Showing papers in "Combustion and Flame in 2016"

An updated experimental and kinetic modeling study of n-heptane oxidation

Abstract: This work presents an updated experimental and kinetic modeling study of n-heptane oxidation. In the experiments, ignition delay times of stoichiometric n-heptane/air mixtures have been measured in two different high-pressure shock tubes in the temperature range of 726–1412 K and at elevated pressures (15, 20 and 38 bar). Meanwhile, concentration versus time profiles of species have been measured in a jet-stirred reactor at atmospheric pressure, in the temperature range of 500–1100 K at φ = 0.25, 2.0 and 4.0. These experimental results are consistent with those from the literature at similar conditions and extend the current data base describing n-heptane oxidation. Based on our experimental observations and previous modeling work, a detailed kinetic model has been developed to describe n-heptane oxidation. This kinetic model has adopted reaction rate rules consistent with those recently developed for the pentane isomers and for n-hexane. The model has been validated against data sets from both the current work and the literature using ignition delay times, speciation profiles measured in a jet-stirred reactor and laminar flame speeds over a wide range of conditions. Good agreement is observed between the model predictions and the experimental data. The model has also been compared with several recently published kinetic models of n-heptane and shows an overall better performance. This model may contribute to the development of kinetic mechanisms of other fuels, as n-heptane is a widely used primary reference fuel. Since the sub-mechanisms of n-pentane, n-hexane and n-heptane have adopted consistent reaction rate rules, the model is more likely to accurately simulate the oxidation of mixtures of these fuels. In addition, the successful implementation of these rate rules have indicated the possibility of their application for the development of mechanisms for larger hydrocarbon fuels, which are of great significance for practical combustion devices.

A comprehensive experimental and modeling study of isobutene oxidation

Abstract: The work at NUI Galway was supported by Saudi Aramco under the FUELCOM program. Collaboration between NUI Galway and LRGP enters in the frame the COST Action CM1404.

A detailed chemical kinetic modeling, ignition delay time and jet-stirred reactor study of methanol oxidation

Abstract: A shock tube (ST) and a rapid compression machine (RCM) have been used to measure new ignition delay times for methanol oxidation over a wide range of pressures (2–50 atm) and equivalence ratios (0.5, 1.0, and 2.0). These measurements include dilute and fuel/‘air’ conditions (1.5–21.9% methanol), over a temperature range of 820–1650 K. The new data has been compared to previously published studies and provides insight into internal combustion engine relevant conditions which are previously un-studied at pressures of 10, 30, 40 and 50 atm. In addition to these ignition delay times, species concentrations have also been measured using a jet-stirred reactor (JSR). In these experiments methanol concentrations of 2000 and 4000 ppm were used at equivalence ratios of 0.2–2.0, at pressures of 1–20 atm, and in the temperature range of 800–1200 K with residence times varying from 0.05–2.00 s. The newly measured experimental data was used to develop a new detailed chemical kinetic model (Mech15.34). This model was also validated using available literature data. The new model is capable of predicting all of the validation data with reasonable accuracy, with some discrepancy in predicting formaldehyde in the JSR data. All of this, results in a robustly validated and accurate, new detailed chemical kinetic model.

Reaction pathway of coal oxidation at low temperatures: a model of cyclic chain reactions and kinetic characteristics

Abstract: Organic chemical reactions cause the temperature rising during coal oxidation; however, because of the complex structure of coal, it is difficult to analyze and characterize the reactions involved in coal low-temperature oxidation. To date, a main reaction pathway describing the heating progress during coal oxidation has not been proposed. Here, a series of cyclic chain reactions is presented to describe the oxidation mechanism during coal spontaneous combustion using quantum chemistry calculations. Main active sites and their molecular models were built. Three interactive modes of active orbitals and detailed reaction sequences of coal oxidation are proposed. The structural parameters and thermodynamic data were calculated and the orders of reactions for transformations between functional groups were identified based on their activation energies. The reaction pathway was constructed based on functional transformation relationships and the order of reactions. The results show that main reactions occurring during coal oxidation can be defined as the reactions of oxygen and hydroxide free radicals reacting with coal active sites. Methyne and carbon free radicals reacting with oxygen is the initial reaction during coal oxidation. The decomposition of peroxides linking the reaction pathway form cyclic chain. Hydroxyl and aliphatic hydrocarbon radicals as key of chain reactions consumes coal active sites and oxygen continuously. Aliphatic hydrocarbons appear to contribute more to heat release during coal oxidation due to greater heat release and lower activation energy of their reactions. Limited spontaneous reactions maintain constant apparent activation energy for the oxidation until the chain reactions are generated; the apparent activation energy then increases. Low-ranking coals have higher apparent activation energies during oxidation due to more oxygen-containing groups and side chains contain more reactions with higher activation energy. Results from this study can improve understanding of mechanism of coal oxidation and provide a guide to forecasting and preventing spontaneous combustion of coal in underground coal mines or coal stockpiles.

A general theory of ignition and combustion of nano- and micron-sized aluminum particles

Abstract: A general theory of ignition and combustion of nano- and micron-sized aluminum particles is developed. The oxidation process is divided into several stages based on phase transformations and chemical reactions. Characteristic time scales of different processes are compared to identify physicochemical phenomena in each stage. In the first stage, the particle is heated to the melting temperature of the aluminum core. Key processes are heat and mass transfer between the gas and particle surface and diffusion of mass and energy inside the particle. The second stage begins upon melting of the aluminum core. Melting results in pressure buildup, thereby facilitating mass diffusion and/or cracking of the oxide layer. Melting is followed by polymorphic phase transformations, which also results in the formation of openings in the oxide layer. These provide pathways for the molten aluminum to react with the oxidizing gas; the ensuing energy release results in ignition of nano-aluminum particles. For large micron-sized particles, ignition is not achieved due to their greater volumetric heat capacity. In the third stage, nanoparticles undergo vigorous self-sustaining reactions with the oxidizing gas. Reactions typically occur heterogeneously in the particle and the burning rate is controlled by chemical kinetics. For large micron-sized particles, polymorphic phase transformations result in the formation of a crystalline oxide layer. The oxide layer melts and particle ignition is achieved. In the fourth stage, the large micron-sized particle burns through gas-phase or surface reactions, depending on the oxidizer and pressure. The burning rate is controlled by mass diffusion through the gas-phase mixture.

An ignition delay time and chemical kinetic modeling study of the pentane isomers

Abstract: Ignition delay times of n -pentane, iso -pentane, and neo -pentane mixtures were measured in two shock tubes and in a rapid compression machine. The experimental data were used as validation targets for the model described in detail in an accompanying study [14]. The present study presents ignition delay time data for the pentane isomers at equivalence ratios of 0.5, 1.0, and 2.0 in ‘air’ (additionally, 0.3 in ‘air’ for n -, and iso -pentane) at pressures of 1, 10, and 20 atm in the shock tube, and 10 and 20 atm in the rapid compression machine, as well as data at an equivalence ratio of 1.0 in 99% argon, at pressures near 1 and 10 atm in a shock tube. An infrared laser absorption technique at 3.39 µm was used to verify the composition of the richest mixtures in the shock-tube tests by measuring directly the pentane isomer concentration in the driven section. By using shock tubes and a rapid compression machine, it was possible to investigate temperatures ranging from 643 to 1718 K. A detailed chemical kinetic model was used to simulate the experimental ignition delay times, and these are well-predicted for all of the isomers over all ranges of temperature, pressure, and mixture composition. In-depth analyses, including reaction path and sensitivity analyses, of the oxidation mechanisms of each of the isomers are presented. To the authors’ knowledge, this study covers conditions not yet presented in the literature and will, in conjunction with the aforementioned accompanying study, expand fundamental knowledge of the combustion kinetics of the pentane isomers and of alkanes in general.

Exploration of combustion instability triggering using Large Eddy Simulation of a multiple injector Liquid Rocket Engine

Abstract: This article explores the possibility of analyzing combustion instabilities in liq- uid rocket engines by making use of Large Eddy Simulations (LES). Calculations are carried out for a complete small-scale rocket engine, including the injection manifold thrust chamber and nozzle outlet. The engine comprises 42 coaxial injectors feeding the combustion chamber with gaseous hydrogen and liquid oxy- gen and it operates at supercritical pressures with a maximum thermal power of 80 MW. The objective of the study is to predict the occurrence of transverse high-frequency combustion instabilities by comparing two operating points fea- turing different levels of acoustic activity. The LES compares favorably with the experiment for the stable load point and exhibits a nonlinearly unstable trans- verse mode for the experimentally unstable operating condition. A detailed analysis of the instability retrieves the experimental data in terms of spectral features. It is also found that modifications of the flame structures and of the global combustion region configuration have similarities with those observed in recent model scale experiments. It is shown that the overall acoustic activity mainly results from the combination of one transverse and one radial mode of the chamber, which are also strongly coupled with the oxidizer injectors.

Compositional effects on the ignition of FACE gasolines

Abstract: As regulatory measures for improved fuel economy and decreased emissions are pushing gasoline engine combustion technologies towards extreme conditions (i.e., boosted and intercooled intake with exhaust gas recirculation), fuel ignition characteristics become increasingly important for enabling stable operation. This study explores the effects of chemical composition on the fundamental ignition behavior of gasoline fuels. Two well-characterized, high-octane, non-oxygenated FACE (Fuels for Advanced Combustion Engines) gasolines, FACE F and FACE G, having similar antiknock indices but different octane sensitivities and chemical compositions are studied. Ignition experiments were conducted in shock tubes and a rapid compression machine (RCM) at nominal pressures of 20 and 40 atm, equivalence ratios of 0.5 and 1.0, and temperatures ranging from 650 to 1270 K. Results at temperatures above 900 K indicate that ignition delay time is similar for these fuels. However, RCM measurements below 900 K demonstrate a stronger negative temperature coefficient behavior for FACE F gasoline having lower octane sensitivity. In addition, RCM pressure profiles under two-stage ignition conditions illustrate that the magnitude of low-temperature heat release (LTHR) increases with decreasing fuel octane sensitivity. However, intermediate-temperature heat release is shown to increase as fuel octane sensitivity increases. Various surrogate fuel mixtures were formulated to conduct chemical kinetic modeling, and complex multicomponent surrogate mixtures were shown to reproduce experimentally observed trends better than simpler two- and three-component mixtures composed of n-heptane, iso-octane, and toluene. Measurements in a Cooperative Fuels Research (CFR) engine demonstrated that the multicomponent surrogates accurately captured the antiknock quality of the FACE gasolines. Simulations were performed using multicomponent surrogates for FACE F and G to reveal the underlying chemical kinetics linking fuel composition with ignition characteristics. A key discovery of this work is the kinetic coupling between aromatics and naphthenes, which affects the radical pool population and thereby controls ignition.

The influence of wall roughness on detonation limits in hydrogen–oxygen mixture

Abstract: In this study, wall roughness is generated by inserting a Shchelkin spiral with different wire diameter ( δ ) and pitch ( L s ). Roughness is defined as the ratio δ / L s . The effect of tube wall roughness on the detonation limits in stoichiometric hydrogen–oxygen mixture is systematically examined. The detonation velocity is determined from optical fibers and shock pins spaced at 10 cm intervals along the tube. Smoked foils are employed to record the cellular detonation structure near the limits. The experimental results indicate that detonation in both smooth and rough sections can be self-sustaining and can propagate with a steady velocity as the conditions are well within the detonation limits. However, the detonation velocity decreases as it transmits into the rough-walled tube. The velocity deficit is more significant in tubes with larger roughness due to the interaction of the detonation reaction zone and the boundary layer formed behind the shock. Single-headed spinning structure is observed as the detonation approaches the limits in the rough-walled tube. Below the minimum initial pressure at which single-headed spinning phenomena occur, detonation fails and decays to deflagration, and the minimum velocity is approximately 0.4 V CJ . It is found that wall roughness can either promote or prohibit the detonation propagation limits. When the roughness is smaller than 0.231, it is believed the turbulence generated from the roughness facilitates detonation and extends the detonation limits. However, when the roughness is larger than 0.333, low-velocity behavior plays a dominant role in prohibiting the detonation, which indicates that roughness above a certain level has a negative effect on detonation limits.

High-pressure oxidation of methane

Abstract: Methane oxidation at high pressures and intermediate temperatures was investigated in a laminar flow reactor and in a rapid compression machine (RCM). The flow-reactor experiments were conducted at 700–900 K and 100 bar for fuel-air equivalence ratios (Φ) ranging from 0.06 to 19.7, all highly diluted in nitrogen. It was found that under the investigated conditions, the onset temperature for methane oxidation ranged from 723 K under reducing conditions to 750 K under stoichiometric and oxidizing conditions. The RCM experiments were carried out at pressures of 15–80 bar and temperatures of 800–1250 K under stoichiometric and fuel-lean (Φ=0.5) conditions. Ignition delays, in the range of 1–100 ms, decreased monotonically with increasing pressure and temperature. A chemical kinetic model for high-pressure methane oxidation was established, with particular emphasis on the peroxide chemistry. The thermodynamic properties of CH3OO and CH3OOH, as well as the rate constants for the abstraction reactions CH3OOH + CH3 = CH3OO + CH4 and CH3OH + CH3 = CH3O + CH4, were calculated at a high level of theory. Model predictions were evaluated against the present data as well as shock tube data (1100–1700 K, 7–456 bar) and flame speeds (1–10 bar) from literature. The model yielded satisfactory predictions for the onset temperature as well as for most major species upon ignition in the flow reactor, but the concentration of particularly CH3OH was severely underpredicted, indicating that further work is desirable on reactions of CH3O and CH3OO. Measured ignition delay times from the RCM tests were reproduced well by the model for high pressures, but underpredicted at 15 bar. For the shock tube and flame conditions, predictions were mostly within the experimental uncertainty. Prompt dissociation of HCO increased predicted flame speeds by up to 4 cm s−1 but had little impact under flow reactor, RCM or shock tube calculations.

Flame macrostructures, combustion instability and extinction strain scaling in swirl-stabilized premixed CH4/H2 combustion

Abstract: In this paper, we report results from an experimental investigation on transitions in the average flame shape (or microstructure) under acoustically coupled and uncoupled conditions in a 50 kW swirl stabilized combustor. The combustor burns CH4/H2 mixtures at atmospheric pressure and temperature for a fixed Reynolds number of 20,000 and fixed swirl angle. For both cases, essentially four different flame shapes are observed, with the transition between flame shapes occurring at the same equivalence ratio (for the same fuel mixture) irrespective of whether the combustor is acoustically coupled or uncoupled. The transition equivalence ratio depends on the fuel mixture. For the baseline case of pure methane, the combustor is stable close to the blowoff limit and the average flame in this case is stabilized inside the inner recirculation zone. As the equivalence ratio is raised, the combustor transitions to periodic oscillations at a critical equivalence ratio of ϕ = 0.65 . If hydrogen is added to the mixture, the same transition occurs at lower equivalence ratios. For all cases that we investigated, flame shapes captured using chemiluminescence imaging show that the transition to harmonic oscillations in the acoustically coupled cases is preceded by the appearance of the flame in the outer recirculation zone. We examine the mechanism associated with the transition of the flame between different shapes and, ultimately, the propagation of the flame into the outer recirculation zone as the equivalence ratio is raised. Using the extinction strain rates for each mixture at different equivalence ratios, we show that these transitions in the flame shape and in the instability (in the coupled case) for different fuel mixtures collapse as a function of a normalized strain rate : κ e x t D U ∞ . We show that the results as consistent with a mechanism in which the flame must overcome higher strains prevailing in the outer recirculation zone, in order to stabilize there.

Skeletal mechanism reduction through species-targeted sensitivity analysis

Abstract: The use of simplifying techniques to obtain skeletal kinetic mechanisms with the required accuracy is often a necessary step when computationally demanding simulations are concerned. In this work, a novel approach for an automatic mechanism reduction, aimed at retaining accuracy on specific target species, is proposed. Starting from the consolidated coupling between flux analysis and sensitivity analysis, a methodology based on curve matching and functional data analysis was developed, through which the importance of a species in the target accuracy is assessed via a proper metric. The error associated with the removal of uncertain species from the detailed mechanism is quantified in terms of distance and similarity indices before and after such a removal, within a Species-Targeted Sensitivity Analysis (STSA) framework. A species ranking is then generated, and the original mechanism is progressively reduced. The whole algorithm also implements several improvements to enhance a faster convergence, and adds a novel criterion to remove unimportant reactions, based on sensitivity analysis to kinetic parameters. The capability of this algorithm was tested through two case studies in this work. A kinetic mechanism for a Toluene Reference Fuel (TRF) was first obtained, with the overall reactivity as reduction target. The numerical procedure allowed to obtain a compact skeletal mechanism (115 species and 856 reactions), able to retain good accuracy in ignition delay time and laminar flame speed predictions of both fuel mixture and pure compounds. More important, two skeletal mechanisms for methane combustion, including chemistry of nitrogen oxides (NO x ), were developed, with different degrees of reduction. The agreement between the original and the skeletal mechanisms in terms of NO formation was successfully assessed with satisfactory results. Attention was also dedicated to the choice of the type of reactor where undertaking reduction, which turned out to play a major role in the overall process.

Laminar flame speeds under engine-relevant conditions: Uncertainty quantification and minimization in spherically expanding flame experiments

Abstract: The spherically expanding flame method is the only approach for measuring laminar flame speeds at thermodynamic states that are relevant to engines. In the present study, a comprehensive evaluation of data obtained under constant pressure and constant volume conditions was carried out through experiments, development of a mathematically rigorous method for uncertainty quantification and propagation, and advancement of numerical models that describe the experiments accurately. The proposed uncertainty characterization approach accounts for parameters related to all measurements, data processing, and finally data interpretation. With the aid of direct numerical simulations, an alternative approach was proposed to derive laminar flame speeds in constant pressure experiments by eliminating the need for using extrapolation equations developed based on simplifying assumptions, which are known to be susceptible to major errors under certain conditions. The propagation of spherical flames under constant volume conditions was investigated through experiments carried out in an entirely spherical chamber and the use of two numerical models. The first involves the solution of the one-dimensional conservation equations of mass, species, and energy while accounting for pressure rise. The second model was developed based on thermodynamics similarly to existing literature, but radiation loss was introduced at the optically thin limit and approximations were made to allow for re-absorption with minimum computational cost. It was shown that neglecting radiation in constant volume experiments could introduce errors as high as 15%. Incorporating the aforementioned techniques, laminar flame speeds were measured and reported with properly quantified uncertainties for flames of synthesis gas for pressures ranging from 3 to 30 atm, and unburned mixture temperatures ranging from 298 to 550 K. Selected measurements were carried out as well for methane and propane flames for pressures ranging from 3 to 7 atm, and unburned mixture temperature of 298 K. The approaches introduced in this study allow for the determination of laminar flame speeds with notably reduced uncertainties under conditions of relevance to engines, which has major implications for the validation of kinetic models of surrogate and real fuels.

Shock tube ignition delay times and methane time-histories measurements during excess CO2 diluted oxy-methane combustion

Abstract: The combustion of methane in air results in large amounts of CO 2 and NO X emissions. In order to reduce the NO X emissions, one possible solution is the oxy-methane combustion with large CO 2 dilution so that the combustion products can be reduced mainly to CO 2 and H 2 O. However, there are very few studies on the chemical kinetics of oxy-methane combustion in a CO 2 diluted environment. In this study, methane time-histories, CH * emission profiles, and pressure time-histories measurements were conducted behind reflected shock waves to gain insight into the effects of CO 2 dilution of the gas mixtures on the ignition of methane. The measurements were carried out for mixtures of CH 4 , CO 2 and O 2 in argon bath gas at temperatures of 1577–2144 K, pressures of 0.53–4.4 atm, equivalence ratios (Φ) of 0.5, 1, and 2, and CO 2 mole fractions (X CO2 ) of 0, 30, and 60%. The laser absorption measurements were conducted using a continuous wave distributed feedback interband cascade laser (DFB ICL) centered at 3403.4 nm. The results showed the decrease of activation energy and the increase of ignition delay time as the amount of CO 2 dilution was increased. However, the changes were minor and within the experimental uncertainties of the measurements. Also, the results were compared to the predictions of two different natural gas mechanisms: GRI 3.0 and AramcoMech 1.3 mechanisms. In general the predictions were reasonable when compared to the experimental data; however, there were discrepancies at some conditions. Three different influences of CO 2 addition to the argon bath gas in regards to chemistry, collision efficiencies, and heat capacities were examined. In addition, the present study included experimentally obtained correlations for absorption cross sections of methane for its P(8) line in the v 3 band in argon bath gas with and without carbon-dioxide dilutions at temperatures between 1200

Optimized reaction mechanism rate rules for ignition of normal alkanes

Abstract: This work was performed within the Cluster of Excellence “Tailor-Made Fuels from Biomass”, which is funded by the Excellence Initiative of the German federal state governments to promote science and research at German universities. The authors also acknowledge funding support from the Clean Combustion Research Center and Saudi Aramco under the FUELCOM program. VR was supported by SERDP Grant WP-2151 with Dr. Robin Nissan as Program Manager. NUI Galway would like to acknowledge the support of the Irish Research Council in funding this work.

Large Eddy Simulation of n-dodecane spray flames using Flamelet Generated Manifolds

Abstract: In the present study the Engine Combustion Network (ECN) “Spray A” target conditions are investigated using the Large Eddy Simulation (LES) and the Flamelet Generated Manifold (FGM) methods. We investigate n -dodecane spray flames at three different ambient oxygen levels in engine relevant conditions. The flamelet database is generated by simulating the counterflow diffusion flamelets for two recently developed n -dodecane mechanisms with 257-species/1521-reactions (Narayanaswamy et al., 2014) and 130-species/2395-reactions (Ranzi et al., 2014). In addition to validation in non-reacting conditions, we evaluate the performance of the newly implemented FGM model by comparing spray ignition delay times and flame lift-off lengths to the available experimental data within the ECN. The obtained ignition delay times agree well with the experimental data for the mechanism by Ranzi et al., 2014 and are over-predicted for the mechanism by Narayanaswamy et al., 2014. This observation is consistent with a respective trend in the observed flame lift-off lengths. Further, we provide evidence of only minor spray realization to realization variation of the ignition delay time in the present configuration. The spray flame structure is noted to consist of two parts: (1) a diffusion flame enveloping the combusting part of the spray close to the stoichiometric isoline, and (2) a premixed combustion regime in the fuel-rich core of the spray. During spray ignition, the model predicts the spatio-temporal phases of ignition. The model also indicates the presence of a ‘cool flame’ between the flame lift-off length and the nozzle. For the first time, we quantify the size of such a topological structure. In general, the flamelet data showed significant differences in the ignition characteristics between the two chemical mechanisms for all three ambient oxygen cases, but indicated little differences for a steady flame.

An overview of coal rank influence on ignition and combustion phenomena at the particle level

Abstract: Individual particles of pulverized coals exhibit strikingly different combustion phenomena depending on their rank (anthracite, semi-anthracite, bituminous, sub-bituminous and lignite). Herein a concise review is presented on pertinent findings in the literature to contrast ignition and combustion behavior of such fuels at the particle-level. Emphasis is given to recent investigations performed in the laboratory of the authors, where combustion of a variety of coal particles of the same size-cut took place in the same apparatus under identical operating conditions. Such behaviors were then compared to those reported in the literature to verify their replication under often different, but yet relevant, conditions. The objective has been to relate the effect of coal rank to a number of key qualitative and quantitative parameters, such as modes of ignition and combustion, ignition temperatures, ignition delay times, combustion temperatures and burnout times (both those encountered in the volatile and the char combustion phases), volatile flame sizes as well as extents of particle fragmentation. Besides reviewing combustion behaviors in air, analogous behaviors under simulated dry oxy-combustion conditions were also highlighted. Then the coal rank dependence of the required oxygen mole fraction in dry O2/CO2 blends to match the intensity of air-fired combustion was examined.

Effects of simultaneous hydrogen enrichment and carbon dioxide dilution of fuel on soot formation in an axisymmetric coflow laminar ethylene/air diffusion flame

Abstract: Effects of simultaneous hydrogen enrichment and carbon dioxide dilution to hydrocarbon fuels on soot formation are of fundamental and practical interest. Previous studies found that addition of either hydrogen or carbon dioxide to fuel reduces soot chemically in addition to the dilution effect in laminar coflow ethylene/air diffusion flames. A numerical study was carried out in this work to investigate the effects of adding hydrogen and carbon dioxide simultaneously to fuel on soot formation in an axisymmetric laminar coflow ethylene/air diffusion flame at atmospheric pressure. Numerical calculations were conducted using detailed gas-phase chemistry and thermal and transport properties. Soot inception is assumed to be the result of collision of two pyrene molecules. The subsequent particle surface growth, soot oxidation, and particle interactions are modeled by the hydrogen abstraction C2H2 addition (HACA) mechanism and a sectional model. Soot surface growth through condensation of pyrene was also taken into account. The flame model is able to reproduce fairly well the chemical effects of adding either hydrogen or carbon dioxide to ethylene observed experimentally in the literature. Addition of hydrogen is more effective on soot inception suppression and addition of carbon dioxide is more effective on soot surface growth suppression. The simultaneous hydrogen enrichment and carbon dioxide dilution to ethylene retains the individual soot suppression benefits of hydrogen enrichment and carbon dioxide dilution. These results suggest that the chemical interactions between hydrogen and carbon dioxide on soot formation are weak.

Kinetic studies of ozone assisted low temperature oxidation of dimethyl ether in a flow reactor using molecular-beam mass spectrometry

Abstract: The ozone assisted low temperature oxidation chemistry of dimethyl ether (DME) from 400 K to 750 K has been investigated in the mixture of DME/O3/O2/He/Ar in an atmospheric-pressure flow reactor coupled with the molecular beam mass spectrometry (MBMS) sampling technique. The mole fraction of ozone was varied from 0 to 0.146% in the mixture to study its enhanced kinetic effect on DME oxidation. The mole fractions of DME, O2, O3, CH2O, H2O2, CO, CO2, and CH3OCHO were quantified as functions of temperature at a fixed total volumetric flow rate. The experimental results revealed that the presence of ozone dramatically enhances the low temperature DME oxidation. Numerical simulations using the existing kinetic models (Kurimoto's model (KM) (Kurimoto et al., 2015), Burke's model (BM) (Burke et al., 2015), and Wang's model (WM) (Wang et al., 2015)) with an ozone sub-mechanism over-predicted the DME oxidation significantly. The observed large discrepancies between models and experiments for DME, CH2O, O2 and CH3OCHO mole fractions suggested that there were large uncertainties in the branching ratios of two competing chain-propagation and chain-branching reaction pairs involving CH3OCH2O2 and CH2OCH2O2H radicals at low temperature.

Three-dimensional direct numerical simulation of turbulent lean premixed methane combustion with detailed kinetics

Abstract: The interaction of maintained homogeneous isotropic turbulence with lean premixed methane flames is investigated using direct numerical simulation with detailed chemistry. The conditions are chosen to be close to those found in atmospheric laboratory experiments. As the Karlovitz number is increased from 1 to 36, the preheat zone becomes thickened, while the reaction zone remains largely unaffected. A negative correlation of fuel consumption with mean flame surface curvature is observed. With increasing turbulence intensity, the chemical composition in the preheat zone tends towards that of an idealised unity Lewis number flame, which we argue is the onset of the transition to distributed burning, and the response of the various chemical species is shown to fall into broad classes. Smaller-scale simulations are used to isolate the specific role of species diffusion at high turbulent intensities. Diffusion of atomic hydrogen is shown to be related to the observed curvature correlations, but does not have significant consequential impact on the thickening of the preheat zone. It is also shown that susceptibility of the preheat zone to thickening by turbulence is related to the ‘global’ Lewis number (the Lewis number of the deficient reactant); higher global Lewis number flames tend to be more prone to thickening.

The role of low temperature chemistry in combustion mode development under elevated pressures

Abstract: Negative Temperature Coefficient (NTC) behavior is an essential feature of low-temperature oxidation for large hydrocarbon fuels, which is of particular relevance to cool flame and auto-ignition. In this study, using n-heptane as a typical fuel exhibiting NTC, combustion phenomena involving both auto-ignition and flame propagation are computationally studied at initial temperatures within and above NTC regime under elevated pressures in a one-dimensional planar constant-volume configuration, with detailed kinetics and transport. Multi-staged flame structures representing cool flame and hot flame are observed, and consequently, different types of auto-ignition are identified during two-staged and single-staged flame propagation scenarios by varying initial temperature. Specially, as the initial temperature increases, the behavior of cool flame is gradually suppressed and auto-ignition position is transferred from the location ahead of flame front to end-wall region, leading to different combustion modes and peak pressure magnitudes. Moreover, attributed to the chemical reactivity processed by cool flame, the flame propagation of the cases within NTC regime is even faster than those beyond NTC regime. A recently developed two-staged Livengood–Wu integral is further utilized to predict these auto-ignition scenarios, yielding good agreement and further demonstrating the significant role of NTC chemistry in modifying the thermodynamic state and chemical reactivity at upstream of a reaction front. Finally, different combustion modes and knocking intensity for these detailed calculations are summarized in non-dimensional diagrams, which suggest that a higher initial temperature does not guarantee a higher knocking intensity, instead, the developing and developed detonation wave initiated by an auto-ignition occurring within NTC regime could even induce higher knocking intensity in comparison to the thermal explosion under the temperatures beyond NTC regime.

Engine Hot Spots: Modes of Auto-ignition and Reaction Propagation

Abstract: Many direct numerical simulations of spherical hot spot auto-ignitions, with different fuels, have identified different auto-ignitive regimes. These range from benign auto-ignition, with pressure waves of small amplitude, to super-knock with the generation of damaging over-pressures. Results of such simulations are generalised diagrammatically, by plotting boundary values of ξ , the ratio of acoustic to auto-ignition velocity, against ɛ. This latter parameter is the residence time of the developing acoustic wave in the hot spot of radius r o , namely r o /a , normalised by the excitation time for the chemical heat release, τ e . This ratio controls the energy transfer into the developing acoustic front. A third relevant parameter involves the product of the activation temperature, E/R , for the auto-ignition delay time, τ i , normalised by the mixture temperature. T , the ratio, τ i / τ e , and the dimensionless hot spot temperature gradient, ( ∂ ln T / ∂ r ¯ ) , where r ¯ is a dimensionless radius. These parameters define the boundaries of regimes of thermal explosion, subsonic auto-ignition, developing detonations, and non-auto-ignitive deflagrations, in plots of ξ against ɛ.The regime of developing detonation forms a peninsula and contours, throughout the field. The product parameter ( E / R T ) ( τ i / τ e ) / ∂ ln T / ∂ r ¯ expresses the influences of hot spot temperature gradient and fuel characteristics, and a unique value of it might serve as a boundary between auto-ignitive and deflagrative regimes. Other combustion regimes can also be identified, including a mixed regime of both auto-ignitive and “normal” deflagrative burning. The paper explores the performances of a number of different engines in the regimes of controlled auto-ignition, normal combustion, combustion with mild knock and, ultimately, super-knock. The possible origins of hot spots are discussed and it is shown that the dissipation of turbulent energy alone is unlikely to lead directly to sufficiently energetic hot pots. The knocking characterisation of fuels also is discussed.

Additional chain-branching pathways in the low-temperature oxidation of branched alkanes

Abstract: Chain-branching reactions represent a general motif in chemistry, encountered in atmospheric chemistry, combustion, polymerization, and photochemistry; the nature and amount of radicals generated by chain-branching are decisive for the reaction progress, its energy signature, and the time towards its completion. In this study, experimental evidence for two new types of chain-branching reactions is presented, based upon detection of highly oxidized multifunctional molecules (HOM) formed during the gas-phase low-temperature oxidation of a branched alkane under conditions relevant to combustion. The oxidation of 2,5-dimethylhexane (DMH) in a jet-stirred reactor (JSR) was studied using synchrotron vacuum ultra-violet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). Specifically, species with four and five oxygen atoms were probed, having molecular formulas of C8H14O4 (e.g., diketo-hydroperoxide/keto-hydroperoxy cyclic ether) and C8H16O5 (e.g., keto-dihydroperoxide/dihydroperoxy cyclic ether), respectively. The formation of C8H16O5 species involves alternative isomerization of OOQOOH radicals via intramolecular H-atom migration, followed by third O2 addition, intramolecular isomerization, and OH release; C8H14O4 species are proposed to result from subsequent reactions of C8H16O5 species. The mechanistic pathways involving these species are related to those proposed as a source of low-volatility highly oxygenated species in Earth's troposphere. At the higher temperatures relevant to auto-ignition, they can result in a net increase of hydroxyl radical production, so these are additional radical chain-branching pathways for ignition. The results presented herein extend the conceptual basis of reaction mechanisms used to predict the reaction behavior of ignition, and have implications on atmospheric gas-phase chemistry and the oxidative stability of organic substances.

A component library framework for deriving kinetic mechanisms for multi-component fuel surrogates: Application for jet fuel surrogates

Abstract: Surrogate fuels are often used in place of real fuels in computational combustion studies. However, many different choices of hydrocarbons to make up surrogate mixtures have been reported in the literature, particularly for jet fuels. To identify the best choice of surrogate components, the capabilities of different surrogate mixtures in emulating the combustion kinetic behavior of the real fuel must be examined. To allow extensive assessment of the combustion behavior of these surrogate mixtures against detailed experimental measurements for real fuels, accurate and compact kinetic models are most essential. To realize this goal, a flexible and evolutive component library framework is proposed here, which allows mixing and matching between surrogate components to obtain short chemical mechanisms with only the necessary kinetics for the desired surrogate mixtures. The idea is demonstrated using an extensively validated multi-component reaction mechanism developed in stages (Blanquart et al., 2009; Narayanaswamy et al., 2010, 2014, 2015), thanks to its compact size and modular assembly. To display the applicability of the component library framework, (i) a jet fuel surrogate consisting of n-dodecane, methylcyclohexane, and m-xylene, whose kinetics are described in the multi-component chemical mechanism is defined, (ii) a chemical model for this surrogate mixture is derived from the multi-component chemical mechanism using the component library framework, and (iii) the predictive capabilities of this jet fuel surrogate and the associated chemical model are assessed extensively from low to high temperatures in well studied experimental configurations, such as shock tubes, premixed flames, and flow reactors.

Characterization of Spray A flame structure for parametric variations in ECN constant-volume vessels using chemiluminescence and laser-induced fluorescence

Abstract: The transient and quasi-steady flame structure of reacting fuel sprays produced by single-hole injectors has been studied using chemiluminescence imaging and Planar Laser-Induced Fluorescence (PLIF) in various constant-volume facilities at different research institutes participating in the Engine Combustion Network (ECN). The evolution of the high-temperature flame has been followed based on chemiluminescence imaging of the excited-state hydroxyl radical (OH*), and PLIF of ground-state OH. Regions associated with low-temperature chemical reactions are visualized using formaldehyde (CH2O) PLIF with 355-nm excitation. We compare the results obtained by different research institutes under nominally identical experimental conditions and fuel injectors. In spite of design differences among the various experimental facilities, the results are consistent. This lends confidence to studies of transient behavior and parameter variations performed by individual research groups. We present results of the transient flame structures at Spray A reference conditions, and include parametric variations around this baseline, involving ambient temperature, oxygen concentration and injection pressure. Key results are the observed influence of an entrainment wave on the transient flame behavior, model-substantiated explanations for the high-intensity OH* lobes at the lift-off length and differences with OH PLIF, and a general analogy of the flame structures with a spray cone along which the flame tends to locate for the applied parametric variations.

Effect of decreasing blockage ratio on DDT in small channels with obstacles

Abstract: Multidimensional numerical simulations of an unconfined, homogeneous, chemically reactive gas were used to catalog interactions leading to the deflagration-to-detonation transition (DDT). The configuration studied was an infinitely long rectangular channel with regularly spaced obstacles containing a stoichiometric mixture of ethylene and oxygen, initially at atmospheric conditions and ignited in a corner with a small flame. The channel height is kept constant at 3200 µm and obstacle heights varied from 2560 µm to 160 µm to decrease the blockage ratio (br) from 0.8 to 0.05. The compressible reactive Navier–Stokes equations were solved by a high-order numerical algorithm on a locally adapting grid. The initially laminar flame develops into a turbulent flame with the creation of shocks, shock-flame interactions, shock-boundary layer interactions, a host of fluid and chemical-fluid instabilities, and DDT. Several different DDT mechanisms are observed as the br is reduced. For br in the range of 0.5–0.35, the shocks in the unburned material diffract over the obstacles and reflect against the channel wall, forming Mach stems that increase in strength with every obstacle traversed. Eventually, the Mach stem strength is sufficient for the unburned mixture to detonate after it reflects from an obstacle. For br outside of this range, DDT may occur either through Mach-stem reflection or through direct initiation due to shock focusing. Stochasticity of the turbulence leading to DDT in channels with low br is considered.

Laminar burning speed measurement and flame instability study of H2/CO/air mixtures at high temperatures and pressures using a novel multi-shell model

Abstract: A new differential based multi-shell model has been developed in conjunction with Schlieren photography to measure laminar burning speeds and to investigate flame structures of H2/CO/air mixtures. The experiments were carried out in two constant volume vessels; one spherical and one cylindrical. Flame instability has been studied using the cylindrical vessel which was installed in a Z-type Schlieren Shadowgraph system equipped with a high speed CMOS camera, capable of taking pictures up to 40,000 frames per second. Flame instabilities such as cracking and wrinkling have been observed during flame propagation and discussed in terms of the hydrodynamic and thermo-diffusive effects. Laminar burning speeds were measured by a novel thermodynamic model using pressure rise during flame propagation in the spherical chamber. Gases in the vessel are divided into two parts; unburned and burned gases. The burned gases are in the center of sphere surrounded by a small preheat zone followed by the unburned gases. The burned gases were divided into multiple shells to determine the temperature gradient precisely. The following energy transfers have been included in the model: Conductive energy loss to the chamber wall and electrodes, energy transfer in the preheat zone, energy transfer between adjacent shells and radiation energy loss from burned gases. The governing equations of unknown variables have been defined by a set of nonlinear ordinary differential equations which were solved using CVODE solver. Power law correlations have been developed for laminar burning speeds of smooth H2/CO/air flames over a wide range of temperatures (298 K up to 617 K), pressures (from sub-atmospheric up to 5.5 atm), equivalence ratios (0.6–5) and three different hydrogen concentrations of 5%, 10% and 25%, respectively. Experimental burning speeds of H2/CO/air mixtures have been compared with available measurements as well as computed values obtained by 1D free flame simulations using three chemical kinetics mechanisms. Comparisons show a very good agreement for the conditions which data is available and the predicted results.

Effect of the length of a plate flame holder on flame blowout limit in a micro-combustor with preheating channels

Abstract: It is challenging to achieve a large blowout limit for micro-combustors due to the increased heat loss ratio and reduced residence time. For this, we recently developed a micro-combustor with a plate flame holder and two preheating channels. In this paper, the effect of the plate length (Lb = 0.5, 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 mm) on the blowout limit of CH4/air flames was numerically investigated. The results show that the flame blowout limit increases firstly and then decreases with an increasing plate length. The largest blowout limit is obtained at Lb = 1.0 mm. Three neighboring cases, i.e., Lb = 0.5, 1.0 and 2.0 mm, are taken to analyze the underlying mechanisms responsible for this non-monotonic trend. The flame blowout process demonstrates that, the flame is extinguished due to "pinch-off phenomenon" at high inlet velocity, and the shorter the distance between the upper and lower flame fronts, the smaller the blowout limit will be. Numerical analysis reveals that the differences in flame blowout limits are a result of the combined effects of heat recirculation and local flow field at the entrance of the combustion chamber. The heat recirculation effect grows stronger with a decreasing length of flame holder, which results in a more obvious volumetric expansion at the entrance of the combustion chamber. Meanwhile, the plate flame holder has a redirection effect on the local flow field. As a result, the gaseous mixture enters the combustion chamber with a smallest acute angle in the case of Lb = 1.0 mm, which leads to a largest distance between the upper and lower flame fronts and a longest recirculation zone. Consequently, the flame blowout limit reaches a peak at Lb = 1.0 mm. The present work provides an important guideline to design such kind of micro combustors.

Turbulent flame stabilization modes in premixed swirl combustion: Physical mechanism and Karlovitz number-based criterion

Abstract: Modern low-emission, lean premixed gas turbine combustion systems often rely on confined swirling flows associated with sudden expansions to enhance flame anchoring. Through the establishment of multiple recirculation zones and shear layers, such complex reacting flows give rise to several possible average flame shapes or macrostructures. Among these, the single conical flame stabilized along the inner shear layer (ISL) and the double conical flame stabilized along both the inner as well as the outer shear layers (OSL) and the outer recirculation zone (ORZ) are of special interest. One of the reasons is that the transition between these two flames has been previously linked to the onset of thermo-acoustic instabilities under acoustically coupled conditions. In this study we investigate the mechanism underlying the flame transition to the ORZ/OSL and propose a criterion for its occurrence, in an acoustically uncoupled combustion system. To reach this goal, the effects of the fuel composition (CH 4 –H 2 ), Reynolds number, swirler blade angle and heat loss were experimentally analyzed. We find evidence that the transition starts with an intermittent inflammation of the ORZ caused by a flame kernel detaching from the ISL. Above a critical equivalence ratio, the flame kernel expands and spins along with the ORZ flow. Next, we explore the effect of the operating conditions on the onset of an ORZ flame. We propose a Strouhal number to describe the ORZ flame spinning frequency ( f ORZ ) also shown to be a predominately hydrodynamic quantity. Finally, we show that the flame transition to the ORZ is governed by a balance between a flame time to a flow time that can be expressed in a form of a Karlovitz number ( Ka ORZ ) defined as the ratio of the ORZ spinning frequency and extinction strain rate; the former is a surrogate for the bulk azimuthal strain rate in the ORZ.

Jet flame heights, lift-off distances, and mean flame surface density for extensive ranges of fuels and flow rates

Abstract: An extensive review and re-thinking of jet flame heights and structure, extending into the choked/supersonic regime is presented, with discussion of the limitations of previous flame height correlations. Completely new dimensionless correlations for the plume heights, lift-off distances, and mean flame surface densities of atmospheric jet flames, in the absence of a cross wind, are presented. It was found that the same flow rate parameter could be used to correlate both plume heights and flame lift-off distances. These are related to the flame structure, jet flame instability, and flame extinction stretch rates, as revealed by complementary experiments and computational studies. The correlations are based on a vast experimental data base, covering 880 flame heights. They encompass pool fires and flares, as well as choked and unchoked jet flames of CH4, C2H2, C2H4, C3H8, C4H10 and H2, over a wide range of conditions. Supply pressures range from 0.06 to 90 MPa, discharge diameters from 4 × 10−4 to 1.32 m, and flame heights from 0.08 to 110 m. The computational studies enabled reaction zone volumes to be estimated, as a proportion of the plume volumes, measured from flame photographs, and temperature contours. This enabled mean flame surface densities to be estimated, together with mean volumetric heat releases rates. There is evidence of a “saturation” mean surface density and increases in turbulent burn rates being accomplished by near pro rata increases in the overall volume of reacting mixture.