Ignition Probability Research Articles

Ignition of Aircraft Combustible Fluids by Hot Surface Under Ventilation

Abstract Flight-fire is a crucial factor that affects the safety of aircraft. It mainly results from combustible fluids leakages on hot surfaces. Hence, this study investigates the ignition probability of a droplet on a hot surface under ventilation through experiments. Specifically, two types of widely used combustible fluids, No.3 Jet Fuel and Mobile Jet Oil II, were investigated at varying surface temperatures ranging from 500°C to 700°C. This range covers the non-ignition to sure-ignition temperatures for the two fluids. The experiments were carried out considering the actual ventilation conditions of an aircraft, wherein the air flow rate ranged from 0 m/s to 3 m/s, and the air flow temperature ranged from room temperature to 380°C. The experimental results indicates that environmental factors have significant influence on droplet ignition probability, ignition probability corresponds with the air flow temperature but is opposite to the air flow rate. Furthermore, the degree of influence on ignition probability varies with the type of combustible liquid. The critical temperatures for non-ignition and sure-ignition on hot surface for No. 3 Jet Fuel and Mobile Jet Oil II under ventilation conditions have been acquired. Additionally, an empirical model is established to determine the ignition probability of No. 3 Jet Fuel under ventilation. The results of this work have instructional significance for aircraft fire protection design and airworthiness certificate of aircraft fire safety.

Ignition of Forest Fires by Cigarette Butts: Using Pinus massoniana Needles as an Example

As a cigarette butt falls onto the forest surface fuel, it first smolders the fuel, then ignites into flames, and spreads as forest fire under certain conditions. In this study, the needles under a typical stand of P. massoniana were used as the research object. Needle beds with different moisture content and packing ratios were constructed indoors. Cigarette butt-ignition experiments were conducted under different wind velocities, and 30 experiment cycles were conducted under different conditions. There was a total of 5 (packing ratio) × 4 (moisture content) × 6 (wind velocity) = 120 sets of conditions, and a total of 3600 ignition experiments were conducted. The results showed that (1) the total ignition probability of the cigarette butts was 2.36%, which only occurred when the fuelbed moisture content was <10% and the wind velocity was >1 m/s. The ignition time of cigarette butts ranged from 2.73 to 7.25 min. (2) The fuelbed moisture content and wind velocity significantly influenced the ignition probability and time. With an increase in moisture content, the ignition probability of cigarette butts decreased, while the time required for ignition showed an increasing trend. Wind velocity had a dual effect on ignition. The ignition effect was optimal at a wind velocity of 4 m/s. With an increase in wind velocity, the ignition probability first increased and then decreased, and the ignition time first decreased and then increased. (3) The packing ratio had no significant effect on the ignition probability; however, the ignition time significantly decreased as the packing ratio increased. (4) The logistic regression method (LRM), general linear method (GLM), and nonlinear regression method (NLM) were used to establish a prediction model of ignition probability. The prediction effect of GLM was the worst, followed by LRM, and the NLM had the best prediction effect. The GLM was selected to establish the ignition time model, and the error was also within the allowance range. This study elucidated the underlying mechanism of factors affecting cigarette butt-based fuel ignition. In addition, the established prediction model provides a reference for human-caused forest fires and is highly significant for forest fire prevention.

Ignition Probability of Turbulent Non-Premixed Swirling Flames in a Double-Swirl Burner

ABSTRACT In this paper, the effect of swirl number on mixing structure and ignition probability in turbulent non-premixed natural gas/air flame has been investigated. The flames are visualized by a digital camera and inverted by Abel’s transformation. Maps of ignition probability are generated by changing the spark position axially and radially while the flow field and mixing topology are established using numerical simulation. Three zones were defined based on the probability of successful ignition. The first one was the ineffective zone where the probability of a successful ignition was less than 20%. The second one was called the transition zone where the probability of a successful ignition was between 20% and 80%, and a successful ignition was highly dependent on the spark location. The third one was called the high-probability zone where the probability of a successful ignition was more than 80%. It was found that although the increment in swirl number improves the rate of mixing, it does not result in a better distribution of successful ignition probability. In high swirl numbers, the high-probability region is very small but at lower swirl numbers, the high-probability region is distributed at the burner exit, resulting IN better ignition characteristics.

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.

Experiment and modeling of stochastic ignition and combustion of fuel droplets impacting a hot surface

The ignition dynamic of liquid fuel droplets impacting hot surfaces is critical for fire-safety analysis in engineering systems, as well as for controlling wall-filming effects in IC engines. The scenario of slow fuel-leakage rates poses challenges associated with the stochastic processes of droplet splashing, break-up, turbulent mixing, and combustion. To address this, we conduct controlled experiments of liquid n-heptane droplets impacting a heated surface in the Leidenfrost regime, targeting the individual-droplet-deposition conditions. The experiment encompasses a range of surface temperatures and droplet-deposition rates. The experiments are complemented by theoretical analysis, where we developed a stochastic low-order numerical model, demonstrating good accuracy for predicting ignition probability and overall combustion dynamics. Notably, we observe a broad region of intermittent combustion behavior, with ignition probability varying based on surface temperature and droplet deposition rate. Additionally, we find that the transition to consistent ignition relies heavily on both surface temperature and deposition rate. Experimental and numerical model results shed light on the roles of the complex interplay between droplet breakup, chemical kinetics, and evaporation and mixing time scales, as well as the interaction among subsequent droplet combustion events, in governing the ignition and combustion of impacting droplet trains. The revealed dynamic of droplet/hot-surface ignition and the proposed stochastic model hold promise for advancing predictive capabilities of hot-surface-induced ignition and combustion arising from accidental leaks in flammable-liquid piping and wall-filming, particularly in the stochasticity-dominated individual-droplet-deposition regime.

Analysis of spontaneous ignition of hydrogen-enriched methane in a rectangular tube

This study investigates the spontaneous ignition of high-pressure hydrogen-enriched methane in air within a rectangular tube. A computationally efficient approach has been adopted, utilizing a reduced reaction mechanism and ignition delay model within a 3D Large Eddy Simulation (LES) framework. This approach overcomes the limitations of traditional 1D and 2D simulations with detailed chemistry models, which are unable to accurately reproduce the complex 3D shock wave structures within the tube. The simulated shock wave behavior during 9 MPa hydrogen leakage (case 1) and 11 MPa 90 vol% hydrogen/10 vol% methane mixture leakage (case 2) are found to agree well with experimental observations. In case 2, the hot spots generated by reflected shock waves and Mach reflections ignite the hydrogen/methane-air mixture, resulting in three sequential spontaneous ignitions. The flame is observed to primarily propagate along the tube corners and wall centers, with the central ignition spreading across the entire cross-section. For the 25 MPa 24 vol% hydrogen/76 vol% methane mixture leakage (case 6), the shock intensity is significantly reduced due to the increased methane proportion, leading to spontaneous ignition only at the tube corners when the hemispherical shock wave reflects from the wall. The flame predominantly forms downstream along the tube corner, gradually spreading along the tube wall. It is indicated that while the probability of spontaneous ignition decreases with increasing methane content, the risk remains significant under sufficiently high pressures. To the best our knowledge, this study represents the first 3D large eddy simulation of spontaneous ignition for high-pressure hydrogen-enriched methane leakage into air, providing valuable insights into the underlying physical phenomena.

The initiation of smouldering peat fire by a glowing firebrand

Background Wildfires represent a significant threat to peatlands globally, but whether peat fires can be initiated by a lofted firebrand is still unknown. Aims We investigated the ignition threshold of peat fires by a glowing firebrand through laboratory-scale experiments. Methods The oven-dried weight (ODW) moisture content (MC) of peat samples varied from 5% ODW to 100% ODW, and external wind (ν) with velocities up to 1 m/s was provided in a wind tunnel. Key results and conclusions When MC < 35%, ignition is always achieved, regardless of wind velocity. However, if MC is between 35 and 85%, an external wind (increasing with peat moisture) is required to increase the reaction rate of the firebrand and thus heating to the peat sample. Further increasing the MC to be higher than 85%, no ignition could be achieved by a single laboratory firebrand. Finally, derived from the experimental results, a 90% ignition probability curve was produced by a logistic regression model. Implications This work indicates the importance of maintaining a high moisture content of peat to prevent ignition by firebrands and helps us better understand the progression of large peat fires.

Ensemble predictions of laser ignition with a hybrid stochastic physics-embedded deep-learning framework

When investigating stochastic ignition phenomena, high-fidelity simulations and experiments for statistical characterization can be a laborious endeavor. Machine learning (ML), in particular deep-learning, offers an accurate and cost-effective approach for reduced-order modeling of forced ignition. In this work, we investigate the potential and limitations of ML in modeling ignition, especially in the presence of limited data and multiple ignition modes. To this end, we introduce a hybrid stochastic physics-embedded deep-learning framework that combines sparse experimental data from Schlieren measurements with inert large-eddy simulations for predicting the spatio-temporal evolution of ignition kernel location and morphology. This model combines a stochastic differential equation for modeling kernel dynamics and a deep-learning model for representing kernel morphology. We evaluate this modeling approach for laser ignition of a gaseous methane/oxygen (CH4/O2) model rocket combustor, where morphological effects of the ignition kernel significantly influence ignition behavior. Results demonstrate that this model can reasonably capture behavior associated with the three dominant ignition modes, namely direct, indirect, and failed ignition, along with statistics associated with kernel growth and position, at lower computational costs than high-fidelity reacting simulations. In addition, we demonstrate that this modeling framework can be employed for generating spatially resolved ignition probability maps by incorporating physics to represent kernel interaction with the turbulent jet. We note that limitations in accuracy can be observed when predicting with vastly out-of-distribution data. Nevertheless, these results demonstrate that this physics-embedded ML approach can statistically characterize forced ignition in a more cost-effective manner than reacting high-fidelity simulations, as long as sufficiently representative data is available.

Evaluation of the combustion process of directly injected methane in a rapid compression machine with a laser-based ignition system and an electrical ignition system

In order to reduce exhaust gas emissions, the application of lean operation in internal combustion engines has great potential. Experiments have been carried out in a rapid compression machine with directly injected methane comparing two laser ignition systems based on Nd:YAG lasers with an electrical ignition system. Three stratified extremely lean mixtures are ignited with up to 3 consecutive ignition pulses. This application-oriented investigation with the novel approach of combining laser ignition with a stratified charge in a rapid compression machine provides essential insights into the combustion process. The results highlight the advantages of laser ignition systems. The maximum pressure is increased by up to 9.5 %. The indicated work is increased by up to 16 % and cyclic variations are reduced by up to 35 %. The advantages increase as the mixture becomes leaner. A positive influence of multiple pulses can be observed as the ignition probability and the cyclic variations improve by up to 20 % per pulse and 25 % per pulse, respectively for the laser ignition. The evaluation shows that shorter pulse durations with a higher power achieve improved values for successful ignitions while longer pulses and a higher pulse energy achieve higher ignition probability values.

Non-equilibrium plasma assisted ignition characteristics in premixed ethylene-air flow

Reliable and stable ignition under lean conditions is essential for safe operation of the engine. Nanosecond pulsed discharge non-equilibrium plasma assisted ignition characteristics of premixed ethylene-air flow in an advective combustion chamber were investigated. The effects of the equivalence ratio, discharge gap distance, flow velocity, discharge frequency or inter-pulse time, and pulse number were quantified in terms of ignition probability. Shadow images of ignition kernel development were captured and used to extracted the averaged kernel projected area. The results indicated that increasing the equivalence ratio, a higher flow velocity, a wider discharge gap distance, and a larger number of pulses are all conducive to the increasing of ignition probability via inducing a larger initial kernel. Increasing inter-pulse time has a non-monotonic effect on ignition probability for multiple nanosecond pulsed discharges ignition. As the inter-pulse time decreases, when neighboring kernel boundaries happen to overlap each other, the partially-coupled regime shows a higher ignition probability. Longer or shorter inter-pulse time both cause the decrease in ignition probability. The shortest inter-pulse time shown as the fully-coupled regime is the most favorable for ignition with the highest ignition probability. A method is proposed to estimate the critical frequency at which partially-coupled regime transitions to fully-coupled regime by 95% of the asymptotic time of flame development time.

Cavity effects on spontaneous ignition of pressurized hydrogen jets

The release of pressurized hydrogen jets can lead to spontaneous ignition, which poses a major challenge to the safe utilization of hydrogen energy. Previous studies have primarily focused on the spontaneous ignition of pressurized hydrogen released into tubes. However, in real industrial scenarios, pressurized hydrogen is usually released into the atmosphere or a complex equipment environment. Therefore, the effect of obstacles on spontaneous ignition needs to be considered. In this study, the spontaneous ignition of sudden releases of pressurized hydrogen entering various cavity shapes was modeled using the standard k-ω turbulence model, the eddy dissipation concept (EDC) model and a detailed 21-step chemistry model. The effects of the cavities on the jet flow fields and the spontaneous ignition were analyzed for various cavity shapes and release pressures. For the pressures considered in this study, the cylindrical cavity did not lead to ignition. The highest hydrogen jet temperature occurred in the contact region due to the shock wave reflection, after which spontaneous ignition could not occur due to the flow divergence. The pressurized hydrogen entering hemispherical and conical cavities enhanced the hydrogen-air mixing which led to combustible regions and the formation of multidimensional shock waves, which significantly increased the spontaneous ignition probability. The cavities created a high-temperature region inside the cavity that experienced ignition but the flames did not spread out of the cavity. The conical cavity produced lower flame temperatures than the hemispherical cavity but prolonged the flame lifetime. Higher initial hydrogen release pressures resulted in more violent multidimensional shock wave interactions and higher contact surface temperatures, which lead to earlier ignition times but with little impact on the initial location of the spontaneous ignition. The results of this study provide a scientific foundation for hydrogen safety codes, safe equipment designs and the development of industry codes and standards.

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.

Geostatistical Modeling of Wildfire Occurrence Probability: The Case Study of Monte Catillo Natural Reserve in Italy

The increasing incidence of wildfires in the Mediterranean region has raised significant scientific and environmental concerns. This study focuses on a retrospective analysis of wildfire ignition and propagation within the context of the Monte Catillo Natural Reserve in Italy. After conducting a comprehensive review of the current state-of-the-art wildfire susceptibility mapping, propagation modeling, probability assessment, forest vulnerability models, and preventive silvicultural measures, we examine the regulatory framework surrounding wildfires in the national context, with a specific focus on prevention, prediction, and active firefighting measures. A geostatistical model of wildfire occurrence was developed, starting with the characterization of the area vegetation and anthropogenic factors influencing wildfire ignition. After that, wildfire observations from the period between 2010 and 2021 were included. The objective is to generate a wildfire hazard map for two distinct vegetation communities. To accomplish this, a statistical analysis was applied using the Poisson Model, assessing its goodness-of-fit by comparing observed frequencies with experimental data through the chi-square test. In conclusion, this model serves as a valuable tool for characterizing wildfire hazards, including ignition probabilities and propagation scenarios, within the Monte Catillo Natural Reserve. The research significantly contributes to enhancing our understanding of wildfire dynamics and plays a crucial role in the development of effective strategies for wildfire risk management.

Laser-induced breakdown spectroscopy in inhomogeneous fuel sprays: A novel calibration methodology for local stoichiometry

This paper presents a novel calibration for estimating the local stoichiometry of inhomogeneous sprays when using Laser-Induced Breakdown Spectroscopy (LIBS). The LIBS spectrum in inhomogeneous sprays appears to consist of coexisting spectra. This renders the traditional curve-fit calibration method inapplicable and impedes the determination of local equivalence ratios. A hypothesis is proposed that the spray LIBS spectra can be regarded as a discrete superposition of regions in the detection area with distinct compositions. A novel calibration approach is then proposed to test the simplest version of this hypothesis by identifying the best correlation between the spray LIBS spectra and a binary superposition of the calibration spectra. A local equivalent stoichiometry can then be estimated for the best correlated match. This approach generates spectra with high correlation to the test spray spectra for the majority of sparks, and hence provides a novel insight into LIBS application for inhomogeneous fuel/air mixtures. Test cases where the leaner region already contains fuel (ϕlean > 0) have high probability of successful ignition.

Post-earthquake fire ignition model uncertainty in regional probabilistic shaking–fire cascading multi-hazard risk assessment: A study of earthquakes in Japan

Fires following earthquakes have significant impact on urban communities in earthquake-prone countries with wooden buildings, in addition to the main earthquake hazard of ground shaking. Regional-scale post-earthquake fire risk assessments are important for developing effective risk reduction strategies. Such risk assessments require the consideration of epistemic uncertainty, that is, uncertainty caused by lack of knowledge concerning the best models, in addition to aleatoric uncertainty. This study focused on epistemic uncertainty in post-earthquake fire ignition estimations, and incorporated it into risk assessments by using alternative empirical ignition models. The key objectives of this study were as follows: (1) correlating the probability of ignition per person to the ground motion intensity based on data for different past large earthquakes in Japan; (2) investigating the impact of using ignition models calibrated on different earthquake events in risk assessments through a realistic case study considering possible large earthquakes. A hierarchical Bayesian Poisson regression analysis revealed that the empirical relationship between the ignition probability and ground motion intensity greatly varies across six major large earthquakes in Japan from 1995 to 2022. Particularly, the effects of ignition prevention measures that have been widely implemented in households since the 1995 Kobe earthquake were inferred from much smaller ignition probabilities in approximately the last 10 years. The effects of this high epistemic uncertainty manifested as the large variability of the loss exceedance curve for a portfolio of buildings. These results indicate that the new ignition models can help to foster risk-informed decision-making under epistemic uncertainty.

On the Role of Continuing Currents in Lightning‐Induced Fire Ignition

AbstractLightning flashes are an important source of wildfires worldwide, contributing to the emission of trace gases to the atmosphere. Based on experiments and field observations, continuing currents in lightning have since a long time been proposed to play a significant role in the ignition of wildfires. However, simultaneous detections of optical and radio signals from fire‐igniting lightning confirming the role of continuing currents in igniting wildfires are rare. In this work, we first analyze the optical signal of the lightning‐ignited wildfires reported by the Geostationary Lightning Mapper over the Contiguous United States (CONUS) during the summer of 2018, and we then analyze the optical and the Extremely Low Frequency signal of a confirmed fire‐igniting lightning flash in the Swiss Alps. Despite data uncertainties, we found that the probability of ignition of a lightning flash with Continuing Current (CC) lasting more than 10 ms is higher than that of cloud‐to‐ground lightning in CONUS. Finally, we confirm the existence of a long CC (lasting about 400 ms) associated with a long‐lasting optical signal (lasting between 2 and 4 s) of a video‐recorded fire‐igniting lightning flash.

Experimental Research and Numerical Analysis of Marine Oil Leakage and Accidental Ignition in Fishing Vessels

The hazard of highly combustible marine oil leakage greatly increases fishing vessel operation risks. This research integrates an experiment to explore the coupling mechanism of a typical heated surface of an engine room as a source to ignite marine oil. A numerical model is established that depicts the dynamic process of and variations in the combined effects regarding multiple factors of oil ignition under actual experiment. The leaked marine oil is ignited with a heated surface, relevant models are applied to reproduce the results, and the influences of specific parameters of a fishing vessel’s engine room are analyzed. The results indicate that the leaked oil boils violently on the heated surface, and a vapor film forms on the oil surface. Increased heated-surface temperatures lead to a significant difference in the initial ignition occurrences of marine oil, and the distance between the ignition height and oil is closely related to the engine room environment. The ignition probability of marine oil shows a gradually increasing trend with elevated heated-surface temperatures. The ignition height presents a downward trend with the increase in the heated-surface temperature, while the engine room’s humidity in air inhibits the upward transfer of heat; however, the degree of inhibition is limited accordingly. The results evidence that this comparative work can be an effective approach to reveal the impacts of marine oil, heat source, ventilation velocity, and humidity on initial ignition characteristics. Additionally, this work provides a basis for setting up emergency planning with appropriate monitoring equipment and further preventing vessel fires due to oil–thermal ignition.

Comparative analysis of thermal and non-thermal discharge modes on ultra-lean mixtures in an optically accessible engine equipped with a corona ignition system

Radio-Frequency corona-based ignition systems are currently being investigated by the engine research community for their capability to ensure robust combustion at challenging operating conditions such as lean mixture and/or high exhaust gas recirculation (EGR) dilution. The low-temperature plasma (LTP) produced by the corona discharge accelerates the early flame development thanks to the production of combustion precursors like radical and excited species. Plasma benefits can be maintained with extensive management of the control parameters, preventing the transition from the non-thermal discharge mode to a thermal one.However, under ultra-lean conditions, where the ignition probability significantly decreases, the LTP discharge may not be suitable for promoting the combustion process because of insufficient thermal energy transfer and scarcity of available fuel, which is essential for radical production. Therefore, it may be necessary to increase the volume in which the plasma interacts to ensure robust ignitions and stable flame propagations. For this reason, the present work aims to assess and quantify the differences, in terms of combustion performance, between a corona igniter operating at LTP mode and with electric arcs occurring, at ultra-lean operating conditions. In arc-mode, the igniter produces volumetric discharges, facilitating extensive mixing of the mixture and resulting in advantageous effects on kernel formation and propagation. Tests were carried out in a single-cylinder optical access engine under low load and low speed, using gasoline as fuel. Close to the leanest stable limit found, the LTP mode shows a lower cycle-to-cycle variability. Above such a threshold, the thermal discharge can guarantee stable combustion events while the LTP discharge leads to unstable working conditions. Indicating, imaging, and pollutant emission analysis are carried out to evaluate the effects of thermal and non-thermal plasma modes on combustion characteristics and their environmental impact.

Experimental investigations on the jet flow field propagation characteristics and ignition mechanisms of stoichiometric CH4/O2/N2 premixed turbulent jet flames

The jet flow field (JFF) and flame propagation of stoichiometric CH4/O2/N2 mixtures under variable thermodynamic conditions were investigated experimentally using Schlieren and natural flame luminosity imaging techniques. A two-color method by wavelength integration was used to characterize the temperature field. The turbulent flame velocity (ST), Damköhler number (Da), Reynolds number (Re), and jet flame regime were explored by a combination of experimental data and theoretical calculations. The flame morphology shows that the propagation process can be subdivided into three stages: first, the JFF enters the main combustion chamber, then the flame propagates in the JFF, and finally, the JFF disappears after the flame front and the JFF front coincide. The vortex motion changes the thin reaction zone and the local high temperature region distribution. The normal jet shifts to the Mach disk jet at higher oxygen content. The ignition location changes from the jet tail to the shear layer. The top secondary vortex is more likely to generate at higher ambient temperatures, which enhances the ignition probability on the flame top. In addition, the degree of orifice blockage determines the jet flame type. Since the initial jet flame is more affected by orifice blockage, the variation in ST is strongly influenced by the change in the length-scale ratio. As the jet flame gradually moves away from the orifice, ST is mainly governed by turbulence intensity. Finally, a strong correlation between the Re number and Da number of the jet flame was found.