David L. Frost

Explosive dispersal of solid particles

Abstract. The rapid dispersal of inert solid particles due to the detonation of a heterogeneous explosive, consisting of a packed bed of steel beads saturated with a liquid explosive, has been investigated experimentally and numerically. Detonation of the spherical charge generates a blast wave followed by a complex supersonic gas-solid flow in which, in some cases, the beads catch up to and penetrate the leading shock front. The interplay between the particle dynamics and the blast wave propagation was investigated experimentally as a function of the particle size (100–925

Effects of ambient pressure on the instability of a liquid boiling explosively at the superheat limit

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Dynamics of explosive boiling of a droplet

m) and charge diameter (8.9–21.2 cm) with flash X-ray radiography and blast wave instrumentation. The flow topology during the dispersal process ranges from a dense granular flow to a dilute gas-solid flow. Difficulties in the modeling of the high-speed gas-solid flow are discussed, and a heuristic model for the equation of state for the solid flow is developed. This model is incorporated into the Eulerian two-phase fluid model of Baer and Nunziato (1986) and simulations are carried out. The results of this investigation indicate that the crossing of the particles through the shock front strongly depends on the charge geometry, the charge size and the material density of the particles. Moreover, there exists a particle size limit below which the particles cannot penetrate the shock for the range of charge sizes considered. Above this limit, the distance required for the particles to overtake the shock is not very sensitive to the particle size but remains sensitive to the particle material density. Overall, excellent agreement was observed between the experimental and computational results.

Particle jet formation during explosive dispersal of solid particles

The effect of ambient pressure on the dynamical behaviour of a single droplet (1-2 mm diameter) of volatile liquid boiling explosively at the limit of superheat is studied experimentally and theoretically. In a series of experiments it is shown that the evaporative instability, observed earlier by Shepherd & Sturtevant (1982) during the rapid vapourization of butane droplets at atmospheric pressure, is suppressed at high pressure. Three other fluids (pentane, isopentane, and ether) are tested to establish the generality of the instability and other transient processes previously observed. Direct evidence is obtained showing that during violently unstable boiling small liquid particles are torn from the liquid-vapour interface. This ejection of fine droplets from the evaporating surface produces a mass flux orders of magnitude greater than that characteristic of ordinary boiling. Raising the ambient pressure lowers the superheat attained at the superheat limit, which decreases the vapourization rate. At high pressure boiling consists of normal slow vapourization from a smooth interface. Observed bubble growth rates show reasonable agreement with theory. At intermediate pressures a transitional regime of stability occurs in which a drop initially vapourizes stably for several milliseconds while incipient instability waves develop on the evaporating interface. When only a small amount of liquid remains in the drop in the shape of a thin cap, heat transfer from the surrounding hot host fluid initiates violent boiling at the edge of the liquid cap. The subsequent rapid vapourization generates a radiated pressure field two orders of magnitude larger than during stable boiling, and sets the bubble into violent oscillation. The bubble is subject to the Rayleigh-Taylor instability and rapidly disintegrates into a cloud of small bubbles. Lowering the ambient pressure decreases the time delay between nucleation and onset of unstable boiling. For example, in ether at atmospheric pressure the instability is triggered less than 8 µsec after nucleation, shortly after the smooth vapour bubble contacts the droplet surface. Heterogeneous nucleation spreads out along the surface of the drop while disturbances (with a length scale of 100 µm) distort the unstably evaporating interface within the drop, substantially enhancing the vapourization rate. At early times, droplets torn from the evaporating surface evaporate before the instability-driven jet impinges upon the surrounding fluid, bulging the bubble surface. The last portion of liquid in a drop boils particularly violently and droplets ejected from the evaporating interface at this time remain intact to splatter the bubble surface. At subatmospheric pressures the most rapid vapourization occurs and temperature gradients within a drop produce spatial variations in vapourization rate. The Landau mechanism for the instability of laminar flames is adapted to the case of evaporation to investigate the effects of variable ambient pressure. A spherical version of the theory, applicable before the vapour bubble contacts the droplet surface, predicts absolute stability at atmospheric pressure. At later times the spherical constraint is inappropriate and planar theory yields results in general agreement with observation. Differences in fluid properties make some fluids more prone to instability than others. The product of the maximum growth rate with the time interval the interface is predicted to be linearly unstable measures the susceptibility to instability. For practical estimates it is suggested that a value of 3 of this parameter be taken as the lower limit for instability. The sensitivity of the instability to temperature suggests that small temperature nonuniformities may be responsible for quantitative departures of the behaviour from predictions.

Particle momentum effects from the detonation of heterogeneous explosives

The dynamical behavior of the unstable explosive boiling of single droplets (1–2 mm diam) of diethyl ether, pentane, and isopentane at the superheat limit has been exhibited in detail. A+high ambient pressures, boiling consists of normal stable growth of a smooth bubble. At intermediate pressures a transitional regime of stability occurs in which a drop initially vaporizes stably for several milliseconds while incipient instability waves develop on the evaporating interface, then increased heat flux from the host liquid initiates violent boiling near the edge of the remnant volatile liquid. Direct evidence has been obtained that during violently unstable boiling, fine liquid particles are torn from the liquid–vapor interface, generating a mass flux orders of magnitude greater than that characteristic of ordinary boiling. In this regime of transitional stability, one of a number of different possible kinds of disturbances could externally trigger a breakdown to violent instability. After the evaporative in...

Fragmentation mechanisms based on single drop steam explosion experiments using flash X-ray radiography

Previous experimental studies have shown that when a layer of solid particles is explosively dispersed, the particles often develop a non-uniform spatial distribution. The instabilities within the particle bed and at the particle layer interface likely form on the timescale of the shock propagation through the particles. The mesoscale perturbations are manifested at later times in experiments by the formation of coherent clusters of particles or jet-like particle structures, which are aerodynamically stable. A number of different mechanisms likely contribute to the jet formation including shock fracturing of the particle bed and particle-particle interactions in the early stages of the dense gas-particle flow. Aerodynamic wake effects at later times contribute to maintaining the stability of the jets. The experiments shown in this fluid dynamics video were carried out in either spherical or cylindrical geometry and illustrate the formation of particle jets during the explosive dispersal process. The number of jet-like structures that are generated during the dispersal of a dry powder bed is compared with the number formed during the dispersal of the same volume of water. The liquid dispersal generates a larger number of jets, but they fragment and dissipate sooner. When the particle bed is saturated with water and explosively dispersed, the number of particle jets formed is larger than both the dry powder and pure water charges. More extensive experiments that explore the effect of particle size, density and the mass ratio of explosive to particles on the susceptibility for jet formation are reported in Frost et al. (Proc. of 23rd ICDERS, Irvine, CA, 2011).

The effect of particle strength on the ballistic resistance of shear thickening fluids

Detonation of a spherical high explosive charge containing solid particles generates a high-speed two-phase flow comprised of a decaying spherical air blast wave together with a rapidly expanding cloud of particles. The particle momentum effects associated with this two-phase flow have been investigated experimentally and numerically for a heterogeneous explosive consisting of a packed bed of inert particles saturated with a liquid explosive. Experimentally, the dispersion of the particles was tracked using flash radiography and high-speed photography. A particle streak gauge was developed to measure the rate of arrival of the particles at various locations. Using a cantilever gauge and a free-piston impulse gauge, it was found that the particle momentum flux provided the primary contribution of the multiphase flow to the near-field impulse applied to a nearby small structure. The qualitative features of the interaction between a particle and the flow field are illustrated using simple models for the part...

Combustion of Aluminum Suspensions in Hydrocarbon Flame Products

Flash X-ray and high-speed regular photography were used to investigate the fragmentation processes during the vapor explosion of single drops of molten metal immersed in water. For relatively low ambient flow velocities ( 45 m/s), vapor bubble growth is diminished and high-speed motion of vapor within the bubble leads to an enhanced fragmentation rate.

Effect of diethylenetriamine sensitization on detonation of nitromethane in porous media

The response of shear thickening fluids (STFs) under ballistic impact has received considerable attention due to its field-responsive nature. While efforts have primarily focused on traditional ballistic fabrics impregnated with these fluids, the response of pure STFs to penetration has received limited attention. In the present study, the ballistic response of particle-based STFs is investigated and the effects of fluid density and particle strength on ballistic performance are isolated. It is shown that the loss of ballistic resistance in the STFs at higher impact velocities is governed by the material strength of the particles in suspension. The results illustrate the range of velocities over which these STFs may provide effective armor solutions.

The use of Hugoniot analysis for the propagation of vapor explosion waves

Stabilized aluminum flames are studied in the products of methane combustion. A premixed methane–air Bunsen flame is seeded with increasing concentrations of micron-size aluminum powder, and scanning emission spectroscopy is used to determine the flame temperature via both the continuous and aluminum monoxide spectra. The flame burning velocity is measured and the condensed flame products are collected and analyzed for unburned metallic aluminum content. It was observed that, below a critical concentration of about 120  g/m3, aluminum demonstrates incomplete oxidation with a flame temperature close to the methane–air flame. Below the critical concentration, the flame burning velocity also decreases similar to a flame seeded with inert silicon carbide particles. In contrast, at aluminum concentrations above the critical value, an aluminum flame front rapidly forms and is coupled to the methane flame. The flame temperature of the coupled methane–aluminum flame is close to equilibrium values with aluminum as...