A traditional Japanese sparkler, senko-hanabi, is consists of paper string wrapping 0.1 g of black powder. The sparkler requires heat production by combustion of contained charcoal with ambient oxygen, showing off beautiful sparks luminous by heat radiation. In the present study, the appearance of the sparks is investigated under various ambient conditions, in which the total pressure is from vacuum to 5.0 bar and the oxygen concentration rate is set up to 40 %. The sparkler is ignited by Joule heating inside a closed vessel. Profound effects are confirmed by the ambient conditions on the sparks. As the total pressure rises, sparks spread actively, because the boundary layer surrounding the fireball, created at the bottom end of the paper string, becomes thinner, and the amount of oxygen supply increases. The oxygen concentration rate directly contributes to the amount of oxygen supply. The criterion given by molar flux of oxygen supply to the fireball is identified as 10^-1 mol · s^-1 · m^-2, rate-controlled by molecular diffusion through the boundary layer, for embodying fragile beauty over the centuries.

View full abstract

The present study describes the propagation characteristics of shock waves discharged from the open end of circular tubes that have obstacles placed in front of the open end. These circular tubes with obstacles were used as simplified models of underground magazines that have dikes installed in front of the open end to prevent blast waves and fragments from directly hitting the surrounding area in case of any accidents. The propagation characteristics of shock waves were analyzed with time-resolved visualized images. The background-oriented schlieren (BOS) technique with a high-speed camera was used for the visualization because of the physical limitation of installing ordinary systems, including schlieren, within the explosion chamber. Model configurations were assessed with three different length/diameter ratios for the tube and three types of obstacles. An explosive, pentaerythritol tetranitrate (PETN) pellet (1.0 g) was ignited inside the model tubes to generate shock waves. R-t diagrams were reconstructed from visualized images and peak overpressure was estimated from the diagrams to allow for comparison with overpressure history, which had previously been obtained. The results showed that while the presence of an obstacle diminished the shock strength, the obstacle size did not have a significant effect on the propagation of shock waves behind the dike model.

View full abstract

Hexanitrostilbene (HNS) is a booster explosive that is usually applied to exploding foil initiators (EFIs) properly in HNS-IV form. The initiation process of HNS-IV explosive or EFI is very difficult to observe due to its small scale and rapid reaction. The interface particle velocity profiles were used to indicate the shock compression and reaction process in this paper. The interface particle velocities between HNS-IV explosive and LiF window were measured using an available Photonic Doppler Velocimetry (PDV). The experiment could get the CJ point correctly for its strong signal, so that the velocity peak infers CJ point in the paper. The experimental results show that the detonation pressure of HNS-IV grows quite rapid relatively. The peak pressure is 15.0 GPa at 0.51 mm thickness, which indicates a fast detonation build-up process. The peak pressure is 17.7 GPa at 1.48 mm thickness, which nearly reaches the steady detonation pressure. To allow practical estimations of detonation properties, the ignition and growth parameters for HNS-IV were developed by comparing the interface particle velocities between the experiments and calculations.

View full abstract

The composite nanofibers with nitrocellulose (NC) and high energy of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) has been successfully fabricated by electrospinning technique. The nanofibers were characterized for their morphology and tested for their chemical performances. The average diameter and morphological characteristics of the electrospun composite nanofibers are strongly affected by RDX concentration. The burning performances of the nanofibers were characterized by a confined pressure cell test. The result shows that the maximum pressure increases after adding RDX. The maximum value of the composite fibers with 30 wt% RDX is 1.3 times higher than that of pure NC nanofibers.

View full abstract