Editorials

South Africa and the World Health OrganisationMultiple AuthorshipCost-benefit analysis of hepatitis B vaccinatio

Experimental Investigation of Near-limit Gaseous Detonations in Small Diameter Spiral Tubing

The near-limit propagation of gaseous detonations in seven explosive mixtures with different reaction sensitivities is investigated. Experiments were performed in transparent tubing of four different inner diameters with relatively long tubing length (l/d > 2500 except l/d > 1000 for the largest diameter) arranged in a spiral configuration. Up to 83 fiber optics spaced at regular intervals along the tube were used to provide high resolution velocity measurement. Up to 8 cycles of the galloping mode were recorded, and the spiral boundary did not influence the persistence of galloping detonations. Results confirm that for mixtures with increasing argon dilution, making the detonation more stable with regular cellular pattern, the occurrence of galloping detonation diminishes. For stable mixtures with sufficiently large amount of argon dilution (e.g., stoichiometric C2H2/O2 with 70%Ar), the galloping mode was not observed in all tested tubing. For unstable mixtures, smaller diameters were necessary to achieve the galloping mode. The range of initial pressures, within which galloping detonations were observed decreases rapidly with increasing tubing diameter. These results suggest that both the instability and the boundary effect are essential for galloping detonations. From the velocity histogram and the probability distribution function, a bimodal behavior was also observed in all galloping regimes of different unstable mixtures, with dominant modes near half of the Chapman-Jouguet detonation velocity (DCJ) and DCJ. With decreasing pressure, the lower velocity mode became more prevalent until no more galloping detonation occurred. The normalized wavelength of the galloping cycle (L/d) ranges from 250 to 450 within experimental variation. Nevertheless, few results show a clear minor trend that the wavelength increases with decreasing initial pressure. By looking at the velocity amplitude in the galloping cycle, the lower value as well as the average is relatively constant, while the upper peak has larger fluctuations

Propagation of near-limit gaseous detonations in rough walled tubes

In this study, experiments were carried out to investigate the detonation velocity behavior near limits in rough-walled tubes. The wall roughness was introduced by using different spiral inserts in 76.2-mm-diameter, 50.8-mm-diameter, 38.1-mm-diameter, and 25.4-mm-diameter tubes. Different pre-mixed mixtures, CH4 + 2O2, C2H2 + 2.5O2, C2H2 + 2.5O2 + 70%Ar, and 2H2 + O2 were tested in the experiments. Different spiral wire diameters were used, and the pitch of each spiral was twice of the diameter to keep the same level of roughness in all experiments for each tube. Fiber optics were used to record the detonation time of arrival to deduce the velocity. The normalized velocity V/VCJ and the velocity deficit δ were computed and analyzed to describe the detonation behavior near the limit. The cellular structure near the limit was recorded by the smoked foils

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

The transmission of detonation waves, propagating in a homogeneous, gaseous, reactive medium, from a tube into an unconfined space is well known to succeed or fail based on the tube diameter. Below a certain diameter, the detonation fails to transition into the unconfined space, while for a large enough geometry, the transition succeeds. This critical diameter is well correlated to the incoming detonation cell size. For common undiluted hydrocarbon mixtures with a strong degree of transverse instability, the ratio of critical tube diameter to cell size has been measured at Dc = 13λ. In this paper, stoichiometric acetylene-oxygen mixture at different initial pressures is detonated in a circular tube that transitions into an effectively unconfined space. The transition is observed with simultaneous schlieren photography and soot foil records to look at the role of transverse cellular instability. Three regimes of transition are observed: supercritical, where the cellular pattern is continuously connected from the donor tube to the larger space; subcritical, where the wave fails and the cellular pattern disappears; and a critical regime, where the wave initially fails, asymptoting to a weakly decoupled shock-reaction front regime, and exhibits a subsequent re-initiation in a critical zone of pre-shocked gas through the onset of an explosion bubble. A substantial amount of transverse instability remains even after the expansion wave reaches the central axis, sustaining the diffracted wave at a critical thermodynamic state for the re-initiation. The location of this critical zone is identified at about 22λ and a small obstacle is used to promote the generation of transverse waves and a re-initiation kernel. The re-initiation is effected by placing an obstacle in the critical region. The role of the resulting instability is also illustrated through a simple numerical simulation using an obstacle in the sub-critical regime to perturb the flow and promote the re-initiation

Measurement and scaling analysis of critical energy for direct initiation of gaseous detonations

In this paper, the critical energies required for direct initiation of spherical detonations in four gaseous fuels (C2H2, C2H4, C3H8 and H2)–oxygen mixtures at different initial pressures, equivalence ratios and with different amounts of argon dilution are reported. Using these data, a scaling analysis is performed based on two main parameters of the problem: the explosion length R o that characterizes the blast wave and a characteristic chemical length that characterizes the detonation. For all the undiluted mixtures considered in this study, it is found that the relationship is closely given by Ro26 , where λ is the characteristic detonation cell size of the explosive mixture. While for C2H2–2.5O2 mixtures highly diluted with argon, in which cellular instabilities are shown to play a minor role on the detonation propagation, the proportionality factor increases to 37.3, 47 and 54.8 for 50, 65 and 70% argon dilution, respectively. Using the ZND induction length Δ I as the characteristic chemical length scale for argon diluted or ‘stable’ mixtures, the explosion length is also found to scale adequately with Ro2320I

The re-initiation of cellular detonations downstream of an inert layer

The current work aims to examine how the nature of cellular instabilities controls the re-initiation capability and dynamics of a gaseous detonation transmitting across a layer of inert (or non-detonable) gases. This canonical problem is tackled via computational analysis based on the two-dimensional, reactive Euler equations. Two differ- ent chemical kinetic models were used, a simplified two-step induction-reaction model and a detailed model for hydrogen-air. For the two-step model, cases with relatively high and low activation energies, representing highly and weakly unstable cellular detonations, respectively, are considered. For the weakly unstable case, two distinct types of re-initiation mechanisms were observed. (1) For thin inert layers, at the exit of the layer the detonation wave front has not fully decayed and thus the transverse waves are still relatively strong. Detonation re-initiation in the reactive gas downstream of the inert layer occurs at the gas compressed by the collision of the transverse waves, and thus is referred to as a cellular-instability-controlled re-initiation. (2) If an inert layer is sufficiently thick, the detonation wave front has fully decayed to a planar shock when it exits the inert layer, and re-initiation still occurs downstream as a result of planar shock compression only, which is thus referred to as a planar-shock- induced re-initiation. Between these two regimes there is a transition region where the wave front is not yet fully planar, and thus perturbations by the transverse waves still play a role in the re-initiation. For the highly unstable case, re-initiation only occurs via the cellular-instability-controlled mechanisms below a critical thickness of the inert layer. Additional simulations considering detailed chemical kinetics demonstrate that the critical re-initiation behaviors of an unstable stoichiometric mixture of hydrogen-air at 1 atm and 295 K are consistent with the finding from the two-step kinetic model for a highly unstable reactive mixture