Effect of acoustically absorbing wall tubes on the near-limit detonation propagation behaviors in a methane-oxygen mixture

Methane (CH4) is the main component of natural gas and produces less carbon dioxide for each unit of heat released and more heat per unit mass than other hydrocarbon fuels, and it is thus considered to be an environmental-friendly fuel. The explosion and detonation hazards associated with methane mixtures deserve special attention because of their potential safety hazards. Acoustically absorbing materials are effective in damping out the transverse wave of a detonation structure. A detonation could be attenuated or prohibited after passing over this material, but the failure mechanism of the detonation still needs further exploration. In this study, tubes made of an acoustically absorbing material (hole diameters of the wall are from 30 mu m to 300 mu m) are inserted in the smooth rigid wall tube to investigate the effect of the porous-walled material on the detonation propagation at the near-limit conditions. Porous-walled tubes with three different scales (L/D = 3.85, 7.69, and 15.38) are studied. Photodiodes and smoked foils are employed to simultaneously measure the local velocity of the combustion waves and record the cellular detonation structures, respectively. The results show that, for shorter porous-walled tubes, the prohibition effect of the absorbing material on the detonation propagation is only prominent at the critical and sub-critical conditions, but the material has a minor effect on the detonation propagation at the super-critical condition. In addition, the prohibition effect of the porous material on the detonation propagation becomes more evident with the increased length of the acoustically absorbing material. This outcome occurs because a transverse wave plays an important role in the propagation of a self-sustained detonation, as it is partially damped during the transmission of the detonation through the porous-walled tube; thus, extending the length of the porous-walled section results in increasing the losses of the incident and reflected shock waves due to a greater expansion and mass divergence into porous material. Therefore, the velocity of the combustion wave decreases faster in the downstream of the porous material with an increased length. On the other hand, the methane-oxygen mixture has a highly irregular cellular pattern and is characterized as an unstable mixture with a high degree of instability. The strong instability leads to an enhanced ability to generate new transverse waves in the far downstream, and therefore, at the super-critical condition (a relatively higher initial pressure), the instability partly compensates for the negative effect of the acoustically absorbing material on the detonation propagation.