The three-dimensional structure of a detonation wave propagating in a round tube with orifice plates

Abstract

Experiments were performed to study detonation propagation in a round tube equipped with repeating orifice plates, equally spaced at two tube diameters. Tests were performed with stoichiometric hydrogen–oxygen over a range of initial pressures up to 60 kPa absolute. The self-luminous high-speed video was used to visualize the detonation for two sizes of orifice plate, i.e., 50 and 75% cross-sectional area blockage. The high-speed videos were used to obtain the average combustion wave velocity, from which the detonation propagation regime was determined, and the propagation mechanism for each orifice plate geometry was identified. For the 50% blockage ratio orifice plate tests, the detonation limit was measured to be 6 kPa, and detonation propagation was characterized by cycles of detonation failure and re-initiation after each orifice plate, resulting in a CJ detonation velocity deficit of the order of 20%. The high-speed video, captured at an oblique camera angle, showed that the detonation wave forms at hot spot(s) on the tube wall just downstream from the orifice plate, following shock reflection. For the 75% blockage orifice plates, the detonation propagation limit was measured to be 20 kPa. For initial pressures between 20 and 40 kPa, a galloping detonation mode was observed, which was not observed in a previous study performed in the same tube and orifice plates but with one-tube-diameter orifice plate spacing. The galloping mode consists of repeating cycles of fast-flame propagation between successive obstacles, followed by detonation wave propagation between the following pair of orifice plates. The experimental results were used to explain why the well-established requirement that the orifice accommodates one cell for detonation propagation (i.e., $d/\lambda >1$) is sufficient for low blockage ratio orifice plates; however, for high blockage ratio plates it is necessary but not sufficient.