Effects of obstacle position and hydrogen volume fraction on premixed syngas-air flame acceleration

There have been many studies concerning on the influence of obstacles on gas explosion, but the understanding of some phenomena is still not clear, such as explosion overpressure or the change of flame velocity with the position of obstacles. This work aims to figure out the coupling effects of obstacle position (L1, L2, L3 from near to far) and hydrogen volume fraction (φ) on syngas-air flame acceleration and overpressure variation in a closed duct. Detailed flame structure evolution in the acceleration process was visualized by a high-speed camera, and overpressure was recorded by pressure transducers. Results show that the effect of obstacles moving backward is not always monotonous on the flame acceleration, which affects the maximum flame speed and maximum overpressure. Such as when φ = 0.5, the flame acceleration that is reflected as the flame propagation speed normalized by the laminar flame speed, ranks as L2 > L3 > L1. Because the obstacles strengthened turbulence on the tulip- or distorted tulip-shaped flame in L2. While when φ = 0.1, the flame acceleration ranks as L1 > L2 > L3 since the effect of turbulence is reduced. It is found that maximum overpressure is related to both flame acceleration and local overpressure. Local overpressure is the transient overpressure before the flame front touches obstacles. Obstacles moving backward is beneficial for local overpressure accumulation. Hence, the maximum overpressure increases with obstacle position moving backward when φ ≥ 0.1. This study clarifies the role of obstacle position and hydrogen volume fraction on flame acceleration, which may help to guide the application of syngas in a complex environment.