The effect of open-end ignition at different positions on the explosion behaviors of H2/CO/Air in variable cross-section pipe

This work is focused on the explosion performance of syngas/air premixed gas in a self-designed variable crosssection pipe under varying ignition positions (IP1, IP2) and hydrogen volume fraction (alpha(H-2)). Results show that alpha(H-2) has a great influence on the propagation of the flame as well as the maximum flame front velocity (FFVmax) and the maximum overpressure (P-max). As alpha(H-2) in the syngas increases, the time the flame stays in the pipe is gradually reduced, the FFVmax as well as the P-max increase regardless of the ignition position. Differently, when alpha(H-2) < 50%, the t(closed) at IP1 is shorter than that at IP2. The opposite is true when alpha(H-2) >= 50%. This is attributed to the difference in flame velocity and the effect of film breakage. Both alpha(H-2) and variable crosssection chamber structure all affect the evolution of the flame morphology. When ignition at IP2, a "M" flame is observed for the first time at alpha(H-2) = 20%, and a flame resembling a "T" shape appears at alpha(H-2) = 70%. Ignition positions have a great influence on overpressure oscillation. In the case of alpha(H-2) = 15%, the Fourier transform is performed for the pressure curve. When ignited at IP1, secondary unstable oscillations occur during the later stage of flame propagation. However, when ignition at IP2, primary instability oscillations occur in both the early and late stages of flame propagation. The variable cross-section chamber structure plays a role in the flame front velocity (FFV) and overpressure. After the flame front enters the variable section chamber, the FFV reduces, and when the flame front leaves the variable section chamber, due to the sudden decrease of the crosssectional area, the FFV increases significantly, and FFVmax and P-max are obtained at this moment.