Homogeneous Ignition Research Articles

Surrogate Fuels Formulation for FACE Gasoline Using the Nuclear Magnetic Resonance Spectroscopy

An efficient surrogate fuel formulation methodology, which directly uses the chemical structure information from nuclear magnetic resonance (NMR) spectroscopy analysis, has been proposed. Five functional groups, paraffinic CH2, paraffinic CH3, aromatic C-CH, olefinic CH-CH2, and cycloparaffin CH2, have been selected to show the basic molecular structure of the fuels for the advanced combustion engines (FACE) fuels. A palette that contains six candidate components, n-heptane, iso-octane, toluene, 2,5-dimethylhexane, methylcyclohexane, and 1-hexene, is chosen for different FACE fuels, based on the consideration that surrogate mixtures should provide the representative functional groups and comparable molecular sizes. The kinetic mechanisms of these six candidate components are chosen to assemble a detailed mechanism of each surrogate fuel for FACE gasoline. Whereafter, the accuracy of FACE A and F surrogate models was demonstrated by comparing the model predictions against experimental data in homogeneous ignition, jet stirred reactor oxidation, and premixed flame.

Dust ignition characteristics of different coal ranks, biomass and solid waste

A vertical furnace into which a portion of a dust/air mixture is blown, was used to study dust cloud ignition. A dust/air cloud was formed in the working chamber space, with an average dust concentration of 4.8·10-2kg/m3. Ignition studies were conducted for one separated particle fraction of 63–90 μm. The purpose of the paper is to determine the parameters characterising the ignition (minimum ignition temperature, i.e. critical temperature of the furnace, where ignition takes place, and ignition time) and to describe the mechanism of ignition of coals with various rank (from brown coal then bituminous coal to anthracite) under conditions of particle interaction (cloud ignition). In addition, for comparison purposes, tests of ignition of various types of biomass, sewage sludge, segregated municipal waste and sewage sludge were carried out under the same experimental conditions. The moment of flame appearance (ignition) was recorded using two optical detectors. Ignition delay time was determined at different furnace temperatures ranging from the ignition temperature to 1100 °C. The tests performed show that the values of minimum ignition temperatures and the ignition delay times for the analysed coals depend to a large extent on the coal rank. The ignition temperature and ignition time decreased when the volatile matter content in the coal increased and the calorific value of the fuel decreased, i.e. when the rank of coal decreased. For the remaining category of the fuels tested, it was observed that, unlike for coals, the ignition temperature and the ignition delay time increases with the increase of volatile matter content of fuel and the decrease of the calorific value of fuel. Based on the research, it was concluded that under experimental conditions, the ignition of dust clouds of fuels with a volatile matter content below 65–70% daf (anthracites, bituminous coals and some brown coal) occurred according to the heterogeneous mechanism. However, for the content of volatile matters in the fuel above this value, volatile matter are ignited in the gaseous environment (homogeneous ignition). On the basis of the experimentally determined ignition characteristics, an attempt is presented in the paper to interpret the test results based on a simplified model of a heterogeneous ignition of a dust cloud and a model of homogeneous ignition of volatile matters in the vicinity of a fuel particle.

Shock tube studies of ethanol preignition

Understanding premature ignition or preignition is of great importance as this phenomenon influences the design and operation of internal combustion engines. Preignition leading to super-knock restricts the efficiency of downsized boosted engines. To gain a fundamental understanding of preignition and how it affects an otherwise homogeneous ignition process, a shock tube may be used to decipher the influence of fuel chemical structure, temperature, pressure, equivalence ratio and bath gas on preignition. In a previous work by Javed et al. (2017), ignition delay time measurements of n-heptane showed significantly expedited reactivity compared to well-validated chemical kinetic models in the intermediate-temperature regime. In the current work, ethanol is chosen as a representative fuel that, unlike n-heptane, does not exhibit negative temperature coefficient (NTC) behaviour. Reactive mixtures containing 2.9% and 5% of ethanol at equivalence ratios of 0.5 and 1 were used for the measurement of ignition delay times behind reflected shock waves at 2 and 4 bar. Effect of bath gas was studied with mixtures containing either Ar or N2. In addition to conventional side-wall pressure and OH* measurements, a high-speed imaging setup was utilized to visualize the shock tube cross-section through a transparent quartz end-wall. The results suggest that preignition events are more likely to happen in mixtures containing higher ethanol concentration and that preignition energy release is more pronounced at lower temperatures. High-speed imaging shows that low-temperature ignition process is usually initiated from an individual hot spot that grows gradually, while high-temperatures ignition starts from many spots simultaneously which consume the reactive mixture almost homogeneously.

Characterization on Ignition and Volatile Combustion of Dispersed Coal Particle Streams: In Situ Diagnostics and Transient Modeling

In this paper, applying OH-PLIF diagnostics, we explored the ignition and volatile combustion of dispersed coal particle streams in a Hencken flat-flame burner. The effects of ambient temperature and oxygen mole fraction on coal combustion behaviors have been systematically examined. First, different coal combustion stages and ignition modes (i.e., heterogeneous ignition, homogeneous ignition, and hetero-homogeneous joint ignition) in different ambiences are characterized by the OH profiles within the coal flame domain, with ignition delay time and volatile combustion time being quantitatively determined. Then, an improved transient model, integrating the gas-phase flame-sheet reaction model and the chemical percolation devolatilization (CPD) model, is established to reveal the underlying physicochemical mechanisms during the early stage of coal combustion. This transient model well captures the effects of ambiences on both the characteristic ignition delay time and volatile combustion time. Finally, base...

Heterogeneous reaction characteristics and their effects on homogeneous combustion of methane/air mixture in micro channels I. Thermal analysis

Catalytic combustion of methane/air mixture in various channels was numerically investigated to illuminate the heat characteristics of heterogeneous reactions (HTRs) and their effects on homogeneous reactions (HRs). A two-dimensional (2D) simulation model of channels with a width (d) of 2.00–2.82 mm was employed, which included detailed gas-phase and surface reaction mechanisms. Further, the heat transfer mechanisms were elucidated. HRs always occupied the main part of over 80% of the total released heat in all the cases. HTR is relatively amplified in narrower channels, and S (the ratio of heat released by HTR to that of all reactions) rises by several percentage as d decreases (S = 17.61% when d = 2.10 mm), and it is more effective to use a catalyst in the microcombustor with a smaller gap size to enhance combustion reaction rate and stability. The onsite-S showed a trend of “stable-decrease–peak–rapid increase–slow increase” along x, corresponding to preheating, homogeneous ignition, heterogeneous ignition (conversion of CO to CO2 and H2 to H2O), intensive HTR stage (consumption of O, H, and OH and production of H2O and H2), and complete consumption of reactants in the gas phase. The analysis about the heat sources for homogeneous ignition shows that homogeneous ignition was dominated by the preheating heat from hot gas downstream, and affected by transverse preheating heat. As d decreased, the preheating heat obtained from the transverse preheating increased by several percentages, even reaching to a similar value to preheating heat from hot gas downstream when d = 2.10 mm. The heat released by weak HTR only takes a small part of preheating heat of inlet mixture and homogeneous ignition. After homogeneous ignition, HR promoted the HTR to the maximum. Generally, HTR would delay the homogeneous ignition, enlarge the flame thickness, increase the peak of flame temperature and the exit gas temperature, and improve the completeness of combustion.

Smoldering and spontaneous transition to flaming over horizontal cellulosic insulation

The transition from smoldering to flaming is basically a homogeneous gas-phase ignition reaction that has a high potential to engulf the entire fuel bed in flames. However, the transition mechanism, especially under windy environment, is poorly understood due to lack of theoretical study. This work proposes a two-dimensional computational model with multiphase formulations to simulate the forward horizontal smoldering and spontaneous transition to flaming of cellulosic insulation. A chemical scheme of three-step solid decomposition reactions combined with one global gas-phase reaction is used in the model. The multi-physical coupling of heat, mass and momentum fields is implemented to reveal the interactions between the packed insulation bed and the surrounding free flow. This model shows that the hotter surface char layer acts as an ignition source for heating the gas mixture, and both the surface char oxidation and sample pyrolysis reactions behave like a gaseous fuel supplier for gas-phase reaction. With favorable conditions of high-temperature combustible fuel gas and sufficient oxygen supply under windy environment, the transition ultimately occurs at 2-3 cm above the leading edge of smolder front on the top surface, which is totally different from the transition within the porous fuel bed under windless condition. The predicted effects of wind velocity and bulk density of fuel bed on the smoldering velocity agree well with literature data. The modeling results for the first time give an interpretation of the spontaneous smoldering to flaming transition over the fuel bed surface.

Hetero-/homogeneous combustion of fuel-lean CH4/O2/N2 mixtures over PdO at elevated pressures

The heterogeneous and homogeneous combustion of fuel-lean CH4/O2/N2 mixtures over PdO was investigated experimentally and numerically at equivalence ratios φ = 0.27–0.44, pressures 1–12 bar and surface temperatures 710–1075 K. In situ Raman measurements of major gas-phase species concentrations across the boundary layer of a channel-flow catalytic reactor assessed the heterogeneous reactivity, while planar laser induced fluorescence (LIF) of the OH radical monitored homogeneous combustion. Simulations were performed using a 2-D code with detailed heterogeneous and homogeneous reaction mechanisms. Comparisons between Raman-measured and predicted transverse profiles of major species mole fractions attested the atmospheric-pressure suitability of a detailed surface mechanism and allowed for the construction of a global catalytic step valid in the range 1–12 bar. The methane catalytic reaction rate exhibited an overall pressure dependence ∼p1−n where the exponent n was itself a monotonically increasing function of pressure, rising from 0.58 at 3 bar to 1.02 at 12 bar. This resulted in a non-monotonic pressure dependence of the catalytic reaction rate in the range 1–12 bar, a behavior in stark contrast to other noble metals (Pt and Rh) where the methane reaction rates always increased with rising pressure. Surface temperatures remained well-below the PdO decomposition temperature at each corresponding pressure, owning largely to the “self-regulating” temperature effect of PdO, and this in turn mitigated homogeneous ignition. Simulations using the PdO decomposition temperatures as boundary conditions for the wall temperatures were further performed for practical CH4/air catalytic reactors in power generation systems. It was shown that for p < 7 bar (a range relevant to microreactors) homogeneous ignition was altogether suppressed. For higher pressures relevant to gas-turbine burners, however, gaseous combustion ought to be considered in the reactor design.

Direct numerical simulations of turbulent catalytic and gas-phase combustion of H2/air over Pt at practically-relevant Reynolds numbers

Three-dimensional direct numerical simulations of turbulent catalytic and gas-phase H2/air combustion at a fuel-lean equivalence ratio φ=0.18 were performed in platinum-coated planar channels at two industrially-relevant flow conditions (inlet friction Reynolds numbers Reτ= 182 and 385) using detailed hetero-/homogeneous chemical reaction mechanisms. The preferential diffusion of hydrogen and oxygen, which was responsible for creating significantly higher surface equivalence ratios φw compared to the bulk gas-phase φ, was appreciably suppressed by turbulence at Reτ= 385. The higher turbulence intensity at this Reτ resulted in larger near-wall hydrogen excess that in turn yielded shorter homogeneous ignition distances compared to the lower Reτ case. Gas-phase ignition proceeded from isolated ignition kernels that subsequently formed axially elongated flames confined close to the catalytic walls. The coupling of catalytic and gas-phase chemistry inhibited homogeneous ignition, since at the vicinity of the ignition kernels the OH, H and O radical fluxes to the underlying catalytic wall were net-adsorptive and furthermore hydrogen was depleted by the catalytic reactions. The flame topology included alternating vigorously-burning and extinguished elongated streamwise stripes at Reτ=182 or islands at Reτ=385. The extinguished gas-phase reaction zones at Reτ=385 were characterized by underlying intense catalytic reaction rates. The flame topology and spatiotemporal correlation of the isolated burning and extinguished gaseous zones indicated that significant surface temperature non-uniformities could be obtained in practical catalytic reactors.

The effects of particle size and reducing-to-oxidizing environment on coal stream ignition

Coal particles experience a transition from a reducing to oxidizing environment in the near-burner region of pulverized coal (pc) boilers. For the first time, we report a fundamental study of ignition of a coal-particle stream experiencing a flame environment that transitions from a reducing to an oxidizing environment (termed reducing-to-oxidizing environment). High-speed videography is used to observe the particles in situ, and scanning electron microscopy is used to characterize the sampled particles. The effects of particle size on ignition are presented for four size bins (63–74 µm, 75–89 µm, 90–124 µm and 125–149 µm) for PRB subbituminous coal at two nominal gas temperatures (1300 K and 1800 K). An oxidizing environment with 20% molar oxygen composition is used as base-case. In contradistinction to single particle studies where particles are reported to ignite heterogeneously at higher temperatures, this study shows that coal streams ignite homogeneously, irrespective of particle size, in the oxidizing environment. By changing nominal gas temperature from 1300 K to 1800 K, ignition time decreases, on average, by a factor of five for each of the particle size bins. For both gas temperatures, the trend in ignition delays as particle size changes is non-monotonic. However, at 1800 K nominal gas temperature, ignition delays are independent of particle size in the reducing-to-oxidizing environment and ignition delays are doubled on average when compared to those in the oxidizing environment. It is more noticeable at the lower gas temperature of 1300 K that homogeneous ignition of coal streams is oxygen-dependent below 90 µm particle size and temperature-dependent above 90 µm. In general, ignition delay is determined by volatile release rate (controlled by the particle temperature) and the local oxygen concentration. Micrographs of particles also confirm that ignition and char burnout times are longer in the reducing-to-oxidizing environments than those in the oxidizing environments.

Ignition and combustion behaviors of single coal slime particles in CO2/O2 atmosphere

Coal slime, a low-calorific-value fuel, is a by-product during coal washing, which can be disposed in quantity and fully utilized in circulating fluidized bed (CFB) boiler. This research paid attention to the ignition and combustion behaviors of single coal slime particles, which affect the normal combustion, operation stability and burning efficiency in circulating fluidized bed boiler. The ignition and combustion behaviors including ignition mechanism, ignition delay, ignition temperature and combustion process of single coal slime particles in a vertical heating tube furnace in CO2/O2 atmosphere were researched under different operation condition parameters, for different gas temperatures (Tg = 923, 1073, and 1173 K), gas flow rates (V = 0–20 L/min), and oxygen mole concentrations (O2% = 5–80%). Coal slime particle had three ignition mechanisms in CO2/O2 atmosphere, namely, homogeneous ignition of volatiles at the windward and leeside of particle, heterogeneous ignition of char and heterogeneous ignition of coal. Only heterogeneous ignition of char and homogeneous ignition of volatiles at the windward of particle occurred in quiescent atmosphere. However, homogeneous ignition region decreased while heterogeneous ignition region increased gradually with the increasing flow rates in the oxygen concentration-gas temperature plane. Different ignition mechanisms were accompanied with various combustion processes. The combustion processes corresponding to heterogeneous ignition of char changed from flameless combustion to flaming combustion as oxygen concentration increased. Moreover, the critical oxygen concentration elevated from 30% to 50% with the increasing flow rate. The ignition temperatures and ignition delays decreased with the increasing gas temperature and oxygen concentration. As the flow rate increased, the trends became more obvious in medium-to-low oxygen concentrations. Compared with the ignition characteristics and combustion processes of coal slime particles in N2/O2 atmosphere, those in CO2/O2 atmosphere were suppressed due to the higher mole heat capacity and lower diffusion rate of oxygen molecule in CO2.

Bottom-up modeling using the rate-controlled constrained-equilibrium theory: The n-butane combustion chemistry

It is shown that when the constrained-equilibrium assumption is used, kinetic calculations can be carried out without the use of a detailed kinetic model in the outset. One starts with conservation of atomic elements, to which more constraints and rate-limiting reactions can be added to improve kinetic predictions to the desired accuracy. The idea is demonstrated for high temperature homogeneous ignition of mixtures of stoichiometric n-Butane in air. An {H/O, C1,2, C3Hx (x ≤ 6)} kinetic model, involving 41 species and 348 reactions, is taken as the foundational fuel model and the gap between the fuel molecule and this base model is bridged at five different models that use the constrained-equilibrium assumption. Comparisons of predictions of this approach with the same models that make no use of the constrained-equilibrium assumption indicate that (a) the chain of reactions could be explicitly broken without compromising the fundamental elementary nature of the model, and (b) the use of the constrained-equilibrium assumption makes the model less sensitive to the missing pathways. The results, therefore, suggest that employing the constrained-equilibrium assumption may significantly reduce the modeling task of heavy hydrocarbon fuels, where the intrinsic dimensionality of the model, multiplicity of pathways and the required rates render the modeling task intractable.

Multidimensional Numerical Simulations of Knocking Combustion in a Cooperative Fuel Research Engine

A numerical approach was developed based on multidimensional computational fluid dynamics (CFD) to predict knocking combustion in a cooperative fuel research (CFR) engine. G-equation model was employed to track the turbulent flame front and a multizone model was used to capture auto-ignition in the end-gas. Furthermore, a novel methodology was developed wherein a lookup table generated from a chemical kinetic mechanism could be employed to provide laminar flame speed as an input to the G-equation model, instead of using empirical correlations. To account for fuel chemistry effects accurately and lower the computational cost, a compact 121-species primary reference fuel (PRF) skeletal mechanism was developed from a detailed gasoline surrogate mechanism using the directed relation graph (DRG) assisted sensitivity analysis (DRGASA) reduction technique. Extensive validation of the skeletal mechanism was performed against experimental data available from the literature on both homogeneous ignition delay and laminar flame speed. The skeletal mechanism was used to generate lookup tables for laminar flame speed as a function of pressure, temperature, and equivalence ratio. The numerical model incorporating the skeletal mechanism was employed to perform simulations under research octane number (RON) and motor octane number (MON) conditions for two different PRFs. Parametric tests were conducted at different compression ratios (CR) and the predicted values of critical CR, delineating the boundary between “no knock” and “knock,” were found to be in good agreement with available experimental data. The virtual CFR engine model was, therefore, demonstrated to be capable of adequately capturing the sensitivity of knock propensity to fuel chemistry.

High-speed imaging of inhomogeneous ignition in a shock tube

Homogeneous and inhomogeneous ignition of real and surrogate fuels were imaged in two Stanford shock tubes, revealing the influence of small particle fragmentation. n-Heptane, iso-octane, and Jet A were studied, each mixed in an oxidizer containing 21% oxygen and ignited at low temperatures (900–1000 K), low pressures (1–2 atm), with an equivalence ratio of 0.5. Visible images (350–1050 nm) were captured through the shock tube endwall using a high-speed camera. Particles were found to arrive near the endwalls of the shock tubes approximately 5 ms after reflection of the incident shock wave. Reflected shock wave experiments using diaphragm materials of Lexan and steel were investigated. Particles collected from the shock tubes after each experiment were found to match the material of the diaphragm burst during the experiment. Following each experiment, the shock tubes were cleaned by scrubbing with cotton cloths soaked with acetone. Particles were observed to fragment after arrival near the endwall, often leading to inhomogeneous ignition of the fuel. Distinctly more particles were observed during experiments using steel diaphragms. In experiments exhibiting inhomogeneous ignition, flames were observed to grow radially until all the fuel within the cross section of the shock tube had been consumed. The influence of diluent gas (argon or helium) was also investigated. The use of He diluent gas was found to suppress the number of particles capable of causing inhomogeneous flames. The use of He thus allowed time history studies of ignition to extend past the test times that would have been limited by inhomogeneous ignition.

Direct numerical simulation study of hydrogen/air auto-ignition in turbulent mixing layer at elevated pressures

Auto-ignition of turbulent stratified mixing layer between hydrogen and hot air under elevated pressures p = 1–30 atm is studied using direct numerical simulations (DNS) in this work. Homogeneous isotropic turbulence is superimposed on the field. Detailed chemical mechanism and multicomponent diffusion model are employed. Other than turbulent mixing ignition (TMI), homogeneous mixing ignition (HMI) and laminar mixing ignition (LMI) are also investigated for comparison. For both laminar and turbulent cases, the onset of auto-ignition always happens at the same most reactive mixture fraction isosurfaces. Most reactive mixture fractions in diffusion auto-ignition are inconsistent with HMI calculations and shift to the rich side owing to diffusion for all pressures. At elevated pressures, auto-ignition chemistry is different from low pressures. The importance of H2O2 and HO2 is highlighted as radical sinks during the ignition process, and can also be used as an indicator for locating the ignition spots. Moreover, OH radicals can be used as a marker variable for the transition of auto-ignition to flame propagation under high pressures. Two stages are involved in the diffusion ignition process: radical explosion and thermal runaway. According to our study, under elevated pressures, turbulence has little influence on the radical explosion stage. The role of turbulence is to accelerate the thermal runaway stage in the kernels to make the ignition delay time (IDT) shorter than laminar cases.

Direct numerical simulation of a temporally evolving air/n-dodecane jet at low-temperature diesel-relevant conditions

We present a direct numerical simulation of a temporal jet between n-dodecane and diluted air undergoing spontaneous ignition at conditions relevant to low-temperature diesel combustion. The jet thermochemical conditions were selected to result in two-stage ignition. Reaction rates were computed using a 35-species reduced mechanism which included both the low- and high-temperature reaction pathways. The aim of this study is to elucidate the mechanisms by which low-temperature reactions promote high-temperature ignition under turbulent, non-premixed conditions. We show that low-temperature heat release in slightly rich fuel regions initiates multiple cool flame kernels that propagate towards very rich fuel regions through a reaction-diffusion mechanism. Although low-temperature ignition is delayed by imperfect mixing, the propagation speed of the cool flames is high: as a consequence, high-temperature reactions in fuel-rich regions become active early during the ignition transient. Because of this early start, high-temperature ignition, which occurs in fuel-rich regions, is faster than homogeneous ignition. Following ignition, the high-temperature kernels expand and engulf the stoichiometric mixture-fraction iso-surface which in turn establish edge flames which propagate along the iso-surface. The present results indicate the preponderance of flame folding of existing burning surfaces, and that ignition due to edge-flame propagation is of lesser importance.. Finally, a combustion mode analysis that extends an earlier classification [1] is proposed to conceptualize the multi-stage and multi-mode nature of diesel combustion and to provide a framework for reasoning about the effects of different ambient conditions on diesel combustion.

The competitive reaction mechanism between oxidation and pyrolysis consumption during low‐rank coal combustion at lean‐oxygen conditions: A quantitative calculation based on thermogravimetric analyses

AbstractThe evolution of the oxidation‐pyrolysis competitive mechanism during low‐rank coal combustion at lean‐oxygen conditions determines the development of the coalfield fires and the burning loss rate of the coal resource. In the present work, the mass increments of oxygen‐chemisorption were measured and calculated firstly. An abstract shrinking core model was built for distinguishing the oxidation and pyrolysis pathways. On this basis, the consumption rates of oxidation and pyrolysis were respectively calculated with the TGA data obtained under the lean‐oxygen gradient of 21–0 %. The results demonstrate that the oxidation path gradually delays in temperature while the pyrolysis exhibits great temperature stability as the oxygen concentration decreases. Besides, the significant transition of the competitive consumption mechanism exists below 9 % for the oxidation phase before ignition and 5 % for the combustion phase after ignition, respectively. Two temperature indicators of TC1 and TC2 for guiding the optimal fire extinguishing schemes based on the actual oxygen concentrations and temperatures are proposed. Furthermore, 1 % is identified as the limiting oxygen concentration for the development of the coalfield fires where the ignition mechanisms of both the HST and WD coal transform into the homogeneous ignition. The WD lignite, which always maintains the coke state as the burnout mechanism, holds higher reactivity but wider dispersion degrees of combustion compared with the HST sub‐bituminous coal.

Kinetic Modeling of the Influence of Cyclohexane on the Homogeneous Ignition of a Gasoline Surrogate Fuel

The importance of adding a naphthene in the form of cyclohexane in surrogate mixtures to emulate homogeneous ignition of fully blended research gasoline has been examined with kinetic modeling. On the basis of a nonlinear by volume octane blending model and a correlation of anti-knock index to simulated ignition delay time, a quinary surrogate mixture, including cyclohexane, primary reference fuel (PRF), toluene, and diisobutylene (DIB-1), has been formulated that matches the research octane number, motor octane number, and H/C ratio of the target fuel. Simulated ignition delay times for the quinary mixture and a quaternary mixture without cyclohexane have been compared to measured data for the target fuel in a shock tube and rapid compression machine. Kinetic analysis shows that there is an increased production of HO2 during the induction period for the quinary mixture. This leads to an increased OH production/consumption ratio for PRF, toluene, and DIB-1 in the quinary mixture. Simulated homogeneous cha...

Direct Numerical Simulations of Hotspot-induced Ignition in Homogeneous Hydrogen-air Pre-mixtures and Ignition Spot Tracking

A systematic study relying on Direct Numerical Simulations (DNS) of premixed hydrogen-air mixtures has been performed to investigate the hotspot ignition characteristics and ignition probability under turbulent conditions. An ignition diagram is first obtained under laminar conditions by a parametric study. The impact of turbulence intensity on ignition delays and ignition probability is then quantified in a statistically-significant manner by repeating a large number of independent DNS realizations. By tracking in a Lagrangian frame the ignition spot, the balance between heat diffusion and heat of chemical reaction is observed as function of time. The evolution of each chemical species and radicals at the ignition spot is checked and the mechanism leading to ignition or misfire are analyzed. It is observed that successful ignition is mostly connected to a sufficient build-up of a HO2 pool, ultimately initiating production of OH. Turbulence always delays ignition, and ignition probability goes to zero at sufficiently high turbulence intensity when keeping temperature and size of the initial hotspot constant.

Estimating fuel octane numbers from homogeneous gas-phase ignition delay times

Fuel octane numbers are directly related to the autoignition properties of fuel/air mixtures in spark ignition (SI) engines. This work presents a methodology to estimate the research and the motor octane numbers (RON and MON) from homogeneous gas-phase ignition delay time (IDT) data calculated at various pressures and temperatures. The hypothesis under investigation is that at specific conditions of pressure and temperature (i.e., RON-like and MON-like conditions), fuels with IDT identical to that of a primary reference fuel (PRF) have the same octane rating. To test this hypothesis, IDTs with a detailed gasoline surrogate chemical kinetic model have been calculated at various temperatures and pressures. From this dataset, temperatures that best represent RON and MON have been correlated at a specified pressure. Correlations for pressures in the range of 10–50 bar were obtained. The proposed correlations were validated with toluene reference fuels (TRF), toluene primary reference fuels (TPRF), ethanol reference fuels (ERF), PRFs and TPRFs with ethanol, and multi-component gasoline surrogate mixtures. The predicted RON and MON showed satisfactory accuracy against measurements obtained by the standard ASTM methods and blending rules, demonstrating that the present methodology can be a viable tool for a first approximation. The correlations were also validated against an extensive set of experimental IDT data obtained from literature with a high degree of accuracy in RON/MON prediction. Conditions in homogeneous reactors such as shock tubes and rapid compression machines that are relevant to modern SI engines were also identified. Uncertainty analysis of the proposed correlations with linear error propagation theory is also presented.

Chemical Explosive Mode Analysis for Local Reignition Scenarios in H2/N2 Turbulent Diffusion Flames

Local reignition scenarios in H2/N2 turbulent diffusion flames are investigated using a one-dimensional turbulence (ODT) model and chemical explosive mode analysis (CEMA). Through analogy with the flame fronts in homogeneous ignition and laminar premixed flames, four reignition scenarios are distinguished and analyzed by CEMA. Results show that the reignition scenario via premixed flame propagation corresponds to a high-temperature explosion index and the reignition process is segmentally dominated by the reactions related to the consumption and production of the hydrogen radical. While reignition mode through an independent flamelet corresponds to a high radical explosion index, the whole reignition process is dominated by the production reaction of the hydrogen radical. When these two reignition processes are terminated by turbulent eddies, a hybrid reignition process or reignition scenario through turbulent flame folding may occur, and the turbulent folding mode corresponds to a high dissipation rate a...