Ignition Event Research Articles

Study of the Effect of Research Octane Number on the Auto-Ignition of Lubricant Oil Surrogates (n-Hexadecane).

Engine oil is considered one of the sources for pre-ignition in downsized boosted direct injection spark-ignited engines. When interacting with fuel sprayed in the combustion chamber, engine oil forms an ignitable mixture and can cause an ignition event before firing the spark plug. Because high research octane number (RON) fuels are difficult to auto-ignite and tend to suppress the knock in an internal combustion engine, studying their interaction with engine oil is essential. Hence, in the current study, a suitable lubricant oil surrogate, namely, n-hexadecane, is mixed with iso-octane and n-heptane at different concentrations to investigate the auto-ignition behavior at elevated pressures. Five sets of fuels (PRF0, PRF20, PRF50, PRF80, and PRF100) were prepared to get a wide range of RONs and blended with n-hexadecane at 15, 25, 35, and 45% mixture concentrations (vol %). These experiments were conducted in a constant volume combustion chamber, keeping the initial temperature constant at 300 °C. A single droplet of the mixture was suspended on a thermocouple bead to record the droplet’s lifetime temperature. It was observed that hexadecane mixed with PRF0, PRF20, PRF50, and PRF80 showed similar auto-ignition behaviors. The time of ignition (TI) for these mixtures initially increased until 25% concentration of the fuel in n-hexadecane, and further addition of fuels to 35% and higher concentrations showed a gradual decrease in TI. Ignition of mixtures with 35% and 45% fuel concentrations is attributed to n-heptane, as its low temperature chemistry is the dominant factor in its high reactivity compared to iso-octane. TI increased with the increasing concentration of PRF100 mixtures in hexadecane, unlike other PRF fuels tested in this study. This is because iso-octane is a high RON fuel with a higher auto-ignition temperature, making it challenging to auto-ignite.

Coupling a Lagrangian–Eulerian Spark-Ignition (LESI) model with LES combustion models for engine simulations

In the United States transportation sector, Light-Duty Vehicles (LDVs) are the largest energy consumers and CO2 emitters. Electrification of LDVs is posed as a potential solution, but SI engines can still contribute to decarbonization. Car manufacturers have turned to unconventional engine operation to increase the efficiency of Spark-Ignition (SI) engines and reduce the carbon emissions of their fleets. Dilute, lean, and stratified-charge engine operation has the potential for engine efficiency improvements at the expense of increased cyclic variability and combustion instability. At such demanding engine conditions, the spark ignition event is key for flame initiation and propagation and for enhanced combustion stability. Reliable and accurate spark ignition models can help design ignition systems that reduce cyclic variability. Multiple computational spark-ignition models exist that perform well under conventional conditions, but the underlying physics needs to be expanded, for unconventional engine operation. In this paper, a hybrid Lagrangian–Eulerian Spark-Ignition (LESI) model is coupled with different turbulent flame propagation models for engine simulations. LESI relies on Lagrangian arc tracking and Eulerian energy deposition. The LESI model is coupled with the Well-Stirred Reactor (WSR), Thickened Flame Model (TFM), and g-equation model and used to simulate several cycles of a Direct-Injection Spark-Ignition (DISI) engine using a commercial Computational Fluid Dynamics (CFD) engine solver. The results showcase the successful coupling of LESI with the combustion models. Global engine metrics, such as pressure and Apparent Heat Release Rate (AHRR), for each simulation setup are compared to experimental engine results, for validation. In addition, results highlight the successful prediction of spark channel movement by comparing simulation images to experimental optical engine images. Finally, the successful coupling of LESI to combustion models, making it a usable model in the engine modeling community, is emphasized and future development details are discussed.

Enhanced burning rates in hydrogen-enriched turbulent premixed flames by diffusion of molecular and atomic hydrogen

Direct numerical simulations (DNS) of fuel-lean turbulent premixed hydrogen-enriched flames are analyzed to improve understanding of local effects of molecular diffusion of fast-diffusing, low-Lewis-number hydrogen species (H2 and H) on the overall burning rates of reactant mixtures. Although it is often assumed that the importance of molecular diffusion decreases with increasing turbulence intensity, our analysis reveals that, on the contrary, diffusion of molecular and atomic hydrogen can remain the rate-controlling processes even at high Karlovitz numbers (Ka≫1). Three-dimensional DNS with detailed chemical kinetics and species transport of turbulent premixed hydrogen-air and hydrogen/ammonia-air flames in the regimes of thin and distributed reaction zones are performed and analyzed at different reactant temperatures and pressure levels. The DNS data reveal a significant impact of H2 and H diffusion in all flames analyzed. In particular, the magnitude of H2 diffusion, occurring from low progress variable near the flame elements of the turbulent reaction front that exhibit convex (positive) curvature towards the fresh mixture, is greatly enhanced at high pressure through increased spatial gradients, resulting in localized equivalence-ratio enrichment and super-adiabatic conditions that accelerate and strengthen the flame front. Moreover, for higher reactant temperatures, back diffusion of atomic hydrogen, transported from the reaction zone into unburnt regions from negatively curved reaction layers surrounding them, facilitates the occurrence of localized spontaneous ignition events at the trailing edges of reaction fronts. For certain conditions these events, induced by differential diffusion, can become the dominant process controlling the overall rate of reactants consumption.

Comparison of ignition and early flame propagation in methane/air mixtures using nanosecond repetitively pulsed discharge and inductive ignition in a pre-chamber setup under engine relevant conditions

An optically accessible pre-chamber setup was used to investigate the ignition occurrence and early flame propagation in methane/air mixtures at density conditions relevant to engine application. Two Schlieren setups coupled with fast recording cameras allowed the visualization of the combustion chamber and a close up of the ignition location, respectively. A spectroscopy-based ignition diagnostic was simultaneously applied to characterize the mixture composition near the spark plug.Nanosecond Repetitively Pulsed Discharge (NRPD) ignition with different pulse patterns in terms of pulse number and repetition frequency was applied. Conventional inductive ignition and NRPD-assisted ignition in a pre-chamber setup were compared for the first time in terms of ignition occurrence and early flame propagation in engine-relevant conditions. The effect of different air to fuel ratios was assessed in both laminar and turbulent conditions.Results showed that the discharge dynamics strongly affects the effectiveness of the ignition event and consequently the later combustion stages, suggesting the path for an optimization of the NRPD control parameters based on the specific case. The NRPD ignition is shown to be advantageous for stable ignition especially in lean laminar conditions due to the increased local concentration of radicals provided by multiple discharges. This suggests that the use of NRPD gives an advantage in ignition onset especially in diluted conditions and where there is a low mixing level due to reduced turbulence.The simultaneous application of spectral and Schlieren techniques allowed gaining fundamental understanding of the processes involved in NRPD ignition of gas mixtures, useful to validate simulations and ultimately for predicting and controlling such ignition systems in practical applications.

Experimental and Modeling Uncertainty Considerations for Determining the First Item Ignited in a Compartment Using a Bayesian Method

Abstract Fire scene reconstruction and determining the fire evolution (i.e., item-to-item ignition events) using the postfire compartment is an extremely difficult task because of the time-integrated nature of the observed damages. Bayesian methods are ideal for making inferences amongst hypotheses given observations and are able to naturally incorporate uncertainties. A Bayesian methodology for determining probabilities to items that may have initiated the fire in a compartment from damage signatures is developed. Exercise of this methodology requires uncertainty quantification of these damage signatures. A simple compartment configuration was used to quantify the uncertainty in damage predictions by firedynamicssimulator (fds) and, a compartment evolution program, jt-risk as compared to experimentally derived damage signatures. Surrogate sensors spaced within the compartment use heat flux data collected over the course of the simulations to inform damage models. Experimental repeatability showed up to 4% uncertainty in damage signatures between replicates. Uncertainties for fds and jt-risk ranged from 12% up to 32% when compared to experimental damages. Separately, the evolution physics of a simple three-fuel-package problem with surrogate damage sensors were characterized in a compartment using experimental data, fds, and jt-risk predictions. A simple ignition model was used for each of the fuel packages. The Bayesian methodology was exercised using the damage signatures collected, cycling through each of the three fuel packages, and combined with the previously quantified uncertainties. Only reconstruction using experimental data was able to confidently predict the true hypothesis from the three scenarios.

Vibration-Induced Heating of Energetic Materials: A Review

AbstractThe transport of energetic materials—whether by truck over rough terrain, or attached to the undercarriage of a high-performance jet aircraft—carries a certain level of inherent risk as the repeatedly applied stresses from vibration may lead to heating, mechanical degradation, and potentially even the triggering of an ignition event. Increasing knowledge of the underlying physics which control ignition is allowing us to better understand, and thus reduce, the risk of a catastrophic event occurring. The Apollo and Space Shuttle programmes provided motivation for research into the topic in the 1960s and 1970s, and some recent studies have focussed on the grain-scale physics of ignition. However, much of the useful insight has arisen from work with other primary applications in mind. Therefore, this review aims to bring together literature from several fields, with the intention of better understanding vibration-induced heating (VIH) phenomena in energetic materials. Sensitivity, VIH in viscoelastic polymers and inert composites, and a technique known as vibrothermography which uses VIH to detect cracks, are all considered where relevant read-across can be found. Often being viscoelastic materials and composites with complex rheology, energetic materials subjected to vibrational loading tend to warm up, with potential for even greater temperature rises due to anisotropy-driven localised heating mechanisms. Binders soften as temperature rises, and the chance of damage increases, which may lead to runaway heating and thermal failure (if mechanical failure does not occur first).

Effects of spatial distribution of CO2 dilution on localised forced ignition of stoichiometric biogas-air mixtures

ABSTRACT The localized forced ignition and subsequent flame propagation have been analyzed for stoichiometric biogas-air mixtures ( blend) with different spatial distributions (uniform, Gaussian and bimodal) and mean levels of dilution (i.e. mole fraction of ) for different flow conditions (e.g. quiescent laminar condition and different turbulence intensities) using three-dimensional Direct Numerical Simulations. A two-step chemical mechanism with sufficient accuracy is used to capture the effects of dilution on the laminar burning velocity of the . A parametric analysis in terms of the mean, standard deviation and integral length scale of the initial Gaussian and bimodal distributions of spatial dilution in the unburned gas has been conducted. Qualitatively similar behavior has been observed for all three initial spatial distributions of . A departure from uniform conditions was found to increase the range of reaction rate magnitudes of , which impacts the burned gas volume and, in turn, increases the variability of the outcomes of the ignition event (successful thermal runaway and subsequent self-sustained propagation or misfire). Turbulence intensity and the mean level of dilution were found to have significant impacts on the outcome of the localized forced ignition, and an increase of either quantity acts to reduce the burned gas volume irrespective of mixture composition due to the enhancement of heat transfer from the hot gas kernel, and the heat sink effects of , respectively. An increase of the integral length scale for the spatial distribution of dilution increased the probability of a successful outcome of the ignition event. A uniform initial spatial distribution was found to be optimal, and in the case of a departure from non-uniform conditions, larger variability in the outcome are obtained for a Gaussian initial distribution of dilution than a bimodal one. The variance of the dilution distribution has been found to have a negligible impact on the outcomes observed, as it was dominated by the effects arising from turbulence intensity, nature of initial distribution and integral length scale of dilution.

Dynamics of ignition kernel in a liquid-fueled gas turbine model combustor studied via time-resolved 3D measurements

This work aims to study the ignition kernel dynamics with spatial-temporal resolved experimental measurements in spark-ignition processes of a liquid-fueled gas turbine model combustor. For this purpose, tomographic chemiluminescence combined with fiber endoscopes are used to measure the time-varying, volumetric CH* distribution of the ignition kernel in different combustor operating conditions at 10 kHz. Based on the time-varying 3D measurements, key properties of ignition kernels, including the 3D size, structure, orientation, growth direction, 3D and 3C (three-component) movement velocity, flame propagation speed, and their temporal evolutions are extracted to provide important insights into the kernel dynamics in such a highly localized transient process. Results show that the growth of the kernel is very sensitive to its motion trajectory. Generally, the kernels move following the flow rotation in the tangential direction, but are oriented along the radial direction and temporally evolve toward the radial direction as the ignition event progresses. Splitting and moving inward are also observed in the radial direction, which is not beneficial to the kernel growth. The evolution of flame propagation speed (ST) during the ignition process shows two distinct stages. In the first stage, ST increases following the power-law of ST=αAtα−1. The rapid growing kernels are with the higher A and lower α. While in the second stage, ST oscillates around a mean value. The transition between the two stages is associated with the corrugation levels of the kernel.

Soil moisture influences on Sierra Nevada dead fuel moisture content and fire risks

Dead fuel moisture influences the risk of fire ignition events, with implications for fire hazards, risk mitigation, and the design of prescribed burning activities. Because direct fuel moisture measurements are rarely available, fuel moisture must be estimated when evaluating fire risks. Most estimates rely primarily on atmospheric conditions and ignore the interaction of fuels with the soil surface with which they are in hydraulic contact. In this study we explore whether dead fuel moisture predictions can be improved with information about surface soil moisture. Despite the likelihood that dead fuels would exchange water with underlying soil, the influence of soil moisture on fuel moisture has been poorly studied. An analysis of 202 observations of co-located soil moisture, 1 and 10 h fuel moisture measurements, with environmental and meteorological covariates, showed that soil moisture had a small but significant effect on fuel moisture across all sampled conditions. The influence of soil moisture on fuel moisture was the most important among all other environmental factors for wet soil conditions and 10-h fuels, where a 1% increase in soil moisture content led to approximately a 0.6% increase in fuel moisture. The effect of soil moisture on 1-h fuel moisture although significant was small. Incorporating spatio-temporal variability of soil moisture into time-series or spatial predictions of fuel moisture tended to (i) reduce the seasonal duration in which fuels had a high probability of ignition by an average of 52–60 days across the tested years, and (ii) increased the heterogeneity of the probability of ignition through space, compared with similar models that did not incorporate soil moisture information.

Turbulent flame-wall interactions for flames diluted by hot combustion products

Flames diluted by combustion products can reduce emissions such as Carbon Monoxide (CO) and Nitrogen Oxides (NOx) in industrial applications. In gas turbines, these flames are confined in a combustor and can interact with relatively cold walls. This interaction can quench the flame, producing incomplete combustion products. In this study, Flame-Wall Interaction (FWI) for methane/air flames diluted by hot combustion products is investigated using direct numerical simulation. A three-Dimensional (3D) turbulent V-flame in a channel with isothermal hot and cold walls is simulated.It is shown that a main reaction zone in the central region between two walls supported by periodic bulk ignition events changes the position of volumetric reaction zones where CO is formed. The cold wall leads to a longer flame, thereby having disproportionately large contribution to the exhaust CO. Near-wall turbulence-flame interaction creates wrinkled and streaky flame surfaces, and localises the near-wall CO distribution. High mean CO mass fraction develops in the free-stream while a high magnitude of the peak RMS CO mass fraction is present closer to the wall. It is also shown that one-dimensional flame solutions can reasonably describe the changes of CO mass fraction as a function of temperature in the free-stream region and some parts of the near-wall region but not close to the wall. Turbulent mixing and diffusion effects contribute to this deviation. The results highlight the complexities involved in CO modelling for diluted flames and set a benchmark for future work.

Large-Eddy Simulation of a Lifted High-Pressure Jet-Flame with Direct Chemistry

ABSTRACT A large-eddy simulation is presented of a challenging high-pressure jet flame case that is representative of state of the art, dry low-NO x and low-CO real gas turbine combustion. A reaction scheme is developed for lifted lean premixed high pressure methane jet flames, and tested by three-dimensional large-eddy simulation of an experiment, for which very detailed data are available. Auto-ignition-delay times of different mixtures of fresh gas and products have been introduced as a novel optimization criterion for the mechanism development. The new mechanism has been developed by a genetic algorithm-based reduction and optimization, and consists of 15 species and 18 reactions. The large-eddy simulations are performed using a finite rate chemistry (FRC) approach and the dynamic thickened flame (DTF) model to investigate a lifted jet flame at high pressure in a gas turbine model combustor. In the simulations, the novel mechanism is compared to a similar mechanism that was generated without this criterion, and the well-established Lu19 mechanism. With the new mechanism, the LES predicts the flame as accurately as with Lu19, at a significantly lower cost. Further post processing with Lagrangian tracer particles confirmed that ignition events occur in the region corresponding to the liftoff height estimated in the experiment, which is corroborated by a chemical explosive mode analysis (CEMA). Overall, the newly developed mechanism with the novel optimization criterion was found to provide a better agreement with the experiments than previous mechanisms of similar cost, or a comparable agreement to a mechanism of significantly higher cost.

Validation of a LES Spark-Ignition Model (GLIM) for Highly-Diluted Mixtures in a Closed Volume Combustion Vessel

<div class="section abstract"><div class="htmlview paragraph">The establishment of highly-diluted combustion strategies is one of the major challenges that the next generation of sustainable internal combustion engines must face. The desirable use of high EGR rates and of lean mixtures clashes with the tolerable combustion stability. To this aim, the development of numerical models able to reproduce the degree of combustion variability is crucial to allow the virtual exploration and optimization of a wide number of innovative combustion strategies.</div><div class="htmlview paragraph">In this study ignition experiments using a conventional coil system are carried out in a closed volume combustion vessel with side-oriented flow generated by a speed-controlled fan. Acquisitions for four combinations of premixed propane/air mixture quality (Φ=0.9,1.2), dilution rate (20%-30%) and lateral flow velocity (1-5 m/s) are used to assess the modelling capabilities of a newly developed spark-ignition model for large-eddy simulation (GLIM, GruMo-UniMORE LES Ignition Model). The model accounts for all the main physical phenomena governing flame kernel growth, including electric circuit and over-adiabatic thermal expansion, which are included in the LES formalism of ECFM-LES combustion model. In the first part of the study the agreement of the simulation results against measurements is carried out to assess a validated non-reacting condition and converged flow statistics. Then, combustion simulations are carried out, and ignition events are repeated at random timings to replicate flow variability at ignition. Finally, optical comparison is carried out for simulated enflamed volume against high-frequency Schlieren images for all the cases, and measurements of flame radius growth are presented. The agreement of the quantitative flame measurements, as well as the qualitative resemblance of flame development, indicate the use of the presented GLIM ignition model as a valuable model for multi-cycle engine simulations, with particular relevance on unstable and CCV-affected conditions</div></div>

Near Real Time Satellite Event Detection, Characterization, and Operational Assessment Via the Exploitation of Remote Photoacoustic Signatures

Current active satellite maneuver detection techniques can resolve maneuvers as quickly as fifteen minutes post maneuver for large Δv when using angles-only optical tracking. Medium to small magnitude burn detection times range from 6 to 24 h or more. Small magnitude burns may be indistinguishable from natural perturbative effects if passive techniques are employed. Utilizing a photoacoustic signature detection scheme can allow for near real time maneuver detection and spacecraft parameter estimation. We define the acquisition of hypertemporal photometric data as photoacoustic sensing because the data can be played back as an acoustic signal. Studying the operational frequency spectra, profile, and aural perception of an active satellite event such as a thruster ignition or any subsystem operation can provide unique signature identifiers that support resident space object characterization efforts. A thruster ignition induces vibrations in a satellite body which can modulate reflected sunlight. If the reflected photon flux is sampled at a sufficient rate, the change in light intensity due to the propulsive event can be detected. Sensing vibrational mode changes allows for a direct timestamp of thruster ignition and shut-off events and thus makes possible the near real time estimation of spacecraft Δv and maneuver type if coupled with active observations immediately post maneuver. This research also investigates the estimation of other impulse related spacecraft parameters such as mass, specific impulse, exhaust velocity, and mass flow rate using impulse-momentum and work-energy methods. Experimental results to date have not yet demonstrated an operator-correlated detection of a propulsive event; however, the application of photoacoustic sensing has exhibited characteristics unique to hypertemporal photometry that are discussed alongside potential improvements to increase the probability of active satellite event detection. Simulations herein suggest that large, potentially destructive modal displacements are required for optical sensor detection and thus more comprehensive vibration modeling and signal-to-noise ratio improvements should be explored.

Auto-ignition characteristics of a near-term light surrogate fuel for marine diesel: An experimental and modeling study

Autoignition characteristics of a four-component surrogate fuel(M2) were investigated in a heated rapid compression machine over a wide range of conditions. The near-term light surrogate M2 developed for marine diesel fuel was formulated in our previous study containing 17.3% n-hexadecane, 5.93% 2,2,4,4,6,8,8-heptamethylnonane, 30% decalin and 46.77% 1-methylnaphthalene by mole fraction. Ignition delay times (IDTs) of gas-phase M2/N2/O2 mixture were measured at pressures of 7, 10, 15 and 20 bar over the temperature range from 675 K to 914 K for the equivalence ratios (ER) of 0.5, 1 and 1.5. NTC behaviours within the temperature of 730–850 K and two-stage ignition phenomena were observed. The effects of operating conditions such as pressure, equivalence ratio and oxygen content were systematically studied, and correlations of total IDTs and first-stage IDTs were performed to further quantitatively reveal the IDTs dependence on the above parameters. Reasonable modifications were made to a literature mechanism to improve its predicting quality. The simulation results using the modified model could satisfactorily reproduce the IDTs under the test conditions, and the model is also able to capture the ignition delay dependence on pressure and fuel/oxygen content. A brute force sensitivity analysis at the low temperature (680 K) and the NTC temperature (780 K) was carried out to identify the key reactions that govern the ignition event of surrogate M2.

Ignition delay times of methane/diethyl ether (DEE) blends measured in a rapid compression machine (RCM)

Diethyl ether (DEE) is an interesting species for combustion for at least two reasons: On the one hand, it is used as a kind of ”worst case” reference substance for studies concerned with the prevention of accidental ignition events. On the other hand, it is also a candidate bio-fuel. For this reason, in this work, auto-ignition of two different ▪ /DEE-mixtures (90/10 and 95/5 mol-% ▪ /DEE) are studied in a rapid compression machine (RCM). In the RCM, the gas mixture is compressed in a piston–cylinder device up to 20bar and held under isochoric conditions at top dead center. Auto-ignition occurs after an ignition delay time (IDT). IDTs are measured for compression temperatures ranging between 515 and 925K, for both, stoichiometric and fuel-rich mixtures (equivalence ratio ϕ=2). The experimental data are compared to results of simulations involving detailed chemistry, as well as to other fuels investigated in the same RCM (results from literature).

Tuning the reactivity and energy release rate of I2O5 based ternary thermite systems

Iodine pentoxide (I2O5) based nanothermites are one of the most promising candidates for biocidal energetic materials due to superior reactivity and high iodine content. However, the tunability of nanothermites, which is important for biocidal performance, has not been fully exploited for I2O5 based nanothermites. In this work, I2O5 with various fuels (Al, Ti, Si) and their mixtures (i.e., a ternary system) have been investigated. The reactivity and flame temperature were evaluated by a pressure cell coupled with a spectrometer. Temperature-Jump time-of-flight mass spectrometry (T-Jump TOFMS) was used to probe the reaction mechanism and iodine release behavior, along with a high-speed camera to capture the ignition event. I2O5 showed distinct reactivity with different fuels. As a result, by varying the fuel composition of ternary systems, the combustion properties can be tuned. Rapid heating experiments revealed that the reaction initiation was shifted from gas phase dominated to condensed phase dominated mechanism after introducing Ti or Si into Al/I2O5 system. Further analysis of the ternary systems found that the energy release rate correlates with burn time instead of flame temperature. This study shows an approach to tune the reactivity and energy release rate of I2O5 based nanothermites without compromising the energy density.

Thermoplastic Polymeric Materials for Spacecraft Applications: Flame Retardant Properties and UV/AtOx Aging Analysis

Space-mission development introduced the problem of human isolation in extreme environments. The integration of architectural concepts such as windows, with their technical implications into human space missions, is necessary especially for long-duration space flights. Such solutions must be subjected to close certification testing in order to establish their compliance with severe space environment conditions. Moreover, projects of long-term missions involve a renewed concern about fire safety in manned space vehicles. The supposed occupancy time of the order of decades, in fact, makes unplanned fire ignition events virtually unavoidable. An experimental test-plan performed to qualify a commercial thermoplastic material for applications as transparent element in spacecraft is reported in the present work. A flame exposure test, as well as ultraviolet radiation and atomic oxygen erosion rate measurements, was carried out on a commercial poly-sulfone material, following ASTM standard procedures. The aim of the research was to develop and put forward flame retardant structures able to withstand the harsh space environment, preventing or mitigating the degradation of their physical and chemical integrity, with particular regard to the visible-light transparency. The results obtained show that the tested poly-sulfone may be considered as a promising material for the claimed application, even if further surface optimization treatments should be conceived in order to gain a full adaptability to the operative constraints.

Reclosing of Distribution Systems for Wildfire Prevention

The risk of catastrophic wildfire has challenged our societies and impacted various industries, with imminent danger to people's lives, financial burden, and irreparable damages to the environment, among other consequences. While progressive wildfires can impact power system operation and customer reliability and resilience, the relationship between power systems and wildfires is mutual: power systems can also contribute to wildfires. Wildfire risk of power systems is related to, in particular, fault-induced ignition events that can quickly turn into wildfires. While some electric utilities are working on approaches to mitigate such risk, the need for further research, development and demonstration of novel methodologies still exists. This paper identifies, analyzes, and compares the practices that can be further employed by utilities to manage the risk and mitigate the impact of power-system generated wildfires.

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.

A Novel One- and Zero-Dimensional Model for Turbulent Jet Ignition

Turbulent jet ignition (TJI) is a promising combustion technology for burning highly diluted air-fuel mixtures. Computationally efficient models to assess the effect of the operating conditions and design parameters on the ignition propensity and timing are of paramount importance for the development of combustion systems employing TJI. To this end, a one-dimensional (1-D) jet model, which is based on the solution of the section integrated mass and momentum conservation equations, is derived in the present study. The model is extended with two additional transport equations for the turbulence intensity and the ignition precursor/tracer, that marks the ignition event. One-dimensional transient flamelet calculations are performed to generate tables for the ignition precursor source term that account for the turbulence and chemistry interaction. Further simplification of the model is carried out to obtain a novel penetration correlation and a computationally inexpensive Lagrangian ignition model. The extended jet model is hierarchically validated using available literature data for non-reactive and reactive jets, as well as experiments conducted in a state-of-the-art optically accessible prechamber. The derived model is able to reproduce both flow-related quantities (velocity and turbulence profiles, jet penetration) and the ignition delay time under different variations. This study also illustrates how numerical simulations in canonical configurations (one-dimensional flamelet) can be used in practical applications of TJI.