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Minimum ignition energy (MIE) has been extensively studied via experiments and simulations. However, our literature review reveals little quantitative consistency, with results varying from 0.324 to 1.349 mJ for ϕ = 1.0 and from 0.22 to 0.944 mJ for ϕ = 0.9. Therefore, there is a need to resolve these discrepancies. This RANS study aims to partially address this knowledge gap. Additionally, it presents other flame evolution parameters essential for robust combustion design. Using the reactingFOAM solver, we predict the threshold energy required to ignite the fuel mixture. For this, the single step using the Arrhenius law is selected to model ignition in the flame kernel of stochiometric and lean CH4/air mixtures, allowing it to develop into a self-sustained flame. The ignition power density, an energy quantity normalised with volume, is incrementally varied, keeping the kernel critical radius rs constant at 0.5 mm in the quiescent mixture of two equivalence ratios ϕ 0.9 and 1.0, for varied operating pressures of 1, 5, and 10 bar at the constant initial temperature of 300 K. The minimum ignition energy is validated with twelve independent 1-bar datasets both numerically and experimentally. The effect of pressure on MIEs, which diminish as pressure rises, is significant. At ϕ = 1.0 (and 0.9), the flame temperature reached 481.24 K (457.803 K) at 1 bar, 443.176 K (427.356 K) at 5 bar, and 385.56 K (382.688 K) at 10 bar. The minimum ignition energy was validated using twelve independent 1-bar datasets from both numerical simulations and experiments. The results show strong agreement with many experimental findings. Finally, a mathematical formulation of MIE is devised; a function of pressure and equivalence ratio shows a slightly curved relationship. [ABSTRACT FROM AUTHOR]

In order to study the factors affecting the ignition risk parameters of pulverized coal cloud, Godbert-Greenwald constant temperature furnace was used to test the minimum ignition temperature of pulverized coal cloud under different particle size, mass concentration and powder injection pressure conditions. The results show that the minimum ignition temperature increases with the increase of particle size. In the range of 0.250−0.667 kg/m3, the mass concentration of pulverized coal decreases with the increase of mass fraction, and the minimum value is 545 ℃. With the increase of dusting pressure, it changes in the shape of “V”. The degree of influence of the three factors on the minimum ignition temperature is as follows: mass fraction > powder injection pressure > coal particle size.

Given the prevalence of Al/RDX energetic dust in industrial production, this study investigates their ignition safety and explosion hazards using Hartmann tubes and 20 L spherical explosion containers. The minimum ignition energy (MIE) increases from 20 mJ to 55 mJ as RDX content increases, indicating that adding RDX enlarges the effective particle size of the dust, thereby reducing its ignition sensitivity. A logistic regression model was used to develop a fitting equation for ignition probability, correlating dust concentration and ignition energy. The flame propagation velocity and explosion pressure of the energetic dust initially increase and then decrease with increasing RDX content. Optimal combustion characteristics were observed at 20 % RDX, with the maximum flame propagation velocity of 101 m s−1. However, the peak flame temperature decreases with increasing RDX content. The maximum explosion pressure and maximum explosion index at 1.77 MPa and 186.99 MPa m s−1 respectively, at 40 % RDX content. These findings highlight that 20 % RDX content yields the best combustion characteristics, while 40 % RDX content offers the maximum explosive potential, providing valuable insights for enhancing safety measures in industrial applications involving energetic dusts.

There are flammable and explosive gases such as gas underground in coal mines. The electromagnetic waves radiated by the 5G wireless communication system base station antenna are absorbed by the underground metal structure, generating discharge sparks at the metal structure breakpoint. When the energy of the electric spark reaches the minimum ignition energy of gas, an explosion may occur, which limits the application of 5G technology in coal mines. In order to evaluate the safety of the RF power of 5G wireless communication base stations, the relationship between RF power, maximum radiation field strength, and distance is obtained by analyzing the coupling of electromagnetic waves with metal structures. Using the minimum ignition energy as the safety criterion, it can be concluded that when the receiving power of the antenna load is less than 2.625 W, it can ensure that it will not cause gas explosions. The analysis shows that 700 MHz should be given priority as the 5G working frequency band in coal mines underground. By analyzing the directional coefficient, it is concluded that a symmetrical oscillator antenna metal structure with an arm length to wavelength ratio of 0.65 should be chosen for research. The safe electric field strength of the symmetrical oscillator antenna metal structure is 202.9 V/m, and the minimum safe distance is 0.2 m. The simulation results show that the electric field distribution is extremely uneven in areas less than 0.2 m away from the transmitting antenna. The electric field distribution is relatively even in areas more than 0.2 m away from the transmitting antenna. The minimum radio frequency power that causes a gas explosion in an area greater than 0.2 m from the transmitting antenna is 27.45 W.

Minimum ignition energy (MIE) has been extensively studied via experiments and simulations. However, our literature review reveals little quantitative consistency, with results varying from 0.324 to 1.349 mJ for ϕ = 1.0 and from 0.22 to 0.944 mJ for ϕ = 0.9. Therefore, there is a need to resolve these discrepancies. This RANS study aims to partially address this knowledge gap. Additionally, it presents other flame evolution parameters essential for robust combustion design. Using the reactingFOAM solver, we predict the threshold energy required to ignite the fuel mixture. For this, the single step using the Arrhenius law is selected to model ignition in the flame kernel of stochiometric and lean CH4/air mixtures, allowing it to develop into a self-sustained flame. The ignition power density, an energy quantity normalised with volume, is incrementally varied, keeping the kernel critical radius rs constant at 0.5 mm in the quiescent mixture of two equivalence ratios ϕ 0.9 and 1.0, for varied operating pressures of 1, 5, and 10 bar at the constant initial temperature of 300 K. The minimum ignition energy is validated with twelve independent 1-bar datasets both numerically and experimentally. The effect of pressure on MIEs, which diminish as pressure rises, is significant. At ϕ = 1.0 (and 0.9), the flame temperature reached 481.24 K (457.803 K) at 1 bar, 443.176 K (427.356 K) at 5 bar, and 385.56 K (382.688 K) at 10 bar. The minimum ignition energy was validated using twelve independent 1-bar datasets from both numerical simulations and experiments. The results show strong agreement with many experimental findings. Finally, a mathematical formulation of MIE is devised; a function of pressure and equivalence ratio shows a slightly curved relationship.

Copyright of Journal of Mine Automation is the property of Industry & Mine Automation Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

GB 3836.1-2021 Explosive atmospheres - Part 1: Equipment-General requirements stipulates that the RF threshold power of RF equipment in explosive environments shall not exceed 6 W. This regulation is derived from EU standards and lacks experimental verification, seriously restricting the application of 5G technology in mines. In order to reassess the safety of electromagnetic wave energy radiated by 5G communication equipment in mines, it is analyzed that the form of discharge generated by metal structures coupling electromagnetic waves should be low voltage breaking circuit arc discharge. The discharge energy generated by metal structures coupling electromagnetic waves is analyzed. The half wave oscillator structure that is most easily coupling with electromagnetic waves is selected as the research object. Through comparison, it is found that the discharge energy generated by the equivalent DC discharge circuit equivalent half wave oscillator is greater than that generated by the equivalent high-frequency discharge circuit. Therefore, the analysis of the safety of metal structure coupling electromagnetic wave discharge can be transformed into the analysis of the safety of the equivalent DC discharge circuit of equivalent half wave oscillator antenna of the metal structure. The safety judgment principle of intrinsically safe DC circuit is selected to judge the safety of the equivalent DC discharge circuit of equivalent half wave oscillator . By calculating the discharge power and energy, it is concluded that 5G RF equipment will not ignite explosive gas when its radiated power is not greater than 10.5 W. Therefore, the safe radiated power of RF base station of 5G communication system can be increased to 10.5 W.

Understanding how dust can ignite and explode in an industrial contest is an important and complex task, and much of the work around this is mainly performed via experimental measurements, in accordance to specific standards. However, those same properties are straightforwardly closely related to the nature of the experimental tests. Among these, the Minimum Ignition Energy (MIE) of a dust cloud, that is usually measured in a MIKE 3 apparatus, can be affected by several factors, as: delay time of the electric spark with respect to the dust-air dispersion formation inside the apparatus, dust concentration, humidity content, dust granulometry, etc. The delay time is one of the worst parameters to adjust, because the fluid-dynamics of the dust-air mixture inside the tube is not easily predictable. Within this work, a study on the characteristic times of all the relevant phenomena occurring within a MIKE 3 apparatus was done by means of slow-motion videos of the tests. Particularly, three different characteristic times were compared referring to a given sample of niacin dust: dust lifting and settling times, effective spark delay time (that is, the time at which the spark is visible) and combustion time (that is, the time at which the flame is visible). According to the results, the effective delay time is almost always quite different with respect to the theoretical one, influencing the effective concentration of dust between the electrodes and, finally, the possibility to have a flame ignition or not within the apparatus. This means that the value of the MIE parameter can be profoundly influenced by the effective delay. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Mine Automation is the property of Industry & Mine Automation Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Forced ignition has broad application in engines. Usually fuel-lean mixtures of large hydrocarbons are used in engines and they have large Lewis numbers, Le. Due to the high positive stretch of the expanding ignition kernel, it is very difficult to ignite a mixture with very large Le. In this study, the forced ignition of a fuel-rich H2/air mixture with Le ≈ 2.3 in a laminar counterflow is investigated via two-dimensional simulations. The emphasis is placed on assessing the flow effects on ignition in mixtures with large Le. The transient ignition kernel development and the minimum ignition energy (MIE) at different strain rates and different ignition positions are studied. The counterflow is found to greatly affect the ignition kernel development and MIE for Le ≈ 2.3. The counterflow compresses the ignition kernel and changes its shape from a sphere to an oblate spheroid. Consequently, the flame propagating in the axial direction is strengthened since its curvature is reduced and Le > 1; while the opposite occurs to the flame propagating in the radial direction. With the increase of strain rate, the radial flame has a negative displacement speed and extinguishes, and thereby the MIE increases. To achieve successful ignition at large Le and at high strain rate, a strong anchored axial flame needs to be formed. Counterintuitively, it is found that moving the ignition position away from the stagnation point can reduce the MIE and thereby promote ignition for the present mixture with large Le. Such ignition enhancement is caused by the reduction of the flame curvature of the flame propagating in the radial direction. The change of the density-weighted displacement speed with the Karlovitz number is also studied for the transient ignition process; and strong unsteadiness is observed. This work provides insights on better understanding of the ignition of mixtures with Le > 2 under strained conditions. [ABSTRACT FROM AUTHOR]

Under some circumstances, fires can be ignited by electric current. The two main mechanisms for this are arcing/sparking and hot surfaces. However, it has been viewed for a long time that this will not happen if the voltage, current, energy, or power are too low. The concept of a minimum ignition energy (MIE) characterizing the ignitability of flammable gas atmospheres is well established, and extensive published data are available. However, a corresponding ignition energy criterion for solids (minimum energy fluence) has been shown not to be valid. Some additional systematic experimental data (minimum voltage, current, power) have been collected for the spark ignition of gas atmospheres. However, it is found that the results are strongly dependent on the test conditions. Exceedingly scant data are available for the minimum electrical conditions for ignition of solid materials. Two concepts—intrinsic safety, and Class 2 or 3 power supplies—have long been available as safety measures against ignition from electrical circuit sources. However, ignition has been demonstrated to be possible with Class 2 power supplies. Ignition of solid material from a 1.2 V battery has been documented in the literature. Wide-ranging experimental research is urged to expand the knowledge base in this important area of electrical safety. [ABSTRACT FROM AUTHOR]

Non-equilibrium plasma has been proven to expand ignition limits and enhance combustion. This paper uses high-voltage nanosecond pulsed discharge (NPD) to generate non-equilibrium plasma and experimentally studies the discharge characteristics at different parameters in a constant-volume combustion bomb. NPD is used to ignite the methane-air mixture, and the effects of pulse power output voltage and pulse interval (PI) on ignition and combustion processes are studied. In addition, the minimum ignition energy (MIE) at different ignition methods (Transistor coil ignition (TCI) and NPD) is also studied. The results show that the first pulse of NPD discharge has a higher breakdown voltage and discharge energy when using multi-pulse discharge. After the second pulse, the highest breakdown voltage and current tend to stabilize, and the discharge energy remains unchanged. As the PI increases, the breakdown peak voltage and the peak current increase slightly. The study on the MIE of NPD shows that at different mixture concentrations, the MIE of NPD is smaller than that of TCI, and NPD can expand the lean burn limit. An increase in initial temperature can significantly reduce MIE. In addition, when the discharge energy is near the MIE, the combustion stability begins to decrease, and the combustion cycle fluctuates wildly. When the ignition energy is increased, the stability improves, and the combustion fluctuation rate can be maintained within 5%. [Display omitted] • The breakdown voltage and current characteristics of NPD at different discharge parameters are investigated. • Minimum ignition energy is studied for different ignition methods. • At the same parameters, the discharge energy of the hundred-nanosecond pulse is higher than that of the ten-nanosecond pulse. • At different mixture concentrations, the MIE of NPD is smaller than that of TCI. [ABSTRACT FROM AUTHOR]

Due to the characteristics of heat absorption and decomposition, endothermic hydrocarbon fuels (EHFs) have been widely used in scramjets for thermal protection and heat recirculation. The understanding of ignition characteristics of EHFs is of great importance for their safe and efficient utilization. In this paper, the ignition processes of EHFs were numerically simulated at atmospheric pressure and with an initial temperature of 500 K. Three different ignition stages were identified based on the chemical heat release and flame kernel propagation. A 3-component kerosene surrogate model composed of n -dodecane, methyl cyclohexane and m -xylene was adopted, as well as the corresponding chemical kinetic model with 369 species and 2691 reactions. Results showed that the discrepant decomposition characteristics of n -alkanes and cycloalkanes affected the chemical heat release and propagation during the ignition process. Two-stage exothermic characteristic was observed in the time evolutions of chemical heat release rate and fuel decomposition. The mass production of molecules and accumulation of radicals dominated the first and second exothermic peaks, respectively. Furthermore, the minimum ignition energies (MIEs) of EHFs with various methyl cyclohexane were determined to quantify the effect of fuel composition on ignition performance. Characteristically, the MIE dramatically decreased from 10.2 to 2.15 mJ when 20% n -dodecane was replaced by methyl cyclohexane. However, it was slightly increased as methyl cyclohexane continued to increase. Analyses from both physical and chemical aspects were conducted to elaborate the dependence of MIE on fuel composition. The dominant effects of flame-dynamic and chemical effects on different ignition stages were analysed. The faster propagation speed and stronger endothermic ability of methyl cyclohexane led to the nonlinear variation of MIEs. The results in this study provide useful guidance for composition optimization and safety evaluation of EHFs. [ABSTRACT FROM AUTHOR]

The flash ignition as a new ignition method has attracted lots of interest from researchers. The flash ignition can successfully achieve distributed ignition in a short time. To study the flash ignition and combustion characteristics of titanium dioxide mixed with iron nanoparticles and aluminum nanoparticles, an appropriate amount of titanium dioxide was added to the iron nanoparticles and aluminum nanoparticles to form the composite material. The ignition phenomenon of mixture materials was recorded by the high-speed camera and the temperature distribution of ignited materials was calculated by using the two-color method. The minimum ignition energy of mixture materials with different content of titanium dioxide and total mass was measured to analyze the method to decrease the minimum ignition energy. The results showed that the effect of the added titanium dioxide was insignificant on the combustion phenomenon of the iron nanoparticles. The temperature was still maintained at approximately 850 K compared with the pure iron nanoparticles. The minimum ignition energy of the mixture materials increased with the increasing content of titanium dioxide. As for the aluminum nanoparticles, titanium dioxide can enhance the explosion phenomenon occurring at the beginning of the flash ignition. In the exposure process. With the content of titanium dioxide in the range of 0%–20%, the minimum ignition energy of the mixture materials decreased greatly. The content increased to the range of 20%–40%, the minimum ignition energy was neglected. When the content was further increased to higher than 60%, the minimum ignition energy gradually increased until it gets the saturation condition.

This study has investigated experimentally the effects of hydrogen addition and turbulence on the ignition and the flame-kernel development characteristics in isotropic and homogeneous turbulence for propane mixtures. In this study, the lean- and rich-fueled hydrogen added propane mixtures with different equivalence ratios (φ=0.5~1.4) and hydrogen additional rates (δH=0.0~1.0) are prepared, while maintaining the laminar burning velocity (SL0=25 or 15 cm/s). First, in order to investigate the ignition and flame-kernel development in quiescence, the minimum ignition energy Eimin and the relationship between the flame radius and the burning velocity of meso-scale laminar flames in the range of flame radii rf approximately from 1 to 5 mm are examined quantitatively by using sequential schlieren photography in a constant volume vessel. Then, the experiments in isotropic and homogeneous turbulence are carried out for two turbulence level with the turbulence intensity u’ being approximately 0.35 and 1.76 m/s. Eimin for each mixture is also defined experimentally at each u’. It is found that the effects of hydrogen addition and turbulence on the ignition and the burning velocity of meso-scale flame characteristics are much different between the lean and the rich hydrogen added propane mixtures. It also becomes clear that there is a good relationship between Eimin characteristics in turbulence and Lewis number or the Markstein number. Additionally, the transition region of the minimum ignition energy could be summarized regardless of φ, δH and u’/SL0 by using the proposed turbulent Karlovitz number based on the burning velocity of the meso-scale flame in quiescence.

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 (φ) and MIE, a physical model of microwave spark ignition is established. The calculation results show that the MIE has a U-shaped relationship with φ, and the MIE value of methane-air is minimum at the point of φ = 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 φ = 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 φ is qualitatively similar to the theoretical prediction under microwave spark ignition in this paper.

Under some circumstances, fires can be ignited by electric current. The two main mechanisms for this are arcing/sparking and hot surfaces. However, it has been viewed for a long time that this will not happen if the voltage, current, energy, or power are too low. The concept of a minimum ignition energy (MIE) characterizing the ignitability of flammable gas atmospheres is well established, and extensive published data are available. However, a corresponding ignition energy criterion for solids (minimum energy fluence) has been shown not to be valid. Some additional systematic experimental data (minimum voltage, current, power) have been collected for the spark ignition of gas atmospheres. However, it is found that the results are strongly dependent on the test conditions. Exceedingly scant data are available for the minimum electrical conditions for ignition of solid materials. Two concepts—intrinsic safety, and Class 2 or 3 power supplies—have long been available as safety measures against ignition from electrical circuit sources. However, ignition has been demonstrated to be possible with Class 2 power supplies. Ignition of solid material from a 1.2 V battery has been documented in the literature. Wide-ranging experimental research is urged to expand the knowledge base in this important area of electrical safety.

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. [ABSTRACT FROM AUTHOR]

Forced ignition is one of the most fundamental and important combustion problems. It is used in advanced engines and closely related to fire safety and accidental explosion. In this study, forced ignition of laminar counterflow diffusion flames with one-step chemistry is studied through two-dimensional simulations. The initial ignition kernel is simply modelled as a hot spot of unburned gas with a specified radius and uniform high temperature inside it. We focus on the subsequent ignition kernel development, during which an expanding axisymmetric triple flame appears under certain conditions. Non-monotonic change of the flame kernel radius with time is observed near the critical ignition conditions. The effects of strain rate and Lewis number on the ignition kernel development and critical ignition conditions are assessed. For the same initial hot spot, successful ignition can be achieved at relatively low strain rate and fuel Lewis number; while the ignition kernel quenches at relatively high strain rate or fuel Lewis number. The strain rate and Lewis number are shown to have great impact on the critical hot spot radius and on the minimum ignition energy. At relatively low strain rates, the minimum ignition energy increases linearly with the strain rate. This is consistent with previous theory on flame ball in a mixing layer. Besides, the optimum ignition position and the influence of the hot spot shape are investigated. It is demonstrated that the optimum ignition position is close to the stoichiometric surface and it depends on the Lewis number and global equivalence ratio. [ABSTRACT FROM AUTHOR]

Laminar and turbulent minimum ignition energies (MIE L and MIE T) of a lean primary reference automobile fuel (PRF95; 95% isooctane/5% n-heptane) and air mixture at the equivalence ratio of 0.8 at 373 K with large Lewis number Le ≈ 2.95 >> 1 using small and large pin-to-pin electrode spark gaps (d gap) are measured in a dual-chamber, constant temperature/pressure, fan-stirred 3D cruciform burner capable of generating near-isotropic turbulence. Each MIE datum is statistically determined at 50% ignitability by 18–30 repeated runs over a range of ignition energy (E ig). We find two ignition transition (IT) modes. (1) A non-monotonic IT at small d gap = 0.8 mm with the lowest MIE T = 13.9 mJ (< MIE L = 30.1 mJ) occurring at a critical u ′ c ≈ 1.82 m/s due to the coupling effects between differential diffusion and turbulence, where u ′ is the r.m.s. turbulent fluctuating velocity. When u ′ > u ′ c , MIE T increases drastically (>> MIE L), because turbulence re-asserts its dominant role. (2) A regular IT at large d gap = 2 mm is found where MIE L is only 2.05 mJ, in which the increasing slopes of MIE T with u ′ change from gradually to exponentially when u ′ > u ′ c ≈ 2.3 m/s. In the post-transition when u ′ ≥ u ′ c , the averaged ratio of MIE at d gap = 0.8-mm and 2-mm (MIE 0.8 /MIE 2.0) is 1.64 > 1, suggesting that using large d gap = 2-mm has a higher ignition probability than using small d gap = 0.8-mm for the lean-burn gasoline surrogate in intense turbulence. Finally, we estimate the uncertainties of burning velocities at u ′ = 0 and 2.76 m/s, which are respectively less than 2% and 6% when using small/large E ig ≈ MIE/70 mJ and/or d gap = 0.8-mm/2-mm. These results are relevant to spark ignition gasoline engines operated in lean-burn turbulent condition. [ABSTRACT FROM AUTHOR]

Effect of water mist on minimum ignition energy (MIE) and lower and upper flammability limits (LFL and UFL) of C 3H8–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 D 32 = 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 < 1.0 and φ > 1.3. However, for 1.0 < φ < 1.3, the suppression effectiveness was scarcely observed. In numerical simulation, water mist was assumed to be an imaginary ideal gas and evaporation process was expressed by a chemical reaction. Ignition energy is added in the flame kernel at the center and the flame propagates spherically if ignited. This model can predict MIE of the mixture without water mist reasonably. MIE increases nonlinearly with the mass fraction of water mist Y 0 and flammability range narrows. At Y 0 = 0.146 and beyond, the mixture of any φ becomes unable to be ignited. Relative suppression effectiveness factors S e and S s are newly introduced for experiments and simulation, respectively. Comparison of S e with S s shows that the water mist is not so effective as expected by simulation. The life time of a droplet before evaporation is much longer for the water mist of D 32 = 21 µm than the residence time in the flame, and only a part of water mist can evaporate in the flame zone. To make full use of the suppression potential of water mist, the mist diameter should be sub-5 µm. [ABSTRACT FROM AUTHOR]

To investigate the parameters of sucrose dust explosion, the minimum ignition energy (MIE) and minimum ignition temperature (MIT) were evaluated. The experiments tested the MIE of sucrose dust under different conditions of dust quantity, ignition delay time (IDT), and powder injection pressure (PIP). The experiments tested the MIT of different particle sizes. The results demonstrate that the MIE of sucrose powder under three conditions was an open-up quadratic polynomial. When the dust quantity, the IDT, and PIP were 0.5 g (417 g/m3), 90 ms, and 150 kPa, respectively, the MIE was 58.9 mJ, 62.6 mJ, and 52.4 mJ. The MIT was positively correlated with the particle size of sucrose dust, and the MIT was 340 °C. At the molecular level, the “O–H” bonds of the sucrose molecule hydroxyl groups were broken by the discharge of electrodes or high temperature to generate H2. The combustion of H2 caused the explosion to spread to the surrounding sucrose dust and made the deposited dust rise, forming an interlocking explosion. The explosion would not stop until the dust concentration dropped below the lowest explosion limit. The results of this study can provide guidance for sucrose enterprises to prevent dust explosion accidents.

This article presents an experimental quantification of the ignition risk of flammable mixtures in conditions relevant to industry and aviation environment. More specifically, the effect of resistive material on ignition of flammable mixtures was investigated by applying a 500-ns duration high-voltage pulse between pin-to-pin electrodes located across a jet flow of ethylene and air, at atmospheric pressure. Fuel-rich, stoichiometric, and fuel-lean equivalence ratios were considered. A parametric study of the effect of the applied voltage and electrode material was performed. The electrical energy deposited in the discharges was determined from current and voltage measurements. Direct visualization of the sparks and the ignition kernel was performed in order to better understand the early stages of the ignition process. The results showed that the use of highly resistive electrodes compared to metallic electrodes: 1) could significantly reduce the energy deposition in the interelectrode area; and 2) could increase the minimum ignition energy. Therefore, the use of highly resistive material wires instead of metallic wires could offer safer conditions with respect to the ignition hazard in flammable environments. [ABSTRACT FROM AUTHOR]

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 were examined. Three were deemed unlikely, and three were either probable or likely root causes. In the end, electrostatic discharge appears to be the most likely root cause. The Case Study serves to remind scientists about the importance of keeping a clutter-free work area and about hazards associated with common reagents. [ABSTRACT FROM AUTHOR]

A novel ignition strategy using multi-channel sparks was developed in this paper. Compared with the typical single spark, the three-channel spark ignition technique enables three spatially separated and temporally synchronized discharges to increase the ignition kernel size while maintaining the same total ignition energy. The three-channel discharge characteristics, ignition kernel development, and ignition probability of lean n-pentane/air mixtures were studied and compared with those of single-channel spark in a spherical combustion chamber with different discharge distances, pressures, and CO 2 dilution levels. The experimental results show that the three-channel sparks increase the ignition probability and dramatically extend the fuel-lean ignition limits compared to the single-channel discharge under all tested conditions. Moreover, ignition enhancement effects of multi-channel sparks increase with the decrease of pressure, reduction of discharge gap, and increase of CO 2 dilution. One-dimensional numerical simulation was performed by using a detailed n-pentane kinetic model (Bugler et al., 2017) and the results revealed that the increase of fuel lean ignition probability and the decrease of the minimum ignition energy by using multi-channel sparks were the outcome of increased ignition kernel size compared to the minimum critical ignition radius. This present study confirms the advantages of using multi-channel sparks on advanced fuel-lean combustion engines. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The aim of this paper is to approximate danger of dust clouds normally occur by determining their explosion characteristics. Nowadays, dusty environment is phenomenon in the industry. In general, about 70% of dust produced is explosive. Dust reduction in companies is the main purpose of the national and European legislative. Early identification and characterization of dust in companies may reduce the risk of explosion. It could be used to identify hazards in industrial production where an explosive dust is produced. For this purpose several standards for identification and characterization of explosion characteristics of industrial dust are being used.

In this study, we develop a simplified theoretical model for flame initiation, propagation and extinction in premixed gas mixture containing water droplets, by considering water droplet evaporation in pre-flame or/and post-flame zones. The Eulerian droplet model with simplified evaporation sub-model is employed, while for gas phase the assumptions of constant-density, quasi-steady and large activation energy are made. Analytical correlations describing different flame regimes and transitions among flame balls, propagating spherical flames and planar flames are then derived to investigate the spherical flame initiation, propagation and extinction, with emphasis on the effects of water droplet evaporation on spherical flames at different Lewis numbers. Five different flame regimes are observed and discussed for droplets evaporation in pre-flame or/and post-flame zones. It is found that the droplets with larger heat exchange coefficient are more effective in reducing flame propagation speed and temperature but increasing the vaporization front. Moreover, the cooling effect of evaporation heat loss plays an important role on flame regimes and their transitions. At the relatively large heat exchange coefficient, the total evaporation heat loss from pre-flame and post-flame zones reaches its maximum at an intermediate flame radius. The cooling effect is strong enough to quench the flame and results in the self-extinguishing flame. In addition, the combined effects of stretch rate and Lewis number compete with the evaporation heat loss from droplet evaporation. For small Lewis number, the flammability limits can be achieved through self-extinguishing flames, whereas for large Lewis number the flames approach their flammability limits in their evolution into planar flames. For ignition of spherical flames, if the droplet-laden mixture is intrinsically non-flammable, although a propagating flame kernel can be initiated, however it would still be quenched due to evaporation heat loss at a critical radius. The minimum ignition energy increases with heat exchange coefficient and Lewis number. [ABSTRACT FROM AUTHOR]

Abstract The minimum energy requirements for ensuring (i) just successful ignition and (ii) successful self-sustained flame propagation without the assistance of an external energy source following a successful ignition event have been analysed for the forced ignition of a homogeneous stoichiometric methane–air mixture under a wide range of turbulence intensities using three-dimensional Direct Numerical Simulation (DNS) data. It has been found that the minimum energy needed for successful ignition is also sufficient to ensure self-sustained flame propagation for small turbulence intensities. However, for large turbulence intensities, the minimum energy for ensuring self-sustained flame propagation can be considerably greater than the minimum energy needed just to successfully ignite the mixture. At low turbulence intensities, the thermal runaway has been obtained for the minimum ignition energy after the end of the energy deposition indicating an autoignition. For larger energy inputs and turbulence intensity, the thermal runaway was obtained during the energy deposition period. It has been found that the minimum energy requirements for ignition and self-sustained flame propagation increase with increasing turbulence intensity but a transition in this behaviour has been observed. There is a critical turbulence intensity such that the increase in the energy demand is significantly more rapid above the critical value than that for turbulence intensities smaller than the critical value. This has been found to be qualitatively consistent with previous experimental findings. The stochastic nature of the ignition event has been demonstrated by considering different realisations of statistically similar turbulent flow fields. The conditions giving rise to a successful ignition have been identified by a detailed analysis of the energy budget. A scaling analysis has been performed for the critical condition for ensuring self-sustained flame propagation and the insights gained from this analysis have been utilised to explain the physical mechanisms behind the transition of the minimum ignition energy. [ABSTRACT FROM AUTHOR]

The paper introduced composition and working mechanism of driving circuit and echo processing circuit of mine-used intrinsic safety ultrasonic ranging circuit system. By analyzing output voltage waveform, it calculated the maximum energy storage of each energy storage element in ranging circuit system. The experimental results show that the largest energy storage of each energy storage element is less than minimum ignition energy in ranging circuit system, which reaches stipulated intrinsic safety request of national standards.

Abstract 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 H 2 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 H 2 in the mixture towards the ignition kernel and formed a locally high H 2 concentration region, which consequently renders lean (rich) mixtures easier (harder) to ignite. It is further shown that Soret diffusion of both H and H 2 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 H 2 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. [ABSTRACT FROM AUTHOR]

It is well established that radiation plays an important role in the combustion process of premixed combustible mixtures, which significantly affects the flame temperature and chemical reaction rates during flame propagation. However, the impact of radiation, especially the radiation reabsorption effect, on minimum ignition energy (E min) still lacks thorough research and understanding. In this study, the ignition processes of spherical NH 3 /H 2 /H 2 O/O 2 /N 2 flames were simulated. Three different computational models, the adiabatic model (ADI), the optically thin model (OTM) and the statistical narrow-band model (SNB), were employed to determine the radiation effect (including both radiative heat loss and reabsorption) on E min. Results indicated that the water content in the premixture suppressed ignition; characteristically, the addition of vol. 9.63% H 2 O in the premixture resulted in a nearly 40% increase of E min under adiabatic condition. Both radiative heat loss and reabsorption effects increased E min. The radiation effects on E min , quantified as the relative differences between the OTM-/SNB- and ADI-obtained E min , were up to 16.47% (for radiative heat loss) and 17.65% (for radiation reabsorption), respectively. It was further demonstrated that radiation increased E min through the combination of three aspects: thermal effect, chemical effect and flame structure effect. The thermal effect, induced by the outward radiative emission, reduced the ignition kernel temperature significantly. The reduced temperature decelerated chemical reactions and then suppressed the chemical heat release, i.e., the chemical effect as reflected by the concentrations of H, O and OH radicals. Furthermore, the larger flame thickness obtained by the radiation models slowed down the diffusion of fuels, which weakened the chemical heat release and eventually increased the E min. Moreover, the reaction-diffusion region simulated by SNB was enlarged due to the radiation reabsorption, and thus, E min obtained by SNB was higher than that by OTM. [ABSTRACT FROM AUTHOR]

Large hydrocarbon fuels exhibit pronounced low-temperature chemistry (LTC) and negative temperature coefficient (NTC) behaviors. During low-temperature combustion, a small amount of heat is released while massive chemical components are generated, which are of great significance in flame ignition and propagation. However, the roles of thermal and chemical effects of the LTC in combustion evolutions have not been thoroughly investigated before. In this work, the LTC-affected flame initiation and propagation were studied, allowing for the thermal and chemical effects as well as their interactions. Dimethyl ether (DME) as a typical LTC fuel was employed for the study and an ignition energy deposition method was adopted to trigger the transient flame initiation and propagation. Besides, the steady laminar flame speed was also investigated to elucidate the LTC-affected combustion mechanisms. The results show that the LTC facilitates the appearance of cool flames in addition to traditional hot flames. Compared to hot flames, cool flames allow lower minimum ignition energy (MIE), especially under fuel-lean and fuel-rich conditions. Cool flames can coexist with hot flames and promote flame initiation and propagation under fuel-lean conditions. To isolate the specific role of the thermal and chemical effects of the LTC, an artificially modified chemical scheme is adopted for comparative study. The results show that the thermal effects of the LTC promote flame initiation while the chemical effects present an opposite impact. Despite this, the LTC shows a promotion impact on flame initiation in comprehensive due to the leading order significance of the thermal effects. Similar observations can also be found for steady laminar flame speed and adiabatic flame temperature. Sensitivity analysis suggests that low-temperature reactions such as CH 2 OCH 2 O 2 H + O 2 = O 2 CH 2 OCH 2 O 2 H and CH 2 OCH 2 O 2 H = OH + 2CH 2 O play an important role in hot flame propagation. The current work provides useful insights into the LTC-affected combustion at elevated temperature conditions. [ABSTRACT FROM AUTHOR]

A parametric exploration of the dynamics of ignition using nanosecond pulsed high frequency discharges (NPHFD) in flowing mixtures of methane and air is conducted to determine the “inter-pulse coupling” effect of a burst of high frequency discharges in the pulse repetition frequency (PRF) range of 1–300 kHz. The impacts of PRF, number of pulses, equivalence ratio, discharge gap distance, and flow velocity are quantified in terms of ignition probability and minimum ignition power, and schlieren images of ignition kernel development are presented. Three regimes of inter-pulse coupling are found for different values of PRF: fully-coupled, partially-coupled, and decoupled. Each regime is characterized by distinct ignition probabilities and kernel structures. The fully-coupled regime occurs for the highest PRF and exhibits complete ignition of the kernel and the highest ignition probability. The partially-coupled regime occurs for intermediate PRF and exhibits only local ignition of portions of the kernel and has the lowest ignition probability. The decoupled regime occurs for the lowest PRF and exhibits multiple non-interacting ignition events with ignition probability being a linear function of the number of pulses. The effect of equivalence ratio is found to increase or decrease the ignition probability without altering the structure of inter-pulse coupling. The electrode gap distance determines the degree of heat and active species quenching to the electrode surfaces, and shifts the transition between the partially-coupled and fully-coupled regimes to higher PRFs as the gap distance is decreased. Flow velocity determines the degree of convective heat loss, with lower velocities increasing the ignition probability and altering the structure of inter-pulse coupling in a non-monotonic fashion. [ABSTRACT FROM AUTHOR]

Spark ignition of ethanol droplet/vapor/air mixture is studied with a Eulerian-Eulerian method and detailed chemical mechanism. The flame kernel-droplet interaction is quantified with an evaporation completion front (ECF). Two categories of spray flames can hence be defined based on the relative location between the ECF and flame front, i.e., homogeneous and heterogeneous spray flames. An element-based equivalence ratio (ER) at the flame front (flame ER for short) is introduced to measure the gas composition in evaporating sprays. For overall fuel-lean mixtures, quasi-stationary spherical flame (QSSF) occurs due to lean flame ER and the composition at the QSSF front is homogeneous. For overall fuel-rich two-phase mixtures, re-ignition, after the spark-ignited kernel fails, is observed when the droplet diameter is 15 μm for fuel sprays with both fuel-lean and fuel-rich background gas. This is due to rich flame ER and/or strong evaporative heat loss. Meanwhile, the kernel is born in a heterogeneous mixture and transition into homogeneous state is found. For both overall lean and rich two-phase mixtures, fuel droplets affect the ignitability and flame trajectories. Moreover, ignition energy affects the flame ER and front distance at the early stage of kernel development. Lastly, the minimum ignition energies (MIE) with different gas and overall ERs are investigated. Three regimes (A, B and C) are identified from the MIE variations with overall ER and the corresponding flame kernel dynamics behind them are summarized. Regime A is characterized by the QSSF phenomenon, whilst regime C embodies the ignition failure and re-ignition transients when the droplet size is relatively large. Furthermore, regime B only appears with a narrow range of overall ER when initial background gas ER is above unity. These regimes are further generalized in parameter space of overall ER versus initial background gas ER. [ABSTRACT FROM AUTHOR]

Spark ignition is considered one of the most difficult and complex problems because it involves complicated physical and chemical processes, and it has not yet been explained sufficiently. The minimum ignition energy (MIE) is an important parameter for judging the ignition ability of combustion systems. In the present study, the spark ignition characteristics of a methane-air mixture were investigated by numerical analysis using detailed chemical kinetics consisting of 53 species and 325 elementary reactions. Two different analytical models, with and without electrodes, were applied to research the effect of electrode temperature and energy channel length on flame propagation and the relationship between the MIE and equivalence ratio. The electrode temperature was set as 300 K, 1000 K, and 2000 K for analytical models with electrodes, and the energy channel length was set as 1 mm, 2 mm, and 3 mm for analytical models without electrodes. The obtained computational results showed good agreement with experimental results. We determined that with increasing electrode temperature, the minima of the curve indicating the relationship between the MIE and equivalence ratio move toward the leaner side, that a leaner mixture is more sensitive to heat loss to the cold surrounding gas, and that heat loss to the electrodes is an unignorable factor for the initial formation of the flame kernel.