Investigation on the detonation propagation limit criterion for methane-oxygen mixtures in tubes with different scales

The detonation propagation limits of CH4-O-2 mixtures (phi = 0.25-2) in a macro-channel (D = 36 mm) and in micro-channels with heights of 2 mm, 4.5 mm and 7 mm were studied. The critical pressure (p(c)) below which detonation cannot maintain steady propagation was measured experimentally for each mixture in different channels. At the condition of p o the scaling between detonation cell size (lambda) and tube dimension (D-H) were also analyzed to explore the detonation failure mechanism in channels of macro- and micro-scale. The experimental results show that the detonation propagation limit criterion lambda = pi D (D is the inner diameter) holds in a macro-channel for the mixture with larger value of instability; however, the detonation instability is reduced as the mixture tends to fuel-rich condition, where this typical relation breaks down. The results indicate that the propagation limit criterion in macro-channel depends less on the boundary conditions of the tube and more on the detonation instability. In the micro-channels, because of the losses from the walls and mass divergence into the boundary layers, detonation propagates in a thinner channel with larger velocity deficit (VD), thereby increasing both the reaction zone length and the detonation cell size; therefore, lambda = pi D is unsuitable as a criterion of the detonation propagation limit, which suggests that for detonation propagating in the micro-channels, the propagation mechanism is mainly governed by the boundary conditions.