Experiments and modeling of ignition delay times, flame structure and intermediate species of EHN-doped stoichiometric n-heptane/air combustion

The combustion of stoichiometric Ethyl-hexyl-nitrate (EHN)-doped n-heptane/oxygen/argon and (EHN)-doped n-heptane/air mixtures, respectively, was investigated in a low-pressure burner with a molecular-beam mass spectrometer and ignition delay-time (τ_ign) measurements were performed in a high-pressure shock tube. The experiments with the low-pressure flame were used for the determination of the flame structure including concentration profiles of reactants, products and important intermediates in the flame. The shock tube experiments provided τ_ign for a temperature range of 690 K ? T ? 1275 K at a pressure of 40 ± 2 bar for stoichiometric and lean mixtures under engine relevant conditions. A chemical mechanism for n-heptane/EHN mixtures was developed from an automatically generated mechanism for n-heptane by manually adding reactions to describe the influence of EHN. This mechanism was validated against the shock-tube data for various temperatures, levels of EHN-doping and equivalence ratios by homogeneous reactor calculations.The ignition delay times predicted by the model agree well with the shock tube results for a large range of temperatures, equivalence ratios and EHN concentrations. The influence of EHN onto ignition delay was largest in the low-temperature regime (770-1000 K).Numerical analysis suggests that the prevalent reason for the ignition-enhancing effect of EHN is the formation of highly reactive heptyl radicals by thermal decomposition of EHN. Due to this comparatively simple and generic mechanism, EHN is expected to have a similar ignition-enhancing effect also for other hydrocarbon fuels.     

2D computations of FREI with cool flames for n-heptane/air mixture

Two-dimensional axisymmetric numerical simulation reproduced flames with repetitive extinction and ignition (FREI) in a micro flow reactor with a controlled temperature profile with a stoichiometric n-heptane/air mixture, which have been observed in the experiment. The ignition of hot flame occurred from consumption reactions of CO that was remained in the previous cycle of FREI. Between extinction and ignition locations of hot flames, several other heat release rate peaks related to cool and blue flames were observed for the first time. After the extinction of the hot flame, cool flame by the low-temperature oxidation of n-heptane appeared first and was stabilized in a low wall temperature region. In the downstream of the stable cool flame, a blue flame by the consumption reactions of cool flame products of CH₂O and H₂O₂ appeared. After that, the hot flame ignition occurred from the remaining CO in the downstream of the blue flame. Then after the next hot flame ignition, the blue flame was swept away by the propagating hot flame. Soon before the hot flame merged with the stable cool flame, the hot flame propagation was intensified by the cool flame. After the hot flame merged with the stable cool flame, the hot flame reacted with the incoming fresh mixture of n-C₇H₁₆ and O₂.     

On the chemical characteristics and dynamics of n-alkane low-temperature multistage diffusion flames

We demonstrate experimentally, perhaps for the first time, the existence of low-temperature multistage diffusion flames of n-alkanes. Multistage diffusion flames of n-heptane, n-decane, and n-dodecane are established in an atmospheric counterflow burner. Planar laser-induced fluorescence, chemiluminescence, and thermometry are used to probe the structures of such flames. In the first flame zone, the majority of the fuel is partially oxidized via low-temperature peroxy chemistry. In the second flame zone, the intermediate species produced are further oxidized via intermediate-temperature chemistry. The two stages of the flame are coupled such that significant fuel and oxidizer leakage occur, respectively, from the first and second reaction zones. The fuel is then further consumed, in the second stage, after the radical pool is replenished by the oxidation of the intermediates. The structure of the n-alkane multistage flame is found to be consistent with that previously observed for acyclic ethers. Owing to the different classes of temperature-dependent chemistries dominating the first and second stages, the reaction zone structure of multistage diffusion flames is dramatically influenced by the reactant concentrations and flame temperatures. The first stage is relatively favored at lower temperatures whereas the second stage is favored at elevated temperatures. Moreover, near extinction where the flame temperature is low, the multistage flame dynamics are controlled by the first oxidation stage, governed by peroxy chemistry, whereas the second oxidation stage, governed by intermediate chemistry, is dominant near high-temperature ignition conditions. Finally, by doping the oxidizer with ozone, we demonstrate the role of ozone doping on the multistage flame structure and the existence of a separate low-temperature ozone-assisted burning mode.     

Low- and high-temperature study of n-heptane combustion chemistry

n-Heptane has been used extensively in various fundamental combustion experiments as a prototypical hydrocarbon fuel. While the formation of polycyclic aromatic hydrocarbon (PAH) in n-heptane combustion has been studied preferably in premixed flames, this study aims to investigate the combustion chemistry of n-heptane in less-studied diffusion flame and highly rich high-temperature homogeneous oxidation configurations by using a counterflow burner and a flow reactor, respectively. This work addresses the formation of higher-molecular species in the mass range up to about 160 u in both configurations. Samples are analyzed by time-of-flight (TOF) molecular beam mass spectrometry (MBMS) using electron-impact (EI) and single-photon ionization (PI). Highly resolved speciation data are reported. Laminar flow reactor experiments cover a wide temperature range. Especially the measurements at low temperatures provide speciation data of large oxygenates produced in the low-temperature oxidation of n-heptane, which are scarce in the literature. Important precursor molecules for PAH and soot formation, such as C₉H₈, C₁₀H₈, C₁₁H_10, and C₁₂H₈, are formed during the high-temperature combustion process in the counterflow flame, while oxygenated growth species are observed under low-temperature conditions, even at the fuel-rich equivalence ratio of ?=4.00.Numerical modeling for both conditions is performed by using a newly developed kinetic model of n-heptane, which includes the n-heptane and PAH formation chemistry with state-of-the-art kinetic knowledge. Good agreement between model predictions and experimental data of counterflow flame and flow reactor is observed for the major species and some intermediates of n-heptane oxidation. While the concentrations of benzene and toluene measured in the counterflow burner are well-reproduced, the numerical results for flow reactor data are not satisfactory. Differences are found between the formation pathways of fulvene, from whose isomerization benzene is produced in diffusion flame and flow reactor.     

Skeletal mechanism generation and analysis for n-heptane with CSP

We use a procedure based on the decomposition into fast and slow dynamical components offered by the Computational Singular Perturbation (CSP) method to generate automatically skeletal kinetic mechanisms for the simplification of the kinetics of n-heptane oxidation. The detailed mechanism of the n-heptane oxidation here considered has been proposed by Curran et al. and involves 561 species and 2538 reactions. After carrying out a critical assessment of important aspects of this procedure, we show that the comprehensive skeletal kinetic mechanisms so generated are able to reproduce the main features of n-heptane ignition at various initial pressures and temperatures and equivalence ratios. A by-product of the algorithm that generates the skeletal mechanisms is the identification of the network of important species and reactions at a given state of the kinetic system. The analysis of this network is carried out by resorting to a visual representation of the pathways at selected time instants of the ignition process. Visual inspection of the pathways enables the identification and comparison of the relevant kinetic processes as obtained at different ignition regimes. The graphs are generated by interfacing the model reduction procedure with the open-source package graphviz.     

Issues arising in the construction of QSSA mechanisms: the case of reduced n-heptane/air models for premixed flames

A model reduction methodology, based on the quasi steady-state approximation (QSSA), is employed for the construction of reduced mechanisms in the case of an n-heptane/air premixed flame. Several issues related to the construction of these reduced mechanisms are discussed; such as the influence of the size of the starting skeletal mechanism, the stiffness reduction, and the truncation/simplification of (i) the expressions of the global rates and (ii) the steady-state relations. The starting point for the reduction is two skeletal mechanisms that involve 177/768 and 66/326 species/reactions, respectively [J. Prager, H.N. Najm, M. Valorani, and D.A. Goussis, Skeletal mechanism generation with CSP and validation for premixed n-heptane flames, Proc. Combust. Inst. 32 (2009), pp. 509–517] and which were derived from the detailed mechanism of Curran et al. [H.J. Curran, P. Gaffuri, W.J. Pitz, and C.K. Westbrook, A comprehensive modeling study of iso-octane oxidation, Combust. Flame 129 (2002), pp. 253–280], which involves 561/2538 species/reactions. From these two skeletal mechanisms, a number of reduced mechanisms of various sizes are produced and analysed. The validity of the reduced mechanism with the minimum size is demonstrated by considering its accuracy regarding the mass fractions of major and minor species, the temperature, and the flame speed, over a wide range of equivalence ratios and pressures.     

Measurements of the critical initiation radius and unsteady propagation of n-decane/air premixed flames

Unsteady flame propagation, the critical radius for flame initiation, and multiple flame regimes of n-decane/air mixtures are studied experimentally and computationally using outwardly propagating spherical flames at various equivalence ratios and pressures. The transient flame speeds, trajectories, and critical radius are measured. The experimental results are compared with direct numerical simulations using detailed high temperature kinetic models. Both experimental and numerical results show that there exist multiple flame regimes in the unsteady spherical flame initiation process. The transition between the flame regimes depends strongly on the mixture equivalence ratio (or Lewis number). It is found that there is a critical flame radius and that it increases dramatically as the mixture equivalence ratio and pressure decrease. The large increase of critical flame radius leads to a dramatic increase of the minimum ignition energy. Furthermore, the flame thickness and the radical pool concentration change significantly during the transition from the ignition flame regime to the self-sustained propagating flame regime. For the same steady state flame speeds, the predicted unsteady flame speeds and the critical flame radius differ significantly from the experimental results. Moreover, different chemical kinetic mechanisms predict different unsteady flame speeds. The existence of multiple flame regimes and the large critical radius of lean liquid fuel mixtures make the ignition of lean mixtures at low pressure and the development of a validated kinetic model more challenging. The unsteady flame regimes, speeds, and critical flame radius should be included as targets of future kinetic model development for turbulent combustion modeling.     

Lean or ultra-lean stretched planar methane/air flames

Lean premixed combustion has potential advantages of reducing pollutants and improving fuel economy. In some lean engine concepts, the fuel is directly injected into the combustion chamber resulting in a distribution of lean fuel/air mixtures. In this case, very lean mixtures can burn when supported by hot products from more strongly burning flames. This study examines the downstream interaction of opposed jets of a lean-limit CH₄/air mixture vs. a lean H₂/air flame. The CH₄ mixtures are near or below the lean flammability limit. The flame composition is measured by laser-induced Raman scattering and is compared to numerical simulations with detailed chemistry and molecular transport including the Soret effect. Several sub-limit lean CH₄/air flames supported by the products from the lean H₂/air flame are studied, and a small amount of CO₂ product (around 1% mole fraction) is formed in a “negative flame speed” flame where the weak CH₄/air mixture diffuses across the stagnation plane into the hot products from the H₂/air flame. Raman scattering measurements of temperature and species concentration are compared to detailed simulations using GRI-3.0, C₁, and C₂ chemical kinetic mechanisms, with good agreement obtained in the lean-limit or sub-limit flames. Stronger self-propagating CH₄/air mixtures result in a much higher concentration of product (around 6% CO₂ mole fraction), and the simulation results are sensitive to the specific chemical mechanism. These model-data comparisons for stronger CH₄/air flames improve when using either the C₂ or the Williams mechanisms.     

DNS Of the ignition process of n-heptane/air premixed combustion with low-temperature chemistry in turbulent boundary layer

In the present work, three-dimensional direct numerical simulation (DNS) of $n$-heptane/air premixed combustion in turbulent boundary layer was performed to explore the near-wall ignition process with low-temperature chemistry. A reduced chemical mechanism with 58 species and 387 elementary reactions for $n$-heptane combustion was used in the DNS. The general characteristics of the ignition process near the wall were examined. It was found that low-temperature ignition (LTI) dominates the upstream region, and high-temperature ignition (HTI) appears in the downstream region. The ignition process and the low-temperature chemistry pathways of the DNS are compared with those of a corresponding laminar case. It was found that the ignition process was affected by turbulence, which results in thickened reaction zones. However, the carbon flow analysis of low-temperature chemistry showed that turbulence rarely affects the low-temperature chemistry pathway. The combustion modes of various regions were scrutinized based on the budget terms of species transport equations and the chemical explosion mode analysis (CEMA). It was shown that the reaction term of ${\text{RO}}_2$ is significant during the LTI process of the upstream region, and the reaction terms of ${\text{CH}}_2\mathrm{O}$ and ${\text{CO}}_2$ are evident in the downstream region, indicating the occurrence of HTI. It was also shown that auto-ignition is dominant in the upstream region. With increasing streamwise distance, the contribution of flame propagation increases, which takes over that of auto-ignition in the near-wall region.     

Structure of partially premixed n-heptane–air counterflow flames

To avoid the complexities associated with the droplet/vapor transport and nonuniform evaporation processes, a fundamental investigation of liquid fuel combustion in idealized configurations is very useful. An experimental–computational investigation of prevaporized n-heptane nonpremixed and partially premixed flames established in a counterflow burner is described. There is a general agreement between various facets of our nonpremixed flame measurements and the literature data. The partially premixed flames are characterized by a double flame structure. This becomes more distinct as the strain rate decreases and partial premixing increases, which also increases the separation distance between the two reaction zones. The peak partially premixed flame temperature increases with increasing premixing of the fuel stream. The peak CO₂ and H₂O concentrations are relatively insensitive to partial premixing. The CO and H₂ peak concentrations on the premixed side increase as the fuel-side equivalence ratio decreases. These species are transported to the nonpremixed reaction zone where they oxidize. The C₂ species have peaks in the premixed reaction zone. The concentrations of olefins are ten times larger than those of the corresponding paraffins. The oxidizer is present in partially premixed flames throughout the combustion system and there are no regions characterized by simultaneous high temperature and high fuel concentration. As a result, pyrolysis reactions leading to soot formation are greatly diminished.     

Ignition limit and shock-to-detonation transition mode of n-heptane/air mixture in high-speed wedge flows

In this work, oblique detonation of n-heptane/air mixture in high-speed wedge flows is simulated by solving the reactive Euler equations with a two-dimensional (2D) configuration. This is a first attempt to model complicated hydrocarbon fuel oblique detonation waves (ODWs) with a detailed chemistry (44 species and 112 reactions). Effects of freestream equivalence ratios and velocities are considered, and the abrupt and smooth transition from oblique shock to detonation are predicted. Ignition limit, ODW characteristics, and predictability of the transition mode are discussed. Firstly, homogeneous constant-volume ignition calculations are performed for both fuel-lean and stoichiometric mixtures. The results show that the ignition delay generally increases with the wedge angle. However, a negative wedge angle dependence is observed, due to the negative temperature coefficient effects. The wedge angle range for successful ignition of n-heptane/air mixtures decreases when the wedge length is reduced. From two-dimensional simulations of stationary ODWs, the initiation length generally decreases with the freestream equivalence ratio, but the transition length exhibits weakly non-monotonic dependence. Smooth ODW typically occurs for lean conditions (equivalence ratio < 0.4). The interactions between shock/compression waves and chemical reaction inside the induction zone are also studied with the chemical explosive mode analysis. Moreover, the predictability of the shock-to-detonation transition mode is explored through quantifying the relation between ignition delay and chemical excitation time. It is demonstrated that the ignition delay (the elapsed time of the heat release rate, HRR, reaches the maximum) increases, but the excitation time (the time duration from the instant of 5% maximum HRR to that of the maximum) decreases with the freestream equivalence ratio for the three studied oncoming flow velocities. Smaller excitation time corresponds to stronger pressure waves from the ignition location behind the oblique shock. When the ratio of excitation time to ignition delay is high (e.g., > 0.5 for n-C₇H₁₆, > 0.3 for C₂H₂ and > 0.2 for H₂, based on the existing data compilation in this work), smooth transition is more likely to occur.     

Effect of fuel/air ratio and aromaticity on the molecular weight distribution of soot in premixed n-heptane flames

Soot growth from inception to mass-loading is studied in a wide range of molecular weights (MW) from 10⁵ to 10¹⁰u by means of size exclusion chromatography (SEC) coupled with on-line UV-visible spectroscopy. The evolution of MW distributions of soot is also numerically predicted by using a detailed kinetic model coupled with a discrete-sectional approach for the modeling of the gas-to-particle process. Two premixed flames burning n-heptane in slightly sooting and heavily sooting conditions are studied. The effect of aromatic addition to the fuel is studied by adding n-propylbenzene (10% by volume) to n-heptane in the heavily sooting condition. A progressive reduction of the MW distribution from multimodal to unimodal is observed along the flames testifying the occurrence of particle growth and agglomeration. These processes occur earlier in the aromatic-doped n-heptane flame due to the overriding role of benzene on soot formation which results in bigger young soot particles. Modeled MW distributions are in reasonable agreement with experimental data although the model predicts a slower coagulation process particularly in the slightly sooting n-heptane flame. Given the good agreement between model predictions and experiments, the model is used to explore the role of fuel chemistry on MW distributions. Two flames of n-heptane and n-heptane/n-propylbenzene in heavily sooting conditions with the same temperature profile and inert dilution are modeled. The formation of larger soot particles is still evident in the n-heptane/n-propylbenzene flame with respect to the n-heptane flame in the same operating conditions of temperature and dilution. In addition the model predicts a larger formation of molecular particles in the flame containing n-propylbenzene and shows that soot inception occurs in correspondence of their maximum formation thus indicating the importance of molecular growth in soot inception.     

Minimum ignition energy and propagation dynamics of laminar premixed cool flames

Laminar premixed cool flames, induced by the coupling of low-temperature chemistry and convective-diffusive transport process, have recently attracted extensive interest in combustion and engine research. In this work, numerical simulations have been conducted using a recently developed open-source reacting flow platform reactingFOAM-SCT, to investigate the minimum ignition energy (MIE) and propagation dynamics of premixed cool flames in a 1D spherical coordinate. Results have shown that when ignition energy is below the MIE of regular hot flames, a class of cool flames could be initiated, which allow much wider flammability limits, both lean and rich, compared to hot flames. Furthermore, the overall cool flame propagation dynamics exhibit intrinsic similarity to those of hot flames, in that, they begin with an ignition kernel propagation regime, followed by two transition regimes, and eventually reach a normal flame propagation regime. However, a spherical expanding cool flame responds completely differently to stretch. Specifically, a regular outwardly propagating hot spherical flame accelerates with increasing stretch rate when the mixture Le < 1 and decelerates when Le > 1. However, it is found that a cool flame always tends to decelerate with increasing stretch rate regardless of mixture composition, exhibiting unique flame aerodynamic characteristic. This research discovers novel features of premixed cool flame initiation and propagation dynamics and sheds light on flame transition, spark-ignition system design, and advanced engine combustion control.     

NOx formation and flame velocity profiles of iso- and n-isomers of butane and butanol

NO formation and flame propagation are studied in premixed flames of iso- and n-isomers of butane and butanol through experimental measurements and direct simulation of experimental profiles. The stabilized flame is realized through the impingement of a premixed combustible jet from a contraction nozzle against a temperature-controlled plate. The velocity field is obtained by means of Particle Image Velocimetry (PIV) and nitric oxide concentration profiles are measured using Planar Laser Induced Fluorescence (PLIF), calibrated using known NO seeding levels. It is found that NO formation in n- and iso-isomers is comparable under the conditions considered, except for rich butanol mixtures, whereby NO formation is higher for iso-butanol. Generally, less NO is formed in butanol flames than in the butane flames. The experiment is simulated by a 1D chemically reacting stagnation flow model, using literature models of C1–C4 hydrocarbons [Wang et al., 2010] and butanol combustion chemistry [Sarathy et al., 2009, 2012]. NO prediction is tested using two of these mechanisms with a previously-published NO_x submechanism added into the butane and butanol models. While a good level of agreement is observed in the velocity field prediction under lean and stoichiometric conditions, discrepancies exist under rich conditions. Greater discrepancies are observed in NO prediction, except for the C1–C4 mechanism which shows good agreement with the experiment under lean and stoichiometric conditions. The current study provides data for further development of mechanisms with NO_x prediction capabilities for the fuels considered here.     

A kinetics and dynamics study on the auto-ignition of dimethyl ether at low temperatures and low pressures

Though the combustion chemistry of dimethyl ether (DME) has been widely investigated over the past decades, there remains a dearth of ignition data that examines the low-temperature, low-pressure chemistry of DME. In this study, DME/‘air’ mixtures at various equivalence ratios from lean (0.5) to extremely rich (5.0) were ignited behind reflected shock waves at a fixed pressure (3.0 atm) over the temperature range 625–1200 K. The ignition behavior is different from that at high-pressures, with a repeatable ignition delay time fall-off feature observed experimentally in the temperature transition zone from the negative temperature coefficient (NTC) regime to the high-temperature regime. This could not be reproduced using available kinetic mechanisms as conventionally homogeneous ignition simulations. The fall-off behavior shows strong equivalence ratio dependence and disappears completely at an equivalence ratio of 5.0. A local ignition kernel postulate was implemented numerically to quantifiably examine the inhomogeneous premature ignition. At low temperature, no pre-ignition occurs in the mixture. A conspicuous discrepancy was observed between the measurements and constrained UV simulations at temperatures beyond the NTC regime. A third O₂ addition reaction sub-set was incorporated into AramcoMech 3.0, together with related species thermochemistry calculated using the G3/G4/CBS-APNO compound method, to explore the low-temperature deviation. The new reaction class does not influence the model predictions in IDTs, but the updated thermochemistry does. Sensitivity analyses indicate that the decomposition of hydroperoxy-methylformate plays a critical role in improving the low-temperature oxidation mechanism of DME but unfortunately, the thermal rate coefficient has never been previously investigated. Further experimental and theoretical endeavors are required to attain holistic quantitative chemical kinetics based on our understanding of the low-temperature chemistry of DME.     

Laser-induced plasma in methane and dimethyl ether for flame ignition and combustion diagnostics

In this paper we report the investigation of the laser-induced breakdown and ignition behaviour of methane/air and dimethyl ether (DME)/air mixtures. Moreover, the optical emission from the induced plasma is utilized for determining the mixture composition quantitatively by means of laser-induced breakdown spectroscopy (LIBS). To the best of the authors’ knowledge, LIBS and laser ignition of DME have not been reported in literature before. The technique under investigation is finally employed for combustion diagnostics in laminar as well as turbulent flames. In the laminar premixed and non-premixed flames the LIBS spectra allow spatially resolved measurements of the equivalence ratio and enable studying the mixing of gases provided through the burner with the surrounding room air. In addition, the breakdown threshold of the applied laser pulse energy yields an estimate for the local temperature. In the turbulent cases single-shot LIBS spectra are recorded at fixed position allowing the derivation of local statistical fluctuations of the equivalence ratio in partially premixed jet flames. The results show that laser-induced breakdowns have a strong potential for flame diagnostics and, under suitable conditions, for the ignition of combustible mixtures.