Simulation of DDT in Obstructed Channels: Wavy Channels vs. Fence-type Obstacles

The capabilities of an OpenFOAM solver to reproduce the transition of stoichiometric H2-air mixtures to detonation in obstructed 2-D channels were tested. The process is challenging numerically as it involves the ignition of a flame kernel, its subsequent propagation and acceleration, interaction with obstacles, formation of shock waves ahead and detonation onset (DO). Two different obstacle configurations were considered in 10-mm high × 1-m long channels: (i) wavy walls (WW) that mimic the behavior of fencetype obstacles but prevent abrupt area changes. In this case, flame acceleration (FA) is strongly affected by shock-flame interactions, and DO often results from the compression of the gas present between the accelerating flame front and a converging section of the channel. (ii) Fence-type (FT) obstacles. In this case, FA is driven by the increase in flame surface area as a result of the interaction of the flame front with the unburned gas flow field ahead, particularly, downstream of obstacles; shock-flame interactions play a role at the later stages of FA, and DO takes place upon reflection of precursor shocks from obstacles. The effect of initial pressure, p0 = 25, 50 and 100 kPa, at constant blockage ratio (BR = 0.6) was investigated and compared for both configurations. Results show that for the same initial pressure (p0 = 50 kPa), the obstacle configurations could lead to different final propagation regimes: a quasi-detonation for WW, and a choked-flame for FT, due to the increased losses for the latter. At p0 = 25 kPa however, while both configurations result in choked flames, WW seem to exhibit larger velocity deficits than FT due to longer flame-precursor shock distances during quasi-steady propagation and to the increased presence of unburnt mixture downstream of the tip of the flame that homogeneously explodes providing additional support to the propagation of the flame.