Effects of porous walled tubes on detonation transmission into unconfined space

Abstract

Experiments were carried out to investigate the failure mechanisms in the critical tube diameter phenomenon for stable and unstable mixtures. It was previously postulated that in unstable mixtures where the detonation structure is highly irregular, the failure during the diffraction is caused by the suppression of the instability responsible for the generation of local explosion centers. In stable mixtures, typically with high argon dilution and where the detonation is characterized by very regular cell, the failure is driven by the excessive global front curvature above which a detonation cannot propagate. To discern these two failure mechanisms, porous wall tubes are used to attenuate the transverse instability before the detonation emerges into the unconfined space. Porous sections with length L/D from 0 to 3.0 are used with two confined tube diameters D = 12.7 and 15.5 mm. The present results show that when porous wall tubes are used, the critical pressure for unstable C2H2 + 2.5O2 and C2H2 + 5 N2O mixtures increases significantly. In contrast, for stable argon diluted C2H2 + 2.5 O2 + 70% mixtures, the results with porous wall tubes exhibit little variation up to L/D = 2.5. For L/D > 2.5 a noticeable increase in critical pressure for argon diluted mixtures is also observed. This is dominantly caused by the slow mass divergence through the porous material inducing a curvature on the detonation front even before it emerges into the open area. The present experiment again demonstrates the importance of the transverse wave instability for typical hydrocarbon mixtures in critical situations such as the critical tube diameter experiment. For special cases such as highly argon diluted mixtures, the instability does not play a significant role in the failure and the propagation is controlled dominantly by the global curvature effect and the shock-ignition mechanism.

The re-initiation mechanism of detonation diffraction in a weakly unstable gaseous mixture

“…Although these calculations are performed for a weakly stable detonation using simplified chemical kinetics, the re-initiation results show many similarities to those observed in the experimental work of Mehrjoo et al (2015). They used porous-walled tubes to suppress transverse instability and demonstrated that successful transmission was reliant on frontal instability.…”

Computational study of gaseous cellular detonation diffraction and re-initiation by small obstacle induced perturbations

“…The importance of cellular instabilities in the outcome of the detonation diffraction has been further elucidated by Mehrjoo et al 16,17 . Their studies show that, by generating artificially flow perturbations using small obstacles or by damping transverse waves using porous media at the tube exit, the transmission of a diffracting detonation wave can be promoted or suppressed, respectively.…”

The role of cellular instability on the critical tube diameter problem for unstable gaseous detonations

“…Experiments were recently conducted by Mehrjoo et al [16,17] to discriminate between the two modes of failure conjectured by Lee [11] and clarify the importance of instability on the critical tube phenomenon. Two series of experiments were conducted: one by generating artificially small flow instabilities using small obstacles with different blockage ratios placed at the tube exit and the other by damping transverse instability using porous media.…”