Effects of methane volume fractions and vent areas on dynamic characteristics of vented methane-air explosion in a half-open duct

Comprehensively revealing the dynamic characteristics of a vented gas explosion is of great significance for designing more effective vents. In the paper, the vented methane-air explosion experiments were conducted in a half-open duct with a length of 500 mm and a cross-sectional area A(c) of 100 mm x 100 mm. The effects of methane volume fractions phi(CH4) and vent areas A(v) on the dynamic characteristics of overpressure and flame propagation speed were investigated. The results demonstrate that there are mainly 5 dynamic stages and 4 overpressure peaks during the explosion when A(v) is equal to A(c). The difference is that the 9.5% methane-air explosion process mainly includes 6 dynamic stages and 5 overpressure peaks when A(v) is less than A(c). It is a remarkable discovery that the degree of multi-wave interaction (between the superposition effect of the incident waves and the reflected waves and the attenuation effect of rarefaction waves) varies with the vent area from 40 mm x 40 mm to 80 mm x 80 mm, which leads to new overpressure peak groups Pmwi with different fluctuation trends. Moreover, the flame propagation speed also shows characteristic fluctuations during the stages of the vent failure and the multi-wave interaction. The maximum overpressure is attributed to the vent failure when the vent area are 100 mm x 100 mm, 80 mm x 80 mm, and 70 mm x 70 mm. However, the maximum overpressure is caused by the multi-wave interaction and the maximum flame area when the vent area are 60 mm x 60 mm, 50 mm x 50 mm, and 40 mm x 40 mm. In addition, a new vent design equation is established based on the functional relationship between static activation pressure P(sta)t and A(v), which mainly involves the functional relationship between maximum overpressure P-red and vent coefficient K-v.