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Ignition of natural fuels by hot metal particles from powerlines, welding and mechanical processes may initiate wildfires. In this work, a hot steel spherical particle (6–14 mm and 600–1100°C) was dropped onto pine needles with a fuel moisture content (FMC) of 6–32% and wind speed of 0–4 m s–1. Several ignition phenomena including direct flaming, smouldering and smouldering-to-flaming transition were observed. The critical particle temperature for sustained ignition was found to decrease with the particle size (d) and increase with FMC as WF16096_IE1.gif(°C), and the maximum heating efficiency of particle was found to be WF16096_IE2.gif. As the particle size increases, the influence of FMC becomes weaker. The flaming ignition delay times for both direct flaming and smouldering-to-flaming transition were measured, and decreased with particle temperature and wind speed, but increased with FMC. The proposed heat-transfer analysis explains the ignition limit and delay time, and suggests that the hot particle acts as both heating and pilot sources like a small flame for direct flaming ignition, but only acts as a heating source for smouldering. This study deepens the fundamental understanding of hot-particle ignition, and may help provide a first step to understanding the mechanism behind firebrand ignition.

Wildland–urban interface fires are a severe fire hazard due to biomass ignition caused by arc beads. This study investigated how the energy, diameter, and number of arc beads affect biomass ignition probabilities. An improved experimental method was used to generate arc beads of various arc energies. Here, α-cellulose materials, which are well-characterised as biomass, were used as fuels. A high-speed camera recorded ignition phenomenology, revealing two ignition behaviours of rolling and embedding. The results revealed that an electrical fault arc energy of approximately 175 J was the most dangerous ignition condition. Ignition phenomenology was categorised into ignition and non-ignition, and it was observed that ignition could only occur during the bead rolling process. Contrarily, non-ignition occurred when its bounce even spun plenty of times. Ignition limits, namely the ignition region, potential ignition region, and non-ignition region, were determined. Furthermore, a novel predictive logistic regression-based ignition probability model was established, which indicated that the ignition occurrence of an arc bead was highly dependent on the diameter of the arc bead. The developed mathematical model can reasonably predict the ignition ability of biomass fuel ignition by arc beads.

This study investigates the minimum ignition temperature of smoldering paper scraps with varying bulk densities and lengths under different external airflow rates. Paper scraps of different lengths were compressed to modify the bulk density within the smoldering fuel bed. The ignition tests were performed using a rod-heater with a controlled temperature range of 340–460 °C. Once the rod-heater reached the preset temperature, the current was turned off, and the rod-heater was inserted into the center of the vertically oriented combustion chamber filled with paper scraps. By recording the temperature variations at different locations within the combustion chamber, the ignition limits of smoldering paper scraps with varying bulk densities under different external airflow rates were determined. The results showed that in the absence of external airflow and with a fixed paper scrap length, the ignition limit of smoldering paper scrap exhibits a clear U-shaped trend as bulk density increases. Furthermore, we found that in the absence of external airflow, the length of paper scraps had no significant effect on the ignition limit in the low bulk density range. However, in the high bulk density range, the ignition limit increased with scrap length. As for cases with external air flows, the ignition limit of paper scrap smoldering combustion once again exhibits a U-shaped trend with varying bulk density. Compared with the condition without forced airflow, however, the inflection point of the U-shaped curve shifts toward the higher-density region. Moreover, within the range of externally forced airflow rates examined in the present study, the length of paper scraps had no significant effect on the smoldering ignition limit.

The patterns of catalytic ignition of deuterium–air mixtures above the surface of metallic rhodium at pressures of 1–2 atm and temperatures of 20–250°C using hyperspectrometers in the range of 400–1650 nm and high-speed filming have been established. It is established that the catalytic ignition of deuterium–air mixtures in the studied temperature range is observed at a deuterium content of more than 12%; and at a deuterium content of less than 12%, only intense heating of the catalytic wire is observed. It is shown that the initial ignition source occurs on the surface of the reactor. In subsequent experiments, under the same conditions, the location of the original center changes. It has been found that the upper limit of the catalytic ignition above the D 2 –air mixture is noticeably lower than the lower ignition limit of the H 2 –air mixture. Thus, D 2 is more combustible than H 2 over the surface of Rh at a pressure above 1 atm. The limits of catalytic ignition are even lower than 20°C, although the flame velocity in hydrogen–air mixtures and the flame temperature in these mixtures of the same composition are much higher than those of deuterium–air mixtures. The nature of the detected kinetic inverse isotope effect is probably determined by the high level of activity of rhodium deuteride in relation to the deuterium oxidation reaction.

The hyperburner is a key component of a TBCC engine, and its reliable ignition and stable operation are critical. The gliding arc plasma igniter driven by differential pressure has the technical advantages of low energy consumption and high jet temperature. In this paper, the electrical and flow characteristics of the gliding arc plasma igniter are studied, and the basic ignition experiment in the hyperburner is carried out. Electrical characteristic experiments show that the discharge duration, the evolution of the gliding arc and the fracture frequency are affected by the pressure difference between the inlet and outlet of the igniter (Δp). With the increase in Δp, the frequency of the trapezoidal envelope in the voltage and current waveforms increases, and the frequency of the evolution and fracture of the gliding arc increases. The continuous discharge time of the gliding arc decreases when Δp = 550 Torr. The flow characteristic experiments show that the velocity of the swirl sheath is increased and the protective effect on the gliding arc is enhanced with the increase in Δp. In the range of 20–550 Torr, the jet length first increases and then decreases with the increase in Δp. The jet length reaches a maximum of 31 mm at Δp = 50 Torr. Basic ignition experiments show that proper Δp can widen the lean ignition limit and shorten the ignition delay time. In the working conditions of this paper, the ignition effect is the best when Δp = 350 Torr, which can widen the lean ignition limit by 37.5% and shorten the ignition delay time by 17%. After increasing the oil–gas ratios, the combustion is more complete and the ignition delay time can be shortened by 93.1% at most.

It is found that the determining factor for the catalytic ignition of mixtures of hydrogen with ethane and ethylene is not only the material of the catalyst but also the chemical nature of the C 2 hydrocarbon in the mixture with H 2 . It is shown that the limits of the catalytic ignition of the synthesis gas over metallic rhodium (Rh) are qualitatively different from the dependences for a hydrogen–hydrocarbon mixed fuel. The dependence of the lower limit of catalytic ignition on temperature has a distinct maximum, which indicates a more complex mechanism of the catalytic process than in the case of hydrogen–methane mixtures; the Arrhenius dependence of ln [H 2 ] lim on 1/ T does not hold. Therefore, the interpretation of the upper and lower limits of catalytic ignition (ULCI, LLCI) used in the literature, taking into account catalyst poisoning by CO molecules, needs to be clarified. The relatively long delay periods of the catalytic ignition of hydrogen– n -pentane mixtures (tens of seconds) and the absence of dependence of the delays on the initial temperature allow us to conclude that the catalytic ignition of hydrogen–propane/ n -pentane mixtures is determined by the rate of transfer of hydrocarbon molecules to the surface of the catalytic wire. Thus, in the oxidation of hydrogen–hydrocarbon mixtures for “short” hydrocarbons, the main factor determining the catalytic ignition is the oxidation reaction of hydrogen on the catalytic surface. With an increase in the number of carbon atoms in the hydrocarbon, the factors associated with the chemical structure, i.e., the reactivity of the hydrocarbon in catalytic oxidation, begin to play a significant role; and then the rate of oxidation is determined by the rate of transfer of the hydrocarbon molecules to (or within) the catalyst surface.

The values of the ignition temperature are experimentally determined and the effective activation energies of the limits of catalytic ignition of mixtures ((40–70%) H 2 + (60–30%) CH 4 ) stoich + air over metallic rhodium at a pressure of 1.7 atm in the temperature range of 20 to 300°C are estimated. Above the rhodium surface treated with ignitions, the catalytic ignition temperature of a mixture of 70% H 2 + 30% CH 4 + air is 62°C, which indicates the possibility of using rhodium to significantly reduce the ignition temperature of fuels based on hydrogen-methane mixtures. The critical nature of the implementation of the bulk reaction is experimentally discovered: the bulk process occurs at [H 2 ] = 45%, but is absent at hydrogen concentrations of ≤40%. If [H 2 ] ≤ 40%, only a slow surface catalytic reaction occurs. This phenomenon is illustrated by a qualitative calculation. It is established that the effective activation energies of both the upper and lower limits of the catalytic ignition of stoichiometric mixtures of H 2 + CH 4 in the linearity range are approximately (2.5 ± 0.6) kcal/mol. This means that the key reactions responsible for the occurrence of the upper and lower limits of catalytic ignition are the same. It is shown that, in the case of catalysis with a rhodium catalyst, the chain propagation process is most likely of a heterogeneous nature, since the effective activation energy is less than 3 kcal/mol.

An experiment has been developed to investigate the ignition characteristics of ultra-lean premixed H₂/air mixtures by a supersonic hot jet. The hot jet is generated by combustion of a stoichiometric mixture in a small prechamber. The apparatus adopted a dual-chamber design in which a small-volume (1% of the main chamber by volume) prechamber was installed within a large-volume main chamber. A small orifice (nozzle) connects the two chambers. Spark initiated combustion inside the prechamber causes a pressure rise and pushes the gases though the nozzle, resulting in a hot jet that would ignite the lean mixture in the main chamber. Simultaneous high-speed Schlieren photography and OH* Chemiluminescence were applied to visualize the jet penetration and the ignition processes inside the main chamber. Hot Wire Pyrometry (HWP) was used to measure temperature distribution of the transient hot jet. A novel velocity measurement technique based on Schlieren PIV (SPIV) has also been developed to characterize the local flow field. Three nozzle geometries (straight, convergent and converging-diverging) have been studied to understand their effect on ignition probability and characteristics. The results show that a supersonic jet by using a converging-diverging nozzle can ignite leaner mixtures than the jet produced by a straight nozzle of the same throat area (e.g., the ignition limit is reduced toϕ= 0.22, from 0.35). Additionally, infrared imaging and OH* Chemiluminescence indicated diamond shock structures in the supersonic jets and a high-temperature zone downstream the shocks. This high-temperature zone is likely the reason why the main-chamber flammability limit can be further reduced. Lastly, combustion instability becomes noticeable near the lean-limit conditions for all three types of nozzles, which affect the structural integrity of the combustion chamber. Two instable frequency modes, natural frequency of the combustor at 180 Hz and a higher mode at 2400 Hz were observed.

The widespread adoption of boosted, downsized SI engines has brought pre-ignition phenomena into greater focus, as the knock events resulting from pre-ignitions can cause significant hardware damage. Much attention has been given to understanding the causes of pre-ignition and identify lubricant or fuel properties and engine design and calibration considerations that impact its frequency. This helps to shift the pre-ignition limit to higher specific loads and allow further downsizing but does not fundamentally eliminate the problem. Real-time detection and mitigation of pre-ignition would thus be desirable to allow safe engine operation in pre-ignition-prone conditions. This study focuses on advancing the time of detection of pre-ignition in an engine cycle where it occurs. Phase space transforms through time-delay embedding of cylinder pressure and principal component analysis were applied to same-cycle detection of pre-ignition and shown to enable detection on the order of a crank degree earlier than deviation in cylinder pressure can be identified through direct statistical observation of the pressure data. Additionally, it appears that the deviation of the trajectory in phase space may offer the opportunity to extend this method to further extend the detection window and allow more time for mitigation actions to occur.

In stationary natural gas engines, lean-burn combustion offers higher engine efficiencies with simultaneous compliance with emission regulations. A prominent problem that one encounters with lean operation is cyclic variations. Advanced ignition systems offer a potential solution as they suppress cyclic variations in addition to extending the lean ignition limit. In this article, the performance of three ignition systems-conventional spark ignition (SI), single-point laser ignition (LI), and prechamber equipped laser ignition (PCLI)-in a single-cylinder natural gas engine is presented. First, a thorough discussion regarding the efficacy of several metrics, besides coefficient of variation of indicated mean effective pressure (COV_IMEP), in representing combustion instability is presented. This is followed by a discussion about the performance of the three ignition systems at a single operational condition, that is, same excess air ratio (λ) and ignition timing (IT). Next, these metrics are compared at the most optimal operational points for each ignition system, that is, at points where λ and IT are optimized to achieve highest efficiency. From these observations, it is noted that PCLI achieves the highest increase in engine efficiency, Δη = 2.1% points, and outperforms the other two methods of ignition. A closer look reveals that the coefficient of variation in ignition delay (COV_ID) was negligible, whereas that in coefficient of variation in combustion duration (COV_CD) was significantly lower by 2.2% points. However, the metrics COV_ID and COV_CD are not well correlated with COV_IMEP.