Ignition Phenomena Research Articles

Characteristics and fire-inducing risk analyses of arc faults in low-voltage electrical systems

Arc fault is a prevalent phenomenon in low-voltage residential power systems and the main cause of electrical fires. This paper explores the fire risks associated with arc faults and employs standards from arc fault detection devices (AFDD) to create an arc fault experimental system complemented by an integrated multi-sensor system for ignition related data collection. The objective is to synthetically examine the dynamic characteristics and ignition mechanisms of arc faults. This paper investigates the generation mechanisms, waveforms, and energy characteristics of arc faults, including waveform distinctions across different load types and patterns of energy variation during arcing. Data from the multi-sensor system become the cornerstone of the correlation analysis between the current, energy, and ignition phenomena, facilitating the construction of an ignition probability model based on maximum likelihood estimation. The reliability of the model is verified across various datasets with a 95 % confidence interval, offering a quantitative fire-inducing risk assessment of arc faults. Furthermore, the paper assesses the AFDD standards’ break time limits and evaluates the performance of seven AFDD prototypes based on the proposed model, demonstrating the effectiveness of this technology in mitigating the risks of fires induced by arc faults. This study systematically analyzes the behaviors and ignition mechanisms of arc faults, thereby providing scientific evidence and data that support the enhancement of arc fault detection technologies and the development of fire prevention strategies.

Optical Investigation of Sparks to Improve Ignition Simulation Models in Spark-Ignition Engines

The use of renewable fuels in place of fossil fuels in internal combustion engines is regarded as a viable method for achieving zero-impact-emission powertrains. However, to achieve the best performance with these fuels, these engines require further optimization, which is achieved through new combustion strategies and the use of advanced ignition systems such as prechambers. Since simulations greatly accelerate this development, accurate simulation models are needed to accurately predict the combustion phenomenon, which requires a deep understanding of the ignition phenomenon as it significantly affects combustion. This work presents a comprehensive experimental methodology to study sparks under engine conditions, providing quantitative data to improve and validate ignition simulation models. The goal was to determine the volume generated by sparks under engine conditions that can initiate combustion and use this information to improve simulation results to match the experimental results. The visible sparks were observed with high-speed cameras to understand their time-resolved evolution and interaction with the flow. The heat transfer from the plasma was also visualized using a modified Background-Oriented Schlieren technique. The information gained from the experimental observations was used to improve an ignition simulation model. Since the velocity of the plasma was found to be slower than the surrounding flow, a user-defined parameter was included to calibrate the velocity of the simulated plasma particles. This parameter was calibrated to match the simulated spark length to the experimental spark length. In addition, since the previous simulation model did not take the heat transfer from the plasma into account, the simulated plasma particles were coupled to have heat transfer to the surroundings. Based on a comparison of the simulation results with the experimental results, the improved approach was found to provide a better physical representation of the spark ignition phenomenon.

Ignition limits of pine wood building material under the coupling effects of thermal radiation and cross winds

The ignition of timber buildings in the context of wildland-urban interface (WUI) fires has emerged as a significant challenge amidst the backdrop of global climate change. However, to the best knowledge of the authors, little attention was paid to the ignition conditions of wood building materials in WUI fires especially in thermal-fluid coupling environments. This study experimentally investigated the smoldering and flaming ignition characteristics of wood under the coupling effects of thermal radiation and cross winds. Surface temperature, internal temperature, mass loss rates, and time to ignition of wood samples were measured to study their variation with the above factors and the threshold between smoldering and flaming ignition phenomena. The results showed that wood exhibited reduced susceptibility to flaming ignition under low radiant heat flux conditions, and the introduction of cross winds posed more challenges for flaming ignition. In addition, both the critical surface temperature and mass loss rate varied with radiant heat flux and cross-wind speeds, so it was not feasible to determine the occurrence of flaming ignition by any of the two factors directly. This is because the coupling effects of thermal radiation and cross winds would result in a competition between radiant heating and convective cooling on wood surfaces. In light of this fact, two dimensionless quantities, namely Damköhler number and heat transfer ratio, were introduced to distinguish between smoldering and flaming ignitions. Damköhler number was used as an indicator of the potential for gas-phase flaming ignition, while the heat transfer ratio, namely the ratio of convective heat loss to radiant heat flux, was used to measure the net thermal energy received by wood surfaces. It has been demonstrated that the two quantities were effective to distinguish between smoldering and flaming ignitions from the perspectives of gas and solid phases with 20–40 kW/m2 in radiant heat flux and 0–1.0 m/s in cross-wind speed. The data and theories disclosed in this study could help predict the ignition behaviors of timber buildings under thermal-fluid coupling conditions, thus providing useful guidance for WUI fire mitigation and control.

A study on misfire detection by calculating crank angular velocity considering in-cylinder gas properties

Misfire detection using a new crank angular velocity calculation was studied for high efficiency and robust engine ignition performance and combustion stability control. An applying production was achieved by applying in-cylinder gas properties prediction and its correction to the misfire index and verifying it under various conditions. The relationship between the misfire index and torque fluctuation was consistent depending on the combustion control factors EGR, injection timing, pilot injection quantity change, and environmental change factors water temperature and intake gas temperature. On the other hand, it was found that the piston speed changes due to the in-cylinder pressure with respect to the intake pressure, and the crank angular speed needs to be corrected. Commonly used sensors for engine cooling water temperature, intake gas temperature, intake pressure, and engine speed were used as representative values for in-cylinder pressure, and the cooling loss was subtracted from the polytropic index and the reduction in specific heat ratio due to EGR was corrected. By building a new model that calculates the compression end pressure model from the polytropic index and adding corrections to the misfire index, we applied logic that can be calculated for each cycle to the ECU onboard. Conventionally, compression end pressure prediction requires calculations that take combustion conditions into account, which requires the number of sensors and their accuracy, and a long calculation time. However, in this study, Authors focused on the fact that the pressure at TDC during a misfire does not include ignition and combustion phenomena and expressed the necessary physical phenomena using the minimum sensor information. As a result of the above, a control structure at a level that can be applied to products was obtained.

Comparing Ignition of Fuel Beds to Firebrand Showers to Ignition of Fuel Beds by Non-Reacting Heaters

ABSTRACT The combustion of various fuels results in the release of firebrands during wildland-urban interface (WUI) fires. Firebrands represent a hazard, since the release of firebrands may lead to multiple ignitions at locations distant from the original combustion source. A newly developed experimental protocol using a non-reacting heater was used to simulate a firebrand. A direct comparison of this simplified non-reacting heater setup was undertaken to realistic fuel bed ignition to actual firebrand showers. Firebrand showers were generated using a custom experimental setup developed for this investigation. The novelty of this research is a detailed comparison for the first time to fuel bed ignition from non-reacting heaters to actual firebrand showers, for the same fuel bed type. The focus here is on smoldering ignition (SI) of the fuel beds, as this is seen as a complex and least understood aspect of firebrand ignition of actual WUI fires. Results indicate that the overall phenomenology of the ignition process is different between a non-reacting cartridge heater and a shower of firebrands. Complicated smoldering ignition phenomena of fuel beds by firebrand showers cannot be simulated by a simple non-reacting heater.

“Fireworks” ignition and combustion of metal nanoparticles: Based on the rapid conversion and transmission of microwave energy by energetic ionic liquids

The metal nanoparticles (MNPs) are widely used to improve the output performance of various explosives and propellants, which ignition is an important link for the energy release of fuel. The microwaves have been recognized as a safer and smart ignition method. However, it is difficult to achieve rapid ignition of MNPs under microwave radiation due to the low heating rate. The ADN energetic ionic liquids could efficiently absorb microwaves owing to excellent polarity. This work proposes a method of fast “fireworks” ignition and combustion of MNPs under microwave radiation, by employing ADN as the “central hub” for microwave energy conversion. The dispersion of Al and Ti NPs in ADN endowed the solution with nanofluid properties and significantly improved the thermal conductivity. The solution boiled violently under microwave radiation, causing the temperature of MNPs to rise continuously. Meanwhile, the acidic components produced by ADN pyrolysis destroyed the oxide shell of MNPs, further accelerating the ignition process. The ignition phenomenon of Al and Ti NPs were different due to distinct microwave absorption properties and ignition temperature. When the expanding liquid film broken, the shock wave blew the Al NPs into the air to realize the “fireworks” ignition and combustion. The Ti NPs was ignited in the solution, which further ignited the ADN solution, allowing the system for controllable ignition and combustion. The “fireworks” ignition and combustion performance of MNPs could be modulated by the input microwave power. In summary, the new ignition and combustion method of MNPs will have a profound impact on the design of ignition sequence, the improvement of microwave energy utilization and the expansion of the application of ADN-based liquid propellants.

Forward projected background oriented schlieren for study of sparks in internal combustion engines

The use of renewable fuels, such as hydrogen, in internal combustion engines requires novel combustion strategies that require a deep understanding of their ignition phenomena. This leads to the need for improved or new measurement techniques that can provide further insight into these phenomena. As part of a study to better understand the ignition phenomenon, this paper presents a modified Background Oriented Schieren method developed to study the heat transfer from sparks generated by spark plugs. This new method is called Forward Projected Background Oriented Schlieren or FPBOS, and this is the first time such a method has been introduced to the best of the authors’ knowledge. FPBOS provides a flexible and more powerful alternative to other BOS techniques, especially for use in micro-calorimetry applications. It can be used similarly to any other BOS method and with different correlation algorithms. This study presents a detailed account of this technique and its use to obtain high resolution, high-speed visualisation of the heat transfer from the spark to the environment and the interaction of the spark with the flow field.

화물차의 제동장치에서 발생하는 과열 예측방안 연구

Recently, due to the increase in domestic and international online e-commerce platforms and the increase in container traffic at domestic ports, the operating ratio of large trucks has increased, and the number of truck fires is continuously increasing. In particular, spontaneous combustion is the most common cause of truck fires. Various academic approaches have been attempted to prevent truck fires, but due to the lack of research on the spontaneous tire ignition phenomenon that occurs during braking, this research directly designed and manufactured an experimental device to establish an environment similar to the braking system of a truck. A non-contact temperature sensor was installed on the brake device of the experimental device to collect temperature data generated from the brake device. Based on the data collected from the temperature sensor of the brake device and the temperature sensor on the tire surface, the ARIMA model among the time series prediction models was used to Appropriate parameters were selected to suit the temperature change trend, and as a result of comparing and analyzing the measured and predicted data, an accuracy of over 90% was obtained. Based on this, a plan was proposed to reduce the rate of fires in trucks by providing real-time warnings and support for truck drivers to respond to overheating phenomena occurring in the braking system.

MILD combustion stabilization issues through the analysis of hysteresis behaviors: The case of new energy carriers

MILD combustion processes are renewed to reveal a strong resilience to extinction phenomena and/or instabilities, whereas the oxidation process is stabilized trough ignition phenomena. Under MILD conditions, igni-diffusive and/or perfectly mixed kernels, forming during the mixing process between hot products and fresh reactants, are so much diluted and pre-heated to escape classical feed-back flammable flames stabilization mechanisms, while ignition and extinction events merge in a unique condition through “anhysteretic” behaviors. So far, considering methane as reference fuel, it has been largely demonstrated the mentioned “anhysteretic” condition is very conservative and defines a sub-domain of MILD combustion processes, following Cavaliere and de Joannon's definition. Furthermore, the coincidence of ignition and extinction phenomena can occur also preserving hysteresis phenomena. In turns, this condition strongly enlarges the stabilization domain of MILD combustion processes, starting from the upper branch of the hysteresis behaviors to the real extinction, with characteristic unstable loci to consider as further/last opportunity to promote stable operative conditions through the formation of local thermo-kinetic conditions in the combustion chamber during hot products/fresh reactants mixing process (injection configuration/burner design), or by forced ignition events. The hysteresis behaviors of renewable/alternative fuels, relevant within the decarbonization policies of several energy sectors, are thoroughly discussed under MILD conditions through numerical studies in model reactors in order to shed light on common and/or different features, and outline practical rules towards the definition of stable MILD combustion domains. Results show that, as MILD combustion is a chemical kinetics-driven processes, stability issues have to be discussed in relation to fuel nature, albeit with common behavior can be derived. The coincidence between extinction/ignition phenomena is reached for extremely diluted conditions, already ascribable to MILD combustion conditions, thus defining a small sub-domain of the process. This condition can be reached through “hysteretic” or “anhysteretic” behaviors.

Study of energy dissipation mechanisms and hotspot formation patterns during impact process in octogen explosives with circular cavities

Considerable focus has been given to hotspot generation and ignition phenomena in impact-induced explosives. Hotspot formation in explosives is typically attributed to internal dissipation and heat transfer occurring within them. This study refines the momentum and energy equations to illustrate the entire process of particle collision, temperature-rise evolution, and hotspot formation in octogen explosive bed under impact. By octogen, we mean the substance known as cyclotetramethylene tetranitramine, which is also commonly referred to as HMX. Dense particles are considered to have pseudo-fluid properties. During the impact of the explosive, we captured the propagation of the stress wave and compared its similarities and differences with the shock wave. The collision force model incorporates a combination of Hertz–Mindlin elastic and Thornton elastoplastic contact theories. The temperature-governing equation includes sliding friction, rolling resistance, and plastic dissipations as energy sources, taking into account the heat transfer processes between particles. Temperature evolution is a spatiotemporally correlated phenomenon divided into three processes: high-temperature bands formation, cavity collapse, and particle bed dispersion, all of which lead to hotspot formation near the cavity and near the wall. Plastic dissipation is the primary source for particle temperature-rise and hotspot formation. Furthermore, the effect of cavity size, impact velocity, and particle size on temperature evolution and hotspot formation patterns is analyzed. It was found that higher impact velocities and smaller cavity sizes are associated with increased hotspot temperatures near the wall, but the hotspot temperature near the cavity does not consistently vary with impact velocity and cavity size. This is attributed to the relationship between energy dissipation rate and void collapse time.

Impact of Sudden Expansion Structure Length on Shock Wave Dynamics and Autoignition in High-Pressure Hydrogen Releases

This study investigates the effects of expansion structure length on shock wave behavior, specifically focusing on pressure dynamics, and subsequent autoignition in high-pressure hydrogen systems. Using configurations EPT1 and EPT2, along with a direct pipe (SP1) as a control, we analyze how different expansion lengths influence pressure profiles, shock wave attenuation, and ignition phenomena.

Laser-induced indirect ignition of non-premixed turbulent shear layers

This study uses direct numerical simulations to investigate the forced ignition of temporally-evolving turbulent subsonic shear layers separating a gaseous stream of methane (CH4) and a stagnant gas environment of molecular oxygen (O2). Ignition is forced by a thermal-energy source that heats up a small volume of gas during periods of time much shorter than the characteristic acoustic time scale. The kernel, including its initial rounded conical shape, resembles experimental observations after optical breakdown is achieved in a gas irradiated by a focused laser. Particular emphasis is placed on ignition phenomena observed when the laser is focused on the O2 environment outside the turbulent shear layer, where the local composition is far beyond the lean flammability limit. This represents an indirect mode of non-premixed ignition that develops after a relatively long period of time has passed since laser-energy deposition, when the eddies near the oxidizer edge of the turbulent shear layer are intercepted by a baroclinically-generated subsonic vortex of hot dissociated O2 ejected from the laser-energy deposition zone. The success of indirect ignition depends on the orientation of the laser beam, the thickness of the shear layer, and the kernel standoff distance from the oxidizer edge of the shear layer. An ignition Damköhler number is defined that accounts for these averaged effects in six different simulation cases. For near-unity ignition Damköhler numbers, the solution is also sensitive to the local instantaneous flow field prevailing near the oxidizer edge of the shear layer, in that a modification of the instantaneous pre-deposition flow field, while preserving its turbulence statistics, can produce different ignition outcomes.Novelty and significance statement This work investigates forced ignition in a canonical flow configuration – a non-premixed turbulent CH4/O2 shear layer – and analyzes mechanisms by which ignition can be unexpectedly achieved in adverse conditions. In particular, an ensuing laser-generated advection of hot dissociated gas is observed to ignite the shear layer even when the mixture at the laser focal point is not flammable, an observation first made in early experiments. This flow phenomenon, whose consequences on ignition of non-premixed turbulent flows have not previously been analyzed by direct numerical simulation, can significantly separate the laser-energy deposition zone from the ignition location and is linked to baroclinically generated vorticity due to the rounded conical shape of the energy kernel. The phenomenology analyzed here may impact the design of laser-based ignition systems for chemical propulsion.

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.

A data-driven reduced-order model for stiff chemical kinetics using dynamics-informed training

A data-based reduced-order model (ROM) is developed to accelerate the time integration of stiff chemically reacting systems by effectively removing the stiffness arising from a wide spectrum of chemical time scales. Specifically, the objective of this work is to develop a ROM that acts as a non-stiff surrogate model for the time evolution of the thermochemical state vector (temperature and species mass fractions) during an otherwise highly stiff and nonlinear ignition process. The model follows an encode-forecast-decode strategy that combines a nonlinear autoencoder (AE) for dimensionality reduction (encode and decode steps) with a neural ordinary differential equation (NODE) for modeling the dynamical system in the AE-provided latent space (forecasting step). By means of detailed timescale analysis by leveraging the dynamical system Jacobians, this work shows how data-based projection operators provided by autoencoders can inherently construct the latent spaces by removing unnecessary fast timescales, even more effectively than physics-based counterparts based on an eigenvalue analysis. A key finding is that the most significant degree of stiffness reduction is achieved through an end-to-end training strategy, where both AE and neural ODE parameters are optimized simultaneously, allowing the discovered latent space to be dynamics-informed. In addition to end-to-end training, this work highlights the vital contribution of AE nonlinearity in the stiffness reduction task. For the prediction of homogeneous ignition phenomena for H2-air and C2H4-air mixtures, the proposed ROM achieves several orders-of-magnitude increase in the integration time step size when compared to (a) a baseline CVODE solver for the full-chemical system, (b) statistical technique – principal component analysis (PCA), and (c) computational singular perturbation (CSP), a vetted physics-based stiffness-reducing modeling framework.

Numerical study on forced ignition and flame propagation in a counterflow of nitrogen-diluted hydrogen versus air

Hydrogen is widely perceived as the next generation of clean energy due to its zero carbon emissions. However, there is an escalating concern regarding hydrogen safety. A deeper understanding of the ignition characteristics of hydrogen in inhomogeneous environments is imperative for enhancing our physical comprehension and conducting accurate risk assessments. The purpose of this study is to delve into the ignition phenomena of hydrogen and air in a non-premixed counterflow. Transient two-dimensional simulations are performed, considering detailed chemistry and transport. The focus is placed on the effects of strain rate, mixture stratification, and ignition position on ignition and flame propagation. The first part studies the ignition process at varying strain rates while the ignition position is fixed at the stagnation point. The results show that a low strain rate has a minimal impact on ignition outcome, whereas a high strain rate hinders ignition by reducing mixing layer thickness and residence time. As a result, the minimum ignition energy (MIE) barely changes at a relatively low strain rate but rises rapidly as the strain rate approaches the extinction limit. Subsequently, ignition is studied at different positions along the symmetry axis, where the flow field and mixture composition vary simultaneously. It is demonstrated that ignition is more feasible on the lean side than on the stoichiometric and rich sides, and a high strain rate can expand the ignitability limit of the mixture fraction under extreme off-stoichiometric conditions. The separate contributions of forced convection and mixture inhomogeneity are further assessed through simulations with and without counterflow and stratified mixture. The results suggest that the significance of the forced convection is contingent upon the timescale competition between flow and ignition, while the mixture stratification determines the evolution of the ignition kernel into either an outwardly propagating premixed flame or a triple flame. The MIEs at premixed and non-premixed conditions are examined, and the influence of fuel stratification is analyzed. In the end, a regime diagram is presented, illustrating the comprehensive roles of strain rate and mixture fraction during ignition. This study provides invaluable insights into the non-premixed ignition of hydrogen in complex flow conditions.

Identifying two ignition modes of polymer insulated wires with continuous excess current in microgravity

Overload ignition of electric wires in microgravity has been investigated for space fire safety. Microgravity experiments from drop towers and parabolic flight have been conducted to observe the overall ignition phenomena. Two-dimensional numerical simulations were performed employing finite-rate chemistries in both condensed and gas phases. As the applied current changes, two distinct ignition modes were found in both experiments and simulations, one away from the wire and the other near the wire surface, named “spontaneous ignition” and “wire assisted ignition”, respectively. After validating ignition delay times by experiments, the simulation results were used to discuss the transition of two ignition modes. With a lower current, the spontaneous exothermic reaction at the pyrolysis gas front led to the far-distance ignition. While with a higher current, the local heat loss suppressed spontaneous ignition; meanwhile, the excessive heat from the exposed core wire caused the surface ignition. In addition, the ignition energy showed a non-monotonical increase with the applied current, where the minimum ignition energy was consumed with 5.5 A current supply implying the high fire risk of the “spontaneous ignition” regime. The control mechanisms of the ignition energy change in different stages were discussed.

Chemical insights into the two-stage ignition behavior of NH3/H2 mixtures in an RCM

This study investigated the two-stage ignition behavior of NH3/H2 mixtures using a rapid compression machine (RCM). The ignition delay times and first-stage ignition delay times of NH3/H2 blends were measured at temperatures ranging from 881 to 1127 K, pressures of 15 and 25 bar, and equivalence ratios (ϕ) of 1.0 and 1.5. Experimental results showed that H2 had a significant promotional effect on the ignition of NH3, and two-stage ignition behavior was observed in some test mixtures. Time-resolved species concentrations were recorded using the gas chromatography (GC) method during the single- and two-stage ignition process. Species evolution suggested that the consumption of NH3 and H2 separated during the two-stage ignition process. The oxidation of H2 primarily occurred at the first-stage ignition point, while NH3 oxidation occurred at the total ignition point. A kinetic model was developed to predict the two-stage ignition behavior of NH3/H2 and the species profiles. Model analysis showed that H2 played a key role in initiating the oxidation process and contributed to the early heat release. When H2 content was high (e.g. 50%), its oxidation led to the simultaneous total oxidation of NH3, resulting in single-stage ignition characteristics. However, when a small amount of H2 was present (e.g. 10%), its oxidation only partially consumed NH3, leading to the two-stage ignition behavior. Sensitivity analyses indicated that the co-oxidation of fuels during the first-stage ignition was primarily dominated by H2 oxidation chemistry, while NH3 oxidation became dominant during the following total ignition stage as H2 was completely consumed. Additional model simulations revealed that the ignition behavior of NH3/H2 mixtures is strongly influenced by temperature and pressure. A distinct threshold for temperature and pressure was identified, demarcating the transition between single-stage and two-stage ignition phenomena. Moreover, the fraction of H2 in the mixture had a significant impact on the ignition behavior, with higher fractions leading to increased intensity of the first-stage ignition and closer proximity to the total ignition point. However, when the H2 fraction exceeds a certain threshold, two-stage ignition behavior disappears.

Viscous shear flow and heating of impact-extruded composite energetic materials

Investigating the macroscopic deformation and localized heating behaviors of high-ductility composite energetic material (CEM) under impact extrusion is crucial to understand the accidental ignition phenomenon of CEM charged weapon systems with structural defects and design the advanced safe munitions. In this study, the rapid viscous shear flow and ignition responses of impact-extruded CEM are first experimentally studied and then simulated using a physically based thermomechanical model. The experimental results show that CEM sample impacted by a 6 kg and ∼2 m/s falling drop-weight could be extruded into the narrow crack with a high flow speed (∼20 m/s) and ignited. A thermomechanical model is developed to describe the continuum viscoelastic-plastic deformation and localized heating due to viscoplastic deformation in solid material, viscous flow in melted liquid and chemical reaction. By simulation, a semicircular extrusion region with a shear concentration is formed in CEM sample above the crack, and high shear stress and strain rate at this region contributes to the localized viscous shear heating and ignition of CEM. The effects of the diameter ratio between crack and sample (0.1∼0.4) on dynamic responses of CEM are further investigated. The simulated main features of CEM sample, including flow history into the crack, ignition time and location, are consistent with the experimental observations.

Ignition Characteristics of Dielectric Barrier Discharge Ignition System under Elevated Pressure and Temperature in Rapid Compression and Expansion Machine

<div>A rapid compression and expansion machine (RCEM) was used to experimentally investigate the ignition phenomena of dielectric-barrier discharge (DBD) in engine conditions. The effect of elevated pressure and temperature on ignition phenomena of a methane/air premixed mixture was investigated using a DBD igniter. The equivalence ratio was changed to elucidate the impact of DBD on flame kernel development. High-speed imaging of natural light and OH* chemiluminescence enabled visualization of discharges and flame kernel. According to experimental findings, the discharges become concentrated and the intensity increases as the pressure and temperature rise. Under different equivalence ratios, the spark ignition (SI) system has a shorter flame development time (FDT) as compared with the DBD ignition system.</div>

Organic superbase-mediated synthesis of borohydride ionic liquids as novel composite hypergolic fuels

This study prepared a series of novel hypergolic fluids based on borohydride ionic liquids and organic superbase using an in situ synthetic method. In these hypergolic fluids, ionic liquids in 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) acted as triggers for the self-ignition of DBN and DBU upon contact with high-concentration hydrogen peroxide (H2O2). These hypergolic fluids had high densities (>1.000 ​g ​cm−3), low viscosities (as low as 34.03 ​cP), and acceptable ignition delay times (IDT). The ignition processes of the hypergolic fluids with 90% H2O2 as an oxidizer were first investigated in this study, and they differed from the previously reported ignition phenomena. Different from the case with white fuming acid (WFNA) as an oxidizer, the ignition processes of hypergolic fluids with 90% H2O2 as an oxidizer did not exhibit secondary rebound and splashing and formed a homogeneous mixed layer when the droplets were in contact with 90% H2O2. The different ignition processes significantly influenced the properties of hypergolic fluids. Compared with the hypergolic fluids with WFNA as an oxidizer, those with 90% H2O2 as an oxidizer showed a shorter IDT (IDTmin[90% H2O2]=28.3 ​ms, IDTmin[WFNA]=126 ​ms) and formed stable flames without secondary combustion. These results demonstrate that the in-situ synthesized fuels in this study hold great promise as green fuels in hypergolic propulsion systems.