Robert Ripley

Shock Interaction of Metal Particles in Condensed Explosive Detonation

Three‐dimensional mesoscale modeling is conducted to study the two‐phase momentum and heat transfer during shock and thin‐detonation‐front interaction with metal particles in liquid nitromethane under various particle packing configurations and inter‐particle spacings. The results showed that the particle velocity after the shock is a strong function of material density ratio of liquid to metal and volume fraction of particles. In the detonation case, it also depends on the expansion rate of the detonation products. The pressure at the leading edge of particles in a closely‐packed particle bed achieves more than twice that of the theoretical 1D transmission pressure and the corresponding fluid temperature reaches 3828 K, providing hot spots that are responsible for detonation propagation in these heterogeneous explosives.

ACCELERATION AND HEATING OF METAL PARTICLES IN CONDENSED EXPLOSIVE DETONATION

For condensed explosives containing metal particle additives, a characteristic parameter, δ, can be defined as the ratio of particle diameter to detonation reaction zone length. Particle heating and acceleration are studied between the interaction limits ranging from frozen shock to thin‐detonation‐front diffraction followed by an expanding products flow. The intermediate case where the particle diameter and reaction zone length scales are comparable is considered as a function of metal mass fraction and particle packing to determine momentum and heat transfer during the detonation interaction time. The present investigation employs 3D mesoscale simulations to further conduct parametric studies in the 0.1⩽δ⩽10 range by varying the particle diameter and reaction zone length. The results are quantified as velocity and temperature transmission factors, which indicate a strong dependence of particle acceleration and heating rate on δ for dilute particles and high metal mass fraction conditions.For condensed explosives containing metal particle additives, a characteristic parameter, δ, can be defined as the ratio of particle diameter to detonation reaction zone length. Particle heating and acceleration are studied between the interaction limits ranging from frozen shock to thin‐detonation‐front diffraction followed by an expanding products flow. The intermediate case where the particle diameter and reaction zone length scales are comparable is considered as a function of metal mass fraction and particle packing to determine momentum and heat transfer during the detonation interaction time. The present investigation employs 3D mesoscale simulations to further conduct parametric studies in the 0.1⩽δ⩽10 range by varying the particle diameter and reaction zone length. The results are quantified as velocity and temperature transmission factors, which indicate a strong dependence of particle acceleration and heating rate on δ for dilute particles and high metal mass fraction conditions.

Critical Conditions for Ignition of Aluminum Particles in Cylindrical Explosive Charges

The critical conditions for the ignition of spherical aluminium particles dispersed during the detonation of long cylindrical explosive charges have been investigated experimentally. The charges consist of packed beds of aluminium particles, ranging in size from 3 – 114 μm in diameter, and saturated with sensitized liquid nitromethane (NM). The ignition conditions depend on both the charge and particle diameters with the most reactive particles corresponding to an intermediate size (∼54 μm dia). With increasing charge diameter, three particle reaction regimes are observed: i) sub‐critical (no particle reaction), ii) near‐critical (reaction at isolated spots, or radial bands or rings), and iii) super‐critical (continuous reaction of the particle cloud). To monitor the onset of aluminum oxidation, visible radiation from the charge is recorded, through a slit, with a line spectrometer.

Jetting instabilities of particles from explosive dispersal

The formation of post-detonation ‘particle’ jets is widely observed in many problems associated with particle dispersal from explosives. This paper analyzes experimental observations to examine the mechanism for formation and growth of such particle jetting instabilities, and to propose a model to address the issues of jetting growth at a macroscopic level. The experiments involve cylindrical charges with a range of central explosive masses for dispersal of dry solid powder, pure liquid, or a hybrid mixture of solid powder and liquid. The results demonstrate that the jets form very early, and that the number of jets is dependent on shock pressure at the charge perimeter, within the range of particle sizes studied. The jetting instabilities may therefore be initiated by shock interaction with the dense solid particle interfaces near the charge surface, followed by early jet growth through transverse particle motion from the interaction of shocked and turbulent wake flow around the particles. From the exper...

CYLINDRICAL EXPLOSIVE DISPERSAL OF METAL PARTICLES

The explosive dispersal of densely‐packed metal particles in cylindrical RDX‐based charges is studied numerically in support of experimental trials. Simulations are conducted using a reactive multiphase fluid dynamic code. Spherical tungsten particles are applied in high metal mass fraction cylindrical and spherical charges in two configurations: a particle matrix uniformly embedded in a solid explosive versus an annular shell of particles surrounding a high‐explosive core. The effect of particle number density is investigated by varying the nominal particle diameter from 27 to 120 □m while maintaining a constant metal mass fraction. Results are compared with steel particles to evaluate the influence of material density on dispersal. The dispersal dynamics are recorded on wave diagrams and are observed at radial locations in terms of arrival time, velocity and particle concentration.

Air blast characteristics of laminated al and NI-AL casings

Air blast characteristics of Al and Ni-Al laminated materials were experimentally investigated in a 23 m3 closed chamber. Ni and Al foils, 50 to 100 micrometers in thickness, were rolled and compacted to form a cylindrical casing with a density of 95% TMD through an explosive formation technique. Charges were prepared using 2 kg C4 explosive packed in the laminated casing to a metal-explosive mass ratio of 1.75. The blast pressure history measured on the chamber wall showed a double-shock front structure with a precursor shock followed by the primary blast. The front peak pressure for the Ni-Al cased charge reaches 1.5-2 times that of the Al cased, consistent with the larger fireball recorded for the Ni-Al cased. The long time quasi-static explosion pressure (QSP) from the Ni- Al cased charge is 0.8 of that of the Al cased, due to half of Al mass in the Ni-Al.

A RELATION FOR FRAGMENT SIZES OF EXPLOSIVE LOADED ALUMINUM CYLINDERS

It is well established that the fragment size from explosively‐loaded metal cylinders can be related to the casing expansion rate driven by high pressure explosive detonation products. During expansion, high strain rates alter material properties, and failure mechanisms influencing the fragment size distribution become dependent on dynamic phenomena such as localized shear banding. The purpose of this paper is to extend known relations for steel fragment size to aluminum cylinders. The present approach utilizes high strain rate properties of aluminum with energy‐based dynamic fragmentation theory to determine the mean fragment size, and a Mott statistical distribution to account for randomness in the natural fragmentation process. To determine the strain rate, the explosive detonation and cylinder expansion are modeled using a two‐way coupled CFD/FEA routine. Further analysis of a series of modeling results is shown to evaluate relationships for aluminum fragment size as a function of detonation energy an...

IMAGING HIGH SPEED PARTICLES IN EXPLOSIVE DRIVEN BLAST WAVES

This research describes a new application of a commercially available particle image velocimetry (PIV) instrument adapted for imaging particles in a blast wave. Powder was dispersed through the PIV light sheet using a right circular cylindrical charge containing aluminum powder filled in the annular space between the explosive core and exterior paper tube wall of the charge. Images acquired from each shot showed particle agglomeration and unique structures with the smaller particle diameters having developed structured appearances.

An Assessment Capability for LNG Leaks in Complex Environments

Pollutants may be introduced into urban or marine settings by various means and could result in an adverse impact to public safety and the environment. Therefore, it is important for emergency management personnel to understand the potential risks and physical extents of a leaked substance, whether it is toxic, flammable or explosive. Traditional tools for predicting the atmospheric dispersion of leaked substances are quick and simple to use, but may not adequately consider the effects of the built environment that includes complex urban and terrain geometries. Alternatively, CFD methods have been increasing in application; although, their superior accuracy is met with commensurate manual effort. The All Hazards Planner is a fast, accurate gas dispersion modelling tool for city and port environments, which employs a full-physics CFD approach but automates the intensive manual effort. In this work, a credible LNG leak from a 12-mm-diameter hole is modelled for two hypothetical case studies: adjacent to an LNG tanker and between a cruise ship and pier during bunkering. The LNG vapour flammability extents are compared to an empirical model in the absence of geometry effects and are contrasted with geometry effects to highlight the importance of the real environment. The free-field extents are invariant, whereas the inclusion of geometry is shown to reduce the flammability extents by spreading at the ground-level and forcing the plume upwards.