Experimental study of detonation limits in methane-oxygen mixtures: Distinguishing tube scale and initial pressure effects

In this paper, detonation limits in stoichiometric methane-oxygen mixtures with varying tube inner diameter and initial mixture pressure were investigated. Detonations in tubes with different inner diameter (D = 36 mm, 25 mm, 20 mm and 13 mm) and low initial pressure from 3.5 ~ 18 kPa were studied. Smoked foils were applied to observe the evolution of the detonation cellular structure for various initial conditions. An alternate length scale at the limits is examined, Ldcs, which is the maximum length from the beginning of the test section after which cellular patterns can no longer be observed. Simultaneous local velocity measurements were obtained by photodiodes to complement the Ldcs results. The study also aims to reveal relation between the near-limit detonation dynamics, the tube geometry, and the thermodynamic properties of the mixture. Past the failure limit, Ldcs decreases with decreasing initial mixture pressure for a given tube diameter, and Ldcs decreases faster in a smaller diameter tube. In the D = 13 mm tube, galloping detonation mode is observed, and the length of the galloping cycle is reduced with an increase in initial pressure. To further characterize the onset of detonation limits, a scaling analysis of Ldcs with tube inner diameter (D) and detonation cell size (λ) was performed. The experimental results show that the decrease of Ldcs/D and Ldcs/λ are more abrupt in smaller diameter tubes with decreasing initial pressure. At low initial pressure, the boundary layer displacement thickness growth is significant in the flow structure. Since the distribution of global curvature over the whole detonation front is faster in smaller tube, it thus leads to a more abrupt decrease sensitive to initial pressure. For increasing pressure closer to the critical failure limit, the boundary layer displacement thickness is becoming less comparable to the tube diameter. The failure mechanism appears to be more dominant by the rate of transverse waves attenuation or cell disappearance. Lastly, by comparing the detonation cell size and the tube scale at the critical limits condition in different tubes, λ= πD is shown to be an appropriate limit criterion of detonation propagation in agreement previous studies