Ignition Parameters Research Articles

On electron transport in fast ignition research and the use of few-cycle PW-range laser pulses

In this contribution, we address laser-driven electron transport in fast ignition research and the need for ultra-fast (<10 fs) pump–probe diagnostics to study, on the microscopic level, current filamentation and related anomalous energy deposition. Rates of instability growth consistent with realistic fast ignition parameters are discussed. A 3D-hybrid-PIC simulation is presented as an illustrative case. The development of 5 fs PW-range laser pulses at MPQ is reported, which may generate ultra-bright VUV-, x-ray, electron and ion beam pulses to be used for ultra-fast plasma probing.

Hot-Spot Ignition of a Reactive Gas in an Inert Porous Medium

The main stages of development of hot-spot ignition of a reactive gas in a high-porous medium with high values of the Peclet number under conditions of natural gas filtration and limited internal heat transfer between the phases are determined. Gas ignition in a U-shaped hot spot is considered within the framework of an asymptotic analysis with high values of the temperature difference and Frank-Kamenetskii parameter. The critical relation of parameters separating the regimes of gas ignition and gradual cooling of the hot spot is determined. The dependence of the ignition time on parameters of the process is found and analyzed. A strong effect of interphase heat transfer on the ignition limit and time is demonstrated. An example of calculating the critical parameters of hot-spot ignition of methane in processed rocks is given.

Influence of ignition parameters on microstructure and microhardness of Zr based alloy prepared by laser induced selfpropagating reaction synthesis

Based on a large negative enthalpy of mixing among major components of bulk metallic glass forming alloy systems, a new technique to fabricate amorphous based composite materials using laser induced selfpropagating reaction synthesis (LSRS) was developed. The LSRS of the Zr55Al10Ni5Cu30 alloy shows that the products mainly consist of the amorphous, Zr3Al2 and Zr2Cu phases. However, the amorphous phase content in the products is closely related to laser power and ignition time: high laser powers and short ignition times increase the content of the amorphous phase. The microhardness of the samples decreases with the increase of the content of the amorphous phase.

Influence of ignition parameters on micropyretic synthesis of NiAl compound

The influence of ignition parameters on micropyretic synthesis of NiAl compound is numerically investigated in this study. The numerical results show that either a lower ignition power or ignition from the middle of specimen increases the ignition time to start the reactions, further transferring the more thermal energy from the ignition spot to the other regions. Thus, the length of pre-heating zone before propagation and the starting propagation velocity at the ignition spot are correspondingly increased. Such increments in the zone length and the starting propagation velocity result in inhomogeneous structures and properties at the ignition spot. The numerical calculation shows that the percentages of the inhomogeneous structures can be reduced by increasing the ignition power. However, a higher ignition power also consumes the additional ignition energy to ignite the specimens. In addition, the effect of the ignition parameters on the length of pre-heating zone is magnified for the systems with higher thermal conductivity of reactants.

Flame retardant composite materials

Flame retardant additives offer a potential short-term solution for reducing the combustibility of composites, and hence the reduction of the associated hazards. A brief review of fire modelling was performed to identify suitable mathematical expressions with which the results of the experimental flame retardant investigation were analysed. These were then used in a limited trial to compare the experimental and calculated ignition parameters. The comparison of simple mathematical equations with fire test results indicated that their ability to reasonably reproduce the experimental ignition parameters of the flame retardant treated composites is dependent on the mechanism of flame retardant activity, particularly the stage of combustion at which it is designed to be active.

Approximate Analytical Method for Calculating the Time Characteristics of Ignition of a Gas Mixture by a Heated Body

Analytical expressions for calculating the time characteristics of ignition of gas mixtures by a heated body are obtained for the first time with the help of the wave theory of ignition. It is shown that ignition can occur in three different modes: kinetic (one‐temperature), diffuse (two‐temperature), and transitional, which combines the properties of both diffusion and kinetic modes. The parametric region of implementation of each possible ignition mode is determined. It is found that the transition from the kinetic to the diffusion mode occurs as the temperature of the heater changes only by one characteristic interval and is accompanied by a jumplike decrease both in the ignition delay and in the amount of energy necessary for combustion initiation. A relation is established between the laws of ignition and diffusion combustion of a single particle and parameters of ignition of the gas mixture of a given sort of particles. It is shown that there is a minimum in the dependences of the time of establishing of the zero gradient of τ0 on Semenov's criterion. The value of τ0 is found to be minimum if the heater temperature is higher than the temperature of ignition of a single particle by one characteristic interval. The numerical solution of the initial system of equation confirms the validity of the basic assumptions and results of the approximate analysis. The error in determining the time characteristics of ignition with the help of approximate formulas is lower than 50%. For the kinetic mode of ignition, a transformation of the time and space scales is found, such that the time characteristics of ignition in new dimensionless variables become independent of the mass concentration of particles.

Comparison of plasma ignition parameters and firing performance with 20and 60-mm electrothermal-chemical guns

Two electrothermal-chemical gun systems of 20- and 60-mm calibers have been operated with different pulse power supplies at 35 and 10 kV, respectively. Dissimilarities of the injected current behavior during plasma ignition by exploding wires are reviewed. The effects of the varied plasma energy and increased loading density of consolidated propellant charges are analyzed with regard to the resulting pressure development. The improvement of the firing performance with the larger caliber still remains less satisfying. However, empirical regression functions have been found that approximate the kinetic muzzle energy as a function of the maximum pressure in the combustion chamber and can be useful for predictions.

Application of the Scott Hydrodynamic Model for Analysis of Results of Impact Testing of High-Explosive Specimens

The solution of the hydrodynamic problem of a non-Newtonian medium flow squeezed by two approaching parallel disks, which was previously obtained by Scott, is used to determine the strain and ignition parameters of explosive specimens in drop tests at the lower threshold of their sensitivity.

Ignition mechanism in combustion synthesis of Ti–Al and Ti–Ni systems

Ignition phenomena in combustion synthesis (CS) of intermetallic compounds, Ti–Al and Ti–Ni systems, were studied. Different compositions for both systems were investigated. The experiments were performed in quasi-adiabatic conditions and carried out in thermal explosion mode. A CO 2 laser was used as external energy source and was set at different powers to investigate the effect of the heating ramps on the ignition parameters. For both intermetallic systems, the ignition times show an exponential decay as a function of the specific power in use. The ignition temperatures were found to be characteristic for each system and constant at 645±5 and 930±10 °C for Ti–Al and Ti–Ni, respectively. A numerical model was developed to estimate the ignition energy. The calculated energy values are 18±1, 19.5±1 and 22.5±1 kJ/mol at for Ti:Al=3:1, 1:1, 1:3, respectively, and 27±0.5 kJ/mol at for all the studied compositions in the Ni–Ti system.

Influence of interface conditions on laser diode ignition of pyrotechnic mixtures: application to the design of an ignition device

This paper treats of numerical modelling which simulates the laser ignition of pyrotechnic mixtures. The computation zone is divided into two fields. The first is used to take account of the heat loss with the outside. It can represent an optical fibre or a sapphire protective porthole. The second field represents the reactive tablet which absorbs the laser diode's beam. A specific feature of the model is that it incorporates a thermal contact resistance R c between the two computation fields. Through knowledge of the thermal, optical and kinetic properties, this code makes it possible to compute the ignition conditions. The latter are defined by the energy E 50 and the time t i of ignition of any pyrotechnic mixture and for various ignition systems. This work was validated in the case of an ignition system consisting of a laser diode with an optical lens re-focussing system. The reactive tablet contains 62% by mass of iron and 38% by mass of KClO 4. Its porosity is 25.8%. After an evaluation of the laser's coefficient of absorption, the variations of the ignition parameters E 50 and t i are studied as a function of the thermal contact resistance R c . Temperature profiles are obtained as a function of time and for various values of the thermal contact resistance R c . More fundamental observations are made concerning the position of the hot spot corresponding to priming. From this study, which concerns the heat exchange between the two media, several practical conclusions are given concerning the design of an ignition device. By evaluation of the thermal contact resistance R c , comparison with test results becomes possible and the results of the computations are in reasonable agreement with the test measurements.

An investigation of ignition behavior in diesel sprays

Ignition behavior in diesel sprays is explored by comparing computed and measured ignition characteristics in diesel sprays injected into a constant-volume chamber. The ignition parameter that are explored include ignition delay times and the location of ignition in the spray. The effect of changes in chamber density and temperature on these parameters is studied. The computations are carried out with a multi-dimensional model for diesel sprays. Heplane is used to represent the fuel employed in the measurements. The chemical kinetics of heptane is incorporated through an interactive flamelet model. The model is able to reproduce the trends in measured ignition delay times qualitatively. Improved quantitative agreement is achieved in cases where the ignition delay period is relatively longer. The quantitative disagreement is believed to be less important from the point of view of predicted overall heat release rates when the ignition delays are shorter. Ignition is prediced to occur closer to the orifice as chamber density is increased and also as chamber temperature is increased. Consistent with the measurement results, ignition in the spray is shown to be a two-stage process. Ignition is predicted to occur in the rich mixture downstream of the maximum liquid penetration length where the equivalence ratio is about 3.0. As the chamber density and temperature increase, the location of ignition moves upstream in the spray and the corresponding equivalence ratio decreases.

508 水素および天然ガス噴流の着火と燃焼に及ぼす点火条件の影響

508 水素および天然ガス噴流の着火と燃焼に及ぼす点火条件の影響

A new ignition criterion based upon a critical thermal energy density

It has long been known that various ignition criteria of energetic materials have been limited in applicability to small regions. In order to explore the physical nature of ignition, we calculated how much thermal energy per unit mass of energetic materials was absorbed under different external stimuli. Hence, data of several typical sensitivity tests were analyzed by order of magnitude estimation. Then a new concept on critical thermal energy density was formulated. Meanwhile, the chemical nature of ignition was probed into by chemical kinetics. Finally, it was considered that perhaps the critical thermal energy density was an important characteristic parameter for energetic material ignition.

Analysis of recycled thermoplasts from consumer electronics by laser-induced plasma spectroscopy

An experimental setup was established for on-line analysis of recycled thermoplasts from consumer electronics by laser-induced plasma spectroscopy (LIPS) along with the optimum experimental parameters. Via an experimental design, the optimum wavelength for plasma ignition and best parameters for a time-resolved detection, i.e. delay of the integration relative to the plasma ignition, gate width, and irradiance were determined. The employed echelle spectrograph allowed a fast multielement analysis due to the superior spectral coverage (200–780 nm) and spectral resolution ( λ/Δ λ>10,000 at 250 nm). Linear calibration plots were obtained by normalization to carbon as internal standard, the limits of detection are in the parts per million range and sufficient for the envisioned on-line application.

Hydrodynamique de l'implosion d'une cible FCI

We consider the hydrodynamic behaviour of an imploding ICF target. After recalling the requirements for thermonuclear ignition, we analyse in detail the two phases of the implosion. First, the acceleration, with its two important ignition parameters, the implosion velocity and the entropy generated in the DT. Second, the slowing down which, through electronic conduction, allows the creation of a central hot spot of sufficient mass. The method of successive shocks allows the détermination of the laser pulse shape which, by insuring an isentropic acceleration, optimises the energy delivered to the DT. We obtain the implosion velocity as a function of the initial capsule parameters and the maximum incident radiation. We clarify the hydrodynamical reason which makes the ignition threshold sensitive to the entropy generated during the acceleration. These elements constitute the basis of a global indirect drive ICF model, allowing target design optimisation.

Characteristics of electrodeless ferrite-free fluorescent lamp operated atfrequencies of 1-15 MHz

A RF electrodeless ferrite-free fluorescent lamp with a re-entrycavity was studied at driving frequencies, f = 1.2-15.0 MHz and RFpower up to 80 W. Induction coils with different numbers of turns, from 6 to26, were used to ignite and maintain the inductively coupled RF dischargein the lamp bulb. The lamp ignition and operational parameters (such ascoil starting and maintaining voltages and currents, lamp impedance andresistance, starting power and coil power losses) were directly measured.The transformer model of an RF inductive discharge was applied to calculatethe discharge parameters: plasma resistance and current, plasma effectiveRF electric field and plasma Q-factor. The results of the calculations arein good agreement with the experimental data published elsewhere. Due tolow power losses in the induction coil with large number of turns (26), thepower efficiency of the lamp at the frequency of ≃2.65 MHz washigh, 92%, which provided high lamp efficacy of 83 lumen perwatt (LPW) at a RF power of 60 W.

Combustion of kerosene in a supersonic stream

Experiments on the organization of the combustion of kerosene in high-enthalpy supersonic air streams is analyzed. The use of promotor additives, as well as improvements in the atomization process, vaporization, and mixing, do not always facilitate efficient combustion development. The existence of a conversion process is found to have a significant effect on the ignition parameters. The burnup intensity can be ensured by adding hydrogen, and the relative position of the fuel injectors is important in that case. The fundamental role of wave structures in determining the length of the combustion zone in the channel is noted. The integral characteristics of combustion for hydrogen and kerosene are compared.

Explosive Materials Combustion by Heated Wires

The knowledge of ignition parameters of explosive materials (EM) presents both the definite scientific interest for developing the ignition kinetics models and the practical interest from the point of view of their danger assessment. The present investigations, as opposed to the known technique for EMs ignition temperature determination, have been performed by using the model explosive material samples of high density which have been produced on the basis of HMX and TATB. Applying the technique of firing ballistic powders by a heated wire, the EM ignition temperature depending on the time (rate) of heating has been investigated. The technique makes it possible to calculate heat pulses and heat flows leading to ignition. By decreasing the heat flow, the time for the EM heating up to ignition increases and temperature falls thereby approaching the critical value, characterising the danger limit under accidents associated with heating. The ignition of EM based on HMX and TATB takes place in a different manner. With the EM on the basis of HMX and with great heat flows. The ignition beginning from the surface in the form of flash is typical but when achieving the critical parameters, the heated layer flash takes place that increases the probability of the explosion realisation. EM based on TATH always ignite in the form of combustion from the surface, independent of the heat flow that points to the higher extent of its safety. These data correlate well with the higher parameters of its ignition.

Effect of Inerts on Layer Ignition Temperatures of Coal Dust

An experimental study into the hot surface ignition of coal dust layers has been conducted. Two coals were examined: Prince coal from the Cape Breton Development Corporation and Pittsburgh coal from the United States Bureau of Mines. The effect of admixed inerts (dolomite and limestone) on the dust layer ignition temperature has been analyzed using a steady-state thermal explosion model. The analytical procedure used for evaluating the ignition temperature of a dust layer, heated from below and losing heat from its upper surface by convection, is an extension of the thermal explosion model of Thomas (Ref. [8]); namely, that of Thomas and Bowes (Ref. [15]). To commission the hot plate apparatus and validate the model predictions, a series of experiments were undertaken using sodium dithionite. This material is known to exhibit self-heating and there have been previous layer ignition temperature studies with which to compare results. It was demonstrated that an adequate estimate of the critical ignition temperature may be readily obtained by this analytical method. Furthermore, computed values of the critical ignition parameters for layers of coal dust admixed with inerts, accounting for changes in thermal conductivity, were in reasonable agreement with experimentally determined values.

Leaf flammability in Mediterranean species

ABSTRACT We studied the flammability of leaves of 17 Mediterranean species collected over a period of 12 months on the promontory of Monte Argentario (Tuscany Region, Italy). The reaction of the leaves to heat varied both in terms of percentage of flammability and ignition parameters (temperature and time). No correlation was found between moisture content of the leaf tissue and ignition parameters. A hierarchy of the species based on heat resistance was created, also including those leaves that caught fire sporadically.