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A port injection engine that supplies hydrogen to the intake manifold or port exhibits a low intake air amount and torque output. The torque output is limited by backfire. In the present study, the performance and efficiency of a port injection-type hydrogen engine using the fuel injector of a natural gas engine is investigated at various engines speeds under wide open throttle conditions and two operation strategies, i.e., limiting nitrogen oxide emission and maximizing the torque. The increase in electrode temperature is reduced by increasing the heat rating number of the spark plug or reducing the ignition energy applied to the spark plug. Even when the ignition energy is reduced, as the minimum ignition energy of hydrogen is very low, it does not eliminate backfire. However, when a cold-type spark plug is used, backfire is effectively suppressed, resulting in an increase in the maximum torque and a decrease in efficiency.

To study the ignition energy characteristics and explosion propagation law of coal dust cloud, a kind of coal dust cloud is studied through experiment and numerical simulation under different conditions. The result indicated that ignition delay time and dust spray pressure have obvious effects on the minimum ignition energy of coal dust cloud. CFD theory is used to simulate the explosion flame propagation. It is found that the simulation error of flame propagation distance is acceptable and the simulation result is consistent with the experimental result. When the spray pressure is 0.06 MPa, the flame propagation distance is the farthest, indicating that the turbulence of coal dust cloud is the largest at this condition. As the ignition temperature increases, the flame propagation distance continues to increase, proving that ignition temperature has an obvious effect on the flame propagation process of coal dust cloud explosion.

In this work, the explosion characteristics of an aluminum (Al)–diethyl ether (DEE)–air mixture were investigated in a 20 L spherical vessel. The effect factors of the explosion characteristics considered were fuel concentration, component proportion, and ignition energy. With the increasing concentration of the mixed fuel (Al/DEE = 1:1), the maximum pressure (Pmax), the maximum rate of pressure rise ((dP/dt)max), and the flame propagation speed (νF) exhibit an inversely “U-shaped” curve. The maximum Pmax, (dP/dt)max, and νF values are 901.2 kPa, 148.3 MPa/s, and 15.3 m/s, respectively, corresponding to an optimum concentration of 600 g/m3. The Pmax, the (dP/dt)max, and the νF increase with the addition of DEE when the proportion of DEE is below 55% but have a decrease tendency when the proportion of DEE is over 55%. As the explosions of Al and DEE were mutually promoting, the studied explosion characteristics of the Al–DEE–air mixture are obviously higher than those of pure Al or DEE in air. The minimum ignition energy (MIE) of the Al–DEE–air mixture is 1.9 mJ, between the MIE of Al and DEE. With the increase of ignition energy, Pmax, (dP/dt)max, and νF all increase, while the minimum explosion concentration presents a linear decreasing trend. This work could provide significant scientific evidence for evaluating the explosion risk of the Al–DEE–air mixture.

Solids industries have inherent dust explosion hazards. Process risk assessment parameters include overpressure, deflagration index, and minimum ignition energy. These parameters are influenced by dust dynamics, such as turbulence and concentration. Cloud dynamics are affected by particle morphology, density, size, and polydispersity, which affect explosion parameters. Influence of properties such as density during explosion parameter measurement is often overlooked. This study examines particle density influence on dust clouds in the Kuhner MIKE3. Two clouds of particles with different density, but similar particle size and morphology were examined with high-speed digital in-line holography, providing velocity and concentration measurements prior to ignition. Results show velocity and concentration of denser dust is lower compared to lighter dust after ignition delay. The results highlight the density effect on cloud dynamics and make a case for particle density to be considered in setting ignition delay in order to get consistent cloud dynamics and test results.

Effect of water mist on minimum ignition energy (MIE) and lower and upper flammability limits (LFL and UFL) of C3H8–air mixtures were investigated by experiments and numerical simulation. Ignition experiments were performed in a combustion tube. Polydisperse water mist with a Sauter mean diameter of D32 = 21 µm was generated by a piezoelectric atomizer. Mixture was ignited by an exploding wire heated instantaneously by electrical discharge. Ignition probability follows the cumulative probability distribution function of energy density, i.e., energy per unit volume of the wire. Water mist increases minimum ignition energy density (MIED) defined at 50% ignition probability when the equivalence ratio φ 1.3. However, for 1.0

Microwave plasma ignition has the potential for energy savings and emission reductions, and the minimum ignition energy (MIE) and the lean limit can be determined by microwave spark plugs (MSPs) under the same microwave power. To predict the relationship among electric field intensity (EFI), equivalence ratio ( $\varphi$ ) and MIE, a physical model of microwave spark ignition is established. The calculation results show that the MIE has a U-shaped relationship with $\varphi $ , and the MIE value of methane-air is minimum at the point of $\varphi =0.7$ . Then, a novel MSP with three different electrode structures and EFIs is enabled. Finally, this paper presents a combustion performance analysis focusing on the flame kernel, combustion pressure, and composition of combustion emissions. The results indicate that a larger flame kernel size, peak pressure in the chamber and higher fuel efficiency are all achieved under the condition of $\varphi =0.7$ for the same MSP. On the one hand, the MIE of MSP 1 with a higher EFI is lower than that of MSPs 2 and 3; on the other hand, the experimental relationship between the MIE and $\varphi $ is qualitatively similar to the theoretical prediction under microwave spark ignition in this paper.

The minimum ignition energy (MIE) requirements for ensuring successful thermal runaway and self-sustained flame propagation have been analysed for forced ignition of homogeneous stoichiometric biogas-air mixtures for a wide range of initial turbulence intensities and CO2 dilutions using three-dimensional Direct Numerical Simulations under decaying turbulence. The biogas is represented by a CH4 + CO2 mixture and a two-step chemical mechanism involving incomplete oxidation of CH4 to CO and H2O and an equilibrium between the CO oxidation and the CO2 dissociation has been used for simulating biogas-air combustion. It has been found that the MIE increases with increasing CO2 content in the biogas due to the detrimental effect of the CO2 dilution on the burning and heat release rates. The MIE for ensuring self-sustained flame propagation has been found to be greater than the MIE for ensuring only thermal runaway irrespective of its outcome for large root-mean-square (rms) values of turbulent velocity fluctuation, and the MIE values increase with increasing rms turbulent velocity for both cases. It has been found that the MIE values increase more steeply with increasing rms turbulent velocity beyond a critical turbulence intensity than in the case of smaller turbulence intensities. The variations of the normalised MIE (MIE normalised by the value for the quiescent laminar condition) with normalised turbulence intensity for biogas-air mixtures are found to be qualitatively similar to those obtained for the undiluted mixture. However, the critical turbulence intensity has been found to decrease with increasing CO2 dilution. It has been found that the normalised MIE for self-sustained flame propagation increases with increasing rms turbulent velocity following a power-law and the power-law exponent has been found not to vary much with the level of CO2 dilution. This behaviour has been explained using a scaling analysis and flame wrinkling statistics. The stochasticity of the ignition event has been analysed by using different realisations of statistically similar turbulent flow fields for the energy inputs corresponding to the MIE and it has been demonstrated that successful outcomes are obtained in most of the instances, justifying the accuracy of the MIE values identified by this analysis.

Precise measurement of the minimum ignition energy (MIE) of combustible dust is crucial to safety in the process industries. The development of comprehensive methods for MIE dust cloud characterization is therefore highly desirable. The objective of this study is to investigate the application of high-speed digital in-line holography (DIH) for volumetric and in-situ characterization of dust clouds near the ignition zone of a Kuhner MIKE3 MIE device. The dispersion behavior of borosilicate glass and soda-lime glass dust clouds are studied at 20 kHz. Size measurements, with successful detection down to 13 μm, are compared with Beckman Coulter particle size analyzer results. The 5-ms hologram videos also yield new transient particle volume, concentration, and velocity measurements. Sample sizes of 15–150 mg of dust powder are sufficient for each high-speed run. This work showcases new diagnostic capabilities that are accessible to dust explosion research and identifies areas for future research.

The effects of droplet diameter, overall (i.e. liquid and gaseous phases) equivalence ratio, and turbulence intensity on the variation of the Minimum Ignition Energy (MIE) for localized forced igni...

A recent near miss involving ignition of a small amount of red phosphorus was investigated. The near miss is discussed in detail along with the root cause investigation. Six possible root causes we...

International audience; The maximum explosion overpressure and the maximum rate of pressure rise, which characterize the dust explosion severity, are commonly measured in apparatuses and under specific conditions defined by international standards. However, those standards conditions, designed for micropowders, may not be fully adapted to nanoparticles. Investigations were conducted on different nanopowders (nanocellulose, carbon black, aluminum) to illustrate their specific behaviors and highlight the potential inadequacy of the standards. The influence of the sample preparation was explored. Various testing procedures were compared, focusing on the dust cloud turbulence and homogeneity. Dust dispersion experiments evidenced the importance of the characterization of the dust cloud after dispersion, due to the fragmentation of agglomerates, using metrics relevant with nanoparticles reactivity (e.g. surface diameter instead of volume diameter). Moreover, the overdriving phenomenon (when the experimental results become dependent of the ignition energy), already identified for micropowders, can be exacerbated for nanoparticles due to their low minimum ignition energy and to the high energy used under standard conditions. It was evidenced that for highly sensitive nanopowders, pre-ignition phenomenon can occur. Finally, during severe explosions and due to a too long opening delay of the ‘fast acting valve’, the flame can go back to the dust container.

This paper demonstrates experimental investigation on the self-ignition and subsequent flame propagation of high-pressure hydrogen-methane mixture release via a tube. The proportion of methane added to hydrogen is 2.5% (vol.). A transparent rectangular tube (d = 15 mm, L = 400 mm) is used in the experiments. It is shown that the minimum burst pressure required for self-ignition increases 1.57 times for only 2.5% methane addition from 2.89 MPa (pure hydrogen) up to 4.68 MPa (2.5% CH4 addition). This is mainly caused by the following reasons: on the one hand, methane addition can result in the decease of shock intensity inside the tube, thereby lowering the temperature of the combustible mixture; on the other hand, the hydrogen-methane mixture has the higher minimum ignition energy than that of pure hydrogen. Besides, 2.5% methane addition can increase the initial ignition time, weaken the flame intensity and reduce the flame propagation velocity relative to tube wall inside the tube. Moreover, for cases with 2.5% methane addition, the complete flame throughout the tube is formed closer to the back end of the tube. When the self-sustained flame exits from the tube, the maximum overpressure in a confined space increases with 2.5% methane addition.

A method for computing minimum ignition energies for gaseous fuel mixtures with detailed and reduced chemistry, by numerical integration of time-dependent conservation equations in a spherically symmetrical configuration, is presented and discussed, testing its general characteristics and accuracy. The method is applied to methanol-air mixtures described by a 38-step Arrhenius chemistry description and by an 8-step chemistry description based on steady-state approximations for reaction intermediaries. Comparisons of predictions with results of available experimental measurements produced reasonable agreements and supported both the robustness of the computational method and the usefulness of the 8-step reduction in achieving accurate predictions. This work was supported by the Spanish MCINN through Projects # CSD2010-00011, ENE2012-33213 and ENE2015-65852-C2-1-R.

Hydrofluoroolefin 1234ze (1,3,3,3-Tetrafluoropropene) is a fluorinated hydrocarbon introduced as a low global warming potential alternative for HFC-134a (1,1,1,2-tetrafluoroethane) and is classified as mildly flammable (A2L). Definition of appropriate engineering controls and electrical area classification requires accurate knowledge of the flammable properties of a material. Gaps and conflicts in published flammability data for HFO-1234ze are a concern for defining safe handling practices and appropriate layers of protection. The data available in literature has been compiled and analyzed to understand the sources of the data variability and the conditions under which HFO-1234ze will pose a flammability hazard. New data on the minimum ignition energy at 25C, flammable limits at 25C, and maximum experimental safe gap at 40C is also reported here and discussed in context with the large body of data from literature. It is proposed that much of the data measured at ambient conditions in the absence of water vapor does not effectively measure the true limits of flame propagation. Additionally, it appears that ignition limitations, and potentially other design factors, in the current standard test methods may result in the inaccurate measurement of flammable limits for HFO-1234ze under some conditions. It is concluded that HFO-1234ze can be safely handled when appropriate engineering controls are implemented based on a thorough process safety risk assessment.

A study on forced ignition characteristics of RP-3 jet fuel-air mixture was conducted around a constant volume combustion vessel and a nanosecond pulsed plasma discharge power supply. Experiments were carried out at different initial pressures (pu = 0.2, 0.3, 0.5 atm), equivalence ratios (ϕ = 0.7, 0.8, 1.1), steam concentrations (ZH2O = 0%, 10%, 15%) and oxygen concentrations (ZO2 = 13.5%, 16%, 21%). The relationship between ignition probability and ignition energy is investigated. The experimental results show that the decrease in pressure, equivalence ratio, oxygen concentration and the increase in steam concentration all lead to an increase in minimum ignition energy (MIE). In order to further analyze the experimental data, one existing fitting equation is reformed with the initial conditions taken into account. Multivariate fitting is carried out for different conditions, and the fitting results of ignition probability are in good agreement with the experiments. The MIE results under different experimental conditions are figured out with the new fitting equation. The impact indexes, which stand for the effects of different factors, are also calculated and compared in present work.

This study aims to build a theoretical model to describe the ignition process of spherical spray flame laden with fuel droplets under overall fuel-lean condition. Two characteristic fronts are introduced to reveal where the evaporation starts and ends in the ignition process. The results demonstrate that the flame has been strengthened with extra fuel droplet adding while the minimum ignition energy (MIE) is decreased. The evolution of the two evaporation fronts in the flame kernel and propagating branches has also be revealed coupled with the evaporation zone length., arXiv admin note: substantial text overlap with arXiv:2103.14227; text overlap with arXiv:2009.12799

Methane is one of the most common gaseous fuels that also exist in nature as the main part of the natural gas, the flammable part of biogas or as part of the reaction products from biomass pyrolysis. In this respect, the biogas and biomass installations are always subjected to explosion hazards due to methane. Simple methods for evaluating the explosion hazards are of great importance, at least in the preliminary stage. The paper describes such a method based on an elementary analysis of the cubic law of pressure rise during the early stages of flame propagation in a symmetrical cylindrical vessel of small volume (0.17 L). The pressure–time curves for lean, stoichiometric and rich methane–air mixtures were recorded and analyzed. From the early stages of pressure–time history, when the pressure increase is equal to or less than the initial pressure, normal burning velocities were evaluated and discussed. Qualitative experiments were performed in the presence of a radioactive source of 60Co in order to highlight its influence over the explosivity parameters, such as minimum ignition energy, maximum rate of pressure rise, maximum explosion pressure and normal burning velocity. The results are in agreement with the literature data.

Knowledge of fire and explosion-related properties of substances including flammability limits, minimum ignition energies, and flash points are critical in process safety research. However, conducting experimental measurements to determine these properties is often time consuming and hazardous. The recent development of quantitative structure-property relationship (QSPR) analysis provides accurate and reliable predictions of such target properties. In this review, upper and lower flammability limits and temperatures (UFL, UFLT, LFL, and LFLT), minimum ignition energy (MIE), auto-ignition temperature (AIT), flash point (FP), and other related properties are selected for investigation. Developments of QSPR methods within the past decade to predict each of these properties are summarized and examined. These developments serve to address the increasing need for improved fire and explosion risk assessment methods in the process industries.

This paper reports laminar and turbulent minimum ignition energies (MIEL and MIET) of hydrogen/air mixtures at two equivalence ratios (ϕ = 0.18 and 5.1) where Lewis numbers Le ≈ 0.3 and 2.3, respectively, over wide ranges of the electrode spark gap (dgap = 0.3–6.5 mm) and the r.m.s. turbulent fluctuating velocity (u′ = 0–8.3 m/s). Depending on the coupling effects of Le, dgap, and u′, we explain what causes two distinct phenomena: Turbulent Facilitated Ignition (TFI) meaning MIEL >> MIET and MIE Transition meaning a change from MIET ≥ MIEL to MIET >> MIEL when u′ is greater than some critical value. High-speed Schlieren imaging shows that the embryonic spark kernel in quiescence is ball (rod) like when dgap 1 mm), demonstrating large (very small or negligible) positive curvature. This explains why TFI, an unusual phenomenon, only occurs at sufficiently small dgap > 1 because large positive curvature stretch weakens reaction rate due to differential diffusion, making successful ignition in quiescence very difficult to achieve. At dgap = 0.58 mm and Le ≈ 2.3, a non-monotonic decrease and increase of MIET with increasing u′ is observed, because the dissipation of ignition kernel by sufficiently intense turbulence re-declares its dominance leading to the increase of MIET. There is no TFI when dgap > 1 mm regardless of Le. The scenario changes to MIE transition when dgap = 2 mm at Le ≈ 2.3, where MIEL

Non-metallic combustible dusts contribute to more than 50% of dust explosion accidents. Almost 38% of dust explosion accidents relate to mechanical malfunction. Compared to electric sparking as an ignition source, the ignition hazard of non-metallic dust clouds exposed to simulated hotspots during mechanical malfunction has received little attention in the literature. Minimum ignition temperature of hotspots (MITH) for corn starch, wood dust, and polymethyl methacrylate (PMMA) dust fall within a narrow range from 710 to 745 °C although large differences in minimum ignition energy (MIE) were evident. A much narrower dust concentration range (around 1500 g/m3) was observed for MITH than for MIE. A longer ignition delay time when exposed to hotspots also indicated lower ignition hazard compared to ignition by electric sparking. Whether exposed to hotspots or electric sparks, average flame spread velocity (FSV) of PMMA dust was much higher than that of corn starch and wood dust. Once a dust cloud was ignited, pulsating flame propagation was similar for hotspots and electric sparking, but average FSV was higher for hotspots than for electric sparks, due to continuous radiation from the ignition source. At higher dust loadings, layer fires could occur due to sedimentation of many ignited and unburned particles exposed to hotspots.

The Minimum Ignition Energy (MIE) is an important dust hazard parameter that guides the elimination of possible ignition sources in solids handling facilities. Partial inerting is a dust explosion mitigation technique implemented in industries, where the dust cloud MIE is increased, reducing the risk of an accidental explosion. Previous work has shown that purging the MIE apparatus Hartmann tube before testing is essential for obtaining accurate MIE values, but have not discussed the importance of the effective purge time required. Through experimentation and CFD modeling, this study has attempted to refine the existing MIE testing standard for partial inerting applications by introducing purge time as an essential parameter. In this study, oxygen sensor measurements were conducted to determine that the purge time for the MIKE3 apparatus should be > 40 s. Additionally, an ANSYS Fluent CFD purging model was developed, which confirmed the experimentally determined purge time. Using this improved methodology, an accurate partial inerting data set was obtained for the combustible dusts Anthraquinone, Lycopodium clavatum and Calcium Stearate.

Recent experiments (Wu et al., 2014) demonstrated that ignition of a combustible mixture by a high-energy kernel can be facilitated by turbulence for certain conditions. This is contrary to the common notion that ignition in turbulence is more difficult than in quiescence because of the increased dissipation of the deposited energy. In this study, we extend our investigation to a larger range of mixtures and turbulence intensities to explore possible limitations of such facilitated behavior. First, we calibrate our ignition system to quantify the deposited ignition energy. Next, using a lean hydrocarbon/air mixture with Le > 1, we show that: (1) it is possible to ignite a mixture in a turbulent environment with an ignition energy that is not sufficient to ignite the same mixture in a quiescent environment, and (2) there exists an upper limit of turbulence intensity beyond which such facilitation is not feasible. Through detailed experiments an ignition map for such mixtures is constructed, which shows a window of ignition energy within which this non-monotonic transition is observed. The study also identifies a minimum ignition energy required to achieve either quiescent or turbulence-facilitated ignition for the Le > 1 mixtures, and demonstrates that Le

This work addressed laser-induced spark ignition (LSI) of dimethyl ether (DME)-air mixtures in a microgravity environment which was simulated in a laboratory-scale drop tower, to study fundamental ignition characteristics in microgravity. A laser-induced spark was adopted as a pilot source because LSI has potential benefits as compared to conventional electrical spark ignition: no wall effects, no heat-loss to electrodes, etc. Ignition characteristics, therefore, can be investigated without effects of ignition source via LSI. DME-air mixtures filled in a combustion chamber on a framework package were ignited during free-fall. The ignition energy and flame kernel development were investigated with varying the equivalence ratio and the laser pulse energy. The minimum ignition energy (MIE) was calculated statistically via the logistic regression method. Comparing MIE and flame kernel development between normal gravity and microgravity, MIE in microgravity was lower than that in normal gravity over tested equivalence ratios ranging from 0.575 to 0.675. The difference of MIE became significant with decreasing the equivalence ratio, the maximum value of which was 7.0 mJ at the lowest equivalence ratio. Flame kernel was deformed in normal gravity due to buoyant force. The flame kernel in the bottom region was often not able to develop against buoyant force. Ignition succeeded consequently when only the top of flame kernel developed into a self-sustaining flame to propagate throughout the entire chamber. This flame deformation was noticeable as the equivalence ratio decreased because the flame propagation speed also decreased. On the other hand, in microgravity, reduced buoyant force allowed the flame kernel to stably develop into a self-sustaining flame. This is the first time that both MIE and flame kernel development via LSI were found experimentally to be well affected by gravity.

This work studied laser-induced spark ignition for dimethyl ether (DME)–air mixtures with low velocities ranging from 10 cm/s to 100 cm/s, with an emphasis on the minimum ignition energy (MIE) and flame kernel development. Part of ignition experiments were conducted in a microgravity environment that was simulated in a laboratory-scale drop tower to eliminate natural convective flow. MIE was significantly affected by flow velocity. MIE first decreased with increasing flow velocity but then increased with further increasing flow velocity. The role of the flowing mixture was to act as a fuel supplier in a low-velocity flow, and in contrast, to increase convective heat loss in a high-velocity flow. The competition between these roles determined MIE. The effect of flow direction on MIE was also investigated by inclining the combustion chamber. The minimum value of MIE was the same in all flow directions. Therefore, changing flow directions was merely intended to vary the flow velocity by adding or removing buoyant flow to or from the forced flow, which caused the MIE distribution to shift from side to side. The flame kernel hardly developed against a forced flow and was often locally quenched. Flame deformation was noticeable as flow velocity increased. Quenching is comparable to the heat loss from the flame kernel. Therefore, flame kernel development should have a significant effect on MIE. Flame stretch was also found at a high velocity. A high-velocity flow did not allow the flame kernel to stably develop into a self-sustaining flame. Thus, higher energy was required to form a large-volume of flame kernel to overcome both conductive heat loss and the effect of flame stretch. Accordingly, flame kernel behaviors were very much a concern regarding the mechanism via which to determine MIE.

Adequate knowledge of minimum ignition energy (MIE) of a flammable chemical compound plays a significant role while handling and characterizing the hazardous materials and further ensures reliable ignition of fuel-air mixtures in many engines. Despite the significances of this parameter (MIE), its experimental determination is very dangerous, expensive and might be time consuming. The challenges associated with the experimental determination of minimum ignition energy are addressed in this present work through hybridization of gravitational search algorithm (GSA) with support vector regression (SVR) for estimating MIE using relatively few descriptors which include the number of carbon and hydrogen atoms as well as molecular weight of the compound. Novelties of this approach as compared with existing methods include (i) hybridization of GSA with SVR for modeling MIE for the first time, (ii) utilization of relatively few (three) descriptors and (iii) the ease with which the descriptors can be assessed. On the basis of root mean square error, the developed hybrid GSA-SVR shows superior performance as compared with the existing Beibei Wang et al. model with performance improvement of 24.03%. The accuracy of the proposed hybrid GSA-SVR model coupled with the ease of its implementation would definitely ensure quick estimation of MIE of compounds, prevent accidental explosion of hazardous chemicals in industry and enhance aviation safety.

Chemical kinetics effects on fundamental laminar combustion processes were investigated for 20% methanol-blended gasoline and 20% hydrogen-blended methanol for a range of equivalence ratios, \( 0.5 < \phi < 2 \) using a detailed reaction mechanism comprising of 2027 species and 8619 reactions. Fundamental laminar combustion properties—flame speed, flame thickness, minimum ignition energy, ignition, and extinction strain rates of the two fuel blends were compared with base fuels, methanol, gasoline, and hydrogen. Mean extinction strain rates were nearly twice the ignition strain rates which indicated a hysteresis behaviour of laminar flames. An in-depth analysis of the thermal and chemical flame structure of lean fuel blends was conducted to explore the significance of intermediate products and radical species on flame behaviour. Chemical kinetics caused a greater impact on the ignition process compared to extinction. A sensitivity analysis of ignition and extinction strain rates showed the importance of key reactions and their role in the improvement of laminar flame characteristics of fuel blends. The sensitivity coefficients for ignition were greater compared to extinction signifying a stronger influence of chemical kinetics on the ignition process compared to extinction. The knowledge obtained from this study can be leveraged in creating tailored fuel blends with superior laminar combustion competencies enabling higher efficiency and lesser pollutant emission for automotive applications.

International audience; In advanced propulsion systems such as scramjet engines, endothermic decomposition of onboard large hydrocarbon fuels can be used effectively for cooling and active thermal protection. During the cooling process, large hydrocarbon fuels absorb heat and decompose into small fragments. Since fuel decomposition changes the chemical and transport properties of the reactants, it is expected to affect the combustion afterwards. In this study, forced ignition in quiescent n-decane/air mixtures with fuel decomposition is investigated via a simplified model and transient numerical simulations considering detailed chemistry and transport. The emphasis is placed on assessing the effects of fuel decomposition on ignition kernel development and minimum ignition energy (MIE) in both homogenous and fuel-stratified mixtures. Fuel decomposition is modelled by a homogeneous ignition process in n-decane/air mixture at constant atmospheric pressure and with an initial temperature of 1300 K. Small fragments appear during the pyrolysis process. The partially-reacted mixture is frozen and cooled to a lower temperature and used as the initial mixture in forced ignition. For homogeneous mixtures, fuel decomposition can greatly promote ignition for fuel-lean decane/air mixtures while it has little effect for the stoichiometric case. Fuel decomposition also affects the duration of unsteady ignition kernel transition. Besides, fuel decomposition and fuel stratification are combined to further promote forced ignition. New flame regimes are observed and an optimum stratification radius is identified. In order to promote the forced ignition, only the fuel within the optimum stratification radius needs to be decomposed. Furthermore, laser and spark ignition experiments are conducted to measure the MIE of n-decane/ethylene/air mixtures with different equivalence ratios and ethylene blending ratios. The MIE measured in experiments cannot be directly compared with simulation results since the simulation model for forced-ignition is simplified. Nevertheless, the experimental results are consistent with simulation results and thus validate some conclusions mentioned above. The present results provide useful guidance to the fundamental understanding of forced ignition in a mixture with fuel decomposition.

The present theory considers the transient evolution of the temperature and fuel mass fraction distributions, and it can determine the critical heating power and minimum ignition energy for successful ignition. The flame initiation process subject to central heating can be approximately decomposed into four stages, i.e., the fast establishment of the ignition kernel, the ignition-energy-supported flame kernel propagation, unsteady transition of the flame kernel, and quasi-steady spherical flame propagation. Comparison between present transient theory and previous quasi-steady theory indicates that the unsteady effects directly lead to the appearance of flame kernel establishing stage and considerably affect the subsequent flame kernel development by lowering the flame propagation speed. Time scale analysis is conducted, and it is found that the transient formulation completely degenerates to the quasi-steady theory either in the neighborhood of stationary flame ball or as approaching the planar flame. Previous quasi-steady theory shows that the critical heating power for successful ignition is proportional to the cube of the critical flame radius. However, the critical heating power predicted by the present transient formulation deviates this scaling law, which can be attributed to the unsteady thermal conduction from heating center to the flame front. Furthermore, in the transient formulation the central heating with finite duration is considered and the minimum ignition energy is obtained. For the first time, the memory effect that persistently drives flame propagation subsequent to central heating switching-off is examined. It is found that as the heating power grows, the memory effect becomes increasingly important and it can greatly reduce the minimum ignition energy.

In this work, the explosion and combustion characteristics of aluminum and some aluminum alloys AlSi7Mg0.6, AlSi10Mg, AlMg5 under powders conditioning were studied. The idea was to compare the combustion of pure aluminum and aluminum alloys. The Minimum Ignition Energy (MIE) and explosion severity ΔPmax and (dP/dt)max which represents the dust explosion parameters were measured for all powders using Hartman tube and 20 L spherical bomb. The particles temperature and flame temperature were determined by using IR pyrometer and spectroscopy respectively. The results showed that pure aluminum was more sensitive and severe than its alloys. MIE were: 4 mJ for pure aluminum, 13–23 mJ for aluminum alloys. For severity parameters, the overpressure ΔPmax were around 7–8 bars with maximum rate of pressure rise at 1170 bar/s for aluminum and 5–7 bars with 250–360 bar/s for alloys. However, it has been observed that flame temperatures were similar for aluminum and alloys and vary around 2800–3300 K as a function of concentration.

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

In this paper, a semi-detailed mechanism is used to calculate the minimum ignition energy (MIE) of methane-air mixtures, with all calculations conducted using ANSYS Fluent software. Ignition behaviors between success and failure were analyzed, and the effect of isothermal and adiabatic wall condition, equivalence ratio and electrode wall temperature on the MIE were also numerically investigated. From time histories of temperature and species profiles it is easy to determine when a successful ignition occurs. There are big differences between the MIE predictions using the adiabatic wall and isothermal wall conditions. For different equivalence ratio results, we consider our numerical results to be considerably more accurate than previous studies. The authors would like to acknowledge the National Science and Technology Major Project (2017 III 0007 0032). Haitao Lu acknowledges financial support from China Scholarship Council. peer-reviewed 2022-09-28

Open Access; International audience; Faced with the problem of reducing greenhouse gas emissions and transitioning toward a greater use of renewable energies, ammonia, as an energy carrier, is increasingly seen as a potential 'green' fuel for transportation, in particular marine applications. However, its combustion characteristics (high minimum ignition energy and auto-ignition temperature, low combustion speed in comparison to usual hydrocarbon fuels) are drawbacks that have so far limited its use. Due to the evolution of different pollutant standards for road transportation, Spark-Ignition engines and thus the combustion process itself have been subjected to many changes over the last 20 years (e.g., gasoline direct injection, downsizing). The objective of this article is to discuss the potential of ammonia as a fuel for spark-ignition engines thanks to the studies carried out so far and to point out directions for future work.

The risks associated with dust explosions still exist in industries that either process or handle combustible dust. This explosion risk could be prevented or mitigated by applying the principle of inherent safety. One effective principle is to add an inert material to a highly combustible material in order to decrease its ignition sensitivity. This paper deals with an experimental investigation of the influence of inert dust on the minimum ignition temperature and the minimum explosion energy of combustible dust. The experiments detailed here were performed in a Godbert&ndash, Greenwald (GG) furnace and a 1.2 L Hartmann tube. The combustible dust (polyethylene&mdash, PE, 800 mesh) and four inert powders (NaHCO3, Na2C2O4, KHCO3, and K2C2O4) were used. The suppression effects of the four inert powders on the minimum ignition temperature and the minimum explosion energy of the PE dust have been evaluated and compared with each other. The results show that all of the four different inert dusts have an effective suppression effect on the minimum ignition temperature and the minimum explosion energy of PE dust. However, the comparison of the results indicates that the suppression effect of bicarbonate dusts is better than that of oxalate dust. For the same kind of bicarbonate dusts, the suppression effects of potassium salt dusts are better than those of the sodium salt. The possible mechanisms for the better suppression effects of bicarbonate dusts and potassium salt dusts have been analyzed here.

In the context of explosion protection, very conservative safety factors need to be considered, e.g. in the design of electrical devices. This is due to standards which are mainly based on empirical data as opposed to a detailed knowledge of the underlying physiochemical processes. In this work, the early phase of ignition of burnable gas mixtures close to their respective minimum ignition energy is investigated experimentally by means of high-speed schlieren imaging. Our data quantifies how the ignition process at such low energies becomes less repeatable which is evidenced by a high scattering of the flame propagation. It was found that, depending on the mixture, the flow field induced by the electrical discharge may exhibit a considerable effect on the ignition process. This effect is more pronounced for mixtures which are characterized by a large Lewis number, thus, leading to a more random flame propagation.

Hydrogen refueling stations associated with on-site hydrogen production systems based on the organic chemical hydride method (HRS-OCH) will process both hydrogen (H2) and toluene generated by the dehydrogenation of methylcyclohexane. In such systems, toluene containing varying levels of dissolved H2 will be stored in on-site tanks, such that H2 could be released into the tank headspace to form flammable toluene/H2/air mixtures. As a means of examining the associated safety hazards, the present work experimentally determined the minimum ignition energy (MIE) values for toluene/H2/air mixtures with varying proportions, using a cylindrical explosion vessel. The MIE for toluene containing 1 vol% H2 was found to be 0.319 mJ (the ignition probability is greater than 3.3 %), and thus similar to the values for pure toluene/air mixtures. H2 proportions above 2 vol% decreased the MIE by an order of magnitude, such that the value approached that for pure H2/air mixtures (0.021 mJ). This low MIE would allow these mixtures to be readily ignited by a static electrical discharge from toluene after accumulating a charge through flow processes. To reduce the ignition risk to an acceptable level, the concentration of H2 in the gas phase of HRS-OCH storage tanks should therefore be less than 1 vol%. Consequently, safe operation of HRS-OCH facilities requires the installation of H2 stripping processes, such as the use of membrane separation units.

Gases, vapors, and dusts are all potential explosion threats; however, mists should also be taken into account. Indeed, dozens of accidents involving hydrocarbon mists were identified in incident surveys. Mist explosions continue to occur, highlighting the need to evaluate and assess the validity of present approaches for assessing mist ATEX risks and to establish reliable standardized safety parameters for fuel mists. In a modified apparatus based on the 20 L explosion sphere, three fluids of industrial interest were investigated. A new siphon injection system comprising a Venturi junction was installed, offering a wide range of dispersion performances. This system was controlled by a specifically developed program, ensuring the apparatus's versatility and adaptability to various tested liquids. It enables precise control of the gas carrier flow, liquid flow, and injection and ignition durations, allowing modification of the dilution rate of a particular droplet size distribution (DSD). The mist cloud dispersed in the 20 L sphere was characterized by determining its DSD using an in-situ laser diffraction sensor and by performing Particle Image Velocimetry (PIV). Mists of kerosene, diesel and ethanol were then subjected to tests to assess their lower explosive limit (LELmist), minimum ignition energy (MIE), maximum explosion pressure (Pmax), and rate of pressure rise (dP/dtmax). For instance, it was found that the LELmist of ethanol, kerosene Jet A1, and diesel fuel for a DSD averaged at 8–10 μm reach 77, 94, and 93 g/m3 respectively. This LELmist was also shown to increase with increasing DSD in the case of Jet A1 mists. A sensitivity study was also performed to emphasize the impact of parameters such as the fuel type, the DSD, and the mist temperature. Findings showed that the explosion severity is strongly influenced by the chemical nature and the volatility of the dispersed fuel. Moreover, controlling the sphere temperature was proven to be a crucial step when using such apparatus for the evaluation of the explosibility of mists. An evaporation model based on the d2 law was also developed to visualize the vapor-liquid ratio before ignition. These findings have already led to the development of a new procedure for determining safety standards for hydrocarbon mists, as well as tools to assess mist explosion risks. They have proven that it is possible to evaluate the ignition sensitivity and explosion severity of fuel mists using a single well-known apparatus.

The Ti6Al4V powder alloy combustion is studied in order to define dust explosion parameters for industrial safety and combustion mechanism for scientific analysis. No results exist in literature of dust explosion concerning titanium alloy like Ti6Al4V. Three different powders with different average particles size (around 13 μm, 26 μm and 55 μm) were studied. The interest comes from its wide use in metallurgy, especially for aeronautical, dental and defense applications. This paper reports the minimum ignition energy (MIE) of the alloy powders as a function of the concentrations. Hartmann tube apparatus was used in this way. Results can be synthetized as: 3.5 mJ for 13 μm, 4 mJ for 26 μm and 357 mJ for 55 μm particle size. When comparing the values of MIE for pure titanium (Ti) and pure aluminum (Al) powders, the alloy sensibility is near pure Ti. Severity parameters and temperature measured by IR pyrometry were investigated during 20 L spherical explosion vessel tests. Results shown 6 bar overpressure and rate of pressure rise at 546 and 474 bar/s for 13 and 26 μm respectively. In case of the powders average diameter around 55 μm, ignition occurred in only one condition: 1500 g/m3. Temperatures were measured during explosion following two different scales: versus time and versus concentration.

Effects of Soret diffusion on the ignition of hydrogen/air mixtures by a heated kernel, and the structure and dynamics of the embryonic flame that is subsequently formed, were investigated numerically with detailed chemistry and transport. Results show that Soret diffusion leads to larger (smaller) minimum ignition energy (MIE) for relatively rich (lean) mixtures, that this effect is mainly engendered by the Soret diffusion of H2 while that of the H radical is almost negligible, and that Soret diffusion also leads to an increase (decrease) of the Markstein length for rich (lean) mixtures. Satisfactory agreement with literature experimental data on the MIE is shown, especially for the critical states near lean and rich flammability limits. Evolvement of the flame structure shows that before the self-sustained flame is formed, the high temperature gradient associated with the ignition kernel has driven the H2 in the mixture towards the ignition kernel and formed a locally high H2 concentration region, which consequently renders lean (rich) mixtures easier (harder) to ignite. It is further shown that Soret diffusion of both H and H2 affect the propagation dynamics of the stretched spherical flame that is subsequently formed, from its embryonic state until that of free propagation, in that Soret diffusion of H2 is the dominant mode at small flame radius with the large strain rate, while that of H is the dominant mode at large flame radius with the small strain rate similar to that of the unstretched adiabatic planar flame.

Due to both environmental concerns and the depletion of the reserves of fossil fuels, alternative and more environmentally friendly fuels, such as biomass and waste products, are being considered for partial or full fossil fuel replacement. The main disadvantage of these products is their lower energy density compared to fossil fuels. To deal with this several heat and power generation facilities are co-firing fuel mixtures. These processes involve mixtures of flammable dusts whose ignitability and explosibility characteristics are not known and therefore present un-quantified safety risk to the new technologies. This study reports on these risks and on the reactivity characteristics of two and three components dust mixtures of coal/sewage-sludge/torrefied-wood-pellet. In particular chemical composition, ignition sensitivity parameters (including minimum ignition energy, minimum ignition temperature on a layer, minimum explosive concentration) and flame speed have been determined. In all cases the measured parameters for the mixtures were within the range defined by the lower and upper value of the constituent. However, the expected values do not agree with the experimentally obtained ones, providing more relaxed values than the ones needed on this facilities.

The influences of carbon nanotubes (CNTs) additions on the flash ignition characteristics of Iron (Fe) and aluminum (Al) nanoparticles (NPs) were presented. CNTs can be used as the additive to these metal nanoparticles for improving the flash ignition and burning processes. Different mass fractions of CNTs additions were considered. The mixture of Al and CNTs could combust in air with obvious giant flame, whereas the mixture of Fe and CNTs combusted under a relative stable condition with slight red light. The temperature distributions were measured using non-contact optical method and showed that Al NPs mixed with CNTs were burning at a higher temperature level than Fe NPs. Although different mass fractions of CNTs cannot significantly change the overall flash ignition phenomenon, CNTs additions influenced the minimum ignition energy (MIE) of mixtures. The appropriate content of CNTs addition can decrease the Fe NPs MIE significantly. However, the Al NPs MIE decreased all along with the increase of CNTs content. The micro- and nano- structures of Fe and Al NPs with CNTs additions before and after ignitions were examined by scanning electron microscope and high-resolution transmission electron microscopy. It was found that the special thermal conductive characteristics of CNTs and the cross-connected features for metal particles with CNTs caused the enhancement of flash ignition.

The first-stage ignition delay of n-heptane/air mixtures is computationally studied using detailed mechanism and theoretically studied using eigenvalue analysis of simplified systems. Results show that the delay has a turnover behavior as temperature increases, being dominated by the competition of low-temperature branching and termination channels as well as the competition of forward and reverse reaction channels. As temperature increases to the intermediate range, the termination and reverse pathways result in a minimum in the delay, the state of which is theoretically derived. Simple analytical solutions for the delay as well as the species evolutions are presented to identify the rate constants that control the first-stage ignition and quantify the influence of the mixture composition, initial temperature and system pressure. It is further demonstrated that the above results also hold for n-octane/air and iso-octane/air mixtures.

It is challenging to efficiently ignite traditional liquid hydrocarbons when they are applied in extreme combustion conditions, e.g., scramjet combustor. In this study, the low-temperature ignition enhancement performance of fuel additives of various properties and concentrations in n-decane were investigated theoretically and experimentally. The theoretical results indicate that there exists an optimized relationship between the effective activation energy and vaporization rate. Based on the optimization study, a novel design method for composite fuel additives was proposed. With this method, a metallic organic compound blended with a low-boiling-point auxiliary, methoxydiethylborane/tetrahydrofuran (MDEB/THF) solution, was employed to modify the ignition performance of n-decane. Thermogravimetry analysis and droplet-hot plate impinging experiment were performed to characterize the vaporization rate, ignition temperature, and ignition delay time of n-decane-based hybrid fuels. The experimental results suggest that, at a high concentration of MDEB/THF fuel droplets, the minimal ignition surface temperature is reduced to 160 °C, which is approximately 500 °C lower than that of pure n-decane. The ignition delay time of the fuel droplet is diminished from 353 ms to 45 ms at a surface temperature of 500 °C. Moreover, controlling the low-temperature ignition performance of an n-decane-based hybrid fuels by mixing various proportions of the additive was confirmed to be feasible. The results obtained from this study are of great significance in jet propellant design.

Trifluoroethylene and its mixtures are useful as next generation refrigerants because of their lower global warming potential (GWP) than that of those ordinarily used. Trifluoroethylene, however, has the potential to disproportionate explosively, as does tetrafluoroethylene. In the present research, we quantitatively searched for conditions in which trifluoroethylene undergoes disproportionation. We modified the ignition apparatus as so it could be performed using electromagnetic force, we simulated spark phenomena in air conditioners and analyzed the disproportionation process of trifluoroethylene using that equipment. The spark energy yielding 1% probability of disproportionation was found to be 6.33 J, at which pressure was 1 MPa and temperature was 373 K. It was revealed that trifluoroethylene had much higher minimum ignition energy than that of ordinary combustible materials such as methane and hydrogen. The reason for the high ignition energy needed to initiate disproportionation of trifluoroethylene was considered to be that the flame kernel needed to have a large size and a long lifetime to propagate the disproportionation reaction because of its lower frequency factor of producing solid graphite. The fact that the delay time of trifluoroethylene disproportionation was 0.1 s also explained the lower frequency factor of producing solid graphite.

This paper measures turbulent spark ignition probability and minimum ignition energy (MIE) of the pre-vaporized iso-octane/air mixture at an equivalence ratio ϕ = 0.8 at 373 K with Le ≈ 2.98 over a wide range of turbulent intensities ( u′ / S L ), where Le is the mixture's effective Lewis number and S L is the laminar burning velocity. Ignition experiments using a fixed 2-mm electrode gap are conducted in a large dual-chamber, constant-temperature/pressure, fan-stirred 3D cruciform burner capable of generating near-isotropic turbulence. Spark discharges having nearly square voltage and current waveforms are created for accurate determination of the ignition energy ( E ig ) across the electrodes. MIE E ig(50%) that is determined statistically from many repeated experiments at a given condition using a range of E ig to identify an overlapping energy band within which ignition and non-ignition coexist even at the “same discharge E ig ”, where the subscript “ig(50%)” indicates 50% ignitability. Results show that the increasing slopes of MIE T /MIE L = Г versus u′ / S L change drastically from linearly to exponentially when u′ / S L is greater than a critical value of 4.8, which is much smaller than previous rich methane data ( Le > 1) at ϕ = 1.2 with ( u′ / S L ) c ≈ 16 and at ϕ = 1.3 with ( u′ / S L ) c ≈ 24, revealing MIE transition. The subscripts T and L represent turbulent and laminar properties. When a reaction zone Peclet number Pe RZ = u ′ η k / α RZ is used for scaling, it is found that both present lean iso-octane and previous methane data can be collapsed onto a general correlation of Г 1 = 1 + 0.4 Pe RZ in the pre-transition and Г 2 ∼ Pe RZ 4 in the post-transition with the transition occurring at ( Pe RZ ) c ≈ 4.2, showing similarity on MIE transition. η k is the Kolmogorov length scale of turbulence and α RZ is the reaction zone thermal diffusivity estimated at the instant of kernel formation.

An approximate analytical model was established in this study to predict the transient mass flux and ignition time of solid combustibles exposed to time-dependent exponentially increasing heat flux. Critical mass flux was employed as the ignition criterion in the developed model. A new approximation strategy was used to simplify the complicated exact solution and to derive explicit correlations between ignition time and heat flux. Linear and quadratic heat fluxes were focused and the conclusions under other heat fluxes can be extended through analogy. An equivalent ignition temperature, involving critical mass flux, thermodynamics and chemical kinetics, was found in this study. The negative square root of ignition time is linearly proportional to the heat flux at ignition time. Under linear and quadratic heat fluxes, the ignition heat flux increases with a 1/3 and a 1/5 ( a is a constant in the heat flux expression), respectively, whereas the total heat absorbed by the solid before ignition is proportional to a −1/3 and a −1/5 , respectively. The capability of the proposed model was validated by another analytical model, numerical simulations and experimental data of black PMMA (Polymethyl Methacrylate) and pine wood. Furthermore, the effect of surface heat loss on the predictions of the proposed model was estimated and parametric study based on critical mass flux was implemented.

An experimental study on dielectric barrier discharge (DBD) plasma-assisted ignition was performed through the use of a plug-type DBD electrode at various pressures p up to 500 kPa. The ignition behaviors of lean methane–air mixtures with high-frequency DBD were investigated near the lean limit in a constant volume chamber. Using a high-speed video camera, the plasma formations of the DBD were observed for air and a mixture of the equivalence ratio ϕ = 0.3. Plasma spectroscopy was also conducted with an echelle grating and imaging spectrometers. The time-averaged detailed plasma emission spectra from 400 nm to 920 nm and the transient spectrograms in the vicinity of the N2 second positive system (372–382 nm) for ϕ = 0.5 were measured. The minimum ignition energy (MIE), minimum discharge duration, and minimum amplitude of the applied voltage were obtained through logistic regression. The results indicated that a plasma emission lifetime became much longer than the discharge time with an application of the high-frequency sinusoidal voltage at high pressure. The longest plasma lifetime was observed at a discharge frequency of 1,180 kHz. The plasma lifetime showed the maximum at a specific pressure. The detailed spectroscopy also showed that continuum spectra at 500 kPa could be fitted to the Planck's curve with temperature above 3000 K at 500 kPa. The profiles of the MIE as a function of pressure exhibited the minimum in the vicinity of 100 kPa. The existence of the optimal frequency for high-efficiency ignition was shown. At the fixed frequency, an increase in the applied voltage amplitude resulted in a decrease in the MIE. In contrast, the effects were saturated over a certain threshold. At 500 kPa, time-resolved plasma spectroscopy showed a rapid local gas heating to the flame temperature over a specific frequency of the applied voltage. This indicated that there was the optimal frequency where the MIE was the minimum.

Most studies, employing ignition temperature as the ignition criterion, utilized surface absorption of radiant incident heat flux in analytical models when investigating the ignition mechanism of solid combustibles. However, in-depth absorption exerts its influence on ignition time significantly for translucent solid, especially at high radiant heat flux. In this work, we extend the previous researches from surface absorption to in-depth absorption to develop an approximate analytical ignition model using critical mass flux instead of critical temperature. An approximation methodology is proposed during derivation to study the in-depth absorption scenario. The comparison among this model, available experimental data of black PMMA in the literature and previous numerical simulations indicates that the proposed model provides relatively high accuracy in predicting ignition time. Furthermore, the pure surface absorption circumstance is also reexamined and compared with the classical ignition theory. The results show that surface absorption hypothesis accelerates the total mass flux, which consequently shortens the ignition time. However, in-depth absorption assumption eliminates the heat accumulation on surface and results in good prediction for ignition time at high heat flux. For in-depth absorption, the absorption coefficient affects the heat penetration depth and temperature distribution in this layer which consequently affects the thermal degradation reaction rate, mass flux and finally ignition time. Meanwhile, the ignition time considering both surface and in-depth absorption is discussed, and the relationship with pure surface and in-depth absorption conditions is obtained.

Ignition delay times were measured behind reflected shock waves under fuel lean, high pressures conditions for conventional (JP-8) and alternative jet fuels — Fischer-Tropsch (F-T), alcohol to jet (ATJ), direct sugar to hydrocarbon (DSHC), and four bio-jet — and a biodiesel-like fuel. The primary goal of this study was to investigate the effect of fuel structure on ignition delay in order to assess differences in fuel ignition delay relative to JP-8. To support this effort, the ignition characteristics of a few hydrocarbons that are proposed as jet fuel surrogate components were also investigated: n-heptane, n-dodecane, m-xylene, n-dodecane/m-xylene blend (77%/23% by volume) and 2,2,4,6,6-pentamethylheptane (iso-dodecane). Two single-pulse shock tubes (heated and non-heated) were used to obtain the current data set. Using argon as the diluent (93% by volume), the experimental conditions covered a pre-ignition temperature range of approximately 980–1800 K at a pressure of 16 ± 0.8 atm and an equivalence ratio of 0.5. Experimental results show negligible differences between the ignition delay times of JP-8, F-T, DSHC, biodiesel, and the four bio-jet fuels. They are very similar to those of n-heptane, n-dodecane, n-dodecane/m-xylene blend and iso-dodecane under similar conditions. However, the ignition delay times of the ATJ were slightly longer at higher temperatures, and similar to iso-dodecane. m-Xylene reported significantly longer ignition delay times than the other fuels tested under identical conditions. Kinetic modeling results show that light branching and carbon chain length of large n-paraffins do not alter ignition delay because the predominant reactions are oxidation of C1-C4 fuel fragments. They also show for the n-dodecane/m-xylene blend that a discernable slowing of ignition delay by fuel decomposition and oxidation of m-xylene occurs at m-xylene concentrations greater than 50% (by mol.). Further testing is required to understand this behavior at stoichiometric and fuel rich conditions. In addition, an empirical correlation was obtained between the global activation energy of the measured ignition delay times for the single paraffinic species and their derived cetane numbers. This contributes to recent efforts aimed to provide relationships between pre-vaporized, homogeneous combustion properties with existing combustion indicators.

In this study, a physical model of the dust cloud ignition process is developed for both cylindrical coordinates with a straight-line shaped ignition source and spherical coordinates with a point shaped ignition source. Using this model, a numerical algorithm for the calculation of the minimum ignition energy (MIE) is established and validated. This algorithm can evaluate MIEs of dusts and their mixtures with different dust concentrations and particle sizes. Although the average calculated cylindrical MIE (MIEcylindrical) of the studied dusts only amounts to 63.9% of the average experimental MIE value due to reasons including high idealization of the numerical model and possible energy losses in the experimental tests, the algorithm with cylindrical coordinates correctly predicts the experimental MIE variation trends against particle diameter and dust concentration. There is a power function relationship between the MIE and particle diameter of the type MIE ∝ dpk with k being approximately 2 for cylindrical coordinates and 3 for spherical coordinates. Moreover, as dust concentration increases MIE(conc) first drops because of the decreasing average distance between particles and, at fuel-lean concentrations the increasing dust cloud combustion heat; however, after the dust concentration rises beyond a certain value, MIE(conc) starts to increase as a result of the increasingly significant heat sink effect from the particles and, at fuel-rich concentrations the no longer increasing dust cloud combustion heat.

At 38th Symposium, Shy and co-workers investigated a turbulence-facilitated-ignition (TFI) phenomenon via both conventional-single-spark-discharge (CSSD) and nanosecond-repetitively-pulsed-discharge (NRPD) at various pulsed-repetitive-frequency (PRF=5–70 kHz) using a fixed inter-electrode gap (dgap=0.8 mm) in a randomly-stirred lean n-butane/air mixture with Lewis number Le≈2.2 ≫ 1 over a range of r.m.s. turbulent fluctuating velocity (u′=0–2.1 m/s). It was found that the ignition probability (Pig) decreases significantly with increasing u′ for most PRFs, except at PRF=60 kHz having turbulent Pig,T = 37% at u′=0.5 m/s > laminar Pig,L = 34%, showing possible TFI. Using the same methodology, this paper presents two new Pig datasets at dgap=0.6 mm and 2.0 mm over wider ranges of PRF=1–90 kHz and u′=0–2.8 m/s. All NRPD experiments apply the same total ignition energy Etot≈23 mJ via a train of 11 pulses, each pulse having about 2.2 mJ except for the first pulse having about 1 mJ when using sufficiently high open-circuit-voltage of 28 kV. Note that Etot≈MIEL≈23 mJ at dgap=0.8 mm as a baseline for comparison; MIEL is the laminar minimum ignition energy measured by CSSD at 50% ignitability and MIEL=26.3 mJ/3.4 mJ at dgap=0.6 mm/2.0 mm. While TFI is noticeably observed at dgap=0.6 mm when PRF=60 kHz with Pig,T = 92% at u′=0.5 m/s > Pig,L = 78%, turbulence reclaims its dominating role when u′ > 1.4 m/s with Pig,T u′c (critical u′ depending on PRF). We also discover that the curves of Pig,T versus PRF are highly non-monotonic with peaks around PRF=40 kHz for u′ > 1.4 m/s; too high PRFs are detrimental for spherical flame ignition. These results are explained by coupling effects of dgap, PRF, u′, and differential diffusion.