Flame acceleration and DDT in large-scale obstructed channels filled with methane-air mixtures

We study the evolution of premixed methane-air flames in large-scale obstructed channels using reactive CFD simulations. We vary the channel height d, the blockage ratio br, and the scaled distance between obstacles L/d to study their effects on the distance to DDT, L_DDT, and the distance to the shock-flame complex, L_SF. The results of simulations show two main effects. On one hand, the increase of br and decrease of L/d promote the flame acceleration and reduce L_SF and L_DDT. On the other hand, some configurations with higher br and smaller L/d prevent the detonation development. As a result, the leading shock and the flame never merge, and continue to propagate as a quasi-steady-state shock-flame complex. A collision of this complex with solid structures generates high pressures and strong reflected shocks that can ignite a detonation. This detonation would propagate in a shock-compressed material and result in extremely high pressures exceeding pressures of a regular detonation. Thus, the distance L_SF at which the shock-flame complex forms provides an important measure of a destructive potential in addition to L_DDT. This is particularly relevant for channels with high br where L_SF can be significantly shorter than L_DDT.