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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.

The next-generation energy economy relies heavily on hydrogen. Handling hydrogen during production, storage, and transportation requires risk mitigation. In this regard, the application of halide-free additives is very crucial. The present study experimentally and theoretically investigates the individual and combined inhibiting effect of propane and propene on hydrogen-air explosions. The experiments are carried out at 298 K and 1 bar, with additives ranging from 0.5 to 5% for the individual and 1–2% each for the combined effect in the 20–50% hydrogen-air mixture. Experimentally, the additive content in the stoichiometric and rich mixture decreases the explosion pressure, maximum pressure rise (MPR) rate, laminar burning velocity (LBV), and flame temperature, while increasing minimum ignition energy. For instance, the addition of 2% propane in a 50% hydrogen-air mixture, reduces the MPR rate by 98%, whereas such an effect is observed with 2.5% propene. Theoretical study shows that 3% equivalent additive augments inhibiting reactions, countering the main flame-promoting reaction R1 (H + O 2 O + OH) and thereby reducing LBV. Due to its low activation energy, propene consumes the active radicals twice the rate of propane, while propane not only scavenges the active radicals but also promotes oxygen consumption in the prior phase of combustion, weakening flame propagation. Overall, this approach exhibits a significant step forward in harnessing the potential of hydrogen while mitigating the safety risks associated with its use. [Display omitted] • The effect of C 3 -Hydrocarbons and their Mixtures on H2-air explosion is explored. • The additives decrease explosion pressure, MPR rate and LBV while increasing MIE. • Propene increases the consumption rate of active radicals in the additive mixtures. • Propane promotes oxygen consumption in the prior phase of combustion, countering R1. [ABSTRACT FROM AUTHOR]

Minimum ignition energy (MIE) is a crucial parameter for identifying the hazards of combustible gases. To rapidly obtain accurate MIE values for various gases, the shape of the ignition source was simplified to cylindrical. Based on theories of heat conduction and chemical reaction kinetics, a physical model of the ignition process was established. The spatial and temporal evolution of temperature was used as a basis for judging the success of ignition. A numerical algorithm was developed and maximum flame temperatures and MIE were calculated for several combustible gases (H 2 , CH 4 , C 2 H 6 , C 3 H 8 , C 4 H 10 , C 2 H 4 , C 2 H 2). This model accurately predicted the U-shaped trend of MIE values with concentration for CH 4 /air and H 2 /air mixtures. It found that the MIE was minimal at the stoichiometric concentration, at 0.37 mJ and 0.017 mJ, closely aligning with experimental data. This numerical model has been validated for its accuracy and promises to make relatively precise theoretical predictions for the MIE values of uncommon combustible gases or mixtures of various gases. [Display omitted] • New prediction model for the minimum ignition energy of gases. • Full consideration of ignition details and comparison with experimental values. • Maximum temperature and minimum ignition energy for multiple gases. • Trends in minimum ignition energy with concentration for H 2 and CH 4. [ABSTRACT FROM AUTHOR]

A numerical model for the calculation of the minimum ignition energy (MIE) of dust clouds is developed based on heat transfer and particle reaction kinetics. The model is an update of previous models relying on the experimental minimum ignition temperature (MIT) of dust clouds. Gas and particle temperature profiles during the spark ignition processes are studied in detail to generate dust cloud MIE prediction. For the 20 dusts studied, with the updated model, the average calculated MIE over experimental MIE ratio is 81.6%, a significant improvement compared with the ratio of 48.1% using a previous model with the MIT. A strong correlation is found between the dust cloud MIE and the MIE of the nearest particle to the spark center in the dust cloud: for all the studied dusts, the latter accounts for over 85% of the dust cloud MIE. Moreover, the minimum spark ignition temperature of a dust particle (SITP) can be defined, which also strongly correlates with the dust particle MIT (MITP) calculated using the Semenov theory. Based on these correlations, simple MIE predictive equations are derived and validated by comparing their MIE predictions with the experimental MIE data. [ABSTRACT FROM AUTHOR]

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.

This work aims to find how coupled energy per pulse influences the ability of a pulsed dielectric barrier discharge (DBD) plasma to ignite fuel-lean methane–air flow. For that, experiments are performed on a custom-built DBD flow reactor with a variable dielectric thickness and the discharge is operated by bursts of 10 ns duration pulses at 3 kHz repetition rate. With an increase in dielectric thickness, we observe that the coupled energy per pulse decreases even though applied voltage conditions are similar and so more pulses are required to ignite the lean mixture. Interestingly, we observe a significant increase in the minimum ignition energy (MIE) with an increase in the thickness beyond 3 mm. Moreover, the ignition kernel growth rate is much slower in the thicker dielectric cases even though total energy coupling per burst is similar. This phenomenon is investigated further by evaluating plasma parameters using electrical and optical diagnostics. Effective dielectric capacitance, discharge current, and voltage drop across the gas gap are derived from an equivalent circuit analysis, whereas plasma gas temperature and effective reduced electric field ( E / N) are estimated from optical emission spectroscopy. From these analyses, we conclude that a thicker dielectric limits the discharge current and so the plasma filament temperature. For more than 3 mm thick dielectric cases, the filament heating per pulse is too low to achieve strong enough plasma pulse-to-pulse coupling which eventually leads to higher MIE and slower ignition kernel growth rate or the inability to ignite at all. [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.)

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]

In technical terms, wood is a natural composite material with a polymer matrix, reinforced with fibres made of uniaxially oriented longitudinal cells. Due to the ease of obtaining and relatively low technological requirements related to its processing, wood is a common material used for various types of construction materials. An experimental investigation was carried out to determine the evolution of the ignition sensitivity and the explosion of three types of softwood dusts. Two of those dusts come from conifers and one from a deciduous tree. Complete fire characteristic requires several parameters describing wood dust behaviour under fire conditions including heat release rate (HRR), ignition time, or fire growth index. To determine those parameters, a cone calorimeter was used. Explosion characteristic was tested for representative wood as a dust sample in 20-L spherical vessel. Minimum ignition energy (MIE) was tested on MINOR II apparatus, which is a modified Hartman's tube. Minimum ignition temperatures were tested with the use of layer ignition temperature apparatus and Godbert–Greenwald apparatus for minimum ignition temperature of a dust layer and dust cloud (MIT), respectively. Dust with the highest KST value, lowest MIE in the widest concentration range, and lowest MIT was pine wood dust. It also shows the shortest ignition time and a timespan between ignition and HRR peak. [ABSTRACT FROM AUTHOR]

This work numerically investigates the effects of non-equilibrium nanosecond plasma discharge pulse repetition frequency, pulse number, and flow velocity on the critical ignition volume, minimum ignition energy, and chemistry in a plasma-assisted H 2 /air flow at 300 K and 1 atm using a multi-scale adaptive reduced chemistry solver for plasma assisted combustion (MARCS-PAC). The interactions between discharges/ignition kernels spanning decoupled, partially-coupled and fully-coupled regimes in a pulse train are studied. For a single pulse discharge, increased flow velocity increases the minimum ignition energy required due to the increase of convective heat loss and flame stretch. The results show that the minimum ignition kernel propagation speed at the critical ignition kernel volume increases with the flow velocity. The minimum critical ignition volume decreases with the increase of plasma discharge energy. For sequential two-pulse discharges, ignition fails at both decoupled and partially-coupled regimes even when the total discharge energy is above the minimum ignition energy, but succeeds only in the fully-coupled regime at a shorter inter-pulse time. Overlap of the OH radical pool between the sequential two-pulse discharges and the increase of the chemistry effect due to the increase of reduced electric field in the fully-coupled regime contribute to the ignition enhancement. In addition, for two-pulse discharges in the fully-coupled discharge regime, the mixture can be ignited at a total energy below the minimum ignition energy of a single pulse with the same flow conditions. Moreover, for a given total discharge energy with multiple pulsed discharges, the enhancement of the ignition kernel volume has a non-monotonic dependence on discharge frequency and pulse number. The effective ignition enhancement can be achieved with an optimal pulse repetition frequency and pulse number. This work provides a new understanding of the mechanism for repetitive plasma ignition and insights for the optimization of plasma ignition in a reactive flow. [ABSTRACT FROM AUTHOR]

The effects of fuel Lewis number on the minimum ignition energy (MIE) requirements for ensuring successful thermal runaway, and self-sustained flame propagation have been analysed for forced ignition of homogeneous fuel–air mixtures under decaying turbulence for a wide range of initial turbulence intensities using three-dimensional direct numerical simulations. The minimum energy demand for ensuring self-sustained flame propagation has been found to be greater than that for ensuring only thermal runaway irrespective of its outcome for large turbulence intensities, and the minimum ignition energy increases with increasing rms turbulent velocity irrespective of the fuel Lewis number. The MIE values have been found to increase more sharply with increasing turbulence intensity beyond a critical value for all fuel Lewis numbers considered here. The variations of the normalised MIE (MIE normalised by its laminar value) with increasing turbulence intensity beyond the critical point follow a power-law and the power-law exponent has been found to increase with an increase in fuel Lewis number. This behaviour has been explained using a scaling analysis. The stochasticity associated with forced ignition has been demonstrated by using different realisations of statistically similar turbulent flow fields for the energy inputs corresponding to the MIE and successful outcomes are obtained in most instances, justifying the evaluation of the MIE values in this analysis. [ABSTRACT FROM AUTHOR]

Copyright of Fire Safety Science is the property of Fire Safety Science Editorial Office 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.)

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.)

The ignition and combustion of hydrogen in air is considered more hazardous compared to other fuels due to the lower minimum ignition energy (MIE) and the wider flammability range. Spark discharge is the most common type of electrostatic ignition hazard. There is a need in validated safety engineering tools to accurately calculate MIE in a wide range of temperatures from atmospheric to cryogenic which are characteristic for hydrogen systems and infrastructure. Current MIE assessment methodologies rely on the availability of experimental data on quenching distance and/or laminar burning velocity and thus are mostly empirical correlations. This prevents their application beyond the limited number of experimental data, i.e. to arbitrary composition of the hydrogen-air mixture at arbitrary temperatures including cryogenic. This work aims at the development of a model able to accurately predict MIE for hydrogen-air mixtures with arbitrary initial composition and temperature. Cantera and Chemkin software are used to calculate the properties and unstretched laminar burning velocity of hydrogen-air mixtures. The flame thickness is found to well represent the critical flame kernel in the suggested model. The model is validated against experimental data on MIE for mixtures at ambient and cryogenic (down to 123 K) temperatures. Results show that the effect of flame stretch and preferential diffusion shall be considered to accurately predict MIE for lean hydrogen-air mixtures, which was not possible for previous models. • Development and validation of model to determine MIE for hydrogen-air mixtures. • The model is applicable for arbitrary concentration and temperature of the mixture. • The model employs the laminar flame thickness to calculate a critical flame kernel. • Effect of flame stretch and preferential diffusion on laminar burning velocity. • Validation against experiments at ambient and cryogenic temperatures. [ABSTRACT FROM AUTHOR]

Understanding flame initiation is of great interests to combustion research. For induced ignition, a minimum ignition energy (MIE) must be provided by an external ignition source for the formation of a flame kernel. In this paper, the minimum ignition energy in lean primary reference fuel (PRF) /air mixtures is investigated by simulations and experiments. The simulations feature a detailed treatment of chemical kinetics and molecular transport to solve the conservation equations for a flame evolving from an ignition source. In the experiments, a cruciform burner is used for the ignition of the fuel/air mixture. After applying the heat source, three qualitatively different types of evolution are observed: ignition failure, flame kernel formation with subsequent flame extinction, and flame propagation. We define two kinds of MIEs: one for flame initiation, and one for flame propagation. The dependence of MIE for flame propagation on the research octane number (RON) in both experiments and simulations exhibits an upturn at a certain RON: Below this RON, the MIE for flame propagation increases weakly, and above this RON, the MIE for flame propagation increases rapidly with increasing RON. The existence of the upturn can be explained by different reaction pathways of the n-heptane and iso-octane flame. At smaller RON, the massive production of C 2 H 5 during the oxidation of n-heptane helps to sustain the critical early phase of flame with strong diffusion. The results in this study highlight the importance of the interaction between chemical reactions and transport for understanding behaviours of practical ignition processes. [ABSTRACT FROM AUTHOR]

The present work investigates forced ignition and oscillating propagation of spray flame in a mixture of fine ethanol droplets and air. The Eulerian-Eulerian method with two-way coupling is used and a detailed chemical mechanism is considered. Different droplet diameters and liquid fuel equivalence ratios (ER) are studied. The evaporation completion front (ECF) is defined to study the interactions between the evaporation zone and flame front. The gas composition at the flame front is quantified through an effective ER. The results show that the kernel trajectory is considerably affected by droplet size and liquid ER. Generally, the flame ER reaches the maximum when the ECF starts to move from the spherical center. It gradually decreases and reaches a constant value when the flame freely propagates. Quasi-stationary spherical flame is observed when the liquid ER is low, whilst kernel extinction/re-ignition appears when liquid ER is high. These flame behaviors are essentially affected by the heat conduction and species diffusion timescales between the droplet evaporation zone and flame front. The dependence of the minimum ignition energy (MIE) on liquid ER is U-shaped and there is an optimal liquid equivalence ratio range (ER o) with the smallest MIE. Long and short ignition failure modes are observed, respectively for small and large liquid ERs. When liquid ER is less than ER o (long failure mode), larger energy is required to initiate the kernel due to very lean composition near the spark. For liquid ER larger than ER o (short mode), ignition failure is caused by strong evaporative heat loss and rich gas composition due to heavy droplet loading. Flame speed oscillation is seen when the initial droplet size is above 10 μm. The oscillation frequency linearly increases with the liquid ER. Meanwhile, larger droplets lead to smaller oscillation frequencies. [ABSTRACT FROM AUTHOR]

Recently, forced ignition by nanosecond-repetitive-pulsed-discharge (NRPD) has received great attention since it can greatly promote ignition. However, there is no theoretical analysis on ignition induced by multiple heating pulses such as NRPD. Therefore, this work attempts to provide a theoretical interpretation on the ignition of a quiescent, flammable mixture by multiple discharges and to assess the effects of repetitive pulse on ignition characteristics. Based on fully transient formulation, analytical expressions describing ignition kernel propagation induced by multiple pulses are derived. The key parameters of multiple pulse heating, such as energy distribution among individual pulse, intermittent duration between neighboring pulse and total pulse numbers are appropriately incorporated, and their effects on ignition characteristics are assessed. It is found that because of memory effect, the flame kernel continues to propagate after switching off the external heating source. Sequentially introducing identical heating pulses at appropriate intermittent duration repetitively exploits the memory effect during ignition kernel development and thus extends propagating distance of the flame front. The repetitive pulse heating can promote ignition capability due to the flame revitalization effect, i.e., ignition reinforcement to the flame front thanks to additional thermal energy supplied by subsequent pulse. This is consistent with the synergistic effect of NRPD observed in previous experimental studies. In particular, as the pulse number increases, the minimum ignition energy decreases and approaches to an asymptotic value. In the limit of large pulse number, it is found that the integration of the implicit expressions describing the ignition kernel evolution does not depend on the pulse number explicitly. This substantiates the invariance of minimum ignition energy with heating pulse number. The present theory explains the effects of multiple discharges on ignition and provides insights on ignition enhancement. [ABSTRACT FROM AUTHOR]

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]

Premixed flame ignition is a fundamental issue in combustion. A basic understanding of this phenomenon is crucial for fire safety control and for the development of advanced combustion engines. Significant efforts have been devoted to understanding the mechanisms of ignition and determining critical ignition conditions, such as critical flame radius, minimum ignition energy, and minimum ignition power, which have remained challenging research topics for centuries. This review provides an in-depth investigation of the forced-ignition of laminar premixed flames in a quiescent flammable mixture, with emphasis on theoretical developments, particularly those based on activation energy analysis. First, the fundamental concepts are overviewed, including spark ignition, characteristic time scales, and critical ignition conditions. Then, the chronological development of premixed flame ignition theories is discussed, including homogeneous explosion, thermal ignition theory, flame ball theory, quasi-steady ignition theory, and, more importantly, transient ignition theory. Premixed flame ignition consists of three stages: flame kernel formation, flame kernel expansion, and transition to a self-sustaining flame. These stages are profoundly affected by the coupling of positive stretch with preferential diffusion, characterized by the Lewis number. Specifically, positive stretch makes the expanding ignition kernel weaker at larger Lewis numbers, consequently increasing the critical ignition radius and MIE. The premixed flame ignition process is dominated by flame propagation dynamics. Both quasi-steady and transient ignition theories demonstrate that the critical flame radius for premixed ignition differs from either flame thickness (by thermal ignition theory) or flame ball radius (by flame ball theory). Particularly, the transient ignition theory appropriately acknowledges the "memory effect" of external heating, offering the most accurate description of the evolution of the ignition kernel and the most sensible evaluation of minimum ignition energy. In addition, the effects of transport and chain-branching reactions of radicals, finite droplet vaporization, and repetitive heating pulses on premixed flame ignition are discussed. Finally, a summary of major advances is provided, along with comments on the applications of premixed flame ignition theory in ignition enhancement. Suggested directions for future research are presented. [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.

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A numerical model is developed for the minimum ignition energy (MIE) calculation of pure and mixture dust clouds, considering the influence of particle size distribution. The original particle number based size distribution is approximated with median sizes (number d 50) of the subdivisions obtained by dividing the original distribution range. The MIE is calculated for 5 pure dusts, 2 mixtures of combustible dusts and 2 mixtures of combustible & inert dusts. For pure dust clouds, as the number of subdivisions and hence d 50 s increases, variation trend of the MIE as a function of dust concentration (MIE(conc)) gradually shifts from a "V" to a "U" or "bathtub" shape, along with an increase in the concentration where the dust cloud MIE (minimum MIE(conc) value) is reached. Furthermore, the calculated MIE fluctuates at lower number of d 50 s but gradually stabilizes as the d 50 number further increases. The calculated MIE variation trends correspond well with the experimental data. However, due to uncertainties in the dust cloud formation and spark release in the experimental tests, differences in the definitions of the d 50 in different studies and high idealization of the numerical model, there are deviations between the absolute experimental and calculated MIE values. [ABSTRACT FROM AUTHOR]

When an aircraft is struck by lightning, internal sparking may occur as a result of current flow along the outer surface of the fuel tanks due to the formation of a large potential difference between its elements. Under certain conditions, spark discharges can become a source of ignition of fuel vapors. Fragments of aircraft fuel systems are tested for direct lightning impact in accordance with the requirements of regulatory documents, while the minimum ignition energy of a mixture of aviation fuel vapors with air is assumed to be 200 μJ. When testing fuel-system elements for sparking during a direct lightning strike, there are problems associated with the use of the photographic technique, as well as providing the parameters of the normalized lightning current pulse for sizable samples. With an increase in the length of ground conductors, their inductance increases in proportion to the length, which leads to a decrease in the amplitude of the test current. This article provides ways to solve these problems using modern digital technologies. [ABSTRACT FROM AUTHOR]

• Effect of NH 3 partial cracking on forced ignition and flame propagation was assessed. • Markstein lengths at different equivalence ratios and cracking levels were evaluated. • ϕ = 0.9 is the theoretical equivalence ratio for efficient ignition of NH 3 /air mixture. • A slight amount of NH 3 cracking (∼20 %) can significantly promote ignition. • Ignition duration time and ignition radius should be adjusted at different pressures. As a zero-carbon fuel, ammonia has promising applications and thereby has received great attention recently. However, compared to traditional hydrocarbon fuels, ammonia has very low laminar flame speed and extremely high minimum ignition energy (MIE). A feasible method to promote ammonia combustion is partial fuel cracking, during which ammonia undergoes decomposition into hydrogen and nitrogen. This work reports a computational study on forced ignition and spherical flame propagation in partially cracked ammonia/air mixtures. The objective is to assess the effects of fuel cracking on the MIE and flame propagation. It is found that even a slight amount of ammonia cracking can significantly reduce the MIE and promote ignition. In addition, the influence of positive stretch rate on the spherical flame propagation speed, which is quantified by the Markstein length, is shown to be greatly affected by ammonia cracking. The mechanism of ignition promotion and change in spherical flame propagation by ammonia cracking is interpreted and discussed. [ABSTRACT FROM AUTHOR]

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.

Fully utilizing the energy generated by the explosion of pulverized coal will contribute to realize the clean and efficient exploitation of coal resources. The pulverized coal explosion characteristics will be a far-reaching and important task to explore. In this paper, ten kinds of low-quality coals such as high sulfur, high ash, and low metamorphic degree coals were investigated and the minimum ignition energy (MIE), lower explosion limit (LEL), and explosion intensity (EI) parameters under different particle sizes and coal powder concentration conditions were also analyzed combined with a 1.2-L Hartmann tube and a 20-L explosion sphere experimental system. Finally, the morphological characteristics of the exploded coal powder surface were evaluated by scanning electron microscopy (SEM). The results show that the particle size is positively correlated with MIE. LEL shows an inverted "U"-shaped trend with the increasing degree of coal deterioration. The low-rank coal is more flammable and explosive. The maximum pressure PMax at the LEL concentration and maximum pressure rise rate (dP/dt)Max overall value is small. Here, optimum pulverized coal particle size (75μm) for explosive utilization of low-quality coal was determined. Within 50–225 g/m3 of pulverized coal concentration range, the explosion intensity increases with increasing concentration. The smaller the particle size of pulverized coal, the greater the possibility of agglomeration of pulverized coal particles. The surface of the exploded coal particles produces more developed pores. They are irregularly shaped and have more rounded edges than the original coal. [ABSTRACT FROM AUTHOR]

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.

In this paper, the effects of 30 wt%, 50 wt% aluminum hydroxide, calcium chloride and ammonium dihydrogen phosphate three explosion inhibitors on the two sensitivity parameters of the minimum explosion concentration and the minimum ignition energy of pulverized coal were investigated. It was found that for the same explosion inhibitor, the higher the content of explosion inhibitor, the higher the increase of MEC and MIE of pulverized coal-explosion inhibitor. At the same time, dust reduction measures should be taken in time when the coal dust concentration will be close to 70 g/m3. It was found that for the same explosion inhibitor, the higher the content of explosion inhibitor, the higher the increase of MEC and MIE of pulverized coal-explosion inhibitor. In the meantime, dust reduction measures should be taken in time when the coal dust concentration will be close to 70 g/m3. Furthermore, 150 g/m3 was the critical concentration of the ignition energy of pulverized coal. In order to prevent the occurrence of coal dust explosion accidents, it was necessary to resolutely prevent the concentration of pulverized coal from approaching this value on site. In addition, when the pulverized coal-explosion inhibitor concentration was 100 g/m3-250 g/m3, whether it was 30 wt% NH4H2PO4 or 50 wt% NH4H2PO4, the ignition energy required for mixing with pulverized coal was greatly increased. Overall, in the concentration range of 30–50 wt%, 50 wt% NH4H2PO4 was more advantageous in the suppression of coal dust explosion. [ABSTRACT FROM AUTHOR]

Based on the study of physical and chemical properties of co-hydrothermal carbonization products of municipal solid waste(MSW) and peanut shell(PS), thermogravimetry(TG) analysis was used to investigate the combustion characteristic and kinetics of co-carbonization products. The results indicated that the TG curves of co-carbonization products presented three weight loss peaks in the combustion, the lost degree of the second weightlessness peak was more than 50% of the total weightlessness. At the same co-carbonization temperature, with the increase of the proportion of peanut shells, combustion reactions were more thorough, TG curves were shifted to the high temperature side gradually. With the increase of heating rate, the ignition, the burnout temperature and the integrated combustion characteristic index improved. There existed synergistic interaction of co-carbonization products in the combustion process. With the increase of co-carbonization temperature( 180 - 260 °C ), both fixed carbon content and combustion characteristic index S increased first and then decreased, the minimum ignition energy(Eα1.) had an opposite trend. In this study, municipal solid waste were mixed with peanut shells at mass ratio of 5 : 5, under the conditions of co-hydrothermal carbonization temperature 220°C, the heating rate 40 °C /min, co-carbonization product had the highest combustion characteristic index(5.727 x 10-6 min⋅2°C-3) and the lowest ignition energy(89.55 kJ/mol). [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.

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. [ABSTRACT FROM AUTHOR]

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]

The issue of global warming prompts us to consider some questions related to energy supply and use. Organic Rankine Cycles (ORC) is a suitable technology for converting low-grade thermal energy into electrical energy, vital for conserving energy and mitigating emissions. Currently, most environmentally friendly working fluids are characterized by their flammability. Analyzing the minimum ignition energy (MIE) of these working fluids is crucial in guaranteeing secure utilization. This paper introduced a novel approach to predicting the MIE of flammable working fluids using microscopic molecular structure analysis. Firstly, the bond energy of molecules was derived by optimizing the microscopic molecular structure of prevalent combustible agents based on the density functional theory (DFT), which was employed to calculate the heat of combustion. Secondly, a mathematical model for predicting the MIE of combustible working fluids was constructed, drawing inspiration from the energy relationship during combustion. Then, the newly formulated model was used to predict the MIE of propane and nine other combustible working fluids, and the results were compared with the existing data. The results show that the predicted values of the MIE exhibit an impressive accuracy rate exceeding 95%. Finally, the MIE prediction model has been modified to provide higher prediction accuracy. This study is of great significance to ensure the safe application of combustible working fluids and to explore environmentally friendly alternatives. [Display omitted] • A new method for predicting the MIE based on molecular structure was proposed. • The molecular structure was optimized based on density functional theory. • The mathematical model has a prediction accuracy of more than 95 %. • The model can predict the ignition energy of substances outside the database. [ABSTRACT FROM AUTHOR]

This study applies the concept of orthogonal experimental design to explore the influence of ignition delay time, dispersal pressure, dust mass, and interaction effects between these factors on the minimum ignition energy (MIE) of sugar dust. Additionally, it aims to establish a comprehensive predictive model for the MIE of sugar dust considering the combined effects of multiple factors. Comparative analyses were conducted between multiple linear regression polynomial models, Back propagation neural network (BPNN), and Generalized regression neural network (GRNN) for data fitting and prediction accuracy. The results demonstrate that the MIE of sugar dust is significantly influenced by dispersal pressure, ignition delay time, and dust mass, while the interaction effects between these factors are not statistically significant. Artificial neural networks exhibit superior predictive performance compared to traditional multiple linear regression polynomial models when considering the combined effects of multiple factors. Specifically, the GRNN model outperforms the BPNN model, highlighting its high fitting accuracy with limited sample data. [Display omitted] • We compared the various factors that affect the MIE of sugar dust. • Dispersion pressure has the most significant impact on the MIE of sugar dust. • We established three models to predict the MIE of sugar dust. • Artificial neural networks exhibit high accuracy in predicting the MIE of sugar dust. [ABSTRACT FROM AUTHOR]

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 Kühner 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. [Display omitted] • Digital In-line Holography technique developed for MIKE3 MIE measurements. • Effect of particle density on cloud dynamics using Digital In-line Holography. • Lighter particles have higher dispersed concentration/velocity than dense particles. • Particle density effects cloud dynamics, should influence MIE ignition delay selection. [ABSTRACT FROM AUTHOR]

Thermal power is the main source of electricity in Taiwan, where coal is the primary fuel. However, concerns for potential fire and explosion hazards caused by coal dust are increasing. The problem regarding the storage or transportation process of coal cannot be neglected. This study explored four coal dust samples obtained from the three regions of South Africa, Russia, and Australia; the particle size D50 of all the samples was < 75 μm. The intrinsic safety characteristics of combustion and explosion in coal dust were studied using the minimum ignition energy (MIE) test, minimum ignition temperature (MIT) of cloud apparatus, and 20-L apparatus. Russian coal exhibited the lowest MIE and MIT values of 550 °C and 120 mJ, respectively. Furthermore, the minimum dust explosion concentration was 30 g m−3 in Australian coal. These experimental results, combined with the on-site investigation, indicated that appropriate countermeasures are sufficient to reduce dust hazards. The results showed that the dust explosion risk of a properly operating thermal power plant is considerably low. [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]

Dust explosions threaten the solids process industry. Parameters influencing dust explosions include particle size, chemical composition, and polydispersity; however, particle morphology is often overlooked. Studies show particle morphology impacts dust cloud minimum ignition energy (MIE) via heat transfer because higher specific surface results in lower conductive heat resistance. However, heat transfer is not the only factor impacting MIE; the effect of particle morphology on cloud dynamics is another important factor. This work focuses on the influence of particle morphology on cloud aerodynamics during MIE testing. A digital in-line holography method has been developed to measure aerodynamics of particles in an MIE apparatus. Results show higher cloud concentration and turbulence for irregularly shaped particles, and the implication on MIE is discussed. The present results aid understanding of particle morphology on cloud dynamics, which is crucial in determining the effect on explosion parameters such as MIE and the resulting risk assessment. Unlabelled Image • Effect of particle morphology on cloud dynamics using Digital In-line Holography. • Irregular particles have greater concentration/velocity in dispersion than spherical. • Irregular particles have lower terminal fall velocity than spherical shaped particles. • Irregular particles have higher explosion probability than spherical particles. [ABSTRACT FROM AUTHOR]

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 ignition of uniformly dispersed mono-sized n-heptane droplet-laden mixtures under homogeneous isotropic decaying turbulence have been analyzed based on Direct Numerical Simulation (DNS) data. The MIE was evaluated just for (i) obtaining thermal runway irrespective of the fate of the resulting flame kernel, and (ii) also for a successful self-sustained flame propagation without the assistance of an external energy source following the energy deposition by the ignitor. It has been found that the MIE requirement increases with increasing turbulence intensity and this trend for the MIE increase is especially significant for large values of turbulence intensity. The MIE requirement increases with increasing initial droplet diameter and with decreasing overall equivalence ratio. The MIE requirements for droplet-laden mixtures have been found to be greater than the corresponding value for homogeneous mixture with the same nominal values of initial turbulence intensity and equivalence ratio for the parameter range considered here. This behavior arises due to the deposited energy being partially utilized to supply the latent heat of evaporation and also due to the predominantly fuel-lean composition of the gaseous flammable mixture. This tendency of obtaining fuel-lean mixture strengthens with increasing (decreasing) initial droplet diameter (overall equivalence ratio). It has also been demonstrated that combustion takes place predominantly in the fuel-lean premixed mode although there is a finite probability of having non-premixed combustion in all cases. Moreover, there is a small probability of fuel-rich combustion occurring for small droplets, especially under fuel-rich overall equivalence ratios. The stochastic nature of the ignition event has been demonstrated by considering different realizations of statistically similar turbulent flow fields. The conditions giving rise to a successful thermal runaway/self-sustained flame propagation have been identified by a detailed analysis of the energy budget. [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]