Vol 53, No 3 (2017)

The effect of a focused pulsed-periodic beam of a CO₂ laser on initiation and evolution of combustion in subsonic and supersonic flows of homogeneous fuel–air mixtures (H₂ + air and CH₄ + air) is experimentally studied. The beam generated by the CO₂ laser propagates across the flow and is focused by a lens at the jet axis. The flow structure is determined by a schlieren system with a slot and a plane knife aligned in the streamwise direction. The image is recorded by a high-speed camera with an exposure time of 1.5 μs and a frame frequency of 1000 s⁻¹. The structure of the combustion region is studied by an example of inherent luminescence of the flame at the wavelengths of OH and CH radicals. The distribution of the emission intensity of the mixture components in the optical discharge region is investigated in the present experiments by methods of emission spectroscopy.

The effects of silane temperature and ambient moisture on the ignition behavior are considered. The critical velocity for delayed ignition is determined for different silane temperatures and moisture contents in ambient air. The logarithm of the critical exit velocity is found to be inversely proportional to silane temperature. It is also observed that moisture in air has a strong inhibiting effect on silane autoignition in air. From a safety perspective, it is concluded that prompt ignition of silane is favored in a high-temperature and low-humidity environment.

A performance analysis and experimental study of a hybrid gas generator to be used in a ducted rocket are presented. Such a system exhibits potential advantages with regard to safety, performance, costs, availability of the fuel components, storability, and thrust control. A combination of a paraffin wax fuel and oxygen in the gas generator ensures a high regression rate and reveals oxidizer-to-fuel ratios as low as 0.14 in the gas generator (compared to the stoichiometric ratio of 3.4). A fuel regression rate correlation versus the oxidizer mass flux is derived, presenting a major advantage for the fuel flow rate management in comparison to control of the solid propellant gas generator burning rate through the pressure exponent, which requires mechanical interference with the hot nozzle flow to ensure a change in the combustor pressure and a corresponding change in the burning rate. Evaluation of the ducted rocket (with different oxidizers) versus pure ramjet performance shows a higher specific thrust for the former, though the latter exhibits a higher specific impulse.

This paper presents the results of experimental studies of the shock compression of samples of vanadium deuterides and hydrides of the following compositions: VX_0.51, VX_0.7–0.9, and VX_≥1.6, where X is H or D. The experiments were carried out in the pressure range 20–140 GPa. The technology of synthesizing samples using electrolytic vanadium of not less than 99.7% purity. The Hugoniots of vanadium deuterides and hydrides were determined using the well-known reflection method. The samples were compressed using shock-wave generators based on the use of explosive charges of different power. The obtained experimental data are described by an equation of state developed using a model in which the specific heats and Gr¨uneisen ratios of ions and electrons are functions of density and temperature. At low temperature, the specific heat changes in accordance with the Debye theory. The removal of the degeneracy of the electron gas at higher temperatures is considered. The influence of ionization processes on the thermodynamic functions is effectively taken into account.

Coal particles when subjected to shock waves can undergo rapid fragmentation, pyrolysis, and combustion, causing enhanced process intensity and efficiency. Particle fragmentation plays a crucial role in this process. Exposure of coal particles to a shock wave is modelled in the present work as combined convection and radiation at the surface and conduction in the interior. Local temperatures within a coal particle and the corresponding thermal stresses are computed to study particle failure. Particle fracture is modelled by a three-parameter Weibull probability to predict the failure location and time. Simulations indicate that pulverized coal of size up to 250 μm subjected to a shock wave for varying operational, thermal, and physical parameters can experience initial failure within 150 μs. Particles of size d ≥ 50 μm or higher wave strengths (with Mach numbers M ≥ 5) mostly trigger exfoliation, while interior fragmentation dominates at smaller sizes (d ≤ 25 μm). An initial fracture study reveals that pulverized coal with predominant sizes d ≤ 100 μm and the coal rank from lignite to bituminous coal is potentially suitable for detonation combustion in waves at Mach numbers M = 3–6. Coal particles under continuous exposure to post-shock conditions undergo recursive exfoliation until the core is 20–40 μm, after which an interior fragmentation phase is seen until the core is about 1–3 μm. Much finer coal particles, of the order of internal fragmenting cores, are hardly fractured due to low thermal stresses caused by rapid uniform heating. The fracture model approach for studying shock-induced combustion is validated by a reasonable match of the computed ignition delay with experiments. The fragmentation history indicates a substantial increase in the particle surface area and temperature under shock exposure, as against conventional combustion, leading to an increased order of the burning rates at the onset of ignition, which can sustain through the entire burning phase.

Reactive materials are a new class of energetic materials that extremely and efficiently release energy under the influence of high impact loading. An impact tester is used in the present study to explore the impact ignition characteristics of Al/PTFE reactive materials and the impact ignition pressure of Al/PTFE reaction materials under different conditions. The experimental result shows that the critical ignition pressure is approximately 1.44 GPa. Meanwhile, it also shows that the material compactness has a much less pronounced effect on the impact ignition pressure for this reactive material if the loading time scales are of the order of several milliseconds. Plastic work and viscous heat both play a significant role in impact ignition. Finally, it is shown that impact ignition ensures a higher energy release rate than surface ignition.

This paper describes the measurement of the detonation velocities close to ideal velocity relative to large charges of highly dispersed ammonium perchlorate (AP) and its mixtures with different explosive substances in thick-walled steel pipes. The relationship of the detonation velocity of AP with its density and the relationship between the detonation velocity of mixtures with the component ratios and oxygen coefficient of the mixtures are determined. The calculation of the detonation velocity of AP/explosive/Al three-component compositions is proposed for the first time.

In this paper, the commercial finite element code LS-DYNA is employed to simulate the process of a certain kind of a linear shaped charge jet penetrating into a TC4 (Ti–6Al–4V) titanium alloy plate of moderate thickness. The fracture profiles agree well with experimental observations, which confirms the validity of the code and the Johnson–Cook material model applied to describe the TC4 plate. The fracture pattern of the plate is drawn based on this comparison.