Deflagration of premixed methane-air in a large scale detonation tube

Methane explosion hazards in pipes are of pivotal concern in chemical plants. Accurate knowledge of flame deflagration and its behaviours are required to reduce the consequences of accidental fires and explosions. Considering a lack of experimental work exists in large scale methane-air deflagration systems, a detonation tube (30 m long) was facilitated at the University of Newcastle to cover the knowledge gap in terms of boosting flame deflagration of low methane concentrations and also examining flame deflagration characteristics with different reactive lengths (3, 6, 12 and 25 m). The feature of injecting methane at varied reactive sections (RS) was achieved using a balloon isolation system, a 50 mJ chemical ignitor used to ignite the initial explosion section. The results revealed that stagnation pressure gradually increased, from 2.03 bar to 3.77 bar then 4.57 bar, with increasing RS length from 3 m to 6 m then 12 m, respectively. There was no significant influence of 1.25% or 2.5% methane concentrations on dynamic or stagnation pressures, however, they extended the travelling flame distance by about 3 m for RS lengths of 12 m and 25 m. At 9.5% methane concentration and for a RS of 12 m a state of fast deflagration was observed, associated with 5 bar pressure rise. The pressure wave up to 6.5 m was only a few milliseconds (about 15 ms) ahead of the flame for almost the full methane concentration range, however, after this point the gap between the pressure wave and the flame significantly varied in accordance to the methane concentration, where the data analysis at 15 m indicated that for 9% methane concentration the flame was only 21 ms behind the pressure wave, and for 5% and 15% methane concentration the flame was behind the pressure wave in the range of 55-93 ms. Due to the limited length of the DT compared with the large volume of methane injected, there was no significant influence on the flame deflagration properties when extending the RS length from 12 m to 25 m, as the mixture initially located after 12 m pushed out through the open end.