Experimental Investigation on the Failure Mechanism for Critical Tube Diameter Phenomenon of Gaseous Detonations

In this thesis, an experimental investigation is carried out to study the mechanism governing the successful transmission or failure on the critical tube diameter phenomenon when a fully developed, self-sustained detonation propagating in the confined tube transmits into an open space. The result of this study contributes to a better understanding of fundamental physical processes on the initiation, propagation and failure of the detonation. To demonstrate the dependence of critical tube diameter dc on combustion chemistry, two kinds of explosive mixtures are studied. The first is typical for common hydrocarbon mixtures characterized by irregular cellular structures and turbulent reactions zones. The other is referred to as stable mixtures particularly with combustibles highly-diluted with argon. A parametric study is carried out to measure critical tube diameters using stoichiometric acetylene-oxygen diluted with varying amount of argon to obtain these two types of mixtures. The present study validates that the well-accepted universal relation dc = 13λ holds for 0% - 30% argon diluted mixtures and breaks down when argon dilution increases up to 40%. Cell size measurement also indicates that the cellular detonation front starts to become more regular (or stable) when the argon dilution reaches above 40 - 50%. These results hence support that the physical process of critical tube diameter phenomenon is related to the stability nature of the detonation front and failure mechanism. Failure mechanisms for the critical tube diameter phenomenon were previously postulated in the literature for the two kinds of mixtures. For unstable mixtures, the failure is based on the inability to form explosion centers in the failure wave when it has penetrated to the charge axis. For stable mixtures, the failure is caused by excessive curvature of the entire detonation front when the corner expansion waves have distributed the curvature over the diverging wave surface. To discriminate between the two aforementioned modes of failure and clarify the importance of instability, two series of experiments are conducted: one by generating artificially small flow instability using small obstacles with different blockage ratios and the other by damping transverse instability using porous media to see how the critical tube diameter phenomenon responds to these perturbations. Results show that both generation and suppression of flow instability leads to a significant change in the critical condition for successful transmission. The critical pressure obtained in unstable mixtures is found lower with flow perturbation by the obstacles but adversely increases with the damping of instability using porous walled tube; while no noticeable effect could be observed in stable, argon-diluted mixtures. The general implications of the present study are that in common unstable mixtures, instability is essential for the critical tube diameter problem and more generally, for the initiation and propagation of detonation, providing an efficient mechanism of gas ignition. For only a very special class of stable mixtures, the propagation of the detonation wave relies solely on the global coupling between the reaction front and the shock and instabilities only play a minor role on the dynamics of the detonation

A technique for promoting detonation transmission from a confined tube into larger area for pulse detonation engine applications

A simple method of detonation transmission from a small tube to a large area is presented. This technique involves placing obstacles which create slight blockages at the exit of the confined tube before the planar detonation emerges into the larger space, thereby generating flow instability to promote the detonation transmission. In this experimental study two mixtures of undiluted stoichiometric acetylene-oxygen and acetylene-nitrous oxide are examined. These mixtures can be characterized by a cellular detonation front that is irregular and representative of those potentially used in practical aerospace applications. The blockage ratio imposed by the obstacles is varied systematically to identify the optimal condition under which a significant reduction in critical pressure for transmission can be obtained. A new perturbation configuration for practical use in propulsion and power systems is also introduced and results are in good agreement with those obtained using thin needles as the blockage ratio is kept constant