DDT experiments were conducted in a 6000 mm long square cross-section (112 mm x 112 mm) tube with various obstacle configurations with hydrogen-air mixtures and ethylene-air mixtures at ambient pressure (101 kPa) and room temperature (298 K). Square orifice plates with inner side 86.8 mm and 70.8 mm (BR = 0.4 and 0.6) and round orifice plates with inner diameter 80.0 mm (BR = 0.6) were used to assemble the obstacle configurations. The plates were installed at 1, 2 and 3 times the tube inner side. Soot foils were placed between the two orifice plates at the end of the tube for S = 3D, where S is the obstacle spacing and D is the tube inner side. The DDT limits were determined based on the flame velocity above the isobaric sound speed of the burnt products. The results show that at the DDT limits, the criterion d(eff)/lambda approximate to 1 is not pervasive, i.e., d(eff)/lambda decreases with the obstacle spacing increase, in which d(eff) and lambda are the effective diameter of the orifice and the detonation cell size. Within the limits, the measured velocity for BR = 0.6 square orifice plates is higher than that for round orifice plates. On the other hand, no obvious difference in the limits can be observed for the BR = 0.6 obstacle configurations. Soot foils provide insights into the detonation propagation mechanism in the orifice plate section. It is shown that hot spots formed via the interactions between the decoupled shock wave and the tube wall can be responsible for the re-initiation of detonation. In addition, overdriven detonations induced by shock focus at the corners, followed by a band of fine cells. For less sensitive mixture and smaller orifice, the re-initiation distance is longer. Near the limits, no cellular structure can be observed, indicating longer cycle period for detonation re initiation. This also accounts for the significant velocity fluctuation for larger spacing (S = 2D and S = 3D) when the limits are approached. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.