Self-luminous, high-speed photography was used to visualize fast-flame and detonation propagation through a transparent round tube equipped with repeating orifice plates, in stoichiometric hydrogen-oxygen mixtures at initial pressures up to 60 kPa. Experiments were conducted in a 1.55 m, 7.6cm inner-diameter plastic tube filled with equally spaced 5.33 cm and 3.81 cm orifice plates (50% and 75% area blockage ratio, respectively). The unprecedented visualization of quasi-detonation propagation in a round tube was used to identify the propagation mechanisms. For both sets of orifice plates, fast-flames were observed below a critical initial pressure. Fast-flame propagation involved the interaction of an uncoupled shock wave and flame with the orifice plates. Detonation propagation involved repeated detonation failure and initiation along the channel length; the limits measured in the 50% and 75% blockage ratio (BR) orifice plates were 7 kPa and 40 kPa, respectively. The orifice diameter-to-detonation cell size ratio (d/lambda) corresponding to these limits are 1.4 and 14, respectively. It is proposed that the significant variance in the d/lambda at the two limits is attributable to the difference in the detonation propagation mechanism. For the 50% BR orifice plates, near the limit, detonation initiation occurred on the tube wall between orifice plates following reflection of the lead shock wave. Whereas, for the 75% BR orifice plates, detonation initiation at the tube wall was not possible for initial pressures up to 40 kPa. This is the result of a weaker shock wave at the time of reflection due primarily to the larger distance from the orifice edge to the tube wall. Steady propagation of a curved detonation wave was observed for the 50% BR orifice plates for an initial pressure of 50 kPa (d/lambda = 25), or greater; a similar propagation was not observed in the 75% BR orifice plates at initial pressures up to 60 kPa (d/lambda =27). Numerical simulations carried out using a single-step reaction model demonstrated the key processes involved in detonation initiation at the tube wall and the orifice plate but could not predict quantitatively the critical initial pressure required for detonation propagation measured in the experiments. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.