Ignition Thresholds and Flame Propagation of Methane/Air Mixtures Ignited via Radiatively Heated Inert Particles

The prevention and evaluation of explosions requires suitable standards of measurement. As such, for this study two ignition thresholds, the ignition temperature and the minimum ignition irradiance were selected as the assessment criteria. These ignition threshold values were experimentally determined by heating stationary inert silicon carbide particles via thermal radiation with a large spot size in order to ignite quiescent methane-air fuel mixtures. A high-speed Schlieren camera was used to capture the progression of the formation and propagation of the flames throughout the experiments. The results of the experiments show that the irradiance and temperature threshold are directly and inversely proportional to the particle size, respectively. Furthermore, the irradiance and temperature thresholds have similar tendencies within the flammability limits; wherein, the minimum value corresponds to fuel mixtures at a stoichiometric ratio, and increases as the equivalence ratio shifts toward the flammability limits. Irradiance thresholds, though, are more sensitive to changes in equivalence ratio than temperature. The temperature histories of the heated particle determined that when the irradiance is lower than its ignition threshold value, the heated particle-fuel mixture system will arrive at a thermal equilibrium, rather than ignition, due to the inability of the particle to reach the ignition temperature. This study also found that longer ignition times will result in a more drastic deformation of the flame fronts caused by natural convection.

Transient Ignition of Premixed Methane/Air Mixtures by a Pre-chamber Hot Jet: a DNS Study

The present study investigates the transient processes controlling ignition by a hot jet issued from a pre-chamber. Direct numerical simulations (DNS) have been performed to study the characteristics of the turbulent jet flow and of the associated flame during the whole ignition process, quantifying the relevant physicochemical interactions between pre-chamber and main chamber. Thanks to a detailed analysis of the DNS results, the transient ignition is found to consist of three main sequential processes: (1) near-orifice local ignition in the main chamber; (2) further flame development supported by the jet flow; and (3) global ignition and propagation of a self-sustained flame in the main chamber, independently from the hot jet. The characteristic time-scale of the hot jet as well as jet-induced effects (local enrichment, supply of radicals and heat) are found to be essential for successful ignition in the main chamber. A more intense turbulence in the main chamber appears to support local ignition. However, it also induces local quenching, thus delaying global ignition. An ignition threshold based on a critical Damköhler number is a promising concept, but is not sufficient to describe the process in all its complexity.

Prediction of Probabilistic Shock Initiation Thresholds of Energetic Materials through Evolution of Thermal-mechanical Dissipation and Reactive Heating

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 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 samples sets (SEMSS). The predictions are in agreement with available experimental data.

Robust and ultralow-energy-threshold ignition of a lean mixture by an ultrashort-pulsed laser in the filamentation regime

Laser ignition (LI) allows for precise manipulation of ignition timing and location and is promising for green combustion of automobile and rocket engines and aero-turbines under lean-fuel conditions with improved emission efficiency; however, achieving completely effective and reliable ignition is still a challenge. Here, we report the realization of igniting a lean methane/air mixture with a 100% success rate by an ultrashort femtosecond laser, which has long been regarded as an unsuitable fuel ignition source. We demonstrate that the minimum ignition energy can decrease to the sub-mJ level depending on the laser filamentation formation, and reveal that the resultant early OH radical yield significantly increases as the laser energy reaches the ignition threshold, showing a clear boundary for misfire and fire cases. Potential mechanisms for robust ultrashort LI are the filamentation-induced heating effect followed by exothermal chemical reactions, in combination with the line ignition effect along the filament. Our results pave the way toward robust and efficient ignition of lean-fuel engines by ultrashort-pulsed lasers.

INVESTIGATION OF IGNITION OF H2-O2, CH4-O2, AND C10H22-AIR MIXTURES DURING PHOTODISSOCIATION OF O2 MOLECULES BY LASER RADIATION

The subject of research is the features of ignition, threshold radiation energies required for ignition, and their dependence on the composition, pressure and temperature of mixtures, as well as the times of development and propagation of ignition in the working mixture.

Source of megaampere current with the rise time ∼100 ns on the basis of explosive magnetic generator

To achieve a thermonuclear ignition threshold in the scheme of indirect irradiation of Z‑pinch by X‑radiation, it is necessary to implode the liner by the current with the amplitude 65 МА for the time 100 ns. The currents with such parameters can be achieved with the use of super-power disk explosive magnetic generators and a two-stage current pulse sharpening system based on foil electrically exploded current opening switches in a form of a serpentine. The implementation of the explosive current source with a rise time of 100 ns is advisable to be carried out in stages by increasing the magnitude of current. The results of the first-stage experiments, in which the current with the amplitude of 5 MA was produced on the basis of the helical explosive magnetic generator in the load of 10 nH for the time of 110 ns, are presented.

Thermal resonance effect by a strong shock wave in D–T fuel side-on ignition by laser-driven block acceleration

Ignition with the help of a shock wave is performed by the interaction of accelerated plasma block by a petawatt-picosecond (PW-ps) laser, with a solid-state density fuel that it is a new possibility for achieving controlled fusion by inertial confinement. The unexpected production of plasma blocks provides new access to the ignition of solid-state density fuel according to the Chu hydrodynamic model. When the produced plasma block by the PW-ps laser hits the main fuel due to the density differences between the plasma block and the main fuel of the shock wave, this progressive wave increases the density of solidified fuels and reduces the energy of the ignition threshold and increases the flammability. In this study, a new discovery of shock waves has been observed leading to the resonance phenomenon. Nuclear heat shock waves resonance in the side-on ignition of fuel in the internal layer of fuel at x ≠ 0 appears from the exact solution of the hydrodynamic equations with respect to the density profile. This important finding achieves the required ignition temperature for solid-state fuel deuterium–tritium (D–T) in certain energies, with a significant increase due to the resonance of thermonuclear waves. This discovery will facilitate practical experiments on the ignition of advanced solid-state fuels with the accelerated plasma blocks by a PW-ps laser at certain energies.