In this investigation, the dynamic detonation parameters for stoichiometric acetylene-oxygen mixtures diluted with varying amount of argon are measured and analyzed. The experimental results show that the critical tube diameter and the critical energy for direct initiation of spherical detonations increase with the increase of argon dilution. The scaling behavior between the critical tube diameter dc and the detonation cell size λ as well as the critical direct initiation energy Ec is systematically studied with the effect of argon dilution. The present results again validate that the relation dc = 13λ holds for 0% -30% argon diluted mixtures and breaks down when argon dilution increases up to 40%. It is found that the explosion length scaling of Ro ~ 26λ becomes also invalid when the mixture contains approximately this same amount of argon dilution or more. This critical argon dilution is indeed close to that found from experiments in porous-walled tubes by which exhibit a distinct transition in the failure mechanism. Cell size analysis in literature also indicates that the cellular detonation front starts to become more regular (or stable) when the argon dilution reaches more than 40 -50%.Regardless of the degree of argon dilution or mixture sensitivity, the phenomenological model developed from the surface energy concept by Lee, which provides a relation that links the critical tube diameter and the critical energy remains valid. The present experimental results also follow qualitatively the observation from chemical kinetic and detonation instability analyses.

In this study, an experimental investigation is carried out to further study the critical tube diameter problem for the transmission of gaseous detonation from a confined tube into a sudden open space in both regular mixtures, those highly diluted with argon and irregular mixtures of which the cellular detonation is highly unstable. The two commonly postulated modes of failure consisting of one by a local failure mechanism that is linked to the effect of instabilities for undiluted mixtures, and the other due to the excessive curvature of the global front in mixtures highly diluted with argon, are further investigated through experiments. To discern between these mechanisms in the different mixtures, flow perturbations are imposed by placing a minute obstacle with small blockage ratio at the tube exit diameter just before the detonation diffraction. Results show that the perturbation only has an effect in undiluted mixtures resulting in the decrease of the critical pressure for successful detonation transmission. In other words, the flow fluctuation caused by the small obstacle produces transmission and this result seems to indicate that local hydrodynamic instabilities are significant for the detonation diffraction in undiluted unstable mixtures. On the other hand, the results appear to be the same for both unperturbed and perturbed cases in highly argon diluted mixtures. The small blockage only produces flow fluctuations and does not substantially influence the global curvature of the

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.

In this investigation, the dynamic detonation parameters for stoichiometric acetylene-oxygen mixtures diluted with varying amount of argon are measured and analyzed. The experimental results show that the critical tube diameter and the critical energy for direct initiation of spherical detonations increase with the increase of argon dilution. The scaling behavior between the critical tube diameter dc and the detonation cell size λ as well as the critical direct initiation energy Ec is systematically studied with the effect of argon dilution. The present results again validate that the relation dc = 13λ holds for 0% -30% argon diluted mixtures and breaks down when argon dilution increases up to 40%. It is found that the explosion length scaling of Ro ~ 26λ becomes also invalid when the mixture contains approximately this same amount of argon dilution or more. This critical argon dilution is indeed close to that found from experiments in porous-walled tubes by which exhibit a distinct transition in the failure mechanism. Cell size analysis in literature also indicates that the cellular detonation front starts to become more regular (or stable) when the argon dilution reaches more than 40 -50%.Regardless of the degree of argon dilution or mixture sensitivity, the phenomenological model developed from the surface energy concept by Lee, which provides a relation that links the critical tube diameter and the critical energy remains valid. The present experimental results also follow qualitatively the observation from chemical kinetic and detonation instability analyses.

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