High Ignition Probability Research Articles

Effects of CO2 dilution on ignition characteristics in lean premixed H2/Air flames

The research has examined the effects of CO2 dilution on ignition characteristics in H2/air mixtures. An ignition kernel was initiated by a laser-induced plasma in H2/air/CO2 mixtures with a bulk velocity of 6.5 m/s. An infrared camera was used to measure radiation intensity emitted from H2O and ignition probabilities. A high-speed schlieren image system was utilized to measure ignition kernel areas and ignition time. High-speed chemiluminescence was performed to examine OH* intensities. Numerical simulations were conducted using GRI 3.0 mechanisms to calculate reaction rates and laminar flame speeds. The ignition probability is highest for the undiluted mixtures and decreases with increasing the CO2 dilution fraction at constant adiabatic temperatures. A prolonged ignition time is observed at elevated CO2 dilution concentrations. Increasing the CO2 mole fraction decreases the ignition kernel growth, the integrated OH* intensity, and the integrated radiation intensity for each adiabatic temperature. CO2 reacts with H radicals and decreases the production of OH*, OH, and H2O. The CO2 dilution in the H2/air flames significantly contributes to the reduced reaction rate and flame speed, resulting in the low ignition probability. The high ignition probability is found at the large integrated OH* intensity in all cases. The ignition probability is high at the large integrated radiation intensity under the constant adiabatic flame temperatures. The OH* intensity is a critical parameter to estimate the ignition probability in the diluted lean hydrogen flames.

Comparing the risk of third-party excavation damage between natural gas and hydrogen pipelines

Existing natural gas pipelines can facilitate low-cost, large-scale hydrogen transportation and storage, but hydrogen may entail safety challenges. These challenges stem from hydrogen’s different properties compared to natural gas, such as higher ignition probability, different flame behavior, and potential for hydrogen embrittlement. Although risk assessments for hydrogen pipelines are increasing, the impact of hydrogen on the risk of third-party excavation damage (TPD), the major cause of pipeline incidents in the U.S., has received little attention. This work presents the SHyTERP model for Safe Hydrogen Transportation and Excavation Risk Prevention for Pipelines. The model incorporates causal models, excavation damage and pipeline failure statistics, and validated physical models of hydrogen and natural gas release and jet flame behavior. Through four case studies, the model compares the TPD risks of hydrogen and natural gas pipelines, offering insights and recommendations for the safe implementation of hydrogen in existing pipelines.

Ignition probability and fuel consumption of boreal ground vegetation fuels – an experimental study in Finland

In boreal forests fires often ignite and spread within the dominant moss and lichen cover of the ground layer vegetation, which thus greatly influences fire hazard. We used an experimental set-up in greenhouse conditions to study the differences in how (1) fuel moisture and (2) wind velocity influence the ignition probability and fuel consumption among four common circumboreal ground vegetation fuels, Pleurozium schreberi (Willd. ex Brid.) Mitt., Hylocomium splendens Schimp., Dicranum spp. and Cladonia rangiferina (L.) F. H. Wigg. Our results show that the reindeer lichen C. rangiferina was clearly the most flammable species, with high ignition probability even at high moisture contents and low wind velocities. Of the mosses, Dicranum was the least flammable, with low ignition probability and mass loss at low wind velocities regardless of moisture content. P. schreberi and H. splendens behaved somewhat similarly with wind velocities quickly increasing the initially low ignition probability and mass loss observed in the absence of wind. However, especially for mass loss, among-species differences tended to disappear with stronger winds. The observed differences can be explained by the different structures and growth forms of the studied species and open a potential avenue for improving forest fire risk predictions.

Modeling Edge Flame Propagation for Flame Particle Tracking in the Forced Ignition Process of Turbulent Nonpremixed Jet Flame

ABSTRACT To ensure safe and reliable operation, ignition performance in propulsion and power devices needs to be thoroughly assessed. Reduced order models (ROMs) for ignition have gained increasing attention, given that first-principles models are restricted by immense computational costs that preclude rapid assessment for the design purpose. Currently, flame self-propagation is generally neglected in the ROM descriptions of ignition process and the impact on the flame front dynamics as well as the ignition probability map is not clear for low swirl or jet flames which are of interest for industrial applications. In this study, an empirical model for flame self-propagation is proposed to augment the capability of Lagrangian flame particle tracking method, a ROM model to predict ignition probability based on nonreacting flow field. The model adopts the classic scaling law of turbulent flame speed together with fuel gradient correction and flame self-propagation to mimic the edge flame dynamics during the ignition process. The model performances in terms of flame front dynamics and ignition probability map are quantified in a turbulent nonpremixed methane-air jet flame. Results reveal that only with flame self-propagation, the flame particle tracking can well capture the transient evolution of the leading flame front as observed in experiments. It is found that the predicted ignition probability map agrees well with experimental data with high ignition probability around the vicinity of the stoichiometric region at the downstream axial distance of 6 to 40 times the jet diameter. This study illustrates the importance to account for flame self-propagation to correctly predict ignition probability in jet dominant nonpremixed flames.

Contrasting Geographic Patterns of Ignition Probability and Burn Severity in the Mojave Desert

The extent and frequency of fire has increased in many arid systems over the last century, with a large proportion of area in some regions undergoing transitions to novel conditions. Portions of the Mojave Desert in southwestern North America have undergone such transitions, most often from woody to herbaceous-dominated systems. These transitions have often been attributed to the proliferation of invasive annual grasses that promote more frequent fire, but recent evidence indicates that transitions can also occur independent of fire frequency if burn severity is high. In addition, high probability of ignition (i.e., potentially high fire frequency) and high burn severity may not always be geographically related. Therefore, our goals were to: (1) map potential burn severity, fire frequency, and probability of ignition across the Mojave; and, (2) evaluate spatial association among predicted burn severity, fire frequency and probability of ignition. We first mapped perimeters of 250 wildfires > 405 ha that occurred from 1972 to 2010, then extracted data on fire frequency (number of times burned from 1972 to 2010), burn severity (the difference Normalized Burn Ratio), and 15 predictor variables representing physiography, climate, ignition, and vegetation. Maximum entropy was used to predict probability of ignition and Random Forest models were used to predict dNBR and fire frequency. Areas with high burn severity and high ignition probability had opposite spatial trends; areas with high burn severity were predicted to predominantly be in the northwest part of the region whereas areas with high ignition probability were predicted to be in the northeast. The models indicate the existence of a number of spatially structured but temporally dynamic fire regimes throughout the Mojave Desert. Two prevalent and ecologically significant regimes include one with frequent fires of low to moderate severity and another with infrequent fire of high severity. Areas with high fire frequency are currently limited in extent (<1% total area). However, cover of invasive grasses can remain high decades after a burn of high or moderate severity, so grass-fire cycles could develop in areas where there may be expectations of infrequent fire as well as those with relatively high fire frequency.

Modelling ignition probability with pairwise mixing-reaction model for flame particle tracking

Reduced order models for ignition analysis can offer insights into ignition processes and facilitate the combustor optimization. In this study, a Pairwise Mixing-Reaction (PMR) model is formulated to model the interaction between the flame particle and the surrounding cell mixture during Lagrangian flame particle tracking. Specifically, the model accounts for the two-way coupling of mass and energy between the flame particle and the surrounding shell layer by modelling the corresponding turbulent mixing, chemical reaction and evaporation process if present. The state of a flame particle, e.g., burnt, hot gas or extinguished, is determined based on particle temperature. This model can properly describe the ignition process with a spark kernel being initiated in a nonflammable region, which is of practical importance in certain turbine engines and has not been rigorously accounted for by the existing models based on the estimation of local Karlovitz number. The model is integrated into an ignition probability analysis platform and is demonstrated for a methane/air bluff-body flame with the flow and fuel/air mixing characteristics being extracted from a non-reacting simulation. The results show that for the spark location being at the extreme fuel-lean outer shear layer of the recirculation zone, PMR can yield ignition events with a significant number of active flame particles. The mechanisms for the survival of the initial flame particles and the entrainment of the survived flame particles into the recirculation zone are analyzed. The results also show that the ignition probability map from PMR agrees well with the experimental observation: a high ignition probability in the shear layer of the recirculation zone near the mean stoichiometric surface, and low ignition probabilities inside the recirculation zone and the top stagnation region of the recirculation zone. The parametric study shows that the predicted shape of the ignition progress factor and ignition probability is in general insensitive to the model parameters and the model is adequate for quantifying the regions with high ignition probabilities.

Two ignition transition modes at small and large distances between electrodes of a lean primary reference automobile fuel/air mixture at 373 K with Lewis number >> 1

Abstract Laminar and turbulent minimum ignition energies (MIEL and MIET) 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 (dgap) 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 (Eig). We find two ignition transition (IT) modes. (1) A non-monotonic IT at small dgap = 0.8 mm with the lowest MIET = 13.9 mJ ( u′c, MIET increases drastically (>> MIEL), because turbulence re-asserts its dominant role. (2) A regular IT at large dgap = 2 mm is found where MIEL is only 2.05 mJ, in which the increasing slopes of MIET 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 dgap = 0.8-mm and 2-mm (MIE0.8/MIE2.0) is 1.64 > 1, suggesting that using large dgap = 2-mm has a higher ignition probability than using small dgap = 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 Eig ≈ MIE/70 mJ and/or dgap = 0.8-mm/2-mm. These results are relevant to spark ignition gasoline engines operated in lean-burn turbulent condition.

Elevated OH production from NPHFD and its effect on ignition

This work explores ignition in flowing mixtures of methane and air at 100 kPa and 295 K initial temperature using a nanosecond-pulsed high-frequency discharge ignition source. Simultaneous OH planar laser-induced fluorescence (PLIF) and schlieren imaging were utilized at a frequency of 50 kHz to examine the time dependent radical generation during and after the plasma discharge. The results indicate that a significant volume of OH was generated in the discharge, and the magnitude of the said volume was a strong function of the pulse repetition frequency (PRF). In addition, it is demonstrated that the intensity of the OH-PLIF signal during and shortly after the discharge is elevated for PRF≥10 kHz, indicating that both the volume and concentration of OH are built up at high PRF. This accumulation of OH radicals in the inter-electrode region is directly correlated with high ignition probability, with higher PRF leading to faster OH accumulation in a step-wise fashion. It is shown that in cases in which the PRF is below a certain threshold (<10 kHz), OH accumulation does not occur, and it is believed that this condition, along with lower discharge temperatures, is directly responsible for reduced ignition probability.

A forced ignition probability analysis method using kernel formation analysis with turbulent transport and Lagrangian flame particle tracking

A forced ignition probability analysis method is developed for turbulent combustion, in which kernel formation is analyzed with local kernel formation criteria, and flame propagation and stabilization are simulated with Lagrangian flame particle tracking. For kernel formation, the effect of turbulent scalar transport on flammability is modelled through the incorporation of turbulence-induced diffusion in a spherically outwardly propagating flame kernel model. The dependence of flammability limits on turbulent intensities is tabulated and serves as the flammability criterion for kernel formation. For Lagrangian flame particle tracking, flame particles are tracked in a structured grid with flow fields being interpolated from a Computational Fluid Dynamics (CFD) solution. The particle velocity follows a Langevin model consisting of a linear drift and an isotropic diffusion term. The Karlovitz number is employed for the extinction criterion, which compares chemical and turbulent timescales. The integration of the above two-step analysis approach with non-reacting CFD is achieved through a general interpolation interface suitable for general unstructured CFD grids. The method is demonstrated for a methane/air bluff-body flame, in which flow and fuel/air mixing characteristics are extracted from a non-reacting simulation. Results show that the computed ignition probability map agrees qualitatively with experimental results. A reduction of the ignition probability in the recirculation zone and a high ignition probability on the shear layer of the recirculation zone near the mean stoichiometric surface are well captured. The tools can facilitate optimization of spark placement and offer insights into ignition processes.

Conventional spark versus nanosecond repetitively pulsed discharge for a turbulence facilitated ignition phenomenon

This work applies both conventional-single-spark-discharge (CSSD) at 500-µs pulse duration time and nanosecond-repetitively-pulsed-discharge (NRPD) at various pulsed-repetitive-frequency PRF = 5–70 kHz to explore a turbulence facilitated ignition (TFI) phenomenon using a pair of pin-to-pin electrodes at an inter-electrode gap of 0.8 mm in randomly-stirred lean n-butane/air mixture with Lewis number ≫ 1. For CSSD, measured laminar and turbulent minimum ignition energies (MIEL and MIET) at 50% ignitability show that MIEL≈ 23 mJ > the smallest MIET≈ 19.7 mJ at u′ = 0.9 m/s (TFI) and then MIET≈ 28.6/30.8/36.8 mJ at u′ = 1.4/2.1/2.8 m/s (no TFI), where u′ is the r.m.s turbulent fluctuating velocity. For comparison, all NRPD experiments apply the same total ignition energy Etot≈ 23 mJ via a fixed train of 11 pulses, each pulse with 2.2 mJ except for the first pulse with 1 mJ. NRPD results show a cumulatively synergistic effect depending on the coherence between PRF and an inward reactant flow recirculation frequency (fRC) inside the torus-like kernel induced by the discharge that could enhance ignition. When PRF is approximately synchronizing with fRC, the synergistic effect is most profound at PRF = 20-kHz/40-kHz with very high ignition probability Pig = 90%/85% > 50% in quiescence, whereas lower values of Pig = 42%/34% are found at PRF = 10-kHz/60-kHz. Further, Pig = 0 at PRF = 5-kHz even when 5000 pulses (Etot≈ 10 J) are applied. We discover that Pig decreases significantly with increasing u′ for most PRFs (no TFI) except at higher PRF ≥ 60 kHz showing possible TFI. These results are attributed to the interactions between turbulent dissipation, differential diffusion, and synergistic influence, which are substantiated by Schlieren images of initial kernel development and the ignition time determined at one half of the flame critical radius that leads to a self-sustained spherical flame propagation.

An experimental investigation on spray, performance and emission of hydrotreated waste cooking oil blends in an agricultural engine

Biomass-based fuels are gaining importance for operating a compression ignition engine as they can curb greenhouse gases and are a key for addressing the energy security. Hydrotreated oil is considered to be a potential drop-in fuel for the compression ignition engine as its cetane number is higher than fossil diesel. In this study, hydrotreated waste cooking oil and its blends (10%, 20%, 30%, 40% and 50% by volume) with diesel were prepared. The ignition probability of the test fuel samples was found using a hot-plate test setup. The neat hydrotreated fuel has higher ignition probability at a particular temperature than the other test fuels. The Sauter mean diameter of the test fuels was also observed using a Malvern Spraytec test setup. The results reveal that the neat hydrotreated fuel has higher Sauter mean diameter due to its high viscosity. As the percentage of the hydrotreated fuel in the blend decreases, the Sauter mean diameter decreases and diesel has the lowest Sauter mean diameter. The test fuels were also used to run a compression ignition engine. The results reveal a decrease in brake thermal efficiency with the increase in the hydrotreated fuel share in the blend. The heat release for the blends starts earlier than diesel and the peak heat release is also lower than diesel. The HC, CO and smoke emissions for the test blends decreases up to 30% blend. When the percentage of the hydrotreated oil is further increased, the emissions starts increasing. The NO emissions were lower than diesel for all the test samples. As compared to diesel, the maximum reduction in NO (neat), HC (30% blend), CO (30% blend) and smoke emissions (30% blend) is 23.2%, 14.4%, 13.83% and 13.3%, respectively.

Comparison of fuel characteristics of hydrotreated waste cooking oil with its biodiesel and fossil diesel

Compression ignition engines powered by diesel are the work horses of developing countries like India. However, burning fossil fuel causes a lot of air pollution and the depletion of fuel at an alarming rate. Fuels produced from biomass or wastes can partially substitute fossil diesel to decrease its consumption. One such feedstock is waste cooking oil (WCO) which can be easily converted into fuel for diesel engines. The hydrotreating process stands out among the methods available for converting WCO into fuel, since its properties are almost similar to fossil diesel with little or no oxygen content. In this study, the physico-chemical properties of the hydrotreated waste cooking oil (HVO), biodiesel of waste cooking oil, diesel and blends of HVO and diesel are compared. The blends were prepared by mixing 10%, 20%, 30%, 40% and 50% of HVO on volume basis in diesel. The evaporation rate and ignition probability of the fuel samples were found by using a hot-plate test setup. HVO had higher ignition probability than all the test sample. As the percentage of HVO increased in the test samples, the ignition probability increased. The Sauter mean diameter (SMD) of the samples was also found using Malvern Spraytec. The SMD of HVO was larger than diesel but smaller than biodiesel. The study shows that blends of HVO up to 30% are feasible for present use in diesel engines, as the viscosity (2.54, 2.59 and 2.62cSt) and calorific value (42.41, 42.29, 42.08MJ/kg) of the three blends (10%, 20% and 30%) is close to diesel (2.51cSt and 42.58MJ/kg). Also, due to high cetane index, neat HVO or blends having higher HVO content (> 30%) cannot be used in the existing engines as the engine power output may be affected. Therefore, to use these fuels, the engine needs to be modified which is not feasible for existing engines. The FTIR and GC-MS analysis shows that the HVO has low oxygen content and high amount of paraffins, whereas biodiesel of waste cooking oil has high unsaturation and high oxygen content.

The wildfire problem in areas contaminated by the Chernobyl disaster

This paper examines the issue of radionuclide resuspension from wildland fires in areas contaminated by the Chernobyl Nuclear Power Plant explosion in 1986. This work originated from a scientific exchange among scientists from the USDA Forest Service, Ukraine and Belarus that was organized to assess science and technology gaps related to wildfire risk management. A wildfire risk modeling system was developed to predict likely hotspots for large fires and where wildfire ignitions will most likely result in significant radionuclide (Cesium, 137Cs) resuspension. The system was also designed to examine the effect of fuel breaks in terms of reducing both burn probability and resuspension. Results showed substantial spatial variation in fire likelihood, size, intensity, and potential resuspension within the contaminated areas. The potential for a large wildfire and resuspension was highest in the Belorussian Polesie Reserve, but the likelihood of such an event was higher in the Ukrainian Chernobyl Exclusion Zone due to a higher predicted probability of ignition. Fuel breaks were most effective in terms of reducing potential resuspension when located near areas that had both high ignition probability and high levels of 137Cs contamination. Simulation outputs highlighted how human activities shape the fire regime and likelihood of a large fire in the contaminated areas. We discuss how the results can be used to develop a fire management strategy that integrates ignition prevention, detection, effective suppression response, and fuel breaks. Specifically, the modeling system can now be used to explore a wide range of fire management scenarios for the contaminated areas and contribute to a comprehensive fire management strategy that targets specific drivers of fire by leveraging multiple tools including fire prevention and long-term fuel management. Wildfire-caused emissions of radionuclides in Belarus, Ukraine, and Russia are a socio-ecological problem that will require defragmenting existing risk management systems and leveraging multiple short- and long-term mitigation measures.

Effects of changing development patterns and ignition locations within Central Texas.

Researchers have a limited understanding of the interactions between development patterns and Ignition Probability. This study explores the variation in Ignition Probability as a result of differing development patterns. Based on LANDFIRE datasets, development changes were mapped for two sets of years (2001 and 2012) and the relationship between development and Ignition Probability was assessed. The study area covered the two adjacent counties, Bastrop and Travis located in Texas, USA. These two counties have a high potential for wildfire, and due to expanding development have high vulnerability. Expanding lateral development was organized into one of five categories: infill, radial, isolated, clustered, and linear. The Maximum Entropy algorithm predicted the spatial distribution of ignition probabilities based on several physical and land use characteristics coupled with historic ignition locations. Variation in Ignition Probability was assessed for each category of development using two-way ANOVA’s and post hoc analysis. Ignition Probability maps indicated a fair sensitivity (Area Under the Curve: 0.77–0.78), suggesting that the spatial configuration of development patterns influenced wildfire ignition. Analyses found that isolated, clustered, and linear outlying development patterns had higher Ignition Probability than infill and radial development, and that fire probabilities fell along a development gradient. This trend between the development gradient and ignition probabilities should be used to direct land use planning to reduce fire risk.

Geospatial Patterns and Drivers of Forest Fire Occurrence in Romania

Timely and accurate spatial explicit forest fire risk assessment and mapping is essential for forest fire prevention and suppression preparedness, firefighting resources allocation, and efficient multi-level fire management policies. This paper describes the application and validation of an approach for forest fire risk analysis and fire risk zoning over Romania in order to identify areas at national scale where fires are most likely to occur and to threat existing values, resources and assets. A modeling approach based on logistic regression using historical fire observations has been developed based on environmental, socio-economic and demographic data availability at national scale. Ignitions were positively related to south-western slopes and occurred mostly in fuel type of xerophyte oaks as well as in areas of heterogeneous (natural/agricultural) landscape. In addition to the human variables the pattern of ignitions was also significantly related to slope and temperature of the driest quarter. The risk zones produced by the multiple logistic regression model presented satisfactory accuracy when compared with historical fire perimeters extracted from MODIS imagery. The findings of this study could be used by fire managers to implement prevention measures at forest areas with high fire risk. Furthermore, attention should be given to areas with high fire ignition probability, where the vulnerability and potential impact is higher.

Ignition enhancement by dual-pulse laser-induced spark ignition in a lean premixed methane-air flow

Ignition of fuel-lean premixed methane-air flow was investigated using single-pulse laser-induced spark (SPLIS) and dual-pulse laser-induced spark (DPLIS). Ignition enhancement was quantified with ignition probability in flow speeds and equivalence ratios of 3.75 m/s–12.5 m/s and 0.53–0.57, respectively. The range of total laser pulse energy per ignition event was 20–30 mJ. For DPLIS, the energy per pulse was half of the total energy and the time interval between pulses varied from 10 ns to 100 ns where pulse-to-pulse coupling increased the spark surface area. Broadband chemiluminescence within the visible wavelength region was used to determine ignition of the combustible mixture. The ignition probability increased with velocity for both SPLIS and DPLIS as more fuel-oxidizer mixture was exposed to the ignition source. For a fixed total energy, ignition probability was higher for DPLIS than SPLIS particularly as the equivalence ratio was increased. Depending on the flow velocity, a change in the total energy determined whether SPLIS or DPLIS had a higher ignition probability at a given flow velocity. The study showed that using DPLIS to ignite fuel-lean combustible mixtures can enhance ignition compared to SPLIS particularly at low total laser energy and high-speed flow conditions.

Reduction of flame development time in nanosecond pulsed high frequency discharge ignition of flowing mixtures

The effects of discharge and flow parameters on ignition kernel development time are explored in flowing methane–air mixtures. A nanosecond pulsed high frequency discharge in a pin-to-pin configuration is used as the ignition source, providing 2.9 ± 0.23 mJ/pulse. The effects of pulse repetition frequency (PRF) in the range of 10–300 kHz, number of pulses in the range of 1–50 (≈2.9–145 mJ), equivalence ratio in the range of 0.55–0.65, gap distance in the range of 0.5–2.5 mm, and flow velocity in the range of 2.5–10 m/s are explored. For all conditions, the ignition events are in the “fully-coupled” regime, in which high ignition probability is achieved and locally extinguished ignition kernels are avoided. It is found that reducing the PRF reduces the kernel development time for fixed total energy deposition due to an increased volume of unburned mixture exposed to the discharge. Increasing the number of pulses at a given PRF also decreases the kernel development time, again by increasing the volume of gas exposed to the discharge. The equivalence ratio only has an effect on the kernel growth rate after the discharge, with identical kernel areas measured at all equivalence ratios while the discharge is active. Increasing gap distance decreases the kernel development time by providing a larger initial kernel volume as well as reducing heat and radical quenching at the electrode surfaces. Finally, the flow velocity has an effect on the kernel growth rate at velocities greater than 5 m/s, with larger flow velocities resulting in shorter kernel development time. This is due to the competing rates of self-propagating flame expansion and kernel growth due to convection past the discharge region. The combined effects of all of the above parameters on the kernel area after the discharge are summarized in a correlation equation, which predicts the trends in kernel growth rate based on an estimated plasma area defined by the discharge duration, flow velocity, and gap distance.

Multi-channel nanosecond discharge plasma ignition of premixed propane/air under normal and sub-atmospheric pressures

Relight of jet engines at high altitude is very difficult due to the relatively low pressure and temperature of inlet air. Currently, advanced ignition technology for high-altitude relight in jet engines is urgently needed. Successful ignition is achieved only when the ignition kernel can propagate outwardly beyond the so-called critical flame initiation radius. At high altitude with low pressure, the critical flame initiation radius becomes large and it cannot be easily reached by the ignition kernel. Therefore, in order to achieve successful ignition at low pressure conditions, large ignition kernel should be generated. In this study, plasma assisted ignition using multi-channel nanosecond discharge (MND) is proposed to induce a large ignition kernel and to achieve successful ignition at low pressures. Ignition experiments for propane/air mixtures at different equivalence ratios (Ф=0.8∼1.6) and under normal and sub-atmospheric pressures (P=0.3∼1.0bar) were conducted in a constant volume combustion chamber. The performance of three ignition methods, spark discharge, single-channel nanosecond discharge (SND) and MND, were assessed; and the advantages of MND for ignition at sub-atmospheric pressures were demonstrated. The ignition kernel development, ignition probability, minimum ignition energy, and flame development for these three ignition methods (spark, SND and MND) were measured and compared. It was found that compared to spark and SND, MND can generate a much larger ignition kernel with stronger flame wrinkling and has much higher ignition probability, especially at low pressures. Therefore, MND has the advantage in achieving successful ignition at low pressure. Besides, it was shown that though the ignition kernel evolution and ignition probability strongly depend on ignition methods, the subsequent flame propagation is not greatly affected by ignition and there is little change in the flame rise time for different ignition methods.

開口部を持つドーム形フランジ付き点火プラグの点火特性

Heat efficiency is improved by making a lean burn in a spark ignition engine, but on the other hand, it brings on problems that ignition probability falls into misfire and/or decreases effective work due to the downslide of flame propagation. It is important to make lean burn succeed in strong turbulence field to avert these problems. In the present study, the ignition and combustion characteristics of the spark plug with dome-shaped flanges that has some holes for exchanging gas were examined by using swirling flow fields. The result shows that the dome flanged spark plugs were high ignition probability compared with conventional plug. It is considered the reason was the effect of the collection of the shock wave energy and the effect of suppressing the flow in vicinity of the spark gap. Especially, in lean mixture condition, the ignition probability of the dome flanged spark plug with three holes was the highest. The combustion characteristics of dome flanged spark plug that has 2.4mm holes was better than conventional and other dome flanged spark plugs. Furthermore, when the direction of the hole was set up in the direction where a flow is introduced, combustion characteristic was improved.

Short-term effects of mastication on fuel moisture and thermal regime of boreal fuel beds

Mechanical mastication is becoming a common fuel management treatment to reduce vertical fuel connectivity, as well as crown fire initiation and potential fire-line intensity, but the moisture dynamics of these novel fuel types have been largely unstudied. We recorded concurrent in situ meteorological observations with moisture and temperature profiles (at depths of 5 and 13 cm) for masticated fuel beds in three treatments in a lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) – black spruce (Picea mariana (Mill.) B.S.P.) boreal stand located in the Upper Foothills of west-central Alberta. Mulch at the 5 cm depth remained at or near 100% moisture content for a majority of the observation period within the treatment that lacked any residual canopy cover. Only during a 10-day rain-free period was substantial drying observed at a depth of 5 cm. At the interface with the underlying duff layer, mulch remained at upwards of 150% moisture content. Drying was typically &lt;10% of potential evaporation rates, except on days after rain, when drying equal to 25%–100% of potential evaporation was observed. Thermal properties of the mulch showed approximately five times the thermal diffusivity of soils, but less than a fuel crib, suggesting that masticated fuel beds have a thermal and moisture regime more similar to soils than fuel cribs, with a diffusion-dominated regime enhanced by minor advection. Low moisture movement observed from depth to the surface promoted highly variable surface moisture that created surface mulch with temperatures far in excess of air temperatures during periods of full sun. Such excess temperatures are shown to be an efficient indicator of surface dryness and high ignition probability.