Ignition Experiments Research Articles

Insight into energy release characteristics of TiH2 dust explosion through ignition experiments and molecular dynamic simulations

TiH2 is an efficient hydrogen storage material with high energy density and relatively stable chemical quality, which is commonly used in metal sponge production, hydrogen storage, battery technology, rocket propellants, etc. In this study, TiH2 dust explosion is systematically investigated by integrating ignition experiments with reactive force field molecular dynamics simulation. Findings show that when the TiH2 dust concentration is 750 g·m−3, both the explosion pressure and explosion pressure rise rate reach their maximum values under airtight and venting conditions. There are three stages in the whole combustion process: the combustion development stage, the stable combustion stage, and the decay stage. The flame length (L) and flame propagation velocity (v) grow with the increase of dust concentration. TiH2 dust explosion with different concentrations in macroscopic tests is approximated by simulating the combustion behaviors of the TiH2 system with different densities. The combustion reaction rate of TiH2 grows with an increase in system density, and the simulation results are consistent with experimental findings.

Ignition of Forest Fires by Cigarette Butts: Using Pinus massoniana Needles as an Example

As a cigarette butt falls onto the forest surface fuel, it first smolders the fuel, then ignites into flames, and spreads as forest fire under certain conditions. In this study, the needles under a typical stand of P. massoniana were used as the research object. Needle beds with different moisture content and packing ratios were constructed indoors. Cigarette butt-ignition experiments were conducted under different wind velocities, and 30 experiment cycles were conducted under different conditions. There was a total of 5 (packing ratio) × 4 (moisture content) × 6 (wind velocity) = 120 sets of conditions, and a total of 3600 ignition experiments were conducted. The results showed that (1) the total ignition probability of the cigarette butts was 2.36%, which only occurred when the fuelbed moisture content was <10% and the wind velocity was >1 m/s. The ignition time of cigarette butts ranged from 2.73 to 7.25 min. (2) The fuelbed moisture content and wind velocity significantly influenced the ignition probability and time. With an increase in moisture content, the ignition probability of cigarette butts decreased, while the time required for ignition showed an increasing trend. Wind velocity had a dual effect on ignition. The ignition effect was optimal at a wind velocity of 4 m/s. With an increase in wind velocity, the ignition probability first increased and then decreased, and the ignition time first decreased and then increased. (3) The packing ratio had no significant effect on the ignition probability; however, the ignition time significantly decreased as the packing ratio increased. (4) The logistic regression method (LRM), general linear method (GLM), and nonlinear regression method (NLM) were used to establish a prediction model of ignition probability. The prediction effect of GLM was the worst, followed by LRM, and the NLM had the best prediction effect. The GLM was selected to establish the ignition time model, and the error was also within the allowance range. This study elucidated the underlying mechanism of factors affecting cigarette butt-based fuel ignition. In addition, the established prediction model provides a reference for human-caused forest fires and is highly significant for forest fire prevention.

Experimental and numerical investigation on the Ignition performance of the cavity flame holder in the integrated afterburner

Integrating components, as a new afterburner concept, is currently an important trend in afterburner development research. Spurred by this trend, this study proposes an inner cone cavity flame holder for an integrated afterburner. The experimental and numerical investigation evaluates the ignition performance under various pneumatic parameters, illustrating that stable flame is successfully achieved in the cavity of the integrated afterburner for a wide range of flow conditions. This is important, as an appropriate cone expansion angle benefits the ignition and flame stability. By comparing the experimental results, the optimum case is obtained, which reduces the ignition FAR. Furthermore, double fuel line can effectively broaden the ignition boundary. The successful completion of our ignition experiment demonstrates that the cavity flame holder of the integrated afterburner has a good structural design and ignition performance.

The Role of Target Melting in Particle Impact Ignition with Inert Particulate

The high gas temperatures and oxygen pressures in the turbine of oxygen-rich turbomachinery put conventional engineering alloys such as IN718 at risk of particle impact ignition, i.e., metal fires initiated when particulate strikes a solid surface. The standard model of particle impact ignition assumes that the impacting particle must first ignite in order to kindle to the target material. Here, we invalidate this belief through particle impact ignition experiments which show that IN718 can ignite when struck by inert Al2O3 particles with supersonic impact velocities. Through post-mortem analysis of non-ignited samples, we find that subsonic particle impact causes minimal crater damage whereas supersonic particle impact leaves extensive crater plasticity and pileup, with evidence of molten ejecta near the impact site. Complementary finite element simulations of supersonic impact events confirm extreme adiabatic heating and localized melting. These findings demonstrate that particle impact can drive target ignition even in the absence of particle burning provided the thermal excursion at impact exceeds the melting point of the target material.

Iron particles ignition in different hot coflow temperatures

Understanding the ignition of iron particle combustion in hot environments is critical for harnessing the use of metal fuels in clean energy production. In this research work, a new single particle burner is implemented to disperse and burn iron particles using hot air coflow. The burner disperses iron particles through Coulomb forces between two air-spaced capacitor electrodes. The oxidizer coflow is heated up as it passes through a newly designed induction gas heater. Electromagnetic induction is used to heat up multiple metallic and porous heat discs by Joule heating. When the coflow gas passes through the pores of the discs, it carries the heat generated within the porous discs and delivers it to the travelling iron particles which provides ultimate heat transfer effectiveness. Coflow temperature homogeneity is examined using Schlieren imaging and confirmed by thermocouple measurements at the outlet of the coflow tube. Particle ignition and combustion occur in open space, providing excellent conditions for optical diagnostics. Two synchronized high-speed cameras were used to simultaneously determine the particle size using green laser shadowgraphy and detect iron particles ignition. Iron particles ignition experiments were conducted for a sieved patch (45–53 μm) at different hot coflow temperatures starting at 700℃ up to 900℃. The burner showed extensive capabilities to maintains a stable and homogeneous hot coflow for a wide range of temperatures up to 900℃. Ignition experiments showed 5.67 % of ejected particles are burnt at 700 ℃, while full ignition (99.33 %) can be achieved at 900 ℃.

Fundamental study on lean operation limit of super lean-burn spark ignition engines: MIE transition and limit prediction

Fuel-lean combustion is challenging because of the difficulty in successful ignition-to-flame propagation transition in intense turbulence conditions. This study aims to elucidate the governing factor of fuel dependence on the lean limit through fundamental ignition experiments and numerical simulations. Previous scaling analysis has reported strong correlations between lean engine operation limit and Minimum Ignition Energy (MIE) transitions. Additionally, the temporal evolution of turbulent intensity in the engine cylinder plotted on Peters diagram suggested that the flame kernel growth occurs only in relatively weak turbulent intensity, u′, the condition under which u′ is lower than the MIE transition. To investigate the behavior of flame kernel growth in the vicinity of the MIE transition condition, we conducted ignition experiments under both laminar and turbulent conditions utilizing a constant volume chamber with counter-rotating fans. Flame initiation was achieved by spark discharge at various turbulent intensities. The results showed notable distinctions in flame kernel growth processes between below and above the MIE transition condition. For u′< MIE transition, flame kernel development is governed by molecular transports showing an apparent Lewis number effect, whereas for u′> MIE transition, the effect seems to disappear. Subsequently, experiments and numerical simulations on spherically propagating flames in quiescent mixtures with various blended fuels were conducted. The results indicated that fuels facilitating rapid flame kernel growth generally exhibited leaner engine operation limits, regardless of engine specifications. The present study successfully demonstrated that the fuels suitable for lean combustion could be predicted by investigation of spherically propagating flames in quiescent mixtures.

Lab-on-PCB solid propellant microthruster with multi-mode thrust capabilities.

In the realm of nano/microsatellite clustering, the demand for microthrusters is steadily growing. Solid propellant microthrusters, recognized for their lightweight build and structural simplicity, carry significant commercial promise. However, existing solid propellant microthrusters manufactured using MEMS technology encounter notable issues such as inconsistent thrust generation positions, limited thrust profiles, and issues related to productivity, scalability, and durability. In this study, we propose a novel shared-chamber solid-propellant microthruster design that consistently produces thrust at a designated position and accommodates multiple thrust modes. The components and fabrication of this thruster were developed using lab-on-printed-circuit-board (PCB) technology and PCB surface mount technology, showcasing enhanced structural stability, scalability, and potential for mass production. Our ignition and combustion experiments confirmed the repeatability of the unit operation, a fundamental feature of this innovative microthruster. Furthermore, we successfully implemented and evaluated the power mode for increased thrust and the continuous mode for prolonged operational duration. Integrating the lab-on-PCB-based shared-chamber solid propellant microthruster with propulsion and electronic control systems holds promising potential for future satellite missions.

Ensemble predictions of laser ignition with a hybrid stochastic physics-embedded deep-learning framework

When investigating stochastic ignition phenomena, high-fidelity simulations and experiments for statistical characterization can be a laborious endeavor. Machine learning (ML), in particular deep-learning, offers an accurate and cost-effective approach for reduced-order modeling of forced ignition. In this work, we investigate the potential and limitations of ML in modeling ignition, especially in the presence of limited data and multiple ignition modes. To this end, we introduce a hybrid stochastic physics-embedded deep-learning framework that combines sparse experimental data from Schlieren measurements with inert large-eddy simulations for predicting the spatio-temporal evolution of ignition kernel location and morphology. This model combines a stochastic differential equation for modeling kernel dynamics and a deep-learning model for representing kernel morphology. We evaluate this modeling approach for laser ignition of a gaseous methane/oxygen (CH4/O2) model rocket combustor, where morphological effects of the ignition kernel significantly influence ignition behavior. Results demonstrate that this model can reasonably capture behavior associated with the three dominant ignition modes, namely direct, indirect, and failed ignition, along with statistics associated with kernel growth and position, at lower computational costs than high-fidelity reacting simulations. In addition, we demonstrate that this modeling framework can be employed for generating spatially resolved ignition probability maps by incorporating physics to represent kernel interaction with the turbulent jet. We note that limitations in accuracy can be observed when predicting with vastly out-of-distribution data. Nevertheless, these results demonstrate that this physics-embedded ML approach can statistically characterize forced ignition in a more cost-effective manner than reacting high-fidelity simulations, as long as sufficiently representative data is available.

Enhancement effect of Li addition on nitriding reaction and combustion of Al-Li alloy in N2

For aluminum-based alloy particles, the addition of alloying element lithium (Li) with the properties of a high chemical activity and a low melting/boiling point can significantly improve the thermo-reactive properties of aluminum (Al) particles. To further reveal the combustion mechanism, this paper presents the nitriding reaction and combustion characteristics of Al-Li alloy in nitrogen (N2) through thermogravimetric analysis and differential scanning calorimetry (TG-DSC) analysis at a low heating rate and laser-induced ignition experiments at a high heating rate, considering the influences of two kinds of Li contents (1.0 wt% @ Al-1.0Li and 2.5 wt% @ Al-2.5Li) on these characteristics and making a comparison to the counterpart Al. Al-Li alloy powders as-prepared by gas atomization method showed the texture of the Li-rich eutectic phase compounds on the surface by scanning electron microscope (SEM) characterization. The TG-DSC results demonstrated that the thermal nitriding reaction of Al-Li alloy in N2 was significantly faster than that of pure Al. The nitriding reaction temperature decreased from 853 °C for pure Al to 245 °C for Al-2.5Li alloy with a reduction of 71 %. The final weight gain of Al-2.5Li alloy was almost three times higher than that of pure Al in N2. The SEM photograph of intermediate nitriding products revealed that the cracks and loose “island” structures in the Li-rich zone on the Al-Li surface were the key to preventing the formation of a “passivation effect” into Al and promoting N2 uptake during melting. In addition, Li also acts as a catalytic carrier and “oxygen absorber” to enhance the Al-N reaction. The combustion of single Al-Li alloy microparticles induced by laser showed two stages: preheating and rapid nitriding. Al-Li alloy particles were ignited more rapidly and showed typical gas-phase / surface combustion modes different from pure Al. According to these findings, the combustion mechanism was proposed to help guide the development of Al-based solid propellants containing Li and the synthesis of AlN/Al composites in nitrogen-rich environments.

Regulating hollow structure of CL-20 microspheres using microjet droplet technology to enhance safety and combustion performance

Hollow energetic microspheres have attracted great attention in the field of materials due to their wide application potential. However, scalable, simple, and low-cost production methods have not been achieved yet. In this study, using nitrocellulose (NC) as a binder, hollow and porous hollow CL-20 microspheres were prepared by the assist of microjet droplet technology. The effects of receiving liquid temperature, solution concentration, and solvent type on the morphology, particle size, shell thickness, and pore size of the CL-20 microspheres were systematically investigated. Furthermore, the influences of shell thickness and pore size on the dispersibility, crystal structure, thermal decomposition, mechanical sensitivity, and combustion behavior of CL-20 microspheres were further studied. The results showed that precise control over the shell thickness and pore size of CL-20 microspheres could be achieved by adjusting the process parameters. Compared to raw CL-20, the CL-20 microspheres exhibited excellent dispersibility and improved thermal and safety performance (raw CL-20 vs CL-20 microspheres, 40.3° vs 27.1° ∼ 29.1°; 240.4℃ vs 241.1 ∼ 246.2℃; 2 J vs 25 ∼ 32.5 J). Ignition experiments demonstrated that the hollow and porous hollow structures significantly enhanced the combustion flame of CL-20, shortened the burning time, and thereby improved energy release efficiency. This preparation method is simple and efficient, providing a new approach for the fabrication of multi-structured hollow energetic microspheres.

Experimental study on the ignition in a scramjet with a self-designed water plasma ignitor

In this paper, aiming at the problematic situation of igniting and improving combustion efficiency in a supersonic combustor, a self-designed water plasma ignitor is designed for ignition in supersonic combustors. For the self-designed water plasma ignitor, deionized water is selected as the coolant. During the ignitor operation, the water coolant cools the cathode and evaporates. Steam is ionized between the cathode and anode, generating a water plasma jet that is injected into the combustor for ignition. The self-designed ignitor has the advantages of long-time operation, repeatable startup, convenient coolant portability, and a low water consumption rate. The temperature of the water plasma jet exceeds 3500 K. The calculation of ignition delay time indicates that OH, H, and other free radicals in water plasma can significantly shorten the ignition delay time of kerosene and extend the ignition equivalence ratio limitation. The ignition experimental study was conducted using the water plasma ignitor in a thin strut equipped supersonic combustor whose design point corresponds to the cruise condition at a Mach number of 6. In the ignition experiment, a flame, which is self-sustaining, was established after the ignitor was activated for approximately 682 ms. The feasibility of the water plasma ignitor in a supersonic combustor has been verified.

Preparation of flame-retardant HNS/DAAF energetic composite microspheres using droplet microfluidic technology

2,2′,4,4′,6,6′-Hexanitrostilbene (HNS) exhibits excellent detonation performance and has the potential to play a significant role in aerospace technology. However, its application is limited by the extreme space environment. In this study, glycidyl azide polymer/nitrocellulose (GAP/NC) was used as a binder to prepare HNS/DAAF energetic composite microspheres with different mass ratios using droplet microfluidic technology. The morphology, particle size, crystal structure, thermal decomposition, bulk density, mechanical sensitivity, and combustion behavior of the microspheres were characterized and tested. The results showed that the HNS/DAAF composite microspheres exhibited high sphericity and narrow particle size distribution while preserving the crystal structure of HNS. Compared to raw HNS, the HNS/DAAF composite microspheres demonstrated improved thermal stability and enhanced safety performance. Additionally, ignition experiments revealed that the addition of DAAF effectively suppressed the combustion of HNS. These findings provide valuable insights for the safe utilization of HNS in high-temperature environments.

Modeling and ignition performance study of Al/Ni multilayer energy-containing microdevices

In this paper, we designed double V-shaped Al/Ni multilayer energy-containing microdevices with different V-angles, and we performed finite element modeling and simulation of the heat transfer process of the designed energy-containing microdevice. Temperature-dependent resistivity was introduced to effectively simulate the phase change during ignition. We simulated the temperature and current density distribution in the central region of the Al/Ni multilayer energy-containing microdevice and predicted the ignition voltage threshold for the specific device structure. Al/Ni multilayer energy-containing microdevices with different V-angles were prepared by electron beam evaporation technology, and ignition experiments on the prepared devices under the excitation of 47 μF capacitance were conducted. The experimental results show that the critical voltage of ignition increases with the increase in the V-angle, which verifies the correctness of the proposed finite element model.

Prediction model of goaf coal temperature based on PSO-GRU deep neural network

The accurate determination of coal temperature in hidden space such as goaf has always been a worldwide problem that needs to be solved. The research of coal temperature prediction model has important practical significance for the accurate detection of loose coal temperature. Based on the coal natural ignition experiment and coal spontaneous combustion gas characterization index as the initial data set GRU neurons are used to mine the nonlinear relationship between the index gas and temperature, and the GRU model parameters are optimized by PSO to obtain the predicted value of coal body temperature. The results show that the predicted MAE value of the PSO-GRU model is 1.37 °C, 6.51 °C, 11.40 °C, 15.90 °C, 20.20 °C lower than that of the PSO-SVM, PSO-BP, BP, RF and SVM prediction models respectively. The RMSE value decreased by 0.45 °C, 4.44 °C, 10.33 °C, 15.71 °C and 24.24 °C respectively. The judgment coefficient R2 on the test set and the training set of the PSO-GRU model is 0.99, and the generalization, prediction accuracy and robustness are all good. The experimental results show that the inversion temperature error of the model is within the range of 3.87 %, the maximum temperature difference is 5.69 °C, and the average temperature difference is 2.83 °C, which can meet the accuracy requirements of the field temperature measurement.

Diagnosing the origin and impact of low-mode asymmetries in ignition experiments at the National Ignition Facility.

Inertial confinement fusion ignition requires high inflight shell velocity, good energy coupling between the hotspot and shell, and high areal density at peak compression. Three-dimensional asymmetries caused by imperfections in the drive symmetry or target can grow and damage the coupling and confinement. Recent high-yield experiments have shown that low-mode asymmetries are a key degradation mechanism and contribute to variability. We show the experimental signatures and impacts of asymmetry change with increasing implosion yield given the same initial cause. This letter has implications for improving robustness to a key degradation in ignition experiments.

Insight into the ignition characteristics of dust layers by mechanical friction sparks

Frictional sparks resulting from malfunctions pose a potential hazard for the accidental ignition of combustible materials in the processing industry. Ignition experiments revealed that mechanical friction sparks generated by TC4 titanium alloy, Q235 steel or 304 stainless steel were unable to ignite corn starch, PMMA, or wood powders. However, friction sparks generated by those materials at a pressure of 3.75 N/mm2 and a disc speed of 12 m/s could ignite a cotton fibrous dust layer. The release of frictional sparks is accompanied by energy dissipation. As friction pressure and rotational speed increase, the initial energy of the friction spark increases. Thus, the possibility of igniting the cotton fibrous dust layer increases, while the ignition time decreases. In terms of the ability to ignite the fibrous dust layer under identical friction conditions, the ranking is as follows: TC4 titanium alloy > Q235 steel >304 stainless steel. Moreover, TC4 friction sparks demonstrate weak ignition capability for dust layers with humidity levels of 2% and 10%. The susceptibility of the cotton fibre dust layer to ignition diminishes with increasing humidity. The distinctive flocculation structure of fibrous dust makes it more prone to ignition by mechanical friction sparks compared to traditional spherical dust. These findings imply that when employing metal alloys as mechanical equipment or tools in fibrous processing, the possibility of mechanically generated sparks serving as an ignition source should be taken into account.

Experimental Studies on Thermal Oxidation and Laser Ignition Properties of Al-Mg-Li Powders.

Powder ramjets are a kind of vehicle propulsion system with high specific impulse and efficiency. They provide significant benefits in terms of extended propulsion and thrust adjustment. The pursuit of a highly reactive fuel appropriate for powder ramjets is likely to stimulate advancements in innovative propulsion systems, which are crucial for deep space exploration and long-term space missions. This work presents experimental studies on the thermal oxidation and laser ignition performance of aluminum-magnesium-lithium powders at atmospheric pressure. TG-DSC curves of powders in three heating rates were obtained. The ignition processes and ignition delay times were recorded by a CO2 laser ignition experiment system at a laser power of 10~60 W. The results show that at a lower heating rate of 10 K/min, the powder's thermal hysteresis is less, and the powder energy released in stage I is more concentrated. However, the degree of heat release concentration approached a similar level at heating rates of 30 K and 50 K. The ignition delay time decreased as the laser flux density increased. When the laser flux density exceeds 80 W/cm2, the effect of laser power on the ignition delay time decreases. At atmospheric pressure, the mathematical relationship between ignition delay time and laser flux density is given. Finally, the powder ignition processes at different laser powers are represented graphically.

Effect of stearic acid coating on the flame propagation and reaction mechanism of AlH3

AlH3 is considered to be the most promising high-energy additive and stearic acid (SA) is a common passivation coating material to enhance stability. To explore the effect of coating of SA on AlH3, flame propagation and reaction mechanisms are compared through ignition experiments and ReaxFF molecular dynamics, respectively. The flame propagation of AlH3/SA slowed down for ∼20 ms and the flame duration is longer for ∼50 ms, indicating that the combustion intensity is reduced. The combustion reaction of AlH3 is initiated through the release of •H by H-Al breaking and further oxidizing to form H2O and Al-O clusters. The presence of SA results in more O and C atoms contained in the system and more reaction paths. The addition of C atoms and O atoms makes the maximum cluster molecular weight of AlH3/SA increase by 0.1 × 10^4 than AlH3. The results of flame propagation and reaction mechanisms verify that the stability of AlH3 is improved by the coating of SA.

Controlling the combustion and agglomeration characteristics of solid propellants via new micro-unit composite fuel Al@AP

New micro-unit composite fuels Al@AP with different mass ratios of Al and AP are designed, prepared, and added to solid propellants in place of the original aluminum powder. Through Thermogravimetric Analysis- Differential scanning calorimetry- Mass Spectrometry (TG-DSC-MS), laser ignition experiment, thermocouple temperature measurement, burning rate test, and combustion diagnostic, we study how Al@AP affects the ignition, combustion, and agglomeration properties of solid propellants. The results indicate that the increase of Al content of Al@AP can promote the advance of thermal decomposition of AP. As the AP content increases in Al@AP, the ignition delay time of Al@AP decreases by three times, and the mass burning rate increases by almost five times. At the same time, Al@AP significantly improves the burning rate and temperature gradient. When compare to the baseline propellant, Al@AP-containing propellants present a maximum increase in burning rate of 33% at low pressures and 19% at high pressures. In terms of agglomeration reduction, the number of large-size agglomerates in the CCPs of propellants containing Al@AP is reduced, the average particle size decreases by 78%, and the combustion efficiency increases to 99.34%. A mechanism is proposed to explain how the micro-unit composite fuel Al@AP alters propellant combustion and agglomeration. In summary, Al@AP provides excellent control over ignition, combustion, and agglomeration characteristics. This study provides guidance on the application and development of new aluminum-based composite fuels used in solid propellants.

Preparing HNS/n-Al heat-resistant microspheres with enhanced combustion performance using droplet microfluidic technology

Reducing the formation of large carbon clusters during the combustion of energetic materials (EMs) and improving their comprehensive performance hold great significance. With fluororubber (F2604) as a binder, this study prepared HNS/n-Al microspheres with different n-Al contents (5%, 10%, and 15%) using droplet microfluidic technology. Then, it characterized and tested the morphology, particle size distribution, dispersibility, crystal structure, thermal properties, mechanical sensitivity, and combustion behavior of the microspheres. The results show that the prepared microspheres had regular shapes, uniform particle sizes, and excellent dispersibility and contained more homogeneous components than physically mixed samples. Furthermore, the microspheres retained the crystal structures of the raw materials, enjoying high safety performance. The thermal analysis shows that HNS/n-Al microspheres had high heat resistance (thermal decomposition temperature: over 354 °C) and that a higher n-Al content was associated with more thorough thermal decomposition reactions of HNS (HNS: 83%, HNS/n-Al: 84%, 86%, and 93%). The ignition experiments show that the HNS/n-Al microspheres possessed excellent and stable combustion performance, as evidenced by more complete combustion reactions and significantly elevated energy release efficiency. Therefore, it is expected to achieve high-energy and high-speed responses of carbon-rich EMs and promote their practical applications.