Critical Ignition Research Articles

A Comparative Study on the Mathematic Models for the Ignition of Titanium Alloy in Oxygen-Enriched Environment

Metallic materials are always suffered from the risk of combustion when serviced under some extreme conditions such as high temperature, oxygen-enriched enrichment, and high-speed friction. Although different mathematic models have been proposed but it is still a challenge for accurately describing the ignition conditions of metals under the extreme conditions, which is of great significance for the safety-use of materials. In this paper, the mathematic models based on Semenov and Frank-Kamenetskii theory were introduced into describing the effects of size, oxygen concentration, and oxygen pressure on the ignition temperature and critical oxygen pressure of TC17 alloy. The results showed that the critical oxygen pressure of TC17 alloy increased with the increase of size, which was fitted well with the Frank-Kamenetskii model. As a comparison, the critical oxygen pressure was size independent in the Semenov model, which was inconsistent with the experimental data. For the Frank-Kamenetskii model, the fitting results showed that the activation energy, reaction order, as well as the adsorption coefficient of TC17 alloy were determined to be 99.23 kJ/mol, 1.69, and 4.01 MPa−1.69 respectively. Based on above, the ignition temperature of TC17 samples with different sizes were predicted well by the Frank-Kamenetskii model with the relative error within 3.58%, which could be suitable for describing the critical ignition conditions of bulk metallic materials under complex environment.

Development of a model for the prediction of the ignition properties of combustible dusts undergoing homogeneous combustion: Application to sulfur dust

The determination of ignition properties such as the minimum explosible concentration (MEC) is critical for the control of the risks associated with handling combustible dust. This work describes a novel approach for the determination of the MEC of combustible dusts undergoing homogeneous combustion, using sulfur dust as a case study. The proposed model is developed based on the heat and mass transfer occurring when a sulfur dust particle in a control volume is exposed to a hot planar surface acting as an ignition source. The model includes radiative and convective heat transfer from the ignition source to the particle and predicts the dust particle’s fusion and vaporization. Ignition criteria of sulfur vapor accumulation in the control volume are based on the material’s lower flammability limit (LFL) and the comparison of accumulated energy in the control volume to the activation energy of the vapor-air combustion initiation reaction. The model also includes a correlation describing a time to ignition which accounts for necessary dynamic changes like the exposure time of the dust to the ignition source. The model is capable of predicting the influence of the dust particle diameter on the MEC of sulfur dust and was validated with experimental data.

Damage-ignition mechanism studies on modified propellant with different crosslinking density under dynamic loading

The study of high-energy and low-vulnerability propellants is important for the power performance and safety of solid propellant rocket motors. The modified split Hopkinson pressure bar (SHPB) tests are performed on two kinds of propellant with different crosslinking density to study the dynamic mechanical responses and damage-ignition mechanism. SHPB apparatus is equipped with a high-performance infrared camera and high-speed camera to capture the deformation, damage-ignition feature and temperature evolution images in the impact process. The results suggested that the mechanical responses and damage-ignition mechanism of the propellants were affected by the strain rates and crosslinking density. The damage-ignition degree is more intense and the reaction occurs earlier with the increase of strain rates. For propellant 1 with higher crosslinking density, the critical ignition strain rate is 4500 s−1. Two kinds of propellants show different ignition mechanism, i.e. crack generation, propagation and final fracture for propellant 1 while viscous shear flow for propellant 2. Meanwhile, the SEM images also reveal the difference of damage-ignition mechanism of the two kinds of propellants. Finally, the ignition mechanism under different strain rates and critical ignition strain rate of propellants are further explained by the theoretical calculation of temperature variations.

The relationship between ignition and oxidation of molten magnesium alloys during the cooling process.

Ignition of magnesium alloys during casting processes limits their processability and applications. For identifying the ignition mechanism of magnesium alloys during solidification, a Mg-Al-Zn alloy was solidified with different cooling rates and pouring temperatures. The oxide scale morphologies and thicknesses were identified by SEM and energy dispersive spectrometer. Based on the experimental results, the oxidation kinetics and heat released were calculated and the relationship between oxidation and ignition was discussed in detail. The calculation results indicate that oxide rupture directly induces combustion of the melt. The rupture route of the oxide scale was determined to be buckling cracks according to the experimental and calculation results. Based on the buckling mechanism of the oxide scale, the ignition criterion during solidification was correlated to the pouring temperature, cooling rate and casting modulus. This work reveals the underlying relationship between ignition and casting process parameters, and it helps to develop new technology for inhibiting ignition of molten magnesium alloys.

Experimental achievement and signatures of ignition at the National Ignition Facility.

An inertial fusion implosion on the National Ignition Facility, conducted on August 8, 2021 (N210808), recently produced more than a megajoule of fusion yield and passed Lawson's criterion for ignition [Phys. Rev. Lett. 129, 075001 (2022)10.1103/PhysRevLett.129.075001]. We describe the experimental improvements that enabled N210808 and present the first experimental measurements from an igniting plasma in the laboratory. Ignition metrics like the product of hot-spot energy and pressure squared, in the absence of self-heating, increased by ∼35%, leading to record values and an enhancement from previous experiments in the hot-spot energy (∼3×), pressure (∼2×), and mass (∼2×). These results are consistent with self-heating dominating other power balance terms. The burn rate increases by an order of magnitude after peak compression, and the hot-spot conditions show clear evidence for burn propagation into the dense fuel surrounding the hot spot. These novel dynamics and thermodynamic properties have never been observed on prior inertial fusion experiments.

Design of an inertial fusion experiment exceeding the Lawson criterion for ignition.

We present the design of the first igniting fusion plasma in the laboratory by Lawson's criterion that produced 1.37MJ of fusion energy, Hybrid-E experiment N210808 (August 8, 2021) [Phys. Rev. Lett. 129, 075001 (2022)10.1103/PhysRevLett.129.075001]. This design uses the indirect drive inertial confinement fusion approach to heat and compress a central "hot spot" of deuterium-tritium (DT) fuel using a surrounding dense DT fuel piston. Ignition occurs when the heating from absorption of α particles created in the fusion process overcomes the loss mechanisms in the system for a duration of time. This letter describes key design changes which enabled a ∼3-6× increase in an ignition figureof merit (generalized Lawson criterion) [Phys. Plasmas 28, 022704 (2021)1070-664X10.1063/5.0035583, Phys. Plasmas 25, 122704 (2018)1070-664X10.1063/1.5049595]) and an eightfold increase in fusion energy output compared to predecessor experiments. We present simulations of the hot-spot conditions for experiment N210808 that show fundamentally different behavior compared to predecessor experiments and simulated metrics that are consistent with N210808 reaching for the first time in the laboratory "ignition."

Theoretical Model for the Impact-Initiated Chemical Reaction of Al/PTFE Reactive Material.

Reactive material (RM) is a special kind of energetic material that can react and release chemical energy under highly dynamic loads. However, its energy release behavior is limited by its own strength, showing unique unsustainable characteristics, which lack a theoretical description. In this paper, an impact-initiated chemical reaction model is proposed to describe the ignition and energy release behavior of Al/PTFE RM. The hotspot formation mechanism of pore collapse was first introduced to describe the decomposition process of PTFE. Material fragmentation and PTFE decomposition were used as ignition criteria. Then the reaction rate of the decomposition product with aluminum was calculated according to the gas-solid chemical reaction model. Finally, the reaction states of RM calculated by the model are compared and qualitatively consistent with the experimental results. The model provides insight into the thermal-mechanical-chemical responses and references for the numerical simulation of impact ignition and energy release behavior of RM.

Study of alpha particle concentration effects as the ash of deuterium-tritium fusion reaction on ignition criteria

In this study, the ignition criteria in the International Thermonuclear Experimental Reactor (ITER) associated with helium concentration is investigated. The main fuels used in this study are deuterium-tritium. Helium as the ash of deuterium-tritium reaction cannot be avoided. In this work, the effects of alpha particle concentration on plasma operation are investigated. The calculations were performed by using zero-dimensional power and particle balance equations. The results of our calculations show that the temporal evolution of alpha particle concentration becomes constant at 0.14. Then, ignition criteria and saddle points have been studied in n e τ E −T, and −T planes for different alpha particle concentrations.

Influences of HMX contents on potential reaction violence and fragmentation degree of PBT-propellants after impact load

The extent of damage that is developed during the production, transportation, storage, and handling of propellants after a low-velocity impact is an important factor in evaluating a dangerous threat such as deflagration-to-detonation transition. In this paper, a friability test was performed to evaluate the critical impact ignition velocity (CIIV), fragmentation degree, and potential reaction violence of four 3, 3-Bis(azidomethyl)oxetane (BAMO) and tetrahydrofuran (THF) copolymer-propellants (PBT-propellants) with various 3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) contents under impact at different velocities. The results showed that as the HMX contents increased, the CIIV decreased and changes of that also decreased. Simultaneously, the fragmentation degree increased with the increase in impact velocity, as it was judged by the initial dynamic activity ratio. Then, we calculated the maximum value of the initial dynamic activity ratio by fitting the data using Boltzmann, which were 27 (HMX = 0), 39 (HMX = 5%), 48 (HMX = 10%), and 56 (HMX = 15%). The ratios of the maximum value of the initial dynamic activity ratio between PBT-propellants containing HMX and those without HMX were 1.44, 1.78, and 2.07. The addition of HMX increased the maximum fragmentation degree of PBT-propellants under impact. The potential reaction violence increased as the impact velocity increased due to the increase in the fragmentation degree of PBT-propellants after impact. Similarly, we also obtained the maximum values of potential reaction violence, which were 62.39, 50.17, 41.89, and 34.65. The maximum value of potential reaction violence decreased with the increase in HMX contents, and the addition of HMX improved the safety of PBT-propellants burning after impact.

Study on the mechanism of molten aluminum droplets igniting EPS foam

Molten metal droplets with higher fire risk may cause large fires. The ignition by molten metal droplets is more complicated than by solid inert spots due to the latent heat release of solidification and fluid spread involved. There are few studies on the ignition by molten metal with liquid behavior at present. The mechanism of molten aluminum droplets igniting EPS foam was studied in this work. A small spherical aluminum particle (4-12 mm in diameter) was heated and melted to a pre-set temperature (740-1000 °C) and was dropped to a foam sample at a fixed height (20 cm). It was observed that the droplet became a splat after a collision due to the flow of liquid aluminum. The splat diameter was recorded to analyze heat transfer during the collision process. Results show that as the diameter increased from 4 to 12 mm, the critical temperature to ignite decreased from 975 to 760 °C. Compared with solid hot particles, the critical temperatures for ignition and fire spread by molten aluminum droplets were much lower. The internal energy of droplets was also investigated, and the energy per unit contact area was found to be a critical parameter (about 2.5 MJ/m2), which determines whether the ignition will occur. The diffusion of combustible gas was numerically studied and was found consistent with the trend of experimental results. Moreover, the effects of temperature and the initial size of droplets on gas mixing time were also theoretically analyzed. The competition between the gas mixing time and the splat residence time explained the variation of critical ignition temperature with droplet size.

Effects of inter-pulse coupling on nanosecond pulsed high frequency discharge ignition in a flowing mixture

This work numerically investigates the effects of non-equilibrium nanosecond plasma discharge pulse repetition frequency, pulse number, and flow velocity on the critical ignition volume, minimum ignition energy, and chemistry in a plasma-assisted H2/air flow at 300 K and 1 atm using a multi-scale adaptive reduced chemistry solver for plasma assisted combustion (MARCS-PAC). The interactions between discharges/ignition kernels spanning decoupled, partially-coupled and fully-coupled regimes in a pulse train are studied. For a single pulse discharge, increased flow velocity increases the minimum ignition energy required due to the increase of convective heat loss and flame stretch. The results show that the minimum ignition kernel propagation speed at the critical ignition kernel volume increases with the flow velocity. The minimum critical ignition volume decreases with the increase of plasma discharge energy. For sequential two-pulse discharges, ignition fails at both decoupled and partially-coupled regimes even when the total discharge energy is above the minimum ignition energy, but succeeds only in the fully-coupled regime at a shorter inter-pulse time. Overlap of the OH radical pool between the sequential two-pulse discharges and the increase of the chemistry effect due to the increase of reduced electric field in the fully-coupled regime contribute to the ignition enhancement. In addition, for two-pulse discharges in the fully-coupled discharge regime, the mixture can be ignited at a total energy below the minimum ignition energy of a single pulse with the same flow conditions. Moreover, for a given total discharge energy with multiple pulsed discharges, the enhancement of the ignition kernel volume has a non-monotonic dependence on discharge frequency and pulse number. The effective ignition enhancement can be achieved with an optimal pulse repetition frequency and pulse number. This work provides a new understanding of the mechanism for repetitive plasma ignition and insights for the optimization of plasma ignition in a reactive flow.

Ignition of supercritical hydrothermal flames in co-flow jets

Supercritical hydrothermal combustion is a new high-pressure combustion technology. Stable ignition of hydrothermal flames is one of the most important issues. To obtain the ignition criterion, the hydrothermal flame in continuous co-flow jets with methanol as auxiliary fuel was investigated in this study. The autoignition model for laminar fuel jets based on the Frank-Kamenetskii theory was modified by considering the jet’s actual specific heat. Piecewise relations between the ignition boundary and jet radius were obtained. Moreover, the eddy dissipation concept simulation for a turbulent fuel jet in the coaxial nozzle indicated the important influence of interfacial reaction in the jet initial region on the ignition of hydrothermal flames. Finally, the analysis and comparison of the two models demonstrated that the autoignition model is suitable to evaluate the ignition of turbulent jets when hydrothermal flames are ignited in the initial region of jet, and provides a judgement scheme.

Detailed chemical kinetics simulation of hydrogen hydrothermal combustion characteristics: Special effects of supercritical H2O/CO2 mixtures

Hydrothermal combustion of hydrogen is a key process in the H2O/CO2 mixed working medium thermal power generation polygeneration technology, a new coal utilization path. The flammability and stability of hydrogen hydrothermal combustion were evaluated using detailed chemical kinetics coupled with the chemical dynamics simulation method in this paper. The findings reveal that in supercritical water, pure hydrogen is difficult to burn; however, CO2 can dramatically reduce the ignition delay time and lower the critical ignition temperature of hydrogen. This is because free radicals produced by CO2 are actively involved in the chain-branching process of hydrogen oxidation, which promotes hydrogen ignition and combustion stability. Compared with gas combustion, hydrogen hydrothermal combustion has lower ignition temperature and can combust at lower parameters. Water is active in the reaction process, which produces the necessary free radicals for ignition, promotes combustion and influences the path of reaction, resulting in a relatively slow reaction rate.

Study on the Ignition Mechanism of Inert Fuel Tank Subjected to High-Velocity Impact of Fragments.

Nowadays, aircraft fuel tanks are protected by measures such as inerting, fire and explosion suppression, which significantly improve their ability to mitigate mechanical damage and prevent fire in the case of an accidental attack. In this study, an equivalent inert fuel tank with fire and explosion suppression was designed according to the vulnerabilities of a typical fighter. Then, a ballistic gun, a 37 mm gun and a two-stage light-gas gun were used to propel different fragments in tank damage experiments at different speeds (1400 m/s–2600 m/s). Experimental results show that the disassembly of a fuel tank is a prerequisite for igniting fuel. When the fragments hit the gas phase of the tank, the fuel tank was not disassembled and the fuel was not ignited. The calculation results show that the internal oxygen concentration was always lower than the limiting oxygen concentration (12%) before the fuel tank was disassembled. In addition, the minimum ignition speeds of inerted fragments with different masses as predicted by the ignition criterion when hitting the liquid fuel are consistent with the test results. This shows that increasing the mass of inert fragments will increase the minimum ignition speed and reduce the probability of ignition of the fuel. However, the implosion effect of the energetic fragments released about 3 times the chemical energy of its own kinetic energy, and the high-temperature and high-pressure products were very beneficial to the disintegration and ignition of the fuel tank compared to inert fragments.

Newly designed ignition circuit to improve the ignition reliability of Hall thruster

This study proposes a newly designed ignition circuit that does not change the ignition starting parameters of a Hall thruster, reduces the critical ignition voltage, and improves ignition reliability. The complexity and weight of the Hall thruster power supply system are reduced by multiplexing the ignition and discharge power supplies. Under the condition that the design parameters are satisfied, that is, R3+REC2 is much larger than RCC1, the residual energy in the circuit after the cathode breakdown can be fed into the thruster discharge channel to improve the ignition energy supply. The experimental results show that when the anode voltage is 98 V, only nine of the 100 ignition tests are successful using the traditional ignition circuit, whereas the number of successes using the newly designed ignition circuit is as high as 99, which provides a simple solution method for improving the ignition reliability.

Community Reaction Network Reduction for Constructing a Coarse-Grained Representation of Combustion Reaction Mechanisms.

A community-reaction network reduction (CNR) approach is presented for mechanism reduction on the basis of a network-based community detection technique, a concept related to pre-equilibrium in chemical kinetics. In this method, the detailed combustion mechanism is first transformed into a weighted network, in which communities of species that have dense inner connections under the critical ignition conditions are identified. By analyzing the community partitions in different regions, we determine the effective functional groups and driving processes. Then, a skeletal model for the overall mechanism is deduced according to the network centrality data, including transition pathway identification and reaction-path flux. The CNR method is illustrated on the hydrogen autoignition system which has been extensively investigated, and a new reduced mechanism involving seven processes is proposed. Dynamics simulations employing the present CNR model show that the computed ignition time and distribution of major species on a wide range of temperature and pressure conditions are in accord with the experiments and results from other methods.

Critical temperature for volume ignition of deuterium–tritium fuel in inertial confinement fusion: Effects of hydrodynamic instabilities

Volume ignition is an alternative approach to inertial confinement fusion. Due to igniting the whole fuel region rather than the central hot spot compared with the central hot-spot ignition, more laser energy is needed for volume ignition. Therefore, it is much desirable to examine the ignition margin for volume ignition. Hydrodynamic instabilities are major factors responsible for degrading inertial confinement fusion implosion performance. Hydrodynamic instabilities usually bring dramatic deformations of the fuel target, and accordingly, more radiation energy loss leaks from the fuel region. Therefore, the focus of this paper is on how they influence the radiation energy loss and increase critical temperatures for volume ignition. The present results show that critical ignition temperature increases both with the perturbation mode number and the perturbation amplitudes. What is more, we find that perturbations with longitudinal mode have a greater impact than those with latitudinal mode, and targets with lower deuterium–tritium mass are more vulnerable to perturbations. The present results are important and offer support for subsequent ignition-target design.

The determination method of the ignition modes of single coal particle with a transient coal ignition and combustion model

A transient model for the ignition and combustion of a quiescent coal particle was developed by coupling the mass, energy and species governing equations between the particle phase and the gas phase. The ignition delay times of single coal particle were calculated by using the transient ignition and combustion model with Kobayashi-Sarofim devolatilization model and Chemical Percolation Devolatilization (CPD) model, respectively. The predicted results were compared with the experimental data to further validate better performance of CPD model. The new ignition criteria of single coal particle were proposed to determine the onset of the ignition and subsequently to define the ignition modes of single coal particle by comparing the magnitude of typical ignition time scales, including the start time and the end time of devolatilization, homogeneous ignition delay time and the heterogeneous ignition delay time based on the thermal explosion theory (TET). The ignition modes map of single coal particle under various ambient oxygen molar fractions (O2 = 0–100%) and gas temperatures (Tg = 800–2000 K) was firstly depicted by using the transient ignition and combustion model with the new ignition criteria. The ignition modes of single coal particle transferred from homogeneous gas-phase ignition (GI) to homo-heterogeneous ignition (GI-HI) when the ambient oxygen molar fraction was increased from 5% to 21% under Tg = 1340 K condition, and then they transformed into hetero-homogeneous ignition (HI-GI) and heterogeneous ignition of coal (HI-coal) when the ambient oxygen molar fraction was further increased to 40% and 70%, respectively. Moreover, the simulated results showed that the critical oxygen molar fraction XO2,cr for the transition of ignition mode from homogeneous to heterogeneous ignition was 15% for the particle size of 50 μm, XO2,cr = 20% for the particle size of 100 μm, and XO2,cr = 30% for the particle size of 200 μm, respectively. The critical parameters for the transition of ignition mode of single coal particle were quantitatively determined to give a deep understanding on coal particle ignition process and to provide the guidance for improving the combustion performance of coal particles.

A new-type multifunctional small caliber projectile and self-powered ignition features under impact load

The new-type projectile with the penetration and arson function has been playing an important role in modern warfare increasingly with new technique development. A new-type multiple-function small caliber projectile with a self-powered ignition system was designed. The miniaturization design of the self-powered ignition system greatly reduces the size of projectile. The ignition process, ignition energy, and ignition delay time of the self-powered ignition system were analyzed by experiment and finite element simulation. The effects of impact velocity and the mass of the mass block on ignition energy were analyzed. The results show that the self-powered ignition system can ignite stably when the impact velocity is higher than 51 m/s and the mass block is 4 g. The slope of the output energy curve decreases with the increase of the mass blocks; the output energy increases first and then decreases with the increase of impact velocity. According to the critical ignition experimental curve, the appropriate mass block is selected to make the self-powered ignition system ignite stably, and the ignition delay is related to impact velocity, and the higher the impact velocity is, the shorter the ignition delay is under the condition of given mass block.

Study on Combustion Characteristics of Cable Based on Cone Calorimeter

Cross-linked polyethylene (XLPE) carries a high fire risk. In this paper, a cone calorimeter is used to carry out radiation ignition experiments, and the heat release rate (HRR), mass loss rate (MLR) and other combustion parameters of XLPE cables under three kinds of radiation intensity are measured. By comparing the ignition time and HRR of samples under different conditions, the following conclusions are drawn: (1) The ignition time of XLPE cables decreases significantly with the increase in external thermal radiation intensity. The critical ignition heat flux (CHF) is about 16.24 kW/m2. (2) The HRR curve of XLPE is consistent with the characteristics of hot, thick material. The HRR rises rapidly to the first peak after ignition and then rapidly decreases. Then, it slowly rises to the second peak. Finally, it slowly decays until the combustion stops. (3) The first peak values of HRR of XLPE under different radiation intensities are almost the same. The time for the second peak of HRR is shorter, and the value is larger with the increase in external thermal radiation intensity. (4) The cable ignition model is established, which can simulate the cable ignition time well under different radiant heat flow conditions. (5) Based on the mathematical model, the ignition time trend with the thickness of sheath layer and conductive core layer as variables is deduced.