Showing papers in "Combustion and Flame in 2018"

An experimental and chemical kinetic modeling study of 1,3-butadiene combustion: Ignition delay time and laminar flame speed measurements

Abstract: Ignition delay times for 1,3-butadiene oxidation were measured in five different shock tubes and in a rapid compression machine (RCM) at thermodynamic conditions relevant to practical combustors. The ignition delay times were measured at equivalence ratios of 0.5, 1.0, and 2.0 in ‘air’ at pressures of 10, 20 and 40 atm in both the shock tubes and in the RCM. Additional measurements were made at equivalence ratios of 0.3, 0.5, 1.0 and 2.0 in argon, at pressures of 1, 2 and 4 atm in a number of different shock tubes. Laminar flame speeds were measured at unburnt temperatures of 295 K, 359 K and 399 K at atmospheric pressure in the equivalence ratio range of 0.6–1.7, and at a pressure of 5 atm at equivalence ratios in the range 0.6–1.4. These experimental data were then used as validation targets for a newly developed detailed chemical kinetic mechanism for 1,3-butadiene oxidation. A detailed chemical kinetic mechanism (AramcoMech 3.0) has been developed to describe the combustion of 1,3-butadiene and is validated by a comparison of simulation results to the new experimental measurements. Important reaction classes highlighted via sensitivity analyses at different temperatures include: (a) ȮH radical addition to the double bonds on 1,3-butadiene and their subsequent reactions. The branching ratio for addition to the terminal and central double bonds is important in determining the reactivity at low-temperatures. The alcohol-alkene radical adducts that are subsequently formed can either react with HȮ2 radicals in the case of the resonantly stabilized radicals or O2 for other radicals. (b) HȮ2 radical addition to the double bonds in 1,3-butadiene and their subsequent reactions. This reaction class is very important in determining the fuel reactivity at low and intermediate temperatures (600–900 K). Four possible addition reactions have been considered. (c) 3O atom addition to the double bonds in 1,3-butadiene is very important in determining fuel reactivity at intermediate to high temperatures (> 800 K). In this reaction class, the formation of two stable molecules, namely CH2O + allene, inhibits reactivity whereas the formation of two radicals, namely Ċ2H3 and ĊH2CHO, promotes reactivity. (d) Ḣ atom addition to the double bonds in 1,3-butadiene is very important in the prediction of laminar flame speeds. The formation of ethylene and a vinyl radical promotes reactivity and it is competitive with H-atom abstraction by Ḣ atoms from 1,3-butadiene to form the resonantly stabilized Ċ4H5-i radical and H2 which inhibits reactivity. Ab initio chemical kinetics calculations were carried out to determine the thermochemistry properties and rate constants for some of the important species and reactions involved in the model development. The present model is a decent first model that captures most of the high-temperature IDTs and flame speeds quite well, but there is room for considerable improvement especially for the lower temperature chemistry before a robust model is developed.

Experimental and numerical study of the laminar burning velocity of CH4–NH3–air premixed flames

Abstract: With the renewed interest in ammonia as a carbon-neutral fuel, mixtures of ammonia and methane are also being considered as fuel. In order to develop gas turbine combustors for the fuels, development of reaction mechanisms that accurately model the burning velocity and emissions from the flames is important. In this study, the laminar burning velocity of premixed methane–ammonia–air mixtures were studied experimentally and numerically over a wide range of equivalence ratios and ammonia concentrations. Ammonia concentration in the fuel, expressed in terms of the heat fraction of NH3 in the fuel, was varied from 0 to 0.3 while the equivalence ratio was varied from 0.8 to 1.3. The experiments were conducted using a constant volume chamber, at 298 K and 0.10 MPa. The burning velocity decreased with an increase in ammonia concentration. The numerical results showed that the kinetic mechanism by Tian et al. largely underestimates the unstretched laminar burning velocity owing mainly to the dominance of HCO (+H, OH, O2) = CO (+H2, H2O, HO2) over HCO = CO + H in the conversion of HCO to CO. GRI Mech 3.0 predicts the burning velocity of the mixture closely however some reactions relevant to the burning velocity and NO reduction in methane–ammonia flames are missing in the mechanism. A detailed reaction mechanism was developed based on GRI Mech 3.0 and the mechanism by Tian et al. and validated with the experimental results. The temperature and species profiles computed with the present model agree with that of GRI Mech 3.0 for methane–air flames. On the other hand, the NO profile computed with the present model agrees with Tian et al.’s mechanism for methane–ammonia flames with high ammonia concentration. Furthermore, the burned gas Markstein length was measured and was found to increase with equivalence ratio and ammonia concentration.

A physics-based approach to modeling real-fuel combustion chemistry - I. Evidence from experiments, and thermodynamic, chemical kinetic and statistical considerations

Abstract: Real distillate fuels usually contain thousands of hydrocarbon components. Over a wide range of combustion conditions, large hydrocarbon molecules undergo thermal decomposition to form a small set of low molecular weight fragments. In the case of conventional petroleum-derived fuels, the composition variation of the decomposition products is washed out due to the principle of large component number in real, multicomponent fuels. From a joint consideration of elemental conservation, thermodynamics and chemical kinetics, it is shown that the composition of the thermal decomposition products is a weak function of the thermodynamic condition, the fuel-oxidizer ratio and the fuel composition within the range of temperatures of relevance to flames and high temperature ignition. Based on these findings, we explore a hybrid chemistry (HyChem) approach to modeling the high-temperature oxidation of real, distillate fuels. In this approach, the kinetics of thermal and oxidative pyrolysis of the fuel is modeled using lumped kinetic parameters derived from experiments, while the oxidation of the pyrolysis fragments is described by a detailed reaction model. Sample model results are provided to support the HyChem approach.

A physics-based approach to modeling real-fuel combustion chemistry – II. Reaction kinetic models of jet and rocket fuels

Abstract: We propose and test an alternative approach to modeling high-temperature combustion chemistry of multicomponent real fuels. The hy brid chem istry (HyChem) approach decouples fuel pyrolysis from the oxidation of fuel pyrolysis products. The pyrolysis (or oxidative pyrolysis) process is modeled by seven lumped reaction steps in which the stoichiometric and reaction rate coefficients are derived from experiments. The oxidation process is described by detailed chemistry of foundational hydrocarbon fuels. We present results obtained for three conventional jet fuels and two rocket fuels as examples. Modeling results demonstrate that HyChem models are capable of predicting a wide range of combustion properties, including ignition delay times, laminar flame speeds, and non-premixed flame extinction strain rates of all five fuels. Sensitivity analysis shows that for conventional, petroleum-derived real fuels, the uncertainties in the experimental measurements of C2H4 and CH4 impact model predictions to an extent, but the largest influence of the model predictability stems from the uncertainties of the foundational fuel chemistry model used (USC Mech II). In addition, we introduce an approach in the realm of the HyChem approach to address the need to predict the negative-temperature coefficient (NTC) behaviors of jet fuels, in which the CH2O speciation history is proposed to be a viable NTC-activity marker for model development. Finally, the paper shows that the HyChem model can be reduced to about 30 species in size to enable turbulent combustion modeling of real fuels with a testable chemistry model.

Premixed flames subjected to extreme levels of turbulence part I: Flame structure and a new measured regime diagram

Abstract: This paper presents high-fidelity flame structure measurements of premixed methane–air Bunsen flames subjected to extreme levels of turbulence. Specifically, 28 cases were studied with longitudinal integral length scales (Lx) as large as 43 mm, turbulence levels (u′/SL) as high as 246, and turbulent Reynolds (ReT,0) and Karlovitz (KaT) numbers up to 99,000 and 533, respectively. Two techniques were employed to measure the preheat and reaction layer thicknesses of these flames. One consisted of planar laser-induced fluorescence (PLIF) imaging of CH radicals, while the other involved taking the product of simultaneously acquired PLIF images of formaldehyde (CH2O) and hydroxyl (OH) to produce “overlap-layers.” The average preheat layer thicknesses are found to increase with increasing u′/SL and with axial distance from the burner (x/D). In contrast, average reaction layer (i.e. CH- and overlap-layer) thicknesses did not increase appreciably even as u′/SL increased by a factor of ∼ 60. Furthermore, the reaction layer thicknesses (based on the CH images only) did not increase with increasing x/D. The reaction layers are also observed to remain continuous; that is, local extinction events are rarely observed. Although based on a sequence of combined CH–OH PLIF images acquired at a rate of 10 kHz, it is apparent that when instances of local extinction do occur they are the result of cool gas entrainment. The results presented here, as well as those from 12 prior experimental and 9 numerical investigations, do not agree with the predicted Klimov–Williams boundary on the theoretical Borghi Diagram. Thus, a new Measured Regime Diagram is proposed wherein the Klimov–Williams criterion is replaced by a metric that relates the turbulent diffusivity ( D T = u ′ L x ) to the molecular diffusivity within the preheat layer ( D * = S L δ F , L ). Justification for this replacement is based on physical reasoning and the fact that the line defined by DT/D* ≈ 180 accurately separates cases with thin flamelets from those with broadened preheat yet thin reaction layers (i.e. BP-TR flames). Additionally, the results suggest that the BP-TR regime extends well beyond what was previously theorized since neither broken nor broadened reaction layers were observed under conditions with Karlovitz numbers as high as 533, which is five times higher than the theoretical boundary.

Measuring and predicting sooting tendencies of oxygenates, alkanes, alkenes, cycloalkanes, and aromatics on a unified scale

Abstract: Databases of sooting indices, based on measuring some aspect of sooting behavior in a standardized combustion environment, are useful in providing information on the comparative sooting tendencies of different fuels or pure compounds. However, newer biofuels have varied chemical structures including both aromatic and oxygenated functional groups, which expands the chemical space of relevant compounds. In this work, we propose a unified sooting tendency database for pure compounds, including both regular and oxygenated hydrocarbons, which is based on combining two disparate databases of yield-based sooting tendency measurements in the literature. Unification of the different databases was made possible by leveraging the greater dynamic range of the color ratio pyrometry soot diagnostic. This unified database contains a substantial number of pure compounds ( ≥ 400 total) from multiple categories of hydrocarbons important in modern fuels and establishes the sooting tendencies of aromatic and oxygenated hydrocarbons on the same numeric scale for the first time. Using this unified sooting tendency database, we have developed a predictive model for sooting behavior applicable to a broad range of hydrocarbons and oxygenated hydrocarbons. The model decomposes each compound into single-carbon fragments and assigns a sooting tendency contribution to each fragment based on regression against the unified database. The model’s predictive accuracy (as demonstrated by leave-one-out cross-validation) is comparable to a previously developed, more detailed predictive model. The fitted model provides insight into the effects of chemical structure on soot formation, and cases where its predictions fail reveal the presence of more complicated kinetic sooting mechanisms. This work will therefore enable the rational design of low-sooting fuel blends from a wide range of feedstocks and chemical functionalities.

Laser diagnostics and chemical kinetic analysis of PAHs and soot in co-flow partially premixed flames using diesel surrogate and oxygenated additives of n-butanol and DMF

Abstract: Effects of oxygenated fuels on soot reduction strongly depend on the base fuel. Interesting candidates from oxygenated fuels in this respect include both n-butanol and 2,5-dimethylfuran (DMF), because they have already been used in diesel engines recently. However, information is rather limited on n-butanol and DMF added into a diesel fuel surrogate in fundamental flames to investigate the mechanism of soot reduction. In the current work, both n-butanol and DMF was successively added into diesel surrogate (80% n-heptane and 20% toluene in volume, named as T20) in co-flow partially premixed flames. The effects of different oxygenated structures on polycyclic aromatic hydrocarbons (PAHs) and soot were investigated at the same oxygen weight fractions of 4% and the same volume fractions of 20%. The diagnostics on PAHs, soot volume fractions and soot sizes were conducted by using both laser-induced fluorescence (LIF) and two-color laser-induced incandescence (2C-LII). A combined detailed kinetic model (n-heptane/toluene/butanols/DMF/PAHs) has been obtained in order to clarify the chemical effects of the different oxygenated fuels on PAHs formation. Results show that the reduced toluene content due to the addition of oxygenated fuels is the dominant factor for the reduction of soot, as compared with the base fuel of T20. The oxygenated structure of n-butanol has a higher ability to reduce PAHs and soot as compared with the addition of DMF. This is due to the fact that the consumption of DMF leads to much formation of C5H5 which enhances the formation of PAHs and subsequent soot. However, the formation of PAHs can be inhibited remarkably as blending n-butanol because only small hydrocarbons like C2H2 and C3H3 etc. are formed. The formation rate of A4 is more similar to that of soot in comparison with the smaller ring aromatics. For the size of soot particles, the distribution range is shrunk from 19–70 nm for T20 to 20–40 nm for the addition of oxygenated fuels. As compared to the effects of oxygenated structures, DMF20 presents a little wider distribution on soot sizes than that of B16.8. Some larger soot particles are detected in DMF20 flame but cannot be found in B20 flame.

A physics-based approach to modeling real-fuel combustion chemistry – IV. HyChem modeling of combustion kinetics of a bio-derived jet fuel and its blends with a conventional Jet A

Abstract: A Hybrid Chemistry (HyChem) approach has been recently developed for the modeling of real fuels; it incorporates a basic understanding about the combustion chemistry of multicomponent liquid fuels that overcomes some of the limitations of the conventional surrogate fuel approach. The present work extends this approach to modeling the combustion behaviors of a two-component bio-derived jet fuel (Gevo, designated as C1) and its blending with a conventional, petroleum-derived jet fuel (Jet A, designated as A2). The stringent tests and agreement between the HyChem models and experimental measurements for the combustion chemistry, including ignition delay and laminar flame speed, of C1 highlight the validity as well as potential wider applications of the HyChem concept in studying combustion chemistry of complex liquid hydrocarbon fuels. Another aspect of the present study aims at answering a central question of whether the HyChem models for neat fuels can be simply combined to model the combustion behaviors of fuel blends. The pyrolysis and oxidation of several blends of A2 and C1 were investigated. Flow reactor experiments were carried out at pressure of 1 atm, temperature of 1030 K, with equivalence ratios of 1.0 and 2.0. Shock tube measurements were performed for the blended fuel pyrolysis at 1 atm from 1025 to 1325 K. Ignition delay times were also measured using a shock-tube. Good agreement between measurements and model predictions was found showing that formation of the products as well as combustion properties of the blended fuels were predicted by a simple combination of the HyChem models for the two individual fuels, thus demonstrating that the HyChem models for two jet fuels of very different compositions are “additive.”

Instantaneous 3D flame imaging by background-oriented schlieren tomography

Abstract: We apply background-oriented schlieren (BOS) imaging with computed tomography to reconstruct the instantaneous refractive index field of a turbulent flame in 3D. In BOS tomography, a network of cameras are focused through a variable index medium (such as a flame) onto a background of patterned images. BOS data consist of pixel-wise “deflections” between a reference and distorted image, caused by variations in the refractive index along the path between the camera and background. Multiple simultaneous BOS images, each from a unique perspective, are combined with a tomography algorithm to reconstruct the refractive index distribution (or optical density) in the probe volume. This quantity identifies the edges of the wrinkled turbulent flame surface. This is the first application of BOS imaging to flame tomography, setting the stage for low-cost 3D flame thermometry. Tomography is carried out within the Bayesian framework, using Tikhonov and total variation (TV) priors. The TV prior is more compatible with the abrupt spatial variation in the refractive index field caused by the flame front. We demonstrate the suitability of TV regularization using a proof-of-concept simulation of BOS tomography on an LES phantom. The technique was then used to reconstruct the instantaneous 3D refractive index field of an unsteady natural gas/air flame from a Bunsen burner using a 23-camera setup. Our results show how BOS tomography can capture and visualize 3D features of a flame and provide benchmark data for simulations.

Large-eddy simulation on the influence of injection pressure in reacting Spray A

Abstract: The Engine Combustion Network (ECN) Spray A target case corresponds to high-pressure liquid fuel injection in conditions relevant to diesel engines. Following the procedure by Wehrfritz et al. (2016), we utilize large-eddy simulation (LES) and flamelet generated manifold (FGM) methods to carry out an injection pressure sensitivity study for Spray A at 50, 100 and 150 MPa. Comparison with experiments is shown for both non-reacting and reacting conditions. Validation results in non-reacting conditions indicate relatively good agreement between the present LES and experimental data, with some deviation in mixture fraction radial profiles. In reacting conditions, the simulated flame lift-off length (FLOL) increases with injection pressure, deviating from the experiments by 4–14%. Respectively, the ignition delay time (IDT) decreases with increasing injection pressure and it is underpredicted in the simulations by 10–20%. Analysis of the underlying chemistry manifold implies that the observed discrepancies can be explained by the differences between experimental and computational mixing processes.

Detailed, sterically-resolved modeling of soot oxidation: Role of O atoms, interplay with particle nanostructure, and emergence of inner particle burning

Abstract: A newly-developed detailed mechanism of soot oxidation was tested against experimental observations. The computations were performed at an atomistic level and with a detailed consideration of soot particle surface sites. Several additional reactions were investigated theoretically and one of them, oxidation of embedded five-member rings by O atoms, was included in the model. The primary focus of the study was on the high-temperature shock-tube experiments of Roth et al. (1991). The reaction model was able to reproduce the experimental results, but required coupling to particle nanostructure: partial oxidation of PAH molecules and the decrease in PAH initial sizes along the oxidation path. The principle reaction mechanism was identified to be the formation of oxyradicals, their decomposition, formation of hard-to-oxidize embedded five-member rings, and oxidation of the latter predominantly by O atoms. The analysis identified O as the most effective oxidizer of the embedded five-member rings, which thus controls the rate of the overall oxidation. The model thus predicts fast oxidation during a brief initial period followed by a slow-oxidation one. The model of partial oxidation of an aromatic molecule and switching to the next intact molecule suggests pore formation and subsequent inner particle burning. We also investigated the ability of the present model to reproduce recent measurements of soot oxidation rates performed by Camacho et al. (2015) at about 1000 K. The initial reaction model failed to predict these results, and no adjustment could reconcile the differences. The only way to bring the model to experimental values was by assuming a catalytic decomposition of water on the reactor wall supplying additional radicals, H and OH, to the reacting gas mixture. Additional chemistry, oxidation through complex formation at neighboring surface sites, was required to fully reproduce the experimental observations. These additional reactions were found to play no role in the high-temperature simulations, nor were they sufficient to reproduce the low-temperature experiment on their own, without the assumed catalytic decomposition of water.

Effects of inflow Mach number on oblique detonation initiation with a two-step induction-reaction kinetic model

Abstract: Oblique detonations induced by two-dimensional, semi-infinite wedges are simulated by solving numerically the reactive Euler equations with a two-step induction-reaction kinetic model. Previous results obtained with other models have demonstrated that for the low inflow Mach number M0 regime past a critical value, the wave in the shocked gas changes from an oblique reactive wave front into a secondary oblique detonation wave (ODW). The present numerical results not only confirm the existence of such critical phenomenon, but also indicate that the structural shift is induced by the variation of the main ODW front which becomes sensitive to M0 near a critical value. Below the critical M0,cr, oscillations of the initiation structure are observed and become severe with further decrease of M0. For low M0 cases, the non-decaying oscillation of the initiation structure exists after a sufficiently long-time computation, suggesting the quasi-steady balance of initiation wave systems. By varying the heat release rate controlled by kR, the pre-exponential factor of the second reaction step, the morphology of initiation structures does not vary for M0 = 10 cases but varies for M0 = 9 cases, demonstrating that the effects of heat release rate become more prominent when M0 decreases. The instability parameter χ is introduced to quantify the numerical results. Although χ cannot reveal the detailed mechanism of the structural shift, a linear relation between χ and kR exists at the critical condition, providing an empirical criterion to predict the structural variation of the initiation structure.

The structure and propagation of laminar flames under autoignitive conditions

Abstract: The laminar flame speed sl is an important reference quantity for characterising and modelling combustion. Experimental measurements of laminar flame speed require the residence time of the fuel/air mixture (τf) to be shorter than the autoignition delay time (τ). This presents a considerable challenge for conditions where autoignition occurs rapidly, such as in compression ignition engines. As a result, experimental measurements in typical compression ignition engine conditions do not exist. Simulations of freely propagating premixed flames, where the burning velocity is found as an eigenvalue of the solution, are also not well posed in such conditions, since the mixture ahead of the flame can autoignite, leading to the so called “cold boundary problem”. Here, a numerical method for estimating a reference flame speed, sR, is proposed that is valid for laminar flame propagation at autoignitive conditions. Two isomer fuels are considered to test this method: ethanol, which in the considered conditions is a single-stage ignition fuel; and dimethyl ether, which has a temperature-dependent single- or two-stage ignition and a negative temperature coefficient regime for τ. Calculations are performed for the flame position in a one-dimensional computational domain with inflow-outflow boundary conditions, as a function of the inlet velocity UI and for stoichiometric fuel–air premixtures. The response of the flame position, LF, to UI shows distinct stabilisation regimes. For single-stage ignition fuels, at low UI the flame speed exceeds UI and the flame becomes attached to the inlet. Above a critical UI value, the flame detaches from the inlet and Lf becomes extremely sensitive to UI until, for sufficiently high UI, the sensitivity decreases and Lf corresponds to the location expected from a purely autoignition stabilised flame. The transition from the attached to the autoignition regimes has a corresponding peak dLf/dUI value which is proposed to be a unique reference flame speed sR for single-stage ignition fuels. For two-stage ignition fuels, there is an additional stable regime where a high-temperature flame propagates into a pool of combustion intermediates generated by the first stage of autoignition. This results in two peaks in dLf/dUI and therefore two reference flame speed values. The lower value corresponds to the definition of sR for single-stage ignition fuels, while the higher value exists only for two-stage ignition fuels and corresponds to a high temperature flame propagating into the first stage of autoignition and is denoted s R ′ . A transport budget analysis for low- and high-temperature radical species is also performed, which confirms that the flame structures at U I = s R and U I = s R ′ do indeed correspond to premixed flames (deflagrations), as opposed to spontaneous ignition fronts which do not have a unique propagation speed.

Predicting heat fluxes received by horizontal targets from two buoyant turbulent diffusion flames of propane burning in still air

Abstract: An analytical method to estimate the heat fluxes received by horizontal targets from two buoyant turbulent diffusion flames was proposed. Two identical square gas burners with side length of 15 cm were used as the fire sources with propane burning in still air. The heat release rate (HRR) and burner edge spacing were changed in experiments. Heat fluxes received by four external horizontal targets were measured. In cases with one burner or two burners with zero spacing, there is one single flame and the vertical centerline temperature distribution were divided into four zones including core zone, constant zone, intermittent flame zone and plume zone. Based on the established piecewise function for predicting the flame temperature, the cuboid flame model with hierarchical temperatures was proposed to determine the flame emissivity, the mean flame temperature of the model and the corresponding blackbody emissive power. The results showed that when the HRR of single flame ranges from 10.8 kW to 64.8 kW, the flame emissivity of 15 cm square burner ranges from 0.125 to 0.387, and the absorption coefficient ranges from 0.99 m−1 to 3.62 m−1. At a given HRR, the flame emissivities of two burners with zero spacing and infinite spacing are nearly identical, suggesting that the influence of spacing on flame emissivity is marginal. For two burners with spacing higher than zero, by assuming that the flame radiative fraction of propane is 0.3 and the flame emissivity is equal to that of the single flame, the flame radiative heat fluxes received by external targets are calculated by modeling the flame shapes as two right or tilted cuboids. The comparison validates that the calculations using the proposed cuboid models for both one and two flames agree well with the experimental results.

Experimental and modeling study of the pyrolysis and combustion of dimethoxymethane

Abstract: The pyrolysis and low-to intermediate temperature oxidation chemistry of dimethoxymethane (DMM), the simplest oxymethylene ether, is studied theoretically and experimentally in a JSR setup. The potential energy surfaces for peroxy species relevant during the low-temperature oxidation of dimethoxymethane are studied at the CBS-QB3 level of theory and the results are used to calculate thermodynamic properties of the main species as well as rate expressions for important reactions. An elementary step model for DMM pyrolysis and oxidation is built with the automatic kinetic model generation software Genesys. To describe the chemistry of small species not directly related to DMM, the AramcoMech 1.3 mechanism developed by Metcalfe et al. is used. If the more recently extended version of this mechanism, i.e. the propene oxidation mechanism published by Burke et al., was used as alternative base mechanism, large discrepancies for the mole fractions of CO2, methyl formate and methanol during the pyrolysis of DMM were observed. The validation of the new DMM model is carried out with new experimental data that is acquired in an isothermal quartz jet-stirred reactor at low and intermediate temperatures. Different equivalence ratios, = 0.25, = 1.0, = 2.0 and = ∞, are studied in a temperature range from 500 K to 1100 K, at a pressure of 1.07 bar and with an inlet DMM mole fraction of 0.01. The experimental trends are well predicted by the model without any tuning of the model parameters although some improvements are possible to increase quantitative agreement. The largest discrepancies are observed at fuel lean conditions for the hydrocarbon mole fractions, and at low-temperatures as can be noticed by the over prediction of formaldehyde and methyl formate. The kinetic model is also validated against plug flow reactor, jet-stirred reactor and lean and rich premixed flames data from the literature. Rate of production analyses are performed to identify important pathways for low-and intermediate-temperature oxidation and pyrolysis.

New insights into the shock tube ignition of H2/O2 at low to moderate temperatures using high-speed end-wall imaging

Abstract: In this work, the effects of pre-ignition energy releases on H 2 O 2 mixtures were explored in a shock tube with the aid of high-speed imaging and conventional pressure and emission diagnostics. Ignition delay times and time-resolved camera image sequences were taken behind the reflected shockwaves for two hydrogen mixtures. High concentration experiments spanned temperatures between 858 and 1035 K and pressures between 2.74 and 3.91 atm for a 15% H 2 \18% O 2 \Ar mixture. Low concentration data were also taken at temperatures between 960 and 1131 K and pressures between 3.09 and 5.44 atm for a 4% H 2 \2% O 2 \Ar mixture. These two model mixtures were chosen as they were the focus of recent shock tube work conducted in the literature (Pang et al., 2009). Experiments were performed in both a clean and dirty shock tube facility; however, no deviations in ignition delay times between the two types of tests were apparent. The high-concentration mixture (15%H 2 \18%O 2 \Ar) experienced energy releases in the form of deflagration flames followed by local detonations at temperatures

Mechanism of cellulose fast pyrolysis: The role of characteristic chain ends and dehydrated units

Abstract: Understanding the fundamental reactions and mechanisms during biomass fast pyrolysis is essential for the development of efficient pyrolysis techniques. In this work, quantum chemistry calculation, kinetic analysis and fast pyrolysis experiment were combined to reveal the cellulose pyrolysis mechanism. During cellulose pyrolysis, the indigenous interior units, reducing end (RE end) and non-reducing end (NR end) initially form various characteristic chain ends and dehydrated units which then evolve into different pyrolytic products. As the rising of the degree of polymerization (DP), reactions occurring at the interior unit and NR end are more competitive than those taking place at the RE end, resulting in distinct pyrolytic product distribution for cellulose and glucose-based carbohydrates. The reactions occurring at the three indigenous units of cellulose chain all favor the formation of levoglucosan-terminated end (LG end) and/or NR end, which then generate levoglucosan (LG). The acyclic d -glucose end (AG end), which mainly derives from the RE end, is essential for the formation of 1,6-anhydro-β- d -glucofuranose (AGF), 1,4:3,6-dianhydro-α- d -glucopyranose (DGP), furfural (FF), 5-hydroxymethyl furfural (5-HMF) and hydroxyacetaldehyde (HAA). Compared with the chain ends, the dehydrated units are not feasible to be generated, and their decomposition favors the formation of HAA.

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

Abstract: 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.

Combustion of boron and boron–iron composite particles in different oxidizers

Abstract: Mechanical milling was used to prepare a composite powder containing 5 wt% of iron in boron. Iron, expected to behave as a catalyst of boron oxidation, was present in the form of nano-sized particles on the agglomerates of primary boron particles. Powders of both the prepared material and as-received boron were burned in different oxidizing environments. The powders were injected in the combustion products of air–acetylene and air–hydrogen flames to expose them respectively to a mixture of CO2, CO and steam as oxidizers. In addition, the powders were fed through a laser beam to be ignited and burned in air. Particle size distributions were obtained for the powders passed through the feeder. Time-resolved optical emission of particles burning in all environments was recorded using photomultipliers filtered at 700 and 800 nm. Additionally, temporally and spectrally resolved emission traces were obtained using a 32-channel spectrometer recording emission in the range of wavelengths of 373–641 nm. The durations of the recorded emission pulses were interpreted as burn times. Statistical distributions of particle burn times and sizes were correlated with each other to obtain the effect of particle size on burn time. Flame temperatures were measured assuming the radiation source behaving as a gray body. Burning boron particles commonly produced double-peak emission pulses, whereas single-peak pulses were produced by the composite boron–iron particles. Both peaks observed in boron particle combustion were assigned to the full-fledged high-temperature reaction unlike the assignment suggested in the earlier research linking the first emission peak with the removal of the boron oxide layer. The emission intensity for the boron–iron composite particles was weaker than that for boron, which is explained by the effect of iron favoring condensed oxidation and suppressing formation of the vapor-phase combustion products, such as boron suboxides. In air, the burn times of boron–iron composite particles were substantially shorter than those of boron. In air–hydrogen flame, boron–iron composites burned slightly faster than boron, while in the air–acetylene flame, there was no clear difference in the burn times for the two materials. The flame temperatures were very similar for the two materials in all oxidizers.

n-Heptane cool flame chemistry: Unraveling intermediate species measured in a stirred reactor and motored engine

Abstract: This work identifies classes of cool flame intermediates from n-heptane low-temperature oxidation in a jet-stirred reactor (JSR) and a motored cooperative fuel research (CFR) engine. The sampled species from the JSR oxidation of a mixture of n-heptane/O2/Ar (0.01/0.11/0.88) were analyzed using a synchrotron vacuum ultraviolet radiation photoionization (SVUV-PI) time-of-flight molecular-beam mass spectrometer (MBMS) and an atmospheric pressure chemical ionization (APCI) Orbitrap mass spectrometer (OTMS). The OTMS was also used to analyze the sampled species from a CFR engine exhaust. Approximately 70 intermediates were detected by the SVUV-PI-MBMS, and their assigned molecular formulae are in good agreement with those detected by the APCI-OTMS, which has ultra-high mass resolving power and provides an accurate elemental C/H/O composition of the intermediate species. Furthermore, the results show that the species formed during the partial oxidation of n-heptane in the CFR engine are very similar to those produced in an ideal reactor, i.e., a JSR. The products can be classified by species with molecular formulae of C7H14Ox (x = 0–5), C7H12Ox (x = 0–4), C7H10Ox (x = 0–4), CnH2n (n = 2–6), CnH2n−2 (n = 4–6), CnH2n+2O (n = 1–4), CnH2nO (n = 1–6), CnH2n−2O (n = 2–6), CnH2n−4O (n = 4–6), CnH2n+2O2 (n = 0–4, 7), CnH2nO2 (n = 1–6), CnH2n−2O2 (n = 2–6), CnH2n−4O2 (n = 4–6), and CnH2nO3 (n = 3–6). The identified intermediate species include alkenes, dienes, aldehyde/keto compounds, olefinic aldehyde/keto compounds, diones, cyclic ethers, peroxides, acids, and alcohols/ethers. Reaction pathways forming these intermediates are proposed and discussed herein. These experimental results are important in the development of more accurate kinetic models for n-heptane and longer-chain alkanes.

Detailed numerical simulation of transient mixing and combustion of premixed methane/air mixtures in a pre-chamber/main-chamber system relevant to internal combustion engines

Abstract: Transient mixing and ignition mechanisms in a simplified pre-chamber/main-chamber system are investigated using direct numerical simulation (DNS) with detailed chemical kinetics. Full ignition and flame propagation processes in the premixed methane/air mixtures are simulated. Ignition, the progress and topology of flame evolution, and the mean burning velocity in the main chamber are analyzed in detail. Four important phases in the ignition and flame propagation processes are identified based on the flame structure development in the main chamber, the pressure and velocity evolution at typical points in both the pre-chamber and main chamber. Results show that the intermediate species OH, CH2O, and HO2 are critical for flame stabilization and propagation in the main chamber due to their high reactivity. This is sorted as the chemical effect that the pre-chamber jet acts on the main chamber. The high temperature jet also brings heat and unburned fuel into the main chamber, which are sorted as thermal and enrichment effect, respectively. The heat release rate is found to be approximately proportional to the product of CH2O and OH mass fractions, which could be regarded as a reliable and effective marker for the heat release rate of methane/air mixtures. It is found that the mean burning velocity in the main chamber can be elevated up to 30 times under the condition investigated.

A minimalist functional group (MFG) approach for surrogate fuel formulation

Abstract: Surrogate fuel formulation has drawn significant interest due to its relevance towards understanding combustion properties of complex fuel mixtures. In this work, we present a novel approach for surrogate fuel formulation by matching target fuel functional groups, while minimizing the number of surrogate species. Five key functional groups; paraffinic CH3, paraffinic CH2, paraffinic CH, naphthenic CH CH2 and aromatic C CH groups in addition to structural information provided by the Branching Index (BI) were chosen as matching targets. Surrogates were developed for six FACE (Fuels for Advanced Combustion Engines) gasoline target fuels, namely FACE A, C, F, G, I and J. The five functional groups present in the fuels were qualitatively and quantitatively identified using high resolution 1H Nuclear Magnetic Resonance (NMR) spectroscopy. A further constraint was imposed in limiting the number of surrogate components to a maximum of two. This simplifies the process of surrogate formulation, facilitates surrogate testing, and significantly reduces the size and time involved in developing chemical kinetic models by reducing the number of thermochemical and kinetic parameters requiring estimation. Fewer species also reduces the computational expenses involved in simulating combustion in practical devices. The proposed surrogate formulation methodology is denoted as the Minimalist Functional Group (MFG) approach. The MFG surrogates were experimentally tested against their target fuels using Ignition Delay Times (IDT) measured in an Ignition Quality Tester (IQT), as specified by the standard ASTM D6890 methodology, and in a Rapid Compression Machine (RCM). Threshold Sooting Index (TSI) and Smoke Point (SP) measurements were also performed to determine the sooting propensities of the surrogates and target fuels. The results showed that MFG surrogates were able to reproduce the aforementioned combustion properties of the target FACE gasolines across a wide range of conditions. The present MFG approach supports existing literature demonstrating that key functional groups are responsible for the occurrence of complex combustion properties. The functional group approach offers a method of understanding the combustion properties of complex mixtures in a manner which is independent, yet complementary, to detailed chemical kinetic models. The MFG approach may be readily extended to formulate surrogates for other complex fuels.

Coupling an LES approach and a soot sectional model for the study of sooting turbulent non-premixed flames

Abstract: Due to their negative impacts on environment and human health, future regulations on soot emissions are expected to become stricter, in particular by controlling the size of the emitted particles. Therefore, the development of precise and sophisticated models describing the soot production, such as sectional methods, is an urgent scientific and industrial challenge. In this context, the first objective of this work is to use for the first time a sectional model to perform an LES of a sooting turbulent flames in order to demonstrate its capacities. For this, the whole LES formalism for this approach is developed. It includes state-of-art models for the description of the gaseous phase and an extension of a soot subgrid intermittency model to the sectional approach, originally proposed for the hybrid method of moments. Then, the LES is used to analyze a turbulent non-premixed ethylene–air jet diffusion flame and results are validated by available experimental data. The quality of results for the gaseous phase is satisfactory and results for the solid phase show a reasonable agreement with the experimental results in terms of localization, intermittency and soot volume fraction magnitude. Once the coupled LES-sectional approach validated, having access to the full information on the spatial and temporal evolution of the soot Particle Size Distribution (PSD), the second objective of this work is to provide a new fundamental insight on soot production in turbulent non-premixed flames. First, it is observed that a one-peak and a two-peak PSD shapes are observed at the bottom and downstream of the flame, respectively. Second, high fluctuations of the PSD distribution is observed all along the flame. In particular, a time bimodal behavior is observed with the presence of a zone with regular transitions between one- and two-peak PSD shapes. By analyzing soot particles Lagrangian paths, these high fluctuations are shown to be linked with the wide range of history paths of soot particles, which are mainly driven by turbulence.

DNS of MILD combustion with mixture fraction variations

Abstract: Direct numerical simulations of Moderate or Intense Low-oxygen Dilution combustion inside a cubical domain are performed. The computational domain is specified with inflow and outflow boundary conditions in one direction and periodic conditions in the other two directions. The inflowing mixture is constructed carefully in a preprocessing step and has spatially varying mixture fraction and reaction progress variable fields. Thus, this mixture includes a range of thermo-chemical states for a given mixture fraction value. The combustion kinetics is modelled using a 58-step skeletal mechanism including a chemiluminescent species, OH*, for methane–air combustion. The study of reaction zone structures in the physical and mixture fraction spaces shows the presence of ignition fronts, lean and rich premixed flames and non-premixed combustion. These three modes of combustion are observed without the typical triple-flame structure and this results from the spatio-temporally varying mixture fraction field undergoing turbulent mixing and reaction. The flame index and its pdf are analysed to estimate the fractional contributions from these combustion modes to the total heat release rate. The lean premixed mode is observed to be quite dominant and contribution of non-premixed mode increased from about 11% to 20% when the mean oxygen mole fraction in the inflowing mixture is reduced from about 2.7% to 1.6%. Also, the non-premixed contribution increases if one decreases the integral length scale of the mixture fraction field. All of these results and observations are explained on physical basis.

A Physics-based approach to modeling real-fuel combustion chemistry – III. Reaction kinetic model of JP10

Abstract: The Hybrid Chemistry (HyChem) approach has been proposed previously for combustion chemistry modeling of real, liquid fuels of a distillate origin In this work, the applicability of the HyChem approach is tested for single-component fuels using JP10 as the model fuel The method remains the same: an experimentally constrained, lumped single-fuel model describing the kinetics of fuel pyrolysis is combined with a detailed foundational fuel chemistry model Due to the multi-ring molecular structure of JP10, the pyrolysis products were found to be somewhat different from those of conventional jet fuels The lumped reactions were therefore modified to accommodate the fuel-specific pyrolysis products The resulting model shows generally good agreement with experimental data, which suggests that the HyChem approach is also applicable for developing combustion reaction kinetic models for single-component fuels

Principal component analysis coupled with nonlinear regression for chemistry reduction

Abstract: Large kinetic mechanisms are required in order to accurately model combustion systems If no parameterization of the thermo-chemical state-space is used, solution of the species transport equations can become computationally prohibitive as the resulting system involves a wide range of time and length scales Parameterization of the thermo-chemical state-space with an a priori prescription of the dimension of the underlying manifold would lead to a reduced yet accurate description To this end, the potential offered by Principal Component Analysis (PCA) in identifying low-dimensional manifolds is very appealing The present work seeks to advance the understanding and application of the PC-transport approach by analyzing the ability to parameterize the thermo-chemical state with the PCA basis using nonlinear regression In order to demonstrate the accuracy of the method within a numerical solver, unsteady perfectly stirred reactor (PSR) calculations are shown using the PC-transport approach The PSR analysis extends previous investigations to more complex fuels (methane and propane), showing the ability of the approach to deal with relatively large kinetic mechanisms The ability to achieve highly accurate mapping through Gaussian Process based nonlinear regression is also shown In addition, a novel method based on local regression of the PC source terms is also investigated which leads to improved results

Probing the low-temperature chemistry of ethanol via the addition of dimethyl ether

Abstract: The work at Xi'an Jiaotong University was supported by the National Natural Science Foundation of China (no. 91541115) and the Fundamental Research Funds for the Central Universities. The work at DRIVE laboratory was supported by French National Research Agency under the ANR project SHOCK (ANR-13-JS09-0013-01) and the Council of Burgundy (Project STM3D).

Effect of crystallinity on droplet regression and disruptive burning characteristics of nanofuel droplets containing amorphous and crystalline boron nanoparticles

Abstract: The present investigation deals with the droplet combustion characteristics of nanofuel droplets containing amorphous and crystalline boron nanoparticles at various particle loadings (0.25%, 1%, 2.5%, 5%, 7.5%, and 10% by weight). Characterization of pre-burnt particles in terms of particle size, morphology, and elemental boron content have been carried out using standard material characterization techniques such as SEM, TEM, XRD and TGA. The droplet burning process has been recorded using a high-speed imaging system. The diameter regression profiles show distinctly different characteristics for amorphous and crystalline particles loaded droplets. Amorphous particles loaded droplets show comparatively smooth regression with minor puffing coupled with shape oscillations at the early stage and micro-explosions at the later stage whereas the crystalline particles loaded droplets show sudden ejections and high-intensity micro-explosions. The morphology of the particle (crystallinity) is considered to be responsible for this difference in burning behaviour. A porous, permeable agglomerate shell forms in case of amorphous boron loaded droplet whereas a densely packed, impermeable agglomerate shell forms in case of crystalline boron loaded droplets during the early stage of burning. The micrographs of post-burning residues indeed reveal that blow holes are present in the agglomerate even at individual particle level for amorphous boron loaded case whereas there are no such blow holes present in crystalline boron loaded case. Thermograms, true colour images of flame and emission spectra show that the amorphous boron particles burn better than their crystalline counterpart.

Effects of free stream flow turbulence on blowoff characteristics of bluff-body stabilized premixed flames

Abstract: In this article we report on an experimental investigation of a bluff-body stabilized lean premixed flame subjected to different levels of free stream turbulence intensities (4, 14, 24 and 30%) at conditions approaching blowoff. The mean flow velocities ranged from 5 to 15 m/s. The turbulence Reynolds number based on integral length scale and rms velocity ranged from 44 to 4280. Simultaneous imaging of hydroxyl (OH) and formaldehyde (CH2O) by planar laser induced fluorescence and particle image velocimetry (PIV) was used to study the interaction between the flame and the flow field and determine the sequence of events leading to flame blowoff. CH2O fluorescence and the pixel-by-pixel multiplication of OH and CH2O fluorescence signals were utilized to mark the preheat and heat release regions of flame front respectively. The flame structure was observed to be strongly modified by the turbulent flow field which affects the lean blowoff limits. The flame blowoff equivalence ratio increased with increasing free stream turbulence levels owing to strong interactions of the turbulent flow with the flame and the resulting modification of flame surfaces and ensuing local flame extinction. For stably burning flames, the flame front predominantly enveloped the shear layer vortices for all the turbulent conditions. As blowoff was approached, the flame front and shear layer vortices entangled inducing high local strain rates on the flame front that exceed the extinction strain resulting in significant breaks along the reaction zone. At conditions near blowoff, wide regions of CH2O and heat release were observed inside the recirculation zone. Velocity vectors near the flame holes indicate the penetration of the reactants into the recirculation zone. Several properties were measured to characterize the near blowoff flames which include the strain rate and curvature statistics along the flame front, burning fraction, asymmetric index and the average duration of the blowoff event.

Soot particles inception and PAH condensation modelling applied in a soot model utilizing a sectional method

Abstract: This work aims at improving the understanding of soot formation in laminar premixed flames with a strong focus on the interactions between soot and polycyclic aromatic hydrocarbons In this context, a soot model based on a sectional method is presented It includes sub-models for the five main processes involved in soot formation and evolution: particle inception, condensation, coagulation, oxidation and surface growth The two sub-models including novelties are the particle inception and the condensation ones The nucleation sub-model proposed in the present paper is based on a dampening factor Concerning condensation, a model taking into account its reversibility is presented and studied This model is then validated against experimental data Five premixed laminar flames have been selected for that purpose They give insights into the model’s ability to predict soot formation depending on fuel, pressure and equivalence ratio The model predictions show a good agreement with the experimental data concerning soot volume fractions as well as mean particle diameters The influence of the modelling parameters is also studied The reversibility of condensation appears to turn condensation into a soot consuming process which may have a significant impact on PAH profiles in premixed laminar flames