-oxygen mixtures was investigated. Experiments were performed in transparent tubes of three different inner diameters (d). Due to the relatively large tube length l ( ld > 2500 except ld > 1000 for the largest diameter), the tube was arranged in a spiral configuration for the convenience of testing. Photodiodes were spaced at uniform intervals along the tube to provide a high-resolution velocity measurement, such that up to eight cycles of the galloping mode were registered. From the velocity histogram and the probability distribution function, a bimodal behavior was observed in all galloping regimes for different unstable mixtures, with dominant modes near 70% of the Chapman-Jouguet detonation velocity (0.7DCJ) and DCJ. With decreasing pressure, the lower-velocity mode became more prevalent until the failure of detonation. The range of initial pressures, within which galloping detonations were observed, decreased rapidly with increasing tube diameter and with increasing mixture stability. These results suggest that both the instability and the boundary effect influence the existence of galloping detonations. The normalized wavelength of the galloping cycle ( Ld) was in the range of 200-450 for the three different mixture compositions and exhibited a general trend that the wavelength increased with decreasing initial pressure.