Showing papers in "Combustion and Flame in 2019"

Experimental and kinetic modeling study of laminar burning velocities of NH3/air, NH3/H2/air, NH3/CO/air and NH3/CH4/air premixed flames

Abstract: Ammonia (NH3) is a promising energy carrier to store and transport renewable hydrogen (H2) that can be generated using, e.g., wind and solar energy. Direct combustion of NH3 is one of the possible methods to utilize the energy by the end users. To understand the combustion characteristics of NH3 as a fuel, the laminar burning velocities of NH3/air, NH3/H2/air, NH3/CO/air and NH3/CH4/air premixed flames were investigated experimentally using the heat flux method. Measurements are reported for a wide range of equivalence ratios and blending ratios. Kinetic modeling was also performed using available chemical kinetic mechanisms, namely the GRI-Mech 3.0, the Okafor et al. and the San Diego mechanisms. The experimental results for NH3/air flames agree well with the literature data and it is found that blending NH3 with H2 is the most effective manner to increase the burning velocity of NH3 based fuel mixtures. None of the kinetic mechanisms used can accurately predict most of the measured data. Sensitivity and reaction path analyses indicate that the oxidation of NH3 blended with the additive fuels considered can be understood as the parallel oxidation processes of the individual fuels, and that the source of discrepancy between the experimental and modeling results is related to the inaccuracy of the rate parameters of the N-containing reactions. In this regard, the present detailed and reliable experimental data is of special value for model development and validation.

Measurement and modelling of the laminar burning velocity of methane-ammonia-air flames at high pressures using a reduced reaction mechanism

Abstract: Ammonia and blends of ammonia with methane are gaining increased interest as fuels for gas turbine applications and hence optimized reduced reaction mechanisms for the fuels are required for the development of combustors. However, there is a scarcity of measured data on the laminar burning velocity of the fuels for optimizing and validating reaction mechanisms especially at high pressures. In this study, an extensive set of measurements of the unstretched laminar burning velocity and Markstein lengths of CH4 NH3-air flames at high pressures are reported for the first time and an optimized reduced reaction mechanism is proposed. The experiments were conducted in a constant volume chamber for various ammonia heat fractions in the fuel ranging from 0 to 0.30, equivalence ratios ranging from 0.7 to 1.3, and mixture pressures ranging from 0.10 to 0.50 MPa. The reduced reaction mechanism was developed from a detailed reaction mechanism for CH4 NH3-air flames by Okafor et al., and optimized against the present measurements and data in the literature. It is shown that the reduced mechanism models the unstretched laminar burning velocity of NH3/air and CH4 NH3-air flames with high fidelity at all studied conditions. It also models satisfactorily NH3, NO and CO concentration in CH4 NH3 O2 N2 oxidation in a laminar flow reactor. Furthermore, the reduced mechanism is demonstrated to predict NO, OH, and NH profiles in a premixed stagnation NH3-air flame satisfactorily. The experimental measurements were also used to validate selected detailed reaction mechanisms. It was found that the significant over-prediction of NO production from NH3 oxidation by GRI Mech 3.0 is primarily due to the influence of the reaction NH+H2O HNO+H2, which in fact may not be important in fuel NO chemistry.

Experimental and kinetic modeling investigation on the laminar flame propagation of ammonia under oxygen enrichment and elevated pressure conditions

Abstract: Ammonia is attracting more and more attentions due to its role as both a carbon-free fuel for gas turbines and an effective H2 carrier. Only a limit number of investigations on the laminar flame propagation and laminar burning velocity of ammonia have been performed on elevated pressures, which were focused on ammonia/air mixtures and suffered strong buoyancy effect. In this work, laminar flame propagation of ammonia/O2/N2 mixtures covering wide ranges of equivalence ratios, oxygen contents and initial pressures was investigated in a high-pressure constant-volume cylindrical combustion vessel. The oxygen enrichment speeds up the spherically expanding flames and consequently reduces buoyancy effect on the laminar flame propagation of ammonia. The laminar burning velocity was observed to increase with the increasing oxygen content, but decrease with the increasing initial pressure. A kinetic model of ammonia combustion consisting 38 species and 265 reactions was constructed from previous models with updated rate constants of important reactions. The present model can reasonably reproduce the laminar burning velocity data in this work and literature, as well as the ignition delay time and speciation data in literature. Based on the model analysis, effects of oxygen enrichment, equivalence ratio and initial pressure on laminar burning velocities of ammonia were analyzed in detail. It is revealed that the enhanced flame propagation with oxygen enrichment is mainly due to the increase of adiabatic flame temperature which in turn leads to higher concentrations of key radicals like H, OH and NH2. For NH3 and its major decomposition products like NH2 and NH, reactions with oxygenated species such as OH, O, O2 and NO are generally more important in the lean flames, while the role of reactions with H, NH and NH2 becomes crucial in the rich flames. The calculated pressure dependent coefficient indicates that NH3/O2/N2 flames exhibit clear pressure dependence, while this pressure dependence is weaker than those of the hydrocarbon and biofuel flames.

Training convolutional neural networks to estimate turbulent sub-grid scale reaction rates

Abstract: This work presents a new approach for premixed turbulent combustion modeling based on convolutional neural networks (CNN).1 We first propose a framework to reformulate the problem of subgrid flame surface density estimation as a machine learning task. Data needed to train the CNN is produced by direct numerical simulations (DNS) of a premixed turbulent flame stabilized in a slot-burner configuration. A CNN inspired from a U-Net architecture is designed and trained on the DNS fields to estimate subgrid-scale wrinkling. It is then tested on an unsteady turbulent flame where the mean inlet velocity is increased for a short time and the flame must react to a varying turbulent incoming flow. The CNN is found to efficiently extract the topological nature of the flame and predict subgrid-scale wrinkling, outperforming classical algebraic models.

Auto-ignition kinetics of ammonia and ammonia/hydrogen mixtures at intermediate temperatures and high pressures

Abstract: Auto-ignition properties of NH3/O2 and NH3/H2/O2 mixtures have been studied in a rapid compression machine at pressures from 20 to 60 bar, temperatures from 950 to 1150 K, and equivalence ratios from 0.5 to 2. The effect of the ammonia/hydrogen ratio in the fuel mixture has been also investigated. The experiments demonstrate that a higher H2 mole fraction in the fuel mixture increases its reactivity, while the equivalence ratio shows different influence as follows. When the fuel mixture contains 20% H2, the fuel-richer mixtures have shorter ignition delay times, while for mixtures containing 1% H2 in fuel the equivalence ratio dependence is opposite. With 5% H2 in fuel, the stoichiometric mixture presents the shortest ignition delay time. In mixtures without hydrogen, i.e., pure NH3, leaner mixtures show higher reactivity. In addition, numerical simulations were performed based on the literature mechanisms of Glarborg et al. (2018), Mathieu and Petersen (2015), and Klippenstein et al. (2011). While these models can predict well the ignition delay time of NH3/O2 mixtures, none of the models can predict the behavior of NH3/H2/O2 mixtures satisfactorily. The predictions are most sensitive to the branching reactions NH2 + NO and to the reaction H2NO + O2 = HNO + HO2. Hydrogen addition enriches the O/H radical pool consuming NH3 and NH2, but it has small effect on NOx emissions.

On the early stages of soot formation: Molecular structure elucidation by high-resolution atomic force microscopy

Abstract: The early stages of soot formation, namely inception and growth, are highly debated and central to many ongoing studies in combustion research. Here, we provide new insights into these processes from studying different soot samples by atomic force microscopy (AFM). Soot has been extracted from a slightly sooting, premixed ethylene/air flame both at the onset of the nucleation process, where the particle size is of the order of 2–4 nm, and at the initial stage of particle growth, where slightly larger particles are present. Subsequently, the molecular constituents from both stages of soot formation were investigated using high-resolution AFM with CO-functionalized tips. In addition, we studied a model compound to confirm the atomic contrast and AFM-based unambiguous identification of aliphatic pentagonal rings, which were frequently observed on the periphery of the aromatic soot molecules. We show that the removal of hydrogen from such moieties could be a pathway to resonantly stabilized π-radicals, which were detected in both investigated stages of the soot formation process. Such π-radicals could be highly important in particle nucleation, as they provide a rational explanation for the binding forces among aromatic molecules.

Comparison study of the ignition and combustion characteristics of directly-written Al/PVDF, Al/Viton and Al/THV composites

Abstract: The aluminum–fluorine reaction is attracting growing interest due to its higher density over aluminum–oxygen. Fluorine rich polymers are particularly interesting for their applications as an energetic binder in advanced additive manufacturing of energetic materials. In this paper, three soluble polymers of PVDF (59 wt% F), Viton (66 wt% F) and THV (73 wt% F) are incorporated with aluminum nanoparticles (Al NPs) and prepared as free-standing films using solvent-based direct writing. The three composite films are compared for their mechanical properties as well as the ignition and combustion performance. Tensile stress was found to order as Al/PVDF > Al/THV > Al/Viton while the elasticity of Al/Viton is much higher than the other two. The burn rate of different composite films increases with Al content, while the flame temperature peaks slightly fuel-rich. The Al/PVDF had the highest burn rate, however, the flame temperature ordered as Al/THV (∼2500 K) > Al/Viton (∼2000 K) > Al/PVDF (∼1500 K), consistent with fluorine content. With higher fluorine and lower hydrogen content, THV releases more CFx gas than HF, which generates higher temperature. However, HF which is predominantly produced from PVDF has the lowest ignition by far and may be responsible for its high flame speed.

Flame inhibition of aluminum dust explosion by NaHCO3 and NH4H2PO4

Abstract: To evaluate the inhibition capacity of NaHCO3 and NH4H2PO4 on the flame in aluminum dust explosions, the inhibition of 5 μm and 30 μm aluminum dust explosions by NaHCO3 and NH4H2PO4 has been studied experimentally, using a high-speed camera for measurement of flame propagation behaviors and a thermocouple for measurement of flame temperatures. The mechanism of flame inhibition is further investigated computationally. It is found that the concentration of NH4H2PO4 required to inhibit the aluminum dust explosion is lower in comparison with NaHCO3. The acceleration and the maximum flame speed significantly decrease and the flame morphology becomes irregular and discrete, as the inhibitor concentration increases. The NH4H2PO4 addition exerts a stronger effect on aluminum flame temperature compared to NaHCO3 addition. The chemical kinetic model indicates that the addition of the inhibitor decreases the concentrations of AlO and O in the reaction zone. This reduction becomes larger with increasing the inerting ratio. Na- and P-containing species promote highly reactive O atoms to recombine and form a stable combustion product O2, which leads to less heat release and a lower flame temperature. For Na-containing compounds, NaO ⇔ Na inhibition cycle is effective to reduce O atoms. NH3 that decomposed by NH4H2PO4 could consume O2 and reduce the flame temperature of aluminum in air. In addition, the gas‒phase chemical effect of inhibitors is significant for aluminum particles in the diffusion-controlled regime.

Yet another kinetic mechanism for hydrogen combustion

Abstract: Recent suggestion by Burke and Klippenstein (2017) that chemically termolecular reactions H + O2 + R may significantly affect kinetic pathways under common combustion situations requires careful analysis, since, if included in contemporary kinetic mechanisms, these reactions affect global reactivity and calculated burning velocities of laminar premixed flames. In the view of their impact, a detailed kinetic scheme for hydrogen combustion was revisited to elucidate how to counterbalance enhanced chain termination caused by chemically termolecular reactions in attempt to keep or improve model performance. First, recent experimental and theoretical kinetic studies of hydrogen reactions were analyzed. In the new mechanism four reactions were introduced and three rate constants were updated. These changes, however, significantly reduce calculated burning velocities of H2 + air flames as compared to experimental data and earlier model predictions with the major impact from chemically termolecular reactions. It was then found that implementation of the new theoretical transport database developed by Jasper et al. (2014) significantly improves the performance of the updated kinetic model. The new kinetic mechanism for hydrogen combustion which includes updated kinetics and new transport properties was found in good agreement with the consistent dataset of the burning velocity measurements for hydrogen flames obtained using the heat flux method at atmospheric pressure for which the behavior of the previous model of the author was not satisfactory.

Large-eddy simulation of dual-fuel ignition: Diesel spray injection into a lean methane-air mixture

Abstract: In the present study, large-eddy simulation (LES) together with a finite-rate chemistry model is utilized for the investigation of a dual-fuel (DF) ignition process where a diesel surrogate (n-dodecane) spray ignites a lean methane-air mixture in engine relevant conditions. The spray setup corresponds to the Engine Combustion Network (ECN) Spray A configuration enabling an extensive validation of the present numerical models in terms of liquid and vapor penetration, mixture distribution, ignition delay time (IDT) and spatial formaldehyde concentration. The suitability of two n-dodecane mechanisms (54 and 96 species) to cover dual-fuel chemical kinetics is investigated by comparing the predicted homogeneous IDTs and laminar flame speeds to reference values in single-fuel methane-air mixtures. LES of an n-dodecane spray in DF conditions is carried out and compared against the baseline ECN Spray A results. The main results of the study are: (1) ambient methane impacts the ignition chemistry throughout the oxidation process. In particular, the activation of the low-temperature chemistry is delayed by a factor of 2.6 with both mechanisms, whereas the high-temperature chemistry is delayed by a factor of 1.6–2.4, depending on the mechanism. (2) The ignition process starts from the spray tip. (3) There exists a characteristic induction time in the order of 0.1 ms between the start of the first high-temperature reactions and the time when maximum methane consumption rate is achieved. (4) The high-temperature ignition process begins near the most reactive mixture fraction conditions. (5) The role of low-temperature reactions is of particular importance for initiation of the production of intermediate species and heat, required in methane oxidation and (6) both applied mechanisms yield qualitatively the same features (1)–(5) in the DF configuration.

Experimental and kinetic modeling study of ignition characteristics of RP-3 kerosene over low-to-high temperature ranges in a heated rapid compression machine and a heated shock tube

Abstract: Autoignition characteristics of RP-3 kerosene were investigated using a heated rapid compression machine and a heated shock tube over a wide range of conditions. Ignition delay times (IDTs) for RP-3 kerosene were measured at pressures of 10, 15 and 20 bar over a range of temperatures from 624 to 1437 K and for equivalence ratios from 0.5 to 1.5. A three-component surrogate fuel (49.8% dodecane, 21.6% iso-cetane and 28.6% toluene by mole) was proposed and a kinetic model was developed to describe the combustion chemistry of RP-3. Negative temperature coefficient (NTC) behavior was observed in the autoignition, of which the temperature range varied within 701−884 K depending on operating conditions. IDT correlations in low and high temperature regions were obtained and then the dependences of IDT on pressure, equivalence ratio, oxygen content and dilution ratio were systemically studied. Comparison between the predictions using the new model and the experimental data shows that this model can accurately describe the autoignition characteristics of RP-3. Brute force sensitivity analyses were carried out to identify the key reactions that govern the ignition event. The large experimental data set and kinetic model provided in the current work will provide insights into the understanding of RP-3 ignition.

Emission characteristics of turbulent non-premixed ammonia/air and methane/air swirl flames through a rich-lean combustor under various wall thermal boundary conditions at high pressure

Abstract: This study is dedicated to understanding the combustion and emission characteristics of turbulent non-premixed ammonia (NH3)/air and methane (CH4)/air swirl flames in a rich-lean gas turbine − like combustor at high pressure under various wall thermal boundary conditions. In this study, the emission characteristics of both flames were obtained through numerical simulations using large − eddy simulations with the finite-rate chemistry technique. In addition, for NH3/air flames, simultaneous NO and OH planar laser − induced fluorescence (PLIF) images were acquired in order to qualitatively verify the numerical results. The results show that the minimum NO emission could be obtained when the primary zone global equivalence ratio (ϕglobal/pri) was 1.1, irrespective of the wall thermal condition, using a rich-lean combustor in NH3/air flames, whereas in CH4/air flames, the maximum NO emission was obtained with a ϕglobal/pri value of 1.0. Moreover, in NH3/air flames, the local NO concentration is largely dependent on the local OH concentration, whereas in CH4/air flames, the local NO concentration is largely dependent on the local temperature. The NO − OH correlation in NH3/air flames was experimentally verified using simultaneous NO and OH PLIF images. Additionally, it was found that, in NH3/air flames, the wall heat losses due to the combustor wall cooling greatly affected the NH3 oxidation and led to significant emissions of unburnt NH3, although lower NO emission resulted from the combustor wall cooling. This was primarily because of the lower OH concentration level in the flame region owing to the wall heat losses.

On the opposing effects of methanol and ethanol addition on PAH and soot formation in ethylene counterflow diffusion flames

Abstract: An experimental and numerical analysis of the effects of methanol and ethanol addition on polycyclic aromatic hydrocarbon (PAH) and soot formation in non-premixed ethylene flames is reported here. Laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques were used to measure soot volume fractions and relative PAH concentrations in counterflow diffusion flames, respectively. A comprehensive chemical kinetic analysis was performed by modeling soot with detailed gas-phase chemistry and a sectional method. The results showed that although both methanol and ethanol are typically regarded as clean fuels, their presence in ethylene diffusion flames had the opposite effects on PAH and soot formation. The LIF and LII signals decreased significantly as methanol fraction increased, suggesting a soot-inhibiting role for methanol. Apart from the fact that methanol addition reduced the carbon supply for soot thus having a fuel-dilution effect (methanol converted primarily to CO), the increased H2 concentration from methanol decomposition was seen to chemically suppress incipient benzene ring formation and subsequent PAH and soot growth processes. In contrast, a small amount of ethanol addition enhanced soot formation, which was well captured by the numerical model. Reaction pathway analysis showed that ethanol decomposition produced a relatively large amount of methyl radicals, enhancing the chemical interaction between CH3 and C2 species and, thereby promoting the formation of propargyl and C4 species. As a result, benzene formation was promoted through reactions between C2H2 and C4 species and via C3H3 recombination reaction, leading sequentially to the enhancement of PAH growth and soot formation.

A promising strategy to obtain high energy output and combustion properties by self-activation of nano-Al

Abstract: Aluminum based energetic nanomaterials attract significant attention for various applications owing to their ultrahigh energy density. The main obstacle in the application of nano-Al based energetic materials is the slow combustion reaction kinetics and reduced energy output resulting from the inert Al2O3 shell. In this paper, an efficient surface self-activation strategy is proposed to significantly improve the combustion performance and energy output of nano-Al based energetic materials. A porous AlF3 shell is formed on the surface of the nano-Al particle by an etching reaction between perfluorododecanoic acid and the Al2O3 dense layer. The porous AlF3 shell provides a new reaction channel for the reaction of Al and the oxidizer, thus significantly improving the energy output and combustion reaction kinetics. The energy output and combustion reaction speed of polytetrafluoroethylene (PTFE)/nano-Al coated with C11F23COOH are 6304 J/g and 670 m/s, which are 3.0 and 2.6 times higher than those of PTFE/nano-Al, respectively. The mechanism of the self-activating process is proposed to explain the enhanced combustion reaction kinetics and energy output of the nano-Al based energetic materials. The proposed surface self-activation strategy for nano-Al particles can efficiently to enhance the reactivity and energy output and promote the development of the nano-Al based energetic materials.

Detailed kinetic modeling of dimethoxymethane. Part II: Experimental and theoretical study of the kinetics and reaction mechanism

Abstract: In this study (Part II), the oxidation of dimethoxymethane (DMM) is investigated and a detailed chemical reaction model developed for a comprehensive description of both high- and low-temperature oxidation processes. The sub-mechanism of DMM is implemented using AramcoMech2.0 as the base mechanism. Rate coefficients are based on analogies with those for dimethyl ether, diethyl ether, and n-pentane oxidation. Furthermore, theoretical studies from recent works are also included in the present model and new calculations for the dissociation kinetics of Q ˙ OOH radicals have been carried out at the CCSD(T)/CBS(aug-cc-pVXZ; X = D, T) // B2PLYP-D3/6-311 + + G(d,p) level of theory. For validation, new ignition delay time experiments have been performed in a shock tube (ST), a rapid compression machine (RCM), and in a laminar flow reactor covering a wide range of conditions (p = 1–40 bar, T = 590–1215 K, φ = 1). In addition, the kinetic model is validated against laminar burning velocities, jet-stirred reactor, plug flow reactor and further ST and RCM experimental datasets from the literature. Pathway and sensitivity analyses were used to identify critical reaction pathways in the DMM oxidation mechanism. These show that the reactivity of DMM at intermediate temperatures is controlled by the branching between pathways initiated on the primary or secondary fuel radical. While primary fuel radicals eventually lead to chain branching, secondary fuel radical consumption is controlled by fast β-scission over a wide range of temperatures, which inhibits reactivity.

The site effect on PAHs formation in HACA-based mass growth process

Abstract: Hydrogen-abstraction/acetylene-addition (HACA) pathway has long been postulated as the dominant pathway for the formation of polycyclic aromatic hydrocarbons (PAHs) and the surface growth of soot. In this study, the site effect on PAH formation following HACA pathway is systematically investigated using density functional theory, transition state theory and premixed flame kinetic modeling. The entire reaction network includes 186 elementary reactions, starting from benzene to pyrene. Analysis of the potential energy surface and kinetic parameters show that H abstraction and C 2 H 2 addition reactions are greatly sensitive to the site position (ortho-, meta- and para-position) relative to the existing C 2 H chain and surface site type (zig-zag, free-edge and armchair). Specifically, H abstraction and C 2 H 2 addition reactions on the ortho-position and armchair surface site are kinetically unsupported due to the relatively high energy barrier and orientation hindrance effect compared with other site options. Therefore, the formation of a new benzene ring by the addition of the second C 2 H 2 molecule on the ortho-position (e.g., 1-ethynylnaphthalene + C 2 H 2 →phenanthrene) or the first C 2 H 2 molecule on the armchair surface site (e.g., phenanthrene + C 2 H 2 →pyrene) is unlikely, as demonstrated by PAH simulations in a premixed C 2 H 4 /O 2 /N 2 sooting flame. The yield distribution of various reaction products has been investigated using a 0-D reactor, where the combustion conditions are taken from experimental data. The results show that the dominant products are di-substituted PAHs in benzene-naphthalene reaction system and PAHs with 5-membered ring structures in larger PAHs reaction systems. The existence of abundant PAHs with 5-membered rings contributes to clarifying the PAHs signal detected using laser induced fluorescence technology. Additionally, the observed sequence of mass peaks at intervals of mass number 26 in C 2 H 2 /C 2 H 4 pyrolysis is reasonably explained by the HACA pathway with considering site effect.

Understanding strong knocking mechanism through high-strength optical rapid compression machines

Abstract: Strong knocking combustion has become the greatest challenge for advanced internal combustion engines to pursue thermal efficiency limits at high power density conditions. Arising from enclosed space and extreme combustion situations, the fundamental mechanism for strong knocking combustion has still not been fully understood. In this study, synchronization measurement was performed through simultaneous pressure acquisition and high-speed direct photography, and knocking experiments were comparatively conducted under spark-ignition (SI) and compression-ignition (CI) conditions in a high-strength optical rapid compression machine (RCM) with flat piston design. Strong knocking phenomena were reproduced through varying initial thermodynamic conditions, and localized autoignition (AI) initiation and reaction wave evolutions were visualized, companied by synchronous pressure and temperature trajectories. The results show that compared with initial temperature, initial pressure and equivalence ratio exhibit greater influence on the variations of knocking severity. The weighting of different contributors can be further quantified by an effective energy density that shows positive but nonlinear correlations with knocking severity. However, the distinctions between CI and SI knocking characteristics at identical effective energy density also reflect the essential role of the interplay between primary flame propagation and end-gas AI progress. Visualized combustion images show that through improving end-gas thermodynamic state and reactivity sensitivity, the primary flame propagation can enhance localized AI initiation and secondary intensive AI evolutions, facilitating combustion mode transitions into developing detonation. The significant influence of primary flame propagation is diminished until ignition delay time becomes sufficiently short. Finally, with estimated thermal heterogeneities in flat-piston RCM configurations, the ignition modes of strong knocking cycles are quantified by a non-dimensional ignition regime diagram, and favorable scaling agreements with strong and mixed ignition regimes are observed.

Shock focusing and detonation initiation at a flame front

Abstract: Multidimensional numerical simulations were performed to study the interaction of focused shock waves and a flame front leading to detonation initiation. The fully compressible Navier–Stokes equations, coupled with a chemical-diffusive model for energy release and conversion of fuel to product in a stoichiometric hydrogen–air mixture, were solved using a third-order method on a dynamically adapting mesh. Preliminary simulations of deflagration-to-detonation transition (DDT) in an obstructed channel, when compared to previous experiments, point to a DDT scenario where detonation initiation arises from multi-shock focusing at a flame front. A detailed examination of an idealized problem showed two mechanisms of detonation formation: (1) direct detonation initiation triggered at the collision spot by focusing shocks at the flame front, and (2) focusing of relatively weak shocks leading to a delayed transition to detonation through the reactivity-gradient mechanism. Comparisons between the detailed analysis of shock-focusing and experimentally observed DDT phenomena suggests that shock focusing plays an important role in the occurrence of DDT for this problem.

Influence of fuel-oxygen content on morphology and nanostructure of soot particles

Abstract: The share of biofuels in the fuel market has increased over the last several decades. This is related to their potential to reduce the emissions including particulate matter. It has been frequently reported that the fuel oxygen content is the main reason for the reduction in particulate matter emissions. To understand the effect of fuel oxygen content on morphology and nanostructure characteristics of soot particles, different fuels such as diesel, coconut biodiesel and triacetin were tested in a diesel engine with various mixing proportions. The fuel blending was done in such a way that overall oxygen content of fuel was kept in range of 0% to 14% (wt.%). The soot particles were sampled from the engine exhaust system and analysed with a transmission electron microscope (TEM) at low and high spatial resolution. The TEM images were post-processed with the help of an in-house developed image analysis program to determine the morphology and nanostructure characteristics. The results show that oxygenated fuel blends emit smaller sized soot particles forming compact aggregates. The investigation of the internal structure of soot particles show disordered arrangement of graphene layers for fuels up to 11.01% fuel oxygen content (pure biodiesel); however, the opposite trend was observed for fuel blends with triacetin which could be related to the presence of oxygen in a different chemical functional group.

Isolating the effect of induction length on detonation structure: Hydrogen–oxygen detonation promoted by ozone

Abstract: The effect of ignition promoters on H2 O2 detonation structure was evaluated with one-dimensional ZND calculations and experimental detonation cell measurements. Test conditions include a sweep of ozone concentration (up to 3000 PPM by mole), initial pressure (10 to 30 kPa), equivalence ratio (0.4–1.5), Ar and N2 dilution (up to 50%), and CF3I concentration (up to 3000 PPM). The ZND calculations demonstrate that ozone addition reduces induction length without affecting heat release length or thermodynamic properties such as CJ speed, allowing for a unique evaluation of the effect of ignition delay on detonation structure. Experimentally and within the conditions tested, ozone addition acts to reduce detonation cell size by up to 70%. In addition, measured cell width is found to better correlate with induction length than with total reaction length, defined as induction length plus exothermic length. The results confirm that the detonation structure is controlled largely by chemical length scales. Also studied is the addition of CF3I, which is traditionally used as a fire suppressant. Although at high concentrations CF3I displays a radical scavenging function, at the concentrations considered in the current work, CF3I also acts as an ignition promotor due to the production of the I atom, which promotes chain branching during ignition. Finally, the ozone and CF3I additives were found to have, at most, a minor effect on cell regularity.

Initial pyrolysis mechanism of cellulose revealed by in-situ DRIFT analysis and theoretical calculation

Abstract: Cellulose is one of the major components of biomass. The study on its pyrolysis process will be beneficial to the in-depth understanding of biomass pyrolysis mechanism. In this work, in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFT) combined with two-dimensional perturbation correlation infrared spectroscopy (2D-PCIS) was first used to characterize the evolution process of the functional groups in cellulose during pyrolysis. The results showed that the degradation of carbon skeleton was prior to the dehydration of free hydroxyls after the destruction of hydrogen bond networks during pyrolysis. The thermal stability of C O in cellulose followed by the order of glycosidic bond O in glucopyranose ring O between glucopyranose ring and hydroxyl. Followingly, micro pyrolysis experiment was performed to analyze the pyrolysis products of cellulose at various temperatures. It was found that the rupture of glucopyranose rings to form 2C and 4C products was more difficult than the dissociation of C6 hydroxymethyls, and required higher pyrolysis temperature. Quantum chemistry calculation was further carried out to study the key reaction pathways including dehydration, cleavage of glycosidic bond, ring opening and fragmentation in the initial stage of cellulose pyrolysis. The result showed that the concerted cleavage of glycosidic bond to form LG-end short chain was the most favored with the lowest activation energy. The ring opening of the glucose unit in the chain occurred via the cleavage of C1-O. The formed ring opening product was more likely to degrade via the dissociation of C6 hydroxymethyl compared with the breakage of C2–C3 to form 2C and 4C products. Besides, the dehydration of hydroxyls in glucose units required high energy barriers and was difficult to occur.

Flame spread: Effects of microgravity and scale

Abstract: For the first time, a large-scale flame spread experiment was conducted inside an orbiting spacecraft to study the effects of microgravity and scale and to address the uncertainty regarding how flames spread when there is no gravity and if the sample size and the experimental duration are, respectively, large enough and long enough to allow for unrestricted growth. Differences between flame spread in purely buoyant and purely forced flows are presented. Prior to these experiments, only samples of small size in small confined volumes had been tested in space. Therefore the first and third flights in the experimental series, called “Saffire,” studied large-scale flame spread over a 94 cm long by 40.6 cm wide cotton-fiberglass fabric. The second flight examined an array of nine smaller samples of various materials each measuring 29 cm long by 5 cm wide. Among them were two of the same cotton-fiberglass fabric used in the large-scale tests and a thick, flat PMMA sample (1-cm thick). The forced airflow was 20–25 cm/s, which is typical of air circulation speeds in a spacecraft. The experiments took place aboard the Cygnus vehicle, a large unmanned resupply spacecraft to the International Space Station (ISS). The experiments were carried out in orbit before the Cygnus vehicle, reloaded with ISS trash, re-entered the Earth's atmosphere and perished. The downloaded test data show that a concurrent (downstream) spreading flame over thin fabrics in microgravity reaches a steady spread rate and a limiting length. The flame over the thick PMMA sample approaches a non-growing, steady state in the 15 min burning duration and has a limiting pyrolysis length. In contrast, upward (concurrent) flame spread at normal gravity on Earth is usually found to be accelerating so that the flame size grows with time. The existence of a flame size limit has important considerations for spacecraft fire safety as it can be used to establish the heat release rate in the vehicle. The findings and the scientific explanations of this series of innovative, novel and unique experiments are presented, analyzed and discussed.

Numerical investigation on the initiation of oblique detonation waves in stoichiometric acetylene–oxygen mixtures with high argon dilution

Abstract: Oblique detonation waves (ODWs) in stoichiometric acetylene-oxygen mixtures, highly diluted by 81–90% argon, are studied using the reactive Euler equations with a detailed chemistry model. Numerical results show that the incident Mach number M0 changes the ODW initiation structure, giving both the smooth transition in the case of M0 = 10 and the abrupt transition in the case of M0 = 7. By comparing results of numerical simulation and theoretical analysis, the initiation processes are found to be chemical kinetics-controlled regardless of M0, different from those in hydrogen-air mixtures which are wave-controlled in the low M0 regime. The argon dilution effect on the initiation morphology is investigated, showing that the structures are determined by the dilution ratio and M0 collectively. However, the initiation length is found to be independent of the dilution ratio and only determined by M0, which is attributed to the competing effect of the high density and high temperature.

Investigation of OH* chemiluminescence and heat release in laminar methane–oxygen co-flow diffusion flames

Abstract: OH* chemiluminescence is one of the major spontaneous emission in flames, and often applied in combustion diagnostics to indicate flame structure, strain rate, equivalence ratio, heat release rate, etc. In this work, OH* chemiluminescence in the laminar methane–oxygen co-flow diffusion flames was investigated. A high resolution ultra-violet imaging system was used to capture the OH* chemiluminescence images. Numerical simulations of the experimental cases were performed based on OH* chemiluminescence reaction mechanism. The numerical results show good agreement with the experimental measurements. It's found that there are two OH* distribution zones in laminar methane–oxygen co-flow diffusion flames. Analysis on the production pathway of OH* chemiluminescence shows that the reaction H + O + M = OH* + M (R1) is the major formation reaction of OH* chemiluminescence in laminar methane–oxygen diffusion flames. The increase of diluent addition in oxidizer will lead to the dominant OH* production pathway changing from the reaction R1 to the reaction CH + O2= OH* + CO (R2). The OH* distribution characteristics under different global oxygen-fuel equivalence ratios indicate that OH* chemiluminescence can be employed as an appropriate indicator to characterize the combustion condition. Moreover, the correlation between integrated heat release rate and integrated OH* concentration is derived for the oxygen-deficient flames. The integrated heat release rate can be predicted in terms of integrated OH* concentration, methane flow rate and global oxygen-fuel equivalence ratio.

Temperature gradient induced detonation development inside and outside a hotspot for different fuels

Abstract: With the dimensionless parameters obtained for syngas/air mixture, Bradley detonation peninsula is often used to determine the detonation development for hotspot autoignition (AI) in reactive flows. In this work, similar numerical simulations were carried out in order to identify the characteristics of detonation peninsula when considering other fuels. Three alternative C0-1 fuels with detailed chemistry and transport were employed in a 1-D reaction wave propagation induced by temperature gradients, and different critical temperature gradients and hotspot sizes were considered. Meanwhile, the role of detonation parameters in the detonation development outside hotspot was addressed. First, the results show that different AI propagation modes can be well depicted using the dimensionless parameters for individual fuel at various critical temperature gradients. However, the quantitative difference in detonation development regime is significantly observed between different fuels with distinct physical–chemical properties even though similar regime distribution is observed. Second, the evolutions of AI reaction wave propagation outside hotspot were further studied, and combustion mode transitions involving detonation termination and formation were observed. The evolutions of the thermodynamic state of different flow particles show that detonation development is found to switch from constant-pressure to constant-volume combustion. Meanwhile, scaling analysis on combustion mode transitions indicates that besides the early-stage propagation controlled by reactivity gradient in hotspot interior, the reactivity of the mixture outside hotspot also plays an important role in detonation development. This can provide great insights into proposing integrated dimensionless parameters for determining detonation development in the whole reactive flows.

Effects of controlled non-equilibrium excitation on H2/O2/He ignition using a hybrid repetitive nanosecond and DC discharge

Abstract: The present work reports on the effects of controlled non-equilibrium excitation of reactant molecules on low temperature H2/O2/He ignition by numerically modeling a hybrid repetitive nanosecond (NSD) and DC discharge at atmospheric pressure. At first, a detailed plasma-combustion kinetic model of H2/O2/He, including non-equilibrium excitation, is developed and validated by experimental data of a repetitively-pulsed nanosecond discharge. Then, the effects of ignition enhancement by NSD and a hybrid NSD/DC discharge, with controlled electron energy distribution for selective non-equilibrium excitation of vibrationally excited H2(v) and O2(v) as well as electronically excited O2(a1Δg) and O(1D), are compared. The results show that H2(v1) contributes significantly to the H production and OH consumption in the hybrid plasma discharge. Moreover, O2(a1Δg) and O2(v1−4) also contribute to the production O and OH. Uncertainty analysis of H2(v) and O2(a1Δg) elementary reactions on ignition delay time is conducted by using several different kinetic models. The comparison of ignition delay time using different plasma kinetic models indicates the selection of accurate rate constants involving excited species is important for plasma assisted ignition modeling. The results of hybrid discharge assisted H2/O2 ignition show that the optimized ignition enhancement is achieved when both excited species and radicals are produced efficiently at an appropriate DC electric field strength. The present modeling provides useful insight into the plasma-combustion model development and the development of controlled plasma discharge to achieve efficient ignition with optimized non-equilibrium excitation of reactants.

High-pressure pyrolysis and oxidation of DME and DME/CH4

Abstract: The pyrolysis and oxidation of dimethyl ether (DME) and its mixture with methane were investigated at high pressure (50 and 100 bar) and intermediate temperature (450–900 K). Mixtures highly diluted in nitrogen with different fuel–air equivalence ratios ( Φ = ∞ , 20, 1, 0.06) were studied in a laminar flow reactor. At 50 bar, the DME pyrolysis started at 825 K and the major products were CH4, CH2O, and CO. For the DME oxidation at 50 bar, the onset temperature of reaction was 525 K, independent of fuel–air equivalence ratio. The DME oxidation was characterized by a negative temperature coefficient (NTC) zone which was found sensitive to changes in the mixture stoichiometry but always occurring at temperatures of 575–625 K. The oxidation of methane doped by DME was studied in the flow reactor at 100 bar. The fuel–air equivalence ratio (Φ) was varied from 0.06 to 20, and the DME to CH4 ratio changed over 1.8–3.6%. Addition of DME had a considerable promoting effect on methane ignition as the onset of reaction shifted to lower temperatures by 25–150 K. A detailed chemical kinetic model was developed by adding a DME reaction subset to a model developed in previous high-pressure work. The model was evaluated against the present data as well as data from literature. Additional work is required to reconcile experimental and theoretical work on reactions on the CH3OCH2OO PES with ignition delay measurements in the NTC region for DME.

Self-propagating combustion of sputter-deposited Al/CuO nanolaminates

Abstract: The complex Al/CuO self-propagating reaction involving multi-phase and multi-species dynamics was studied in order to investigate the very high flame temperature around the vaporization temperature of alumina, even under a neutral environment. Experiments were performed on different sputter-deposited Al/CuO multilayers coupling optical spectroscopy with high speed camera measurements. The clear presence of both AlO and Al signatures in gas phase suggests that the redox reaction starts in the bulk nanolaminate, which then rapidly tear off the substrate to continue burning in a heterogeneous (condensed and gas) phase in the environment. The flame temperature increases with the stoichiometry but is independent of the bilayer thickness. In addition to the confirmation of the effects of stoichiometry and the bilayer thickness on the characteristics of the self-propagating reaction, the predominant role of process-induced residual stress was highlighted for the first time; it can lead to an early disruption of the multilayer long before the completion of the redox reaction.

A chemical kinetic modeling study of indene pyrolysis

Abstract: An improved polycyclic aromatic hydrocarbon (PAH) model is developed to predict the decomposition of indene and the formation of large PAHs under pyrolytic conditions. This model is developed based on experimental study of pyrolytic kinetics of indene in a flow reactor at low and atmospheric pressures (30 and 760 Torr) by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A general map of PAH growth is presented according to the observations in this study and those in literature. Indene dissociates via indanyl forming mono-cyclic aromatics and small intermediates, while its dominant decomposition product is indenyl. As a resonantly stabilized radical, indenyl serves as a platform molecule in PAH growth process which links small unsaturated hydrocarbons and mono-aromatic species to multi-cyclic ones. Reactions of indenyl radical are proposed to form commonly studied and recently observed PAHs. Rate constants of these reactions are evaluated by analyzing literature data of rate constant measurements, quantum chemical calculations and analogy to cyclopentadienyl radical. The main PAH formation pathways are the bi-molecular addition reactions of indenyl radical with indene and a series of intermediates, forming C10 C18 and larger PAHs. Meanwhile, radical chain reactions provide huge passage for PAH growth form one resonantly stabilized radical (RSR) to larger ones. Particular contribution has been found from the reactions of RSRs that have five-member ring in their molecular structures, such as fluorenyl, benz-indenyl, cyclopenta-phenanthrenyl and benzo-fluorenyl.

Chemical and nanostructural characteristics of the particulate matter produced by renewable diesel fuel in an automotive diesel engine

Abstract: Renewable diesel (RD), a paraffinic fuel produced by the hydrotreating of palm oil, was used neat and blended at 10% and 30% (by volume) with ultra-low sulfur diesel (ULSD) to generate particles in an automotive diesel engine operating at two engine speeds (1890 and 2410 min−1) under the same engine load (95 Nm). Particulate matter was characterized using thermogravimetric analysis (TGA), Raman spectroscopy, X-Ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), and X-Ray photoelectron spectroscopy (XPS). Oxidation profiles showed that diesel soot is slightly more reactive than the soot produced by RD and its blends, independently of the engine speed (the maximum mass loss rate temperature –MLRTmax– was 6.6 °C lower for ULSD than neat RD). This behavior was in agreement with the active surface area (ASA) of the particles, which varied between 10.8 m2/g for RD to 13.9 m2/g for ULSD at the same engine speed. Soot nanostructure (ratio of Raman peaks) and interlayer distance show a slightly higher degree of order of the particles when RD was added into diesel fuel. The mean primary particle diameter of neat RD soot was around 26 nm and fractal dimension of the agglomerates was around 1.66, which were both lower in comparison with ULSD (32 nm and 1.97, respectively). Fringe analysis applied to HRTEM micrographs revealed no clear trend in the fringe length and tortuosity among soot samples. Finally, it was found that, independently of the fuel tested, all particle samples gathered at 2410 min−1 were slightly more reactive and smaller than those collected at 1890 min−1. From this engine configuration and specific experimental setting, it can be expected that the use of RD blends would not markedly affect the performance of aftertreatment devices like diesel particulate filters.