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Hot surface ignition of combustible gas mixtures poses a safety threat in many engineering applications. Gaseous mixtures ignite when hot surface temperature reaches the ignition threshold. In the present work, two-dimensional numerical simulations with detailed reaction mechanism are performed to simulate the flow of stoichiometric hydrogen-air mixture over a stationary hot wire. The effect of heating rates, wire diameters, mixture inlet velocities, and mixture equivalence ratios on the ignition threshold is investigated. In all the cases investigated, ignition is found to occur at the rear stagnation point, and wire heating rate did not influence the ignition phenomenon significantly. With an increase in mixture inlet velocity and mixture equivalence ratio, the ignition threshold increases, whereas the threshold has been found to decrease with increasing wire diameters. The role of the local equivalence ratios at the ignition point and reaction rates prior to the ignition process has been studied to help better understand the ignition phenomenon under different conditions. • Wire heating rate does not influence the ignition threshold significantly. • The ignition threshold values increased with an increase in mixture inlet velocity. • With an increase in wire diameter, the ignition threshold value decreases. • At lower velocities in the range of 0.1 m/s, buoyancy forces also affect the flow. [ABSTRACT FROM AUTHOR]

A multiple (from 20 to 3 kV) decrease in the ignition threshold of commercial flash lamps for solid-state lasers under HF pumping at a megahertz frequency has been detected. It has been found that, upon water cooling of the lamp, the cathode glow is developed during HF pumping on both electrodes, in contrast to the pulsed ignition for the capacitance discharge, since both electrodes alternately act as the anode and cathode. At the stage of ignition development, the Faraday dark space is observed in the middle part of the lamp and its size decreases with the increasing HF pumping voltage until the lamp is ignited by a solitary streamer between the electrodes. It has been established that the main factors that determine a decrease in the full lamp ignition threshold are the high-frequency field amplitude and the coolant impedance, as well as the ignition range. The HF ignition of the lamp with a discharging gap of up to 60 cm (an ignition threshold of ~5 kV) has shown the universality of the developed ignition mode. [ABSTRACT FROM AUTHOR]

The ignition threshold of an energetic material (EM) quantifies the macroscopic conditions for the onset of self-sustaining chemical reactions. The threshold is an important theoretical and practical measure of material attributes that relate to safety and reliability. Historically, the thresholds are measured experimentally. Here, we present a new Lagrangian computational framework for establishing the probabilistic ignition thresholds of heterogeneous EM out of the evolutions of coupled mechanical-thermal-chemical processes using mesoscale simulations. The simulations explicitly account for microstructural heterogeneities, constituent properties, and interfacial processes and capture processes responsible for the development of material damage and the formation of hotspots in which chemical reactions initiate. The specific mechanisms tracked include viscoelasticity, viscoplasticity, fracture, post-fracture contact, frictional heating, heat conduction, reactive chemical heating, gaseous product generation, and convective heat transfer. To determine the ignition threshold, the minimum macroscopic loading required to achieve self-sustaining chemical reactions with a rate of reactive heat generation exceeding the rate of heat loss due to conduction and other dissipative mechanisms is determined. Probabilistic quantification of the processes and the thresholds are obtained via the use of statistically equivalent microstructure sample sets (SEMSS). The predictions are in agreement with available experimental data. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Contributions of plasticity and friction to hotspots in PBX9501 are analyzed. • Overall, dissipation due to plasticity outweighs frictional dissipation. • Frictional dissipation is localized and very important in development of hotspots. • Constituent plasticity is much less localized and very important. • Increased constituent plasticity results in lower ignition sensitivity. Abstract The ignition of energetic materials (EM) under dynamic loading is mainly controlled by localized temperature spikes known as hotspots. Hotspots occur due to several dissipation mechanisms, including viscoplasticity, viscoelasticity, and internal friction along crack surfaces. To analyze the contributions of these mechanisms, we quantify the ignition probability, energy dissipation, damage evolution, and hotspot characteristics of polymer-bonded explosives (PBXs) with various levels of constituent plasticity of the energetic phase and internal crack face friction. Using PBX9501 consisting of HMX (Octahydro-1,3,5,7-Tetranitro-1,2,3,5-Tetrazocine) and Estane as a reference material, we analyze variants of this material with several values of the yield stress of the energetic phase and coefficients of internal crack face friction, while other parameters are kept unchanged. The impact loading involves piston velocities between 200 and 1200 m/s. The analysis uses a Lagrangian cohesive finite element framework that explicitly accounts for finite-strain elastic-viscoplastic deformation of the grains, viscoelastic deformation of the binder, arbitrary crack initiation and propagation in the grains and the binder, debonding between the grains and the binder, contact between internal surfaces, friction and frictional heating along internal surfaces, heat generation resulting from inelastic bulk deformation, and heat conduction. To determine the ignition status of the material or "go" or "no-go" state, we use a criterion based on a criticality threshold obtained from chemical kinetics calculations. For PBX with various levels of HMX plasticity and friction, the probability of ignition, the evolution of dissipation caused by plasticity and friction, the density of cracks, and the locations of cracks are quantified. Results show that samples with higher levels of constituent plasticity (lower yield strengths) or lower levels of internal friction are less likely to ignite. The relative importance of plasticity and friction depends on load intensity, with frictional heating decreasing as load intensity increases. Although the overall viscoplastic heating outweighs the overall frictional heating, friction plays a very important role in hotspot development at all load intensities analyzed, owing to the fact that frictional heating is more localized than viscoplastic heating. The predicted thresholds and ignition probabilities are expressed in a load intensity-load duration relation for PBX with different constituent properties. [ABSTRACT FROM AUTHOR]

In this work, the {210} facets of cyclotrimethylenetrinitramine (RDX) single crystals with different quality were studied by scanning electron microscopy and atomic force microscopy. Their laser sensitivity was then assessed using a direct laser ignition test irradiated with ultraviolet laser (wavelength: 355 nm, pulse width: 6.4 ns). Quantitative relationships between laser sensitivity and surface defects of RDX (210) and ( 2 ¯ 1 ¯ 0 ) facets were investigated. It is determined that the laser sensitivity exhibits significant correlation with the surface roughness, size of which is comparable with scales of laser wavelength. 3D FDTD simulations disclose that this relationship can be well explained with light intensity modulation effects induced by micro-defects on the initial plane wave. [ABSTRACT FROM AUTHOR]

Effects of hollow carbon nanospheres on the combustion behavior of Al/Fe 2 O 3 -based thermite sticks are studied in this work. The results from laser ignition experiments confirm that the presence of hollow carbon nanospheres not only can reduce the ignition threshold because of the excellent laser energy absorption capacity, but also play a positive role in heightening the heat transfer efficiency by introducing pores and increasing heat-flow output during the combustion process. The video snapshots indicate that the combustion propagation stages, including ignition, acceleration, and steady combustion, all improved significantly with the suitable inclusion of hollow carbon nanospheres. The modified composites containing 4 wt% hollow carbon nanospheres exhibit a minimum ignition threshold (28 mJ), shorter acceleration duration (76 ms), higher combustion rate (10.10 cm/s) and steady combustion temperature (~2850 K). Furthermore, it exhibits a remarkable enhancement in energy release (537.36 J/g) and pressure output (0.91 Mpa), which are 1.67 and 1.34 times than that of pristine thermite sticks, respectively. Overall, this work demonstrates that the high porosity structure and heat-flow release of thermite sticks facilitate convective heat transfer and eventually improve the combustion behavior, which can provide some reference in the design and optimization of microstructure and composition in micro-energetic devices. [Display omitted] • Hollow carbon nanospheres are first employed into Al/Fe 2 O 3 -based thermite sticks as additives. • Effects of the hollow carbon nanospheres on combustion behavior of thermite sticks are studied. • The modified thermite sticks exhibit the lower ignition threshold. • The introduction of gas-producing additives is beneficial to improve heat transfer capacity. • The modified thermite sticks show the higher burning rate, energy and pressure release, and combustion zone temperature. [ABSTRACT FROM AUTHOR]

The plasma block (piston) with pressure P 1 is generated as a result of the nonlinear (ponderomotive) force in laser–plasma interaction. The plasma block can be used for the ignition of a fusion flame front in a solid density deuterium–tritium (DT) target by compressing the fuel that creates an ion shock propagating with velocity u ion− shock in the inside of a solid DT target. The ignition is achieved by creating an ion shock during the final stages of the implosion. We estimated the effect of an ion shock in solid DT target at an early stage with no compression and at the last stage with compression, where density increases by a factor of solid-state density. According to the theoretical model, a large target with a very thin layer of fuel (high-aspect ratio target) would be ideal to obtain the very strong shocks. Results indicate that the maximum compression even by an infinitely strong single shock can never produce more than four times the initial density of DT fuel. The results reported that the threshold ignition energy in a solid DT target is reduced by a factor of 4. [ABSTRACT FROM AUTHOR]

Abstract: Using the apparatus for the determination of the MIE a wide series of experiments have been carried out in hydrogen/air, ethene/air, propane/air and acetone/air mixtures. The transferred charge as a criterion to judge the ignition potential is determined to verify the thresholds of transferred charge given in the standards. The stored charge in the capacitance before the discharge is compared to the transferred charge in the spark. The correlation of ignition energy and transferred charge is examined and the thresholds of the transferred charge are discussed. The MIE of the above-mentioned mixtures are reviewed taking into account the measurement uncertainty. [Copyright &y& Elsevier]

The energy thresholds and ignition delays of pressed samples of a stoichiometric pyrotechnical mixture of ammonium perchlorate and ultrafine aluminum are studied as functions of the diameter of the laser spot created by a millisecond laser pulse. A clearly expressed size effect is found. A characteristic size determining the boundary between the wide and narrow beams of the laser pulse applied is determined. The influence of optical properties of the examined composition on the behavior of the size dependences is discussed. The relation between the nature of the size effect and the laws of light scattering in the volume of powdered pyrotechnical mixtures is considered. [ABSTRACT FROM AUTHOR]

Initiation of slightly compacted powders of lead azide of density not greater than 1.2 g/cm3 by laser radiation with a wavelength of 1.06 µm was studied experimentally over a wide range of pulse lengths (10−5–10−3 sec). It is shown that the ignition threshold of the slightly compacted samples does not change as the laser pulse length is increased. It is found that the explosion products contain molten lead particles whose sizes are an order of magnitude larger than the sizes of similar particles from explosion of samples of density 4.0 g/cm3. [ABSTRACT FROM AUTHOR]

We have investigated the acceleration of quasi-monoenergetic ion beams. LPIC++ code has been used to simulate the radiation pressure acceleration hole-boring mechanism. To reach the quasi-monoenergetic ion beam used for ionic fast ignition, the beams with low energy spread and suitable peak energy are the best choice. Effective parameters including intensity for laser, thickness, density and charge state for foil have been evaluated to have a stable acceleration. Based on the results, optimal quasi-monoenergetic ion beams of the laser with an intensity of 1.9 × 1021 W / cm2, pure carbon foil with 0. 56 μ m thickness, and density 15 have been selected with the average energy 624 MeV and 720 MeV. Also, by applying the Deira-4 code, the potential of the aforementioned beams with different ignition energies from 8.5-11 kJ has been studied. For the pre-compressed DT fuel capsule with the density 300 g/cm3 and initial temperature 1keV, the ignition conditions have been considered. Subsequently, we found that the ignition threshold conditions for multi-peak beams are more favorable than those of the ideal single-peak beams. It is shown that the ignition energy threshold of multi-peak beams is about 10 kJ, which is more than the ignition energy for the ideal single-peak beams that are close to 7 kJ. [ABSTRACT FROM AUTHOR]