High-speed flames and DDT in very rough-walled channels

An experimental investigation of flame acceleration in a narrow rectangular cross-section channel with various lower boundary conditions is presented. The boundary conditions investigated include a smooth surface and a rough surface with and without porosity. Tests were performed with a mixture of nitrogen diluted stoichiometric methane–oxygen, i.e., CH4 + 2(O2 + 2/3N2), at initial pressures in the range of 9–15 kPa. The explosion front velocity was measured via ionization probes and pressure transducers and the explosion wave front structure was visualized by high-speed shadowgraph video. For tests performed with a porous rough surface the flame achieves a velocity that approaches the speed of sound of the combustion products. In the case of the rough surface without porosity, a globally quasi-steady high-speed flame is produced with a velocity between the combustion products speed of sound and the Chapman–Jouguet detonation velocity. It is proposed that flame acceleration beyond the speed of sound of the combustion products is possible due to enhanced burning that is driven by chemical–acoustic interactions. Such interactions occur as the turbulent flame brush is traversed by transverse pressure waves produced by the passage of the explosion front shock wave over the rough surface. It is also observed that under certain conditions a smooth transition from a diffusion driven flame to a shock ignition detonation is possible.