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Combustion and Flame
Journal Prestige (SJR): 2.427
Citation Impact (citeScore): 5
Number of Followers: 136  
 
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ISSN (Print) 0010-2180
Published by Elsevier Homepage  [3158 journals]
  • A Bayesian approach to calibrating hydrogen flame kinetics using many
           experiments and parameters
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): John Bell, Marcus Day, Jonathan Goodman, Ray Grout, Matthias Morzfeld First-principles Markov Chain Monte Carlo sampling is used to investigate uncertainty quantification and uncertainty propagation in parameters describing hydrogen kinetics. Specifically, we sample the posterior distribution for thirty-one parameters focusing on the H2O2 and HO2 reactions resulting from conditioning on ninety-one experiments. Established literature values are used for the remaining parameters in the mechanism as well as other thermodynamic and transport data needed to specify fluid properties. The samples are computed using an affine invariant sampler starting with broad, noninformative priors. Autocorrelation analysis shows that O(1M) samples are sufficient to obtain a reasonable sampling of the posterior. The resulting distribution identifies strong positive and negative correlations and several non-Gaussian characteristics. Using samples drawn from the posterior, we investigate the impact of parameter uncertainty on the prediction of two more complex flames: a 2D premixed flame kernel and the ignition of a hydrogen jet issuing into a heated chamber. The former represents a combustion regime similar to the target experiments used to calibrate the mechanism and the latter represents a different combustion regime. For the premixed flame, the net amount of product after a given time interval has a standard deviation of less than 2% whereas the standard deviation of the ignition time for the jet is more than 10%. The samples used for these studies are posted online. These results indicate the degree to which parameters consistent with the target experiments constrain predicted behavior in different combustion regimes. This process provides a framework for both identifying reactions for further study from candidate mechanisms as well as combining uncertainty quantification and propagation to, ultimately, tie uncertainty in laboratory flame experiments to uncertainty in end-use numerical predictions of more complicated scenarios.
       
  • Experimental effective metal oxides to enhance boron combustion
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Sidi Huang, Sili Deng, Yue Jiang, Xiaolin Zheng Thermite reactions between metal fuels and oxide oxidizers are highly exothermic and self-sustaining, so they find wide applications in the explosion, pyrotechnics, thermal batteries, micro-actuator, and material synthesis and process. Compared to the well-studied aluminum (Al)-based thermites, boron (B)-based thermites are thermodynamically attractive due to boron's higher volumetric and gravimetric energy densities and they received limited attention. Previous studies have compared the effect of metal oxide on the reaction onset temperature of B-based thermites and identified that B/Bi2O3 and B/CuO thermites have lower reaction onset temperatures than other B/metal oxides. Nevertheless, there is no systematic study on the effect of metal oxide on both ignition and combustion of B-based thermite. In addition, no study has investigated the effect of binary metal oxide mixtures for B-based thermite. Herein, we experimentally tested five common metal oxides (CuO, Bi2O3, MoO3, Co3O4, and Fe2O3) on the ignition and combustion characteristics of sub-micron sized B particles using Xenon flash ignition, constant-volume pressure vessel and bomb calorimeter experiments. We observed that Bi2O3 and CuO are the most effective oxidizer for ignition and combustion of boron, respectively. We further identified that the binary oxide mixture (75 wt% B–CuO + 25 wt% B–Bi2O3) is more effective than all the single metal oxide for the ignition and combustion of boron particles. The results suggest that mixed oxides are potentially beneficial for ignition and combustion of other metal fuels as well.
       
  • Temperature gradient induced detonation development inside and outside a
           hotspot for different fuels
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Jiaying Pan, Sheng Dong, Haiqiao Wei, Tao Li, Gequn Shu, Lei Zhou 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.
       
  • Combustion regimes in sequential combustors: Flame propagation and
           autoignition at elevated temperature and pressure
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Oliver Schulz, Nicolas Noiray This numerical study investigates the combustion modes in the second stage of a sequential combustor at atmospheric and high pressure. The sequential burner (SB) features a mixing section with fuel injection into a hot vitiated crossflow. Depending on the dominant combustion mode, a recirculation zone assists flame anchoring in the combustion chamber. The flame is located sufficiently downstream of the injector resulting in partially-premixed conditions. First, combustion regime maps are obtained from 0-D and 1-D simulations showing the co-existence of three combustion modes: autoignition, flame propagation and flame propagation assisted by autoignition. These regime maps can be used to understand the combustion modes at play in turbulent sequential combustors, as shown with 3-D large eddy simulations (LES) with semi-detailed chemistry. In addition to the simulation of steady-state combustion at three different operating conditions, transient simulations are performed: (i) ignition of the combustor with autoignition as the dominant mode, (ii) ignition that is initiated by autoignition and that is followed by a transition to a propagation stabilized flame, and (iii) a transient change of the inlet temperature (decrease by 150 K) resulting into a change of the combustion regime. These results show the importance of the recirculation zone for the ignition and the anchoring of a propagating type flame. On the contrary, the autoignition flame stabilizes due to continuous self-ignition of the mixture and the recirculation zone does not play an important role for the flame anchoring. These findings are important for the design and operation of practical sequential combustion systems.
       
  • High-temperature laminar flame speed measurements in a shock tube
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Alison M. Ferris, Adam J. Susa, David F. Davidson, Ronald K. Hanson High-temperature methane and propane laminar flame speed measurements were conducted behind reflected shock waves in a shock tube. A high-power Nd:YAG laser was used to spark-ignite the shock-heated gas mixtures and initiate laminar flame propagation. High-speed, OH* endwall imaging was used to record the propagation of the spherically expanding flames in time, and a non-linear stretch correlation was applied and used to determine the unburned, unstretched laminar flame speed. “Low-temperature” (750 K) flame speed results are presented for a propane/21% O2-47% N2-32% He mixture (ϕ = 0.8) at initial unburned gas conditions of 764–832 K, 1 atm. The high-temperature measurements fall between kinetic model predictions, but the kinetic model results show significant disagreement, highlighting the need for high-temperature flame speed validation data of this kind. We believe that these results represent the first laminar flame speed measurements conducted in a shock tube, and that the high-temperature results are the highest-temperature, 1-atm flame speed measurements available in the literature.
       
  • Role of radicals in carbon clustering and soot inception: A combined EPR
           and Raman spectroscopic study
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Giuseppe Vitiello, Gianluigi De Falco, Francesca Picca, Mario Commodo, Gerardino D'Errico, Patrizia Minutolo, Andrea D'Anna Some aspects of soot formation from gas phase molecules at high temperatures are still unclear. Aromatic π-radicals may be key elements for a change in the model of carbon clustering and particle formation. However, experimental investigations are still needed on the radical nature of molecules and particles in flames and on their roles in the transition from molecules to incipient molecular clusters and their further evolution to mature soot.In this paper, we present electron paramagnetic resonance (EPR) and Raman spectroscopy measurements of particles collected in an ethylene-rich premixed flame at various residence times during nucleation. The experimental results, combined with measurement of the particle size distribution by a differential mobility analyzer, are used to investigate the role of radicals in particle nucleation and rearrangement of aromatic molecules in just-nucleated particles. For all the sampled particles, an EPR signal typical of persistent carbon-centered aromatic radicals is measured. An abrupt change in the EPR signal intensity, which is characteristic of stronger supramolecular interactions, is observed when the size distribution changes from monomodal to bimodal, as confirmed by Raman spectroscopy. Our experimental results indicate strong involvement of π-radicals during particle nucleation and growth. The paramagnetic/radical nature of the sampled particles is discussed on the basis of recent studies on the role of resonantly stabilized radicals in soot nucleation. The presence of localized π-electrons on the edge of aromatic soot constituents and their role in carbon clustering are also discussed.
       
  • Preferential vaporization impacts on lean blow-out of liquid fueled
           combustors
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Sang Hee Won, Nicholas Rock, Seung Jae Lim, Stuart Nates, Dalton Carpenter, Benjamin Emerson, Tim Lieuwen, Tim Edwards, Frederick L. Dryer Recent experimental works have shown that the global equivalence ratio defining lean blow-out (LBO) in model gas turbine combustors correlates with the derived cetane number (DCN) of the tested fuel, which represents the chemical reactivity potential of the fuel, but additional physical and kinetic parameters of the fuel also have influence. The current work explores the significance of preferential vaporization impacts on LBO behaviors; i.e., rather than parameterizing the fuel by overall averaged fuel properties, it looks at DCN correlations based upon distillation properties prior to full vaporization. Preferential vaporization potentials of six fuels are evaluated by measuring the DCN values of five distillation cuts (each of 20% liquid distillation volume recovered). In spite of relatively large disparities in total fuel DCN values (∼9.1), two petroleum-derived jet fuels are found to have nearly the same LBO equivalence ratios, which is attributed to the relatively indiscernible difference of DCN values (∼2) for the initial 20% distillation cut of each fuel. Trade-off impacts between fuel chemical and physical properties are demonstrated by comparing n-dodecane and Gevo-ATJ, which do not have preferential vaporization potential. LBO results suggest that fuel physical properties (particularly fuel boiling characteristics) predominantly control LBO behaviors at low air inlet temperature conditions, whereas fuel chemical properties appear to gain significance with increasing air inlet temperature. Further evidence of preferential vaporization effects on LBO is discussed with two surrogate mixtures formulated to emulate the fully pre-vaporized combustion behaviors of Jet-A, but having drastically different preferential vaporization potentials. Finally, the relationship between DCNs and LBO equivalence ratios is re-examined using the DCN values of initial 20% distillation cuts of all six fuels. The results display a significantly improved correlation, suggesting that the relevance of preferential vaporization on LBO can be significant for fuels that exhibit significant departure of the DCN for high volatile fractions (i.e., the initially vaporized constituents) in comparison to the overall fuel DCN.
       
  • The breakdown of self-similarity in electrified counterflow diffusion
           flames
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Mario Di Renzo, Giuseppe Pascazio, Javier Urzay This work addresses the question of the validity of self-similar formulations in describing the structures of methane/air laminar counterflow diffusion flames subjected to incident sub-breakdown DC electric fields. The electric field is induced by two flat porous electrodes located on the oxidizer and fuel sides of the burner and arranged parallel to the mixing layer. Both experiments and numerical simulations of this configuration in recent work suggest the presence of a strong coupling effect between the aerothermochemical and electric fields whereby the velocity field is significantly modified by the momentum carried by a bi-directional ionic wind directed axially outwards from the diffusion flame. However, as shown in this study, such strong coupling is incompatible with standard self-similar formulations of the problem. An a-priori analysis of the steady axisymmetric numerical simulations results in Di Renzo et al. (2018) [1], which employ multi-component transport and detailed chemical kinetics, is presented in this study in order to address the suitability of self-similar descriptions in the present configuration. It is shown that, while self-similarity is preserved in unelectrified conditions along radial distances similar to one orifice radius, it breaks down profusely in electrified conditions as the applied voltage increases and nears saturation conditions, where the electric force field becomes two-dimensional and non-conservative in the close vicinity of the burner axis. As a result, for the purposes of self-similarity, increasing electrification counteracts the slenderness of the counterflow burner and decreases its effective aspect ratio. Counterflow burners should therefore be extra slender if preservation of self-similar conditions is sought under incident electric fields.
       
  • Impact of conformational structures on primary decomposition of
           cis-1,2-dimethylcyclohexyl isomers: A theoretical study
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Huiting Bian, Lili Ye, Jing Li, Jinhua Sun, Tianshui Liang, Wei Zhong, Jun Zhao The different orientations of two methyl groups in “strain-free” cyclic structure generate multiple conformational structures for dimethyl-substituted cyclohexanes. These conformational structures are most likely to affect the radical stabilities, activation energies, and rate coefficients of key types of reactions in dimethyl-substituted cyclohexane combustion. The conformational inversion-topomerization mechanism among various conformers for cis-1,2-dimethylcyclohexyl isomers has been explored by applying high-level quantum electronic-structure methods and transition state theory (TST). Intramolecular H-transfers and β-scissions were also investigated to fundamentally unravel the way how the conformational structures impact their initial decomposition. The present kinetic predictions show that conformational changes are much more rapid compared with the primary decomposition of cis-1,2-dimethylcyclohexyl isomers. It contributes to the establishment of quasi-equilibrium condition for various conformers retained in each radical and ensures the coexistence of all conformers over 300–2500 K. For the primary decomposition, the intramolecular H-transfers are greatly influenced by the conformational structures. Of particular interest is to observe that 1,4 and 1,5 H-transfers that shift the radical site between side chain and ring are only feasible for chair and twist-boat conformers with the radical site locating in axial side chain. Additionally, the β-scissions of cis-1,2-dimethylcyclohexyl isomers also exhibit the dependence on the conformational structures in aspect of steric energy and substituent effect. Furthermore, facilitated by the speedy equilibration among distinct conformers for each isomer, the contribution of each conformer to kinetic predictions for the initial decomposition was systemically evaluated in terms of the temperature-dependent population for diverse conformers obtained by Boltzmann distribution, and then the appropriate rate parameters for each decomposition type were finally recommended.
       
  • Soot inception in laminar coflow diffusion flames
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Daniel Bartos, Mariano Sirignano, Matthew J. Dunn, Andrea D'Anna, Assaad Rachid Masri This paper focuses on the soot inception region in laminar coflow diffusion flames of methane and ethylene stabilised on the Yale diffusion burner. Earlier studies of these flames have focused on the downstream regions where soot has already developed. Laser-induced fluorescence (LIF) and elastic scattering measurements from 266 nm excitation are combined with laser-induced incandescence (LII) excited at 1064 nm. The structure and evolution of the soot precursor particles are characterised using the LIF intensity, decay time and the relative spectral emission in the ultraviolet and visible. The LIF decay times indicate that the majority of 266 nm excited LIF originates from nanostructures rather than gas phase polycyclic aromatic hydrocarbons (PAH). A similarity in the particle evolution for the low and high sooting flames in the upstream regions is found, indicating a general transition towards larger structures with more aromatic features as the nanostructures advect downstream in the fuel rich pyrolytic conditions. Higher nanostructure concentrations are found to precede the higher soot volume fractions (SVF) found in fuel rich sootier flames, although not proportionally, suggesting that surface growth strongly contributes to SVF in the high sooting flames. In the heavier sooting flames the majority of particle formation shifts from the centreline to the wings at the outer edges of the flame closer to stoichiometry. Particle formation in the wings of the flames occurs in the presence of oxygen and higher temperatures, resulting in particles with spectroscopic properties resembling those formed toward the oxidiser side of counter-flow diffusion flames. In the heavier sooting flames, particles produced in the wings appear to mix with particles formed along the centreline of the flame at the flame tip, resulting in a broad range of nanostructures and soot occurring in this region.
       
  • Influence of fuel-oxygen content on morphology and nanostructure of soot
           particles
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Puneet Verma, Edmund Pickering, Mohammad Jafari, Yi Guo, Svetlana Stevanovic, Joseph F.S. Fernando, Dmitri Golberg, Peter Brooks, Richard Brown, Zoran Ristovski 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.
       
  • Occurrence of multiple flame fronts in reheat combustors
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Francesco Gant, Alessandro Scarpato, Mirko R. Bothien Reheat flames are mainly stabilized by autoignition processes. In this paper their unsteady response to temperature fluctuations is investigated. To this end, a simple one dimensional duct is considered as an analytic surrogate model of a reheat combustion chamber. A chemistry-based correlation for the autoignition delay time is used to predict the flame position. In order to analyze the flame response to temperature fluctuations, the inlet boundary condition is harmonically excited. Depending on the amplitude and frequency of excitation, nonlinear phenomena are observed, in particular the co-existence of multiple flame fronts at different axial locations. The analytic formulation of the problem allows us to predict the onset of nonlinearities and to evaluate maps separating the linear from the nonlinear behavior, as function of the system parameters. Amplitude and frequency thresholds triggering different nonlinear properties of the system are identified. Verification of the obtained results for different excitation states is made by comparison to previously conducted Large Eddy Simulations of a simplified reheat combustor geometry represented by a backward-facing step. The simplified model is able to correctly predict the appearance of nonlinear phenomena as observed in the simulations suggesting that the governing factors are well represented.
       
  • Oxidation chemistry of four C9H12 isomeric transportation fuels:
           Experimental and modeling studies
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Yue-Xi Liu, Zhen-Yu Tian A comparison study of the oxidation of n-propylbenzene (NPB), iso-propylbenzene (IPB), 1,3,5-trimethylbenzene (T135MB), 1,2,4-trimethylbenzene (T124MB) in a same jet-stirred reactor was performed to find a most suitable transport fuel as a surrogate component. The experimental results show that IPB with the weakest CH bond tends to be the most active fuel among the four C9H12 fuels, while T135MB is the slowest. Ten common intermediates detected in all experiments and eight characteristic intermediates were comprehensively compared, including hydrocarbons, aromatics and oxygenated species. NPB and IPB tend to produce more hydrocarbons and benzene, while T124MB and T135MB generate more toluene and acrolein. Based on the previous studies, an universal mechanism was presented by involving the four sub-mechanisms with good prediction on the measured results. The rate-of-production analysis shows that H-abstraction on the methyl or propyl is the dominant consumption pathway of C9H12 fuels, while C9H11 radicals maintains bigger differences than the initial reactions. Sensitivity analysis shows that CH3 is the key intermediate in the consumption of propylbenzenes, and OH plays similar role in the oxidation of trimethylbenzenes. Moreover, the production pathways of the aldehydes and PAHs in the oxidation were also discussed to understand the pollutant formations. Benzyl and styrene are believed to be the main precursors of aldehydes and PAHs, respectively. In general, these results provide better understanding of the oxidation and combustion of C9H12 fuels as potential surrogate fuel constituents for kerosene and diesel. Furthermore, the comprehensive experimental and modeling studies of C9H12 fuels could also offer the guidance function for the surrogate component selection.
       
  • Buoyancy effect in sooting laminar premixed ethylene flame
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Warumporn Pejpichestakul, Alberto Cuoci, Alessio Frassoldati, Matteo Pelucchi, Alessandro Parente, Tiziano Faravelli Polycyclic aromatic hydrocarbons (PAHs) are known as soot precursors, but their formation/consumption is not fully understood. A recent comprehensive experimental study of premixed laminar ethylene flame [23] investigated the transition from gas-phase to soot particles. The complex fluid dynamics of this system is taken into account to compare model predictions with experimental measurements and thus further validate a detailed kinetic mechanism of soot formation. The relatively low inlet velocity and the large distance between the burner and the stagnation plate lead to significant influence of buoyancy, which requires a 2-D simulation. The observed constricted (necking) flame structure can be reproduced only using a comprehensive 2-D simulation, which includes buoyancy effects, radiative heat losses, and thermal diffusion. Predicted axial gaseous and PAH species profiles obtained from the CRECK mechanism are in good agreement with the measurements, especially even-carbon-number aromatics. Reasonable agreement of the predicted soot volume fraction profiles is also observed. Additionally, simulation results from different literature kinetic mechanisms are also discussed to highlight similarities and differences. The largest discrepancies among the predictions of the mechanisms are observed for phenylacetylene, a key-species representing the first building block of PAHs synthesis in flames.A comprehensive analysis of relevant physical sub-models is also carried out in 2-D simulations. Additionally, predicted profiles from 2-D and 1-D simulations are compared. Following the literature, a 1-D simulation with imposed mass flux from the 2-D model was carried out to account for buoyancy effects. This approach provides an axial predicted flame temperature profile similar to the 2-D case. However, the predicted mole fraction profiles are quite different, especially for hydrogen and aromatics species because of the failure in accounting for the interplay of enhanced diffusion due to Soret effect, flame stretch, and large radial velocities in the proximity of the stagnation plane.
       
  • A comment on papers by Zhou et al. (CNF, 2018) and Zhou et al. (CST,
           2019): Flame displacement speed, flame front velocity, and edge
           (reactants) velocity
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Sina Kheirkhah, Ömer L. Gülder In two recent articles published in Combustion and Flame (Zhou et al., 2018) and Combustion Science and Technology (Zhou et al., 2019), Zhou et al. misinterpreted the flame front velocity and flame displacement speed definitions. The mistake in these papers are discussed in this short communication.
       
  • Conditions for formation of the blue whirl
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Yu Hu, Sriram Bharath Hariharan, Haiying Qi, Michael J. Gollner, Elaine S. Oran This paper presents a laboratory study of the relation between blue whirls and fire whirls in terms of circulation (swirl) and energy-release rate. The blue whirl is a small, completely blue, soot-free flame that was originally seen when it evolved from more traditional fire whirls burning liquid hydrocarbons on water. The experimental apparatus consists of two offset quartz half-cylinders suspended over a water surface, with fuel injected onto the water surface from below. The flow circulation is calculated using the diameter of the enclosure and hot-wire velocity measurements made at the inlet gap between the half-cylinders. The heat-release rate was varied by adjusting the volumetric supply rate of liquid n-heptane, and is calculated assuming complete combustion. Results show that stable blue whirls form in a narrow range of circulation and energy-release rate close to a previously cited extinction limit. A scaling law derived from the data, based on the length scale of the enclosure, shows that the transition to a blue whirl depends on the gap size between the half-cylinders of the enclosure.
       
  • On the early stages of soot formation: Molecular structure elucidation by
           high-resolution atomic force microscopy
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Mario Commodo, Katharina Kaiser, Gianluigi De Falco, Patrizia Minutolo, Fabian Schulz, Andrea D'Anna, Leo Gross 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.
       
  • Pre-stressing aluminum nanoparticles as a strategy to enhance reactivity
           of nanothermite composites
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Rohit J. Jacob, Kevin J. Hill, Yong Yang, Michelle L. Pantoya, Michael R. Zachariah Aluminum (Al) fuel particles are used in a variety of energetic formulations yet harvesting their full chemical potential energy and increasing their energy release rate upon ignition have been a challenge and are key motivators to advancing energy generation technologies. One approach to improving combustion performance is to alter the mechanical properties of the Al particle by inducing an elevated stress state through prestressing. This study examines the combustion performance of prestressed nanoscale aluminum (nAl) particles that were annealed to temperatures ranging from 200 to 400 °C and quenched at slow (exponential) and faster (linear) cooling rates. Powder X-ray diffraction measurements show that prestressing nAl particles at 300 °C increases the strain by an order of magnitude. Constant volume combustion cell tests on nAl combined with copper oxide nanopowder (nAl + CuO) revealed higher peak pressures and pressurization rates for prestressed nAl + CuO composites compared to their untreated counterpart. High speed emission spectroscopy was employed to deduce condensed phase temperatures from the reaction confined within the combustion cell. Burn time measurements, obtained by integrating the emission spectra, were observed to correlate inversely with generated pressure. High heating rate (∼5 × 105 K/s) in-situ TEM results augment the combustion cell results. The results imply that prestressing mechanically alters the nanoparticles which subsequently accelerate the release of aluminum core through outward diffusion. This results in the rapid loss of nanostructure which was observed at the nanoscale through in-situ electron microscopy. The released aluminum thus reacts rapidly with the oxidizer in the condensed phase resulting in a faster and more violent reaction. Improved performance of prestressed nAl coupled with the simplicity of processing provides a low cost and scalable approach to improving metal fuel particle combustion.
       
  • Non-linear response of the flame velocity to moderately large curvatures
           in laminar jet flames of methane–air mixtures
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Gabriel Garcia-Soriano, Sergio Margenat, Francisco J. Higuera, Jose L. Castillo, Pedro L. Garcia-Ybarra An experiment was set up to investigate the correlation between flame velocity and flame stretch in moderately stretched laminar jet flames of methane–air mixtures at different equivalence ratios. Gas flow velocities were obtained by Particle Image Velocimetry (PIV) from images of axial sections of seeded flows, and the location of the flame front was related to the gradient of luminosity. Flame stretch was determined and shown to be mainly due to flame curvature, with a small contribution from the gas strain rate. Inlet gas velocities not much larger than the planar flame velocity led to short rounded flames for which the classical Markstein relation was experimentally validated at any point of the flame fronts. However, the Markstein relation ceases to be uniformly valid for larger inlet gas velocities, in the order of ten times the planar flame velocity, for which the flame adopts a slender shape. These tall flames are nearly conical and obey the Markstein relation far from their tips, but a superlinear increase of flame velocity with stretch is observed when the tip is approached. Furthermore, the local velocity of these flames depends not only on the stretch but also on the inlet velocity of the gas. This behavior has been analyzed in terms of an empirical correction to the Markstein relation that is quadratic in the flame stretch with a coefficient that depends on the gas inlet velocity. The experimental results suggest that this coefficient is zero below a certain threshold value of this velocity and increases as the square root of the excess of gas inlet velocity above this threshold. This kind of sharp transition controlled by the velocity of the incoming gas is reminiscent of a typical critical behavior and is tentatively taken to reveal a stability exchange between rounded (weakly curved) and slender (strongly curved) flames.
       
  • High-pressure pyrolysis and oxidation of DME and DME/CH4
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Hamid Hashemi, Jakob M. Christensen, Peter Glarborg 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.
       
  • Enhancing ignition and combustion characteristics of micron-sized aluminum
           powder in steam by adding sodium fluoride
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Baozhong Zhu, Fan Li, Yunlan Sun, Yuxin Wu, Wei Shi, Weikang Han, Qichang Wang, Qi Wang Micron-sized aluminum powder exhibits difficulties in ignition and burnout in steam. To overcome these problems, we added various sodium fluoride contents to micron-sized aluminum powder in steam at 800, 900, and 1000 °C to assess their effects on ignition and combustion performances. The ignition delay times, ignition temperatures, and combustion characteristics of all samples were measured primarily using two high-temperature tube resistance electric furnace systems. The experiments showed that adding sodium fluoride to micron-sized aluminum powder decreases its ignition delay time and temperature. Moreover, the ignition delay time and temperature of aluminum powder with addition of sodium fluoride decreases considerably when the temperature increases. For this study, the components and morphology of solid combustion products were obtained using X-ray diffraction and scanning electron microscopy. The combustion efficiency was measured volumetrically using a specially designed apparatus. The obtained combustion efficiency was found to increase with the amount of sodium fluoride added and increase in the temperature. The relationship among the parameters related to combustion characteristics and product characterization was examined in detail to reveal the combustion mechanism.
       
  • Microgravity diffusion flame spread over a thick solid in step-changed
           low-velocity opposed flows
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Feng Zhu, Zhanbin Lu, Shuangfeng Wang, Yongli Yin We report results from a microgravity combustion experiment conducted aboard the SJ-10 satellite of China, focusing on the structure and dynamics of diffusion flames spreading over a thick PMMA in low-velocity opposed flows. The width of the PMMA sample is chosen to be as large as possible in order to minimize the side diffusion effects of oxygen, and for each of the four oxygen concentration cases considered, four decrementally changing gas flow velocities are imposed such that a wide range of parameter values are spanned near the quenching limit. Two distinct flame spread modes are identified near the quenching limit, namely the continuous flame mode for gas flow velocities greater than an oxygen-concentration dependent critical value, and the flamelet mode for subcritical gas flow velocities. The transition process between these two spread modes due to a step change in the gas flow velocity is usually accompanied by flame oscillations, and diffusive-thermal instability of the leading flame front is identified as the mechanism controlling such transition. A correlation of the flame spread rate data among different oxygen concentrations indicates that, in the presently considered radiation-controlled regime the normalized flame spread rate deviates from the predictions of the thermal theory and decreases monotonically with the increase in the flame Damköhler number. Meanwhile, with the decrease in the flame spread rate, the standoff distance and the inclination angle at the flame leading edge show an increasing and decreasing trend, respectively. An energy balance analysis across the fuel surface beneath the flame leading edge indicates that the variation of the heat absorbed by the solid for vaporization is sub-linear with respect to the flame spread rate, thereby implying that the fuel regression depth has a tendency to increase with decreasing flame spread rate. Moreover, the energy balance analysis suggests that the quenching boundary and the marginal stability boundary identified on the flammability map are, respectively, intrinsically associated with a certain specific ratio of the overall heat losses to the total heat conducted from the flame, or equivalently, associated with a certain specific value of the flame spread rate.
       
  • A species-clustered splitting scheme for the integration of large-scale
           chemical kinetics using detailed mechanisms
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Jian-Hang Wang, Shucheng Pan, Xiangyu Y. Hu, Nikolaus A. Adams In this study, a species-clustered integrator for chemical kinetics with large detailed mechanisms based on operator-splitting is presented. The ordinary differential equation (ODE) system of large-scale chemical kinetics is split into clusters of species by using graph partition methods which have been intensely studied in areas of model reduction, parameterization and coarse-graining, e.g., diffusion maps based on the concept of Markov random walk. The definition of the weight (similarity) matrix is application-dependent and follows from chemical kinetics. Each species cluster is integrated by the variable-coefficient ODE solver VODE. The theoretically expected speedup in computational efficiency is reproduced by numerical experiments on three zero-dimensional (0D) auto-ignition problems, considering detailed hydrocarbon/air combustion mechanisms at varying scales, from 53 species with 325 reactions of methane to 2115 species with 8157 reactions of n-hexadecane. Optimal clustering weighing both prediction accuracy (for ignition delay and equilibrium temperature) and computational efficiency is implied with the clustering number N=2 for the 53-species methane mechanism, N=4 for the 561-species n-heptane mechanism and N=8 for the 2115-species n-hexadecane mechanism.
       
  • Filtered Wrinkled Flamelets model for Large-Eddy Simulation of turbulent
           premixed combustion
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Renaud Mercier, Cédric Mehl, Benoît Fiorina, Vincent Moureau Models for combustion LES based on a geometrical description of the reactive layer are well suited to capture the turbulent flame front displacement speed, but do not predict the filtered chemical flame structure. This article aims to discuss and model the impact of the flame sub-filter wrinkling level on the species production, with a focus on carbon monoxide emission. For that purpose, 2-D wrinkled flames with a sinusoidal pattern, which include detailed chemistry effects, are manufactured. Three controlling parameters are identified: the flame filter size, the sub-filter flame wrinkling and the number of flame patterns contained within the sub-filter volume. This new flame archetype, named Filtered Wrinkled Flamelets (FWF), may be embedded in various combustion modeling frameworks. In the present paper, it is used to build-up a filtered chemical look-up table in order to model the unclosed terms of the filtered progress variable equation. A priori tests are conducted by analyzing an existing turbulent premixed flame database. A posteriori tests consist in modeling the swirling bluff-body stabilized Cambridge flame. Results analysis shows that accounting for sub-filter flame wrinkling on the chemical flame structure is mandatory to predict intermediate species such as CO.
       
  • Intrinsic thermoacoustic feedback loop in turbulent spray flames
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Abdulla Ghani, Thomas Steinbacher, Alp Albayrak, Wolfgang Polifke This paper investigates low-frequency thermoacoustic instabilities of a turbulent spray flame, a phenomenon known as ‘rumble’. Based on experimental data, a network model analysis is performed, which suggests an intrinsic thermoacoustic (ITA) feedback loop as the root cause of instability. At first, the ITA nature of the observed instability is confirmed by a parametric analysis, which reveals the sensitivity of the instability frequency to the time delay of the flame. Then, we investigate pure acoustic modes and pure ITA modes, which couple to the full system modes such that the origin and the trajectory of each mode is trackable. Finally, we show that the unstable mode frequency scales with the inlet bulk velocity: a feature that clearly separates the observed instability from classical cavity modes. The network model results are corroborated by phasor plots, in which all relevant phase information is compiled. It reveals the phase of the Flame Transfer Function (FTF) contributing to the instability and provides an estimate of the ITA frequency, which agrees well with the dominant peak in the experimental pressure spectrum. Additionally, the obtained flame phase is used to infer the instability frequency by the experimentally measured droplet burning time τB, which reproduces similar trends as experimental and network model results. This theoretical study confirms that (1) ITA feedback loops are important for spray flames, (2) that the ITA instability of the experiment scrutinized is controlled by the droplet dynamics and (3) ITA modes appear also in acoustically closed systems.
       
  • Providing effective constraints for developing ketene combustion
           mechanisms: A detailed kinetic investigation of diacetyl flames
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Wenyu Sun, Jiaxing Wang, Can Huang, Nils Hansen, Bin Yang Ketene is an important combustion intermediate, but due to its high reactivity, it has been difficult to validate the corresponding kinetic sub-mechanisms with direct experimental measurements. It is shown here the possibility to refine the ketene combustion model through relevant experimental and modeling analyses of diacetyl [2,3-butadione, (CH3CO)2] combustion. The consumption of diacetyl leads to abundant ketene under high-temperature oxidation conditions. From the aspect of the hierarchical structure of a kinetic mechanism, the ketene sub-mechanism acts as a part of a secondary mechanism in the kinetic scheme exploring diacetyl combustion. Therefore, this strategy to understand the kinetics of ketene through studying the combustion chemistry of diacetyl should be based on a good understanding of the diacetyl combustion kinetics. To this end, we investigated the flame chemistry of diacetyl through experimental, modeling, and theoretical approaches. Flame-sampling molecular-beam mass spectrometry with single-photon ionization was employed to measure the detailed chemical structure of a premixed diacetyl flame stabilized at the pressure of 18.0 Torr and the equivalence ratio of 1.2. The quantitative speciation measurements were essential in identifying important reactions in the fuel sub-mechanism, and the corresponding rate coefficients were obtained through high-level theoretical calculations. A kinetic model was proposed, which showed satisfactory performances in predicting the measured flame structure. With the diacetyl flame chemistry better understood, global sensitivity analyses were carried out to suggest conditions for future experiments potentially facilitating the development of ketene sub-mechanisms.
       
  • Role of ozone doping in the explosion limits of hydrogen-oxygen mixtures:
           Multiplicity and catalyticity
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Wenkai Liang, Yiru Wang, Chung K. Law Effects of ozone doping on the Z-shaped explosion boundary of stoichiometric hydrogen-oxygen mixtures are computationally studied. Results show that with increasing ozone doping and within a small range of increment, the explosion limit transitions from the Z-shaped response with two turning points to responses exhibiting four, six, four, two and none turning points. By modifying the rate coefficients of the sensitive reactions within the ozone sub-mechanism, four reactions are identified to control the above highly non-monotonic behavior, and that the system transitions to the H2O3 chemistry as the limit becomes monotonic and overall more explosive. The practical implication is that there exists a critical range of ozone doping such that the mixture is more sensitive to unstable burning for lower levels of doping, with the associated multiple transition states, while the facilitating role of ozone catalyticity is fully realized with just slightly higher levels of doping, resulting in overall stronger explosive burning.
       
  • Observed dependence of characteristics of liquid-pool fires on swirl
           magnitude
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): Wilfried Coenen, Erik J. Kolb, Antonio L. Sánchez, Forman A. Williams One dozen vertically oriented thin rectangular vanes, 62 cm tall and 15.2 cm wide, were placed 27 cm from the center of heptane and ethanol pool fires in continuously fed, floor-flush pans 3.2 cm and 5.1 cm in diameter in the laboratory. The vanes were all oriented at the same fixed angles from the radial direction, for 9 different angles, ranging from 0∘ to 85∘, thereby imparting 9 different levels of circulation to the air entrained by each pool fire. The different swirl levels were observed to engender dramatically different pool-fire structures. Moderate swirl suppresses the global puffing instability, replacing it by a global helical instability that generates a tall fire whirl, the height of which increases with increasing circulation. Except for the largest heptane pool, higher swirl levels produced vortex breakdown, resulting in the emergence of a bubble-like recirculation region with a ring vortex encircling the axis. Measured burning rates increase with increasing swirl levels as a consequence of the associated increasing inflow velocities reducing the thickness of the boundary layer within which combustion occurs right above the liquid surface, eventually forming detached edge flames in the boundary layer that move closer to the axis as the circulation is increased. Still higher circulation reduces the burning rate by decreasing the surface area of the liquid covered by the flame, thereby reducing the height of the fire whirl. Even higher circulation causes edge-flame detachment, resulting in formation of the blue whirl identified in recent literature, often meandering over the surface of the liquid in the present experiments. This sequence of events is documented herein.
       
  • Experimental investigation of aerodynamics and structure of a
           swirl-stabilized kerosene spray flame with laser diagnostics
    • Abstract: Publication date: July 2019Source: Combustion and Flame, Volume 205Author(s): P. Malbois, E. Salaün, A. Vandel, G. Godard, G. Cabot, B. Renou, A.M. Boukhalfa, F. Grisch A gas turbine model combustor was equipped with an industrial Lean Premixed fuel injection system operating with liquid commercial kerosene (Jet-A1) at atmospheric pressure. Large optical accesses enable joint Particle Image Velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) measurements at repetition rates up to 5 kHz and 10 kHz, respectively. Using these diagnostics, flame topologies and non-stationary events were investigated in operating conditions representative of the ones encountered in real aeronautic propulsion systems. The flame shape was analyzed in terms of interactions between the different flame zones responsible for flame stabilization in confined swirled flames. Data processing of the strain rate and vorticity fields highlighted the existence of two shear layers that interfere differently with the inlet air/fuel mixing jet. An inner shear layer (ISL) between the Inner Recirculation Zone (IRZ) and the fresh inlet flow is located at the upper base of the fuel spray. An Outer Shear Layer (OSL) is also identified between the Outer Recirculation Zone (ORZ) and the fresh incoming flow. Spanwise-oriented vortices are produced from this latter, with a growth rate function of the free stream speed ratio (fresh incoming reactants and the flow circulating inside ORZ). The detailed analysis of the shear layers gives new insight on the flame structures obtained from OH-PLIF data. Finally, high-speed simultaneous measurements of flow velocity and OH distributions highlighted unusual flame pinching mechanisms leading to the release of subsequent unburned pockets propagating in the burned gases.
       
  • The effects of pressure treatment on the flamelet modeling of supersonic
           combustion
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Foluso Ladeinde, Zhipeng Lou, Wenhai Li The flamelet method has been used extensively as an affordable turbulence-combustion interaction model that obviates the need to solve the evolution equations for the species mass fractions during a large-eddy or Reynolds-averaged Navier–Stokes calculation of a reactive flow field, leading to substantial savings in the simulation time and enabling modeling with relatively complex kinetic mechanisms. The canonical problem analyzed and stored in a look-up array in the flamelet procedure usually assumes some baseline fields; in particular, the pressure is often specified at a fixed value that is characteristic of the examined configuration. However, pressure in supersonic combustion has significant dynamical roles, unlike in low-Mach number or incompressible flows, and a constant pressure field will not be adequate for the former. To remedy this problem, reaction rate in the combustor is often assumed to scale squarely with pressure. This approach, which is probably acceptable for low-speed, high pressure combustors, is not suitable for dealing with the variable pressure conditions in supersonic combustion. This paper focuses on the assessment of the aforementioned scaling, in absolute sense, and also relative to an approach where pressure is added as a control parameter in the flamelet library. To achieve this, three classes of reactive systems with different levels of modeling complexities are investigated to show that representative chemical variables do not scale squarely with pressure. For the case of supersonic combustion, the scaling treatment in general leads to over-prediction of pressure and combustion and also tends to stabilize the flame. To the knowledge of the authors, no previous studies have reported on the issues addressed in the present paper.
       
  • A split random time-stepping method for stiff and nonstiff detonation
           capturing
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Jian-Hang Wang, Shucheng Pan, Xiangyu Y. Hu, Nikolaus A. Adams In this paper, a new operator splitting method is proposed for capturing stiff and nonstiff detonation waves. In stiff cases, an incorrect propagation of discontinuities might be observed for general shock-capturing methods due to under-resolution in space and time. Previous random projection methods have been applied successfully for stiff detonation capturing at under-resolved conditions. Not relying on random projection of the intermediate state onto two presumed equilibrium states (completely burnt or unburnt) as with the random projection method, the present approach randomly advances or interrupts the reaction process. Each one-way reaction is decoupled from the multi-reaction kinetics by operator splitting. The local temperature is compared with a random temperature within a temperature interval to control the random reaction. Random activation or deactivation in the reaction step serves to reduce the accumulated error of discontinuity propagation. Extensive numerical experiments demonstrate the effectiveness and robustness of the method. For nonstiff problems, the proposed random method recovers the accuracy of general operator splitting methods by adding a drift term.
       
  • Reaction pathways, kinetics and thermochemistry of the
           chemically-activated and stabilized primary methyl radical of methyl ethyl
           sulfide, CH3CH2SCH2•, with 3O2 to CH2CH3SCH2OO•
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Guanghui Song, Joseph W. Bozzelli Oxidation of Methyl ethyl sulfide (CH3SCH2CH3, methylthioethane, MES) under atmospheric and combustion conditions is initiated by reaction with hydroxyl radicals. Methyl ethyl sulfide (MES) radicals generated after losing an H atom via OH abstraction subsequently reacts with O2 to form chemically activated and stabilized peroxyl radical adducts. The kinetics of the chemically activated reaction between the CH2•SCH2CH3 radical and molecular oxygen are analyzed using quantum Rice–Ramsperger–Kassel (QRRK) theory for k(E) with master equation analysis and a modified strong-collision approach to account for further reactions and collisional deactivation. Thermodynamic properties of reactants, products and transition states are determined by the CBS-QB3 composite and M062X/6-311+G(2d, p) DFT methods. The reaction of CH2•SCH2CH3 with O2 forms an energized peroxy adduct •OOCH2SCH2CH3 with a calculated well depth of 26.4 kcal/mol at the CBS-Q//M062x/6-311+g(2d,p) levels of theory. Thermochemical properties of reactants, transition states and products obtained under CBS-QB3 level are used for calculation of the thermochemical and kinetic parameters. The temperature and pressure dependent rate coefficients for both the chemically activated reactions of the energized adduct and the thermally activated reactions of the stabilized adducts are presented. Stabilization and isomerization of the •OOCH2SCH2CH3 adduct are important under high pressure and low temperature. At temperatures between above 600–800 K reactions of the chemically activated peroxy adduct become important relative to stabilization under the atmospheric pressure. Two new pathways are observed, in addition to conventional hydrogen atom transfer reactions from the three carbons to the peroxy oxygen radical. One of these new paths is formation of a Criegee intermediate plus CH2•OO• plus CCS•. A second new path involves the peroxy oxygen radical addition to the sulfur moiety followed by carbon-sulfur bond cleavage with formation of carbon–oxygen and oxygen–sulfur double bonds: CH2O + S•(O)CC. These are potentially important new pathways other alkyl-sulfide peroxy radical systems under thermal or combustion conditions.
       
  • Ignition delay times of ethane under O2/CO2 atmosphere at different
           pressures by shock tube and simulation methods
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Yang Liu, Jia Cheng, Chun Zou, Lixin Lu, Huixiang Jing Pressurized oxy-fuel combustion is a promising oxy-fuel technology owing to its high efficiency and low emission. The ignition delay times of ethane under O2/CO2 atmosphere were determined in a shock tube at different pressures, equivalence ratios, and C2H6 and CO2 concentrations. The results suggested that the ignition delay times decrease with the increasing ethane concentration at 0.8, 2.0, and 10 bar, while the effect of the fuel concentration on the ignition delay times is not sensitive to the pressure. The ignition delay times increased with the increasing equivalence ratio at 0.8 and 2.0 bar, while the effect of the equivalence ratio decreased with the increasing pressure from 0.8 to 2.0 bar. At 10 bar, the effect of the equivalence ratio on the ignition delay times further weakened at high temperatures, while the ignition delay times decreased with the increasing equivalence ratio in the low-temperature range. An updated model (OXYMECH) was developed and updated on the basis of our previous work, providing yields in good agreement with the experimental data under all conditions, while Aramco 2.0 showed poor prediction of the experimental results at 10 bar. Analysis of the sensitivity and the rate of production indicated that updating the rate constants of the reactions C2H6 + HO2 ⇔ C2H5 + H2O2, H + O2 (+M) ⇔ HO2 (+M), CH3 + HO2 ⇔ CH3O + OH, 2HO2 ⇔ H2O2 + O2, C2H4 + H (+M) ⇔ C2H5 (+M), and H + O2 ⇔ O + OH improves the performance at 10 bar.
       
  • Numerical investigation on the initiation of oblique detonation waves in
           stoichiometric acetylene–oxygen mixtures with high argon dilution
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Yuhang Zhang, Yishen Fang, Hoi Dick Ng, Honghui Teng 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.
       
  • Characterization of flameless combustion in a model gas turbine combustor
           using a novel post-processing tool
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Ehsan Fooladgar, Pál Tóth, Christophe Duwig Flameless combustion is a very promising technology for the future gas turbines. It is clean and stable—without large oscillations, noise and flashback. To facilitate the adoption of this technology in gas turbines, advanced design tools are needed. In this paper, a recently developed unsupervised post-processing tool is used to analyze the large amount of high-dimensional data produced in a series of Large Eddy Simulations (LES) of a model gas turbine operating in flameless mode. Simulations are performed using Finite Rate Chemistry (FRC) combustion modeling and a detailed description of chemistry. The automatic post-processing reveals important features of the combustion process that are not easily recognizable by other methods, making it a complementary step for the already established FRC–LES approach, and a potential design tool for advanced combustion systems.
       
  • Investigating the effect of oxy-fuel combustion and light coal volatiles
           interaction: A mass spectrometric study
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Martina Baroncelli, Daniel Felsmann, Nils Hansen, Heinz Pitsch Given the multi-physical nature of coal combustion, the development and validation of detailed chemical models reproducing coal volatiles combustion under oxy-fuel conditions is a crucial step towards the advancement of predictive full-scale simulations. During the devolatilization process, a large variety of gases is released and undergoes secondary pyrolysis and oxidation reactions. Therefore, the ability to capture their interactions is a prerequisite for each chemical model used in its detailed or reduced form to simulate these processes. In this work, a high-resolution time-of-flight molecular-beam mass spectrometer was employed to enable fast and simultaneous detection of stable and unstable species in counterflow flames of typical light volatiles. Following an approach of increasing complexity, carbon dioxide and methane were progressively added to an argon diluted acetylene base flame. For the three flames investigated here, results showed a significant increase in the concentration of C2 and C3 hydrocarbons and oxygenated compounds caused by methane addition to the acetylene flame. By hindering the production of the butadienyl radical, the addition of methane induces the reduction of benzene which triggers the decrease of aromatic species. Conversely, CO2 addition did not have significant effects on intermediates. To guide and interpret the measurements, numerical simulations with two existing chemical models were performed and the results were found to be consistent with the experimental data for small hydrocarbons. Some discrepancies were found between the two model predictions and between simulations and experiments for C4 and C5 species. Additionally, numerical simulations were found to overestimate the role of the methyl radical in aromatics formation.
       
  • Measurement of methane autoignition delays in carbon dioxide and argon
           diluents at high pressure conditions
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Miad Karimi, Bradley Ochs, Zefang Liu, Devesh Ranjan, Wenting Sun The directly fired supercritical carbon dioxide (sCO2) power cycle has high efficiency while allowing nearly complete carbon dioxide (CO2) capture. The operating conditions of sCO2 power cycle (100–300 bar) combustors are dramatically different from conventional gas turbine combustors. However, combustion properties such as autoignition delay are not well understood at these conditions. This study reports methane autoignition delay measurements for diluted carbon dioxide environments at 100 and 200 bar and at temperatures within the range of 1139–1433 K using a high pressure shock tube. To study the effect of CO2 on ignition, similar experiments are conducted at 100 and 200 bar by replacing carbon dioxide with argon. The experimental data is then compared with calculations using different chemical kinetics models. For the conditions of this study, predictions of the Aramco Mech 2.0 show the overall best agreement with experimental measurements, while predictions of the GRI 3.0 kinetic model have the largest (by a factor of 3) deviation with experiments. Sensitivity and reaction pathway analyses reveal that methyl (CH3) recombination to form ethane (C2H6) and oxidation of CH3 to form methoxide (CH3O) are the most important reactions controlling the ignition behavior at temperatures greater than approximately 1250 K. However, at temperatures below approximately 1250 K, an additional reaction pathway for methyl radicals is found through CH3+O2+M = CH3O2+M which leads to formation of methyldioxidanyl (CH3O2). This reaction pathway plays a distinct role in dictating the ignition trends at lower temperature conditions.
       
  • Soot temperature characterization of spray a flames by combined extinction
           and radiation methodology
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Tiemin Xuan, Jose M. Desantes, Jose V. Pastor, Jose M. Garcia-Oliver Even though different optical techniques have been applied on ‘Spray A’ in-flame soot quantification within Engine Combustion Network in recent years, little information can be found for soot temperature measurement. In this study, a combined extinction and radiation methodology has been developed with different wavelengths and applied on quasi-steady Diesel flame to obtain the soot amount and temperature distribution simultaneously by considering self-absorption issues. All the measurements were conducted in a constant pressure combustion chamber. The fuel as well as the operating conditions and the injector used were chosen following the guidelines of the Engine Combustion Network. Uncertainty caused by wavelength selection was evaluated. Additionally, temperature-equivalence ratio maps were constructed by combining the measurements with a 1D spray model.Temperature fields during the quasi-steady combustion phase show peak temperatures around the limit of the radiation field, in agreement with a typical diffusion flame structure. Effects of different operating parameters on soot formation and temperature were investigated. Soot temperature increases dramatically with oxygen concentration, but it shows much less sensitivity with ambient temperature and injection pressure, which on the other hand have significant effects on soot production.
       
  • Aircraft and MiniCAST soot at the nanoscale
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Iman Marhaba, Daniel Ferry, Carine Laffon, Thomas Z. Regier, François-Xavier Ouf, Philippe Parent Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) and Fourier-transform infrared spectroscopy (FTIR) have been used to compare the nanoscale characteristics of aircraft soot collected at the exhaust of a recent PowerJet SaM146 jet engine, with those of soot generated by a MiniCAST burner. Analyses show that some MiniCAST operating conditions enable generating soot particles of morphology, internal nano-structure and chemical structure close to those of aircraft soot. However, MiniCAST soot have gyration diameters systematically larger compared to aircraft soot. Provided that this imperfect agreement is not critical for the studied properties, MiniCAST soot might be used as a relevant analogue of aircraft soot for studying some of their physical or chemical properties, offering a convenient and affordable way to conduct laboratory studies on the environmental impacts of aviation emitted particles.
       
  • Ion current and carbon monoxide release from an impinging methane/air
           coflow flame in an electric field
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Yu-Chien Chien, David Escofet-Martin, Derek Dunn-Rankin This research examines changes in the ion current and the release of carbon monoxide (CO) from surface-impinging coflow diffusion flames when subjecting those flames to high DC electric fields. Carbon monoxide results from the incomplete oxidation of hydrocarbon fuels and, while CO can be desirable in some syngas processes, it is usually a dangerous emission from forest fires, gas heaters, stoves, or furnaces where the core reaction insufficiently oxidizes the fuel to carbon dioxide and water. Electrical aspects of flames, specifically, the production of chemi-ions in hydrocarbon flames and the convective flows driven by these ions, have been investigated in ranges of application, and this research examines the use of electric fields as one mechanism for governing combustion as flames are partially extinguished when impinging on nearby surfaces. We evaluate the ion current and study the flame behavior at saturation current in response to the impinging plate location. We also measure the changes in CO emission, as correlated with variations in flame structure observed using OH chemiluminescence and OH planar laser induced fluorescence (PLIF), as a function of burner-to-plate distance and electrical potential applied to the flame. Three major saturation current impact regions in plate distance is observed, and it uses chemiluminescence with PLIF to illuminate the relative locations of the peak heat release zone and the extended oxidative zone changing by external electric fields. The results show that CO release correlates strongly with changes in location and extent of high concentration regions of OH in the surface-impinging diffusion flames. The results also show that electric fields affect the CO emission but that the burner-to-plate distance has the dominant influence. The detailed findings indicate that a continuous monitor of ion current from the flame to the impinging surface can be an effective sensing index of flame behavior and CO emission.
       
  • Structural effects on the growth of large polycyclic aromatic hydrocarbons
           by C2H2
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Abhijeet Raj Soot particles are composed of planar and curved polycyclic aromatic hydrocarbons with different types of reactive sites, where their growth and oxidation reactions occur. This study presents the effect of curvatures in PAHs present in soot on their growth in the flame environment. For this, six planar and curved model PAH molecules having five to nine aromatic rings with armchair and zigzag sites are selected. Density functional theory (B3LYP functional and 6-311G (d,p) basis set) is used to study the reaction energetics for their growth through hydrogen-abstraction-C2H2-addition mechanism that is primarily used in all soot models for their chemical growth. The rate constants evaluated using transition state theory for the reactions involved in the mechanism are provided. Through energetics and kinetics comparison, the differences in the reactivity of planar and curved PAHs at different site types are observed. The growth at armchair site was found to be sensitive to the PAH structure, with the curved one having higher growth rate at all temperatures between 1000 and 2500 K studied in this work. The PAH curvature had less impact on the growth at zigzag sites of the model PAHs. The fast conversion of planar PAH to a curved one through ring addition and the high growth rate of curved PAHs at low temperatures suggests that tortuous PAHs can also be present in nascent soot.
       
  • A regularized deconvolution method for turbulent closure modeling in
           implicitly filtered large-eddy simulation
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Qing Wang, Matthias Ihme Turbulence closure models that are based on a regularized deconvolution method (RDM) are proposed for application to implicitly filtered Large-Eddy Simulation (LES). This method reconstructs sub-grid scale contributions by approximately recovering filtered quantities through an optimization approach. Physical principles on the boundedness and conservation of reconstructed quantities are preserved by constraining the optimization problem. To account for effects of scales that are not resolved in implicitly filtered LES, a reconstruction method is proposed that combines sub-grid projection and energy-similarity to enrich unresolved scales. With this method, closure models are developed for turbulent scalar fluxes and turbulence–chemistry interaction that are intrinsically consistent with the LES formulation. These closure models are evaluated in application to a piloted turbulent jet flame with inhomogeneous inlet composition, in which combustion is represented by a two-scalar manifold model. The RDM-formulation is benchmarked against simulations obtained using a presumed PDF method and eddy diffusivity closure model. In partially-premixed flame regions, it is observed that RDM improves the prediction of turbulent mixing, and faster grid convergence is observed for results with RDM compared to simulations employing a presumed PDF method.
       
  • The chemical structure effects of alkylbenzenes on soot formation in a
           laminar co-flow flame
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Carson Chu, Tongfeng Zhang, Murray J. Thomson Alkylbenzene isomers are often used in transportation fuel surrogates but have different sooting tendencies. There is a need to understand how their chemical structures affect the soot formation mechanism. In this study, the chemical structure effects of alkylbenzenes (1,2,4-trimethylbenzene (124TMB) and n-propylbenzene (PBZ)) on soot formation in a laminar diffusion flame were experimentally and numerically investigated. In the experiment, the optical time-resolved laser-induced incandescence (LII) and spectral soot emission (SSE) diagnostics were used to measure radial soot volume fraction, primary particle diameter and flame temperature profiles. The radial number density was also experimentally derived. The results are consistent with the literature as 124TMB exhibits higher soot concentration than PBZ. From the analyses of the primary particle diameters and the derived number density, this is caused by the higher soot nucleation rate for 124TMB. This conclusion is supported by numerical modeling, which utilized the detailed CoFlame code with a moderately reduced CRECK mechanism. The simulation results show that 124TMB has earlier soot inception and Polycyclic Aromatic Hydrocarbon (PAH) addition than PBZ. Consistent with both earlier soot inception and PAH addition, the 124TMB model predicts earlier pyrene (A4) formation, suggesting 124TMB has alternative reaction pathways for pyrene formation. The reaction pathway analysis suggests that pyrene is formed via the aromatic radical recombination route, as opposed to the conventional HACA mechanism. Bypassing the formation of the second ring could be the reason for 124TMB having earlier soot nucleation. In contrast, the PBZ model predicts that the formation of pyrene follows the slower HACA pathways, leading to later soot nucleation.
       
  • Ignition and detonation onset behind incident shock wave in the shock tube
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): A.D. Kiverin, I.S. Yakovenko The paper analyses in details and describes the process of ignition kernel formation and subsequent detonation onset behind the shock wave propagating in the shock tube. To get the overall pattern of the process a series of one- and two-dimensional calculations are carried out with the use of a dissipation-free numerical technique. It is shown that one of the leading roles in the process of ignition kernel formation belongs to the non-steady flow dynamics establishing behind the shock wave. The development of the boundary layer determines both the temperature re-distribution in the near-wall region and the conditions for gas-dynamic acceleration of the flow. With an account of thermal runaway, the most intensively heated region corresponds to the area between the inner margin of the boundary layer and the contact surface separating driver gas and the test mixture. After localized ignition takes place the forming reaction wave propagates out from the ignition epicenter. Reaction wave propagates behind the outrunning compression wave through the already reacting mixture. Shock-induced compression of the test mixture provides conditions for the self-sustained acceleration of the combustion wave, and finally, the detonation onset takes place.
       
  • A promising strategy to obtain high energy output and combustion
           properties by self-activation of nano-Al
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Jun Wang, Yanyang Qu, Feiyan Gong, Jinpeng Shen, Long Zhang 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.
       
  • Pressure-dependent rate rules for intramolecular H-migration reactions of
           normal-alkyl cyclohexylperoxy radicals
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): XiaoXia Yao, JingBo Wang, Qian Yao, YongQing Li, ZeRong Li, XiangYuan Li For hydrocarbon fuels, reliable chemical mechanisms play an essential role towards improving the simulations of combustion process. However, in most of the previous modeling studies for alkylcycloalkanes combustion, the kinetic parameters are taken from acyclic alkanes based on the similarities between the reaction classes of alkylcycloalkanes and acyclic alkanes. Among the reaction families for alkylcycloalkanes, the intramolecular H-migration reactions of normal-alkyl cyclohexylperoxy radicals are one of the most important reaction families in the low temperature oxidation mechanisms. High-pressure limit rate rules and pressure-dependent rate rules for this reaction family have been developed based on quantum chemical computations at the CBS-QB3 level in combination with the transition state theory (TST) and Rice–Ramsberger–Kassel–Marcus/Master-Equation (RRKM/ME) theory. The reactions in this family involve a bicyclic transition state and can be divided into classes depending upon the cycle size of the newly formed cycle in the transition states and the types of the carbons from which the H atoms are migrated and if the H-migration occurs on the alkyl side chain, or on the cycle, or from the side chain to the cycle, or from the cycle to the side chain. For each reaction class, a representative set of reactions are chosen from methylcyclohexane to n-butylcyclohexane and the energy barriers of all chosen reactions are calculated and the average value for each class is obtained and compared with the value of the similar reaction class in alkanes. The high-pressure limit rate constants and the pressure-dependent rate constants of all reactions at pressures varying from 0.01 to 100 atm are calculated and the high-pressure limit rate rules and pressure-dependent rate rules for each class are derived from the average rate constants of reactions within each class. All calculated rate constants are fitted by a nonlinear least-squares method to the form of a modified Arrhenius rate expression.It is shown that there are large differences of the rate constants between alkylcycloalkanes and alkanes no matter if the H-migration occurs on the alkyl side chain, on the cycle or between the side chain and the cycle. Therefore, the construction of rate rules for alkylcycloalkanes, instead of taking values from similar reactions in alkanes, is necessary and significant to the low-temperature combustion modeling of alkylcyclohexanes.
       
  • Measurement and modelling of the laminar burning velocity of
           methane-ammonia-air flames at high pressures using a reduced reaction
           mechanism
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Ekenechukwu Chijioke Okafor, Yuji Naito, Sophie Colson, Akinori Ichikawa, Taku Kudo, Akihiro Hayakawa, Hideaki Kobayashi 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 CH4NH3-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 CH4NH3-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 CH4NH3-air flames with high fidelity at all studied conditions. It also models satisfactorily NH3, NO and CO concentration in CH4NH3O2N2 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+H2OHNO+H2, which in fact may not be important in fuel NO chemistry.
       
  • Active subspace-based dimension reduction for chemical kinetics
           applications with epistemic uncertainty
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Manav Vohra, Alen Alexanderian, Hayley Guy, Sankaran Mahadevan We focus on an efficient approach for quantification of uncertainty in complex chemical reaction networks with a large number of uncertain parameters and input conditions. Parameter dimension reduction is accomplished by computing an active subspace that predominantly captures the variability in the quantity of interest (QoI). In the present work, we compute the active subspace for a H2/O2 mechanism that involves 19 chemical reactions, using an efficient iterative strategy. The active subspace is first computed for a 19-parameter problem wherein only the uncertainty in the pre-exponents of the individual reaction rates is considered. This is followed by the analysis of a 36-dimensional case wherein the activation energies and initial conditions are also considered uncertain. In both cases, a 1-dimensional active subspace is observed to capture the uncertainty in the QoI, which indicates enormous potential for efficient statistical analysis of complex chemical systems. In addition, we explore links between active subspaces and global sensitivity analysis, and exploit these links for identification of key contributors to the variability in the model response.
       
  • Flame stabilization of supersonic ethylene jet in fuel-rich hot coflow
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Bing Liu, Guoqiang He, Fei Qin, Qingchun Lei, Jian An, Zhiwei Huang The stability limit of a supersonic ethylene jet flame in a fuel-rich hot coflow was examined by investigating the influence of the injection pressure, which was varied from 2.0 atm to 4.5 atm, and of the equivalence ratio of the coflow, which was varied from 1.2 to 1.6. The flames were investigated with time-resolved chemiluminescence and schlieren images, as well as a large-eddy simulation of combustion. The results show that, with increasing injection pressure, the flame state changes from stable to unstable and blow-off, and the flame brush thickness, heat release, and height of coflow decrease. The flame stability limits decrease as the equivalence ratio of coflow increases. Lastly, a large-eddy simulation was performed to investigate the mechanism of flame stabilization, and the numerical simulation results are in good agreement with the experimental results. It was found that the stability of a supersonic flame is affected by the chemical time scale and flow time scale.
       
  • Turbulent flame–shock interaction inducing end-gas autoignition in a
           confined space
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Haiqiao Wei, Jianfu Zhao, Xiaojun Zhang, Jiaying Pan, Jianxiong Hua, Lei Zhou To understand the detonation phenomenon in a confined space, especially in a downsized spark ignited engine and in explosion disasters, an experiment on the turbulent flame–shock interaction inducing end-gas autoignition was carried out in an improved constant volume combustion chamber equipped with a perforated plate. In the present work, the entire detonation formation process, including the turbulent flame acceleration, shock wave formation and enhancement process, motion and reflection of the shock wave, and detonation formation and propagation, was observed. A very strong pressure oscillation with a peak value of 5.7 MPa and a maximum amplitude of pressure oscillation of 3.8 MPa are achieved for the end-gas autoignition with detonation development.
       
  • Numerical study of the transition between slow reaction and ignition in a
           cylindrical vessel
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): J. Melguizo-Gavilanes, P.A. Boettcher, R. Mével, J.E. Shepherd A numerical study of a transiently (uniformly/non-uniformly) heated cylindrical reactor was performed using a computationally inexpensive one-step model capable of capturing the experimentally observed transition behavior from slow to fast reaction. The methodology used to find the kinetic parameters of the simplified model was described in detail. A parametric study using a control volume (0-D) thermal ignition model provided transition maps due to changes in heating rate, initial pressure and composition. Two-dimensional reactive Navier-Stokes equations were used to examine the fluid mechanics and chemical reaction leading to slow or fast consumption of the mixture. During uniform heating, a dynamic buoyancy flow is induced in which the mixture rises along the walls and turns at the centerline creating two well defined vortical structures. Once significant chemical heat release is generated, the flow reverses. During non-uniform heating, the flow field is composed of two large vortices in the center of the vessel, and two sets of smaller vortices trapped at the top and bottom of the reactor. Depending on the heating rate, and irrespective of the mode of heating, the mixture undergoes either slow oxidation or ignition whereby a flame that propagates from the top of the vessel consumes the mixture.
       
  • Calculated concentration distributions and time histories of key species
           in an acoustically forced laminar flame
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Kae Ken Foo, Michael J. Evans, Zhiwei Sun, Paul R. Medwell, Zeyad T. Alwahabi, Graham J. Nathan, Bassam B. Dally A numerical study of the fluid-chemical interactions in a steady and time-varying laminar non-premixed jet flame was conducted to advance understanding of the complex interplay between the flame chemistry, fluid dynamics and soot evolution. Modelling of the steady flame is performed with two alternative reduced mechanisms and compared with the significant body of experimental data that are now available to provide confidence in the calculated values of mixture fraction, which was not previously available. A Method-of-Moments soot model with a 47-species mechanism provides much better agreement with the measured soot volume fraction than does a 32-species mechanism, but both mechanisms predict both the temporal and spatial profiles of mixture fraction to agree within 6%. Nevertheless, neither scheme predicts a reduction in temperature that coincides approximately with the location immediately upstream from the measured soot field. The calculations of the unsteady flame also reveal new insights about the cause of the pinch-off point and the neck zone, together with the role of buoyancy at the flame tip.
       
  • Exploration of chemical composition effects on the autoignition of two
           commercial diesels: Rapid compression machine experiments and model
           simulation
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Liang Yu, Sixu Wang, Wenyu Wang, Yue Qiu, Yong Qian, Yebing Mao, Xingcai Lu Chemical composition difference widely exists in real fuels, and the composition difference will affect the fuel autoignition and heat release in HCCI-based advanced engines. This study aims to explore the composition effects on autoignition by comparing the autoignition characteristics of two commercial diesels (China Stage-V and Stage-VI). The main composition difference is that Stage-V diesel has a higher paraffin content and a lower naphthene content than Stage-VI diesel. Ignition delay times (IDTs) of the two diesels were measured in a heated rapid compression machine at equivalence ratios of 0.37–1.25, pressures of 10–20 bar, and temperatures of 687–865 K. It is found that the difference in the total IDTs of the two diesels varies with the temperature range, and the first-stage IDTs of Stage-V diesel are much shorter than those of Stage-VI diesel. The IDT discrepancies were appropriately explained using the composition difference between the two diesels. Model simulation was carried out using a ternary and a five-component diesel surrogate coupled with an updated kinetic model. Simulation results show that the composition effects on the autoignition of the two diesels can be well captured by the two surrogates, where the ternary and five-component surrogates agree well with Stage-V and Stage-VI diesels, respectively. To further reveal the intrinsic mechanism of the composition effects, low-temperature reactivity difference between the two surrogates was interpreted from a kinetic perspective. Rate of production (ROP) analysis on OH radical confirms that the addition of decalin in the five-component surrogate is primarily responsible for the longer first-stage IDT compared to the ternary surrogate. Since the two surrogates capture the composition difference and autoignition characteristics of the two diesels, the conclusion from the kinetic analysis will help understand the composition effects on the autoignition of the diesels.
       
  • Conditional relationships for the layered brush structure of turbulent
           premixed flames in statistical steadiness
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Dohyun Kim, Kang Y. Huh A turbulent premixed flame is decomposed into the three layers of internal region of fluctuating flamelets and leading and trailing edges with negligible density variation in the nonflamelet regime. Conditional averaging is performed in terms of the successively higher order differentials of c, (1−c), Σf′(=−∂c/∂n) and ∂2c/∂n2 to derive conditional relationships through the layered brush structure. The leading edge is defined as the region of negligible mean reaction rate to avoid the cold boundary difficulty for existence of a steadily propagating flame. The leading and trailing edges are identified in terms of the length scales of exponential decay, LLE and LTE, for c¯ and (1−c¯) becoming equal to that for Σf, as c¯ approaches zero and unity respectively. Analytical expressions are derived for LLE and LTE which are in good agreement with DNS results except for LTE interacting with the wall boundary layer in the stagnating flame. The turbulent burning velocity, ST, is given by the total diffusivity, (Dm+Dt), divided by LLE with its two limiting forms at strong and weak turbulence. The new c¯ transport equation is given in terms of the turbulent diffusivity, Dts, which is defined for the flux, 〈v′(Σf′)′〉, free from countergradient diffusion due to volume expansion. A rigorous expression is derived for the derivative, dΣf/dc¯, in terms of the mean orientation vectors and curvature, 〈n〉f, 〈n〉K and 〈∇ · n〉f. It is consistent with a familiar parabolic profile of Σf for approximately uniform 〈n〉f and 〈 ∇ · n 〉f in the c¯ space. The conditional velocities show the asymptotic behavior of 〈v〉u approaching 〈v〉 and 〈v〉b approaching 〈v〉f at the leading edge and 〈v〉b approaching 〈v〉 and 〈v〉u approaching 〈v〉f at the trailing edge. Good agreement is shown for the analytical expressions of ST and the integrated profiles of Σf with DNS results of the two test flames in statistical steadiness.
       
  • Experimental and numerical investigations of the unscavenged prechamber
           combustion in a rapid compression and expansion machine under engine-like
           conditions
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Guoqing Xu, Maria Kotzagianni, Panagiotis Kyrtatos, Yuri M. Wright, Konstantinos Boulouchos Even though the unscavenged prechamber has been extensively applied in lean premixed natural gas engines, the limited understanding of the fundamentals and the lack of predictive modeling tools (3D RANS CFD) place obstacles in the way of prechamber design and optimization. The present study investigates unscavenged prechamber combustion of lean methane/air mixtures in a Rapid Compression Expansion Machine (RCEM) by combining optical diagnostics (high-speed OH*-chemiluminescence and Schlieren imaging) and 3D Computational Fluid Dynamic (CFD) simulations. Data from the former is used to develop and validate the modeling approach for the specific application and the latter aims to provide indispensable interpretation of the experimental observations. Initially, the comparison of the Schlieren and the OH* images confirm the hypothesis that inherent reacting flame jets exit the prechamber, which justifies the applicability of a level set combustion modeling framework for the investigated operating conditions. The employed G-equation combustion model has been extended to account for the specifics of spark ignition and flame wall interaction present in the prechamber configuration studied. Validation of the developed model by means of the experimental data shows good agreements in terms of (i) jet exit timing, (ii) main chamber heat release rate (HRR) and (iii) projected reactive flame area, evidencing encouraging predictive capability of the proposed modeling approach. The combined insights from experiments and CFD simulations suggest two phases of the main chamber heat release rate, dominated by the jet penetration and the turbulent flame propagation respectively. The subsequent analysis on a single flame jet, using OH*-chemiluminescence and CFD images, indicates that the jet head tends to be more reactive due to a higher turbulence levels and larger eddy size. Moreover, the entire dataset reveals an inverse correlation between the initial reactive jet speed and the early phase combustion duration (5% of total cumulative heat release). Overall, this research provides useful guidelines for the future unscavenged prechamber design.
       
  • Experimental investigation of entropy waves generated from acoustically
           excited premixed swirling flame
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Guoqing Wang, Xunchen Liu, Sirui Wang, Lei Li, Fei Qi Entropy waves play an important role in the production of indirect combustion noise and thermoacoustic instability. The characteristics of entropy waves from swirling flames have not been systematically investigated. Here, a premixed methane/air swirl burner was built with upstream acoustic excitation from a loudspeaker. A joint infrared imaging and tunable diode laser absorption spectroscopy (TDLAS) thermometry measurement was carried out to investigate the entropy waves generated in this burner. The infrared imaging technique provides qualitative images of the distribution, propagation and dissipation of entropy waves while the TDLAS technique provides quantitative measurement of temperature fluctuation. Time resolved measurements of the swirling flame bulk velocity, CH* chemiluminescence, gas temperature and infrared images were simultaneously obtained, demonstrating that entropy waves were generated from the premixed swirling flame under external acoustic excitation. Entropy waves were shown to be greatly influenced by the amplitude and frequency of acoustic waves. They also showed dissipation as the entropy waves propagate downstream according to the attenuated temperature fluctuation and infrared radiation intensities. Simultaneous high speed infrared imaging and particle image velocimetry measurements showed that the temperature non-uniformities arise from engulfment and mixing through periodic vortex roll-up.
       
  • Non-premixed flame dynamics excited by flow fluctuations generated from
           Dielectric-Barrier-Discharge plasma
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Yong Tang, Jiankun Zhuo, Wei Cui, Shuiqing Li, Qiang Yao Plasma-induced flow perturbation and its subsequent dynamic effects on a flame are decoupled, both experimentally and theoretically, from the plasma assisted combustion system. A coaxial Dielectric-Barrier-Discharge (DBD) plasma generator is designed, and the discharge is characterized using probes and cameras. Particle tracing method is elaborately demonstrated to map the velocity of flow fluctuation induced by the plasma, with uncertainty estimated to be less than 5%. Then the plasma generator is coupled with the fuel nozzle of the non-premixed counterflow burner. By using multiple diagnostics, several turbulent-like features are observed from the upstream laminar flow (Re ≈ 300) and the downstream non-premixed flat flame, including the distorted velocity profile, fluctuation intensity above 50%, wrinkled flame sheet, and near −5/3 slope of frequency spectra for both fluctuation velocity and CH* chemiluminescence intensity. The aerodynamic effect on the flame is resolved by more than 90% over frequency spectra and then, characterized using flame transfer functions (FTFs). The experimental results show a negative linear correlation between the FTFs’ gain and perturbation frequency on the logarithmic plot, which is then verified by a theoretical model derived from the Z-equation of non-premixed counterflow flame. Further, the model indicates that for small perturbations, the influence of the global stretch rate on the FTFs is linear, while the effect of the imposed amplitude is negligible, and the dimensionless perturbation frequency (St) scaled by the global stretch rate becomes the only variable.
       
  • Combustion characteristics of solid propellants under high-temperature
           dense-particle erosion conditions
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Jiang Li, Kai Ma, Xiang Lv, Peijin Liu, Guoqiang He, Yang Liu, Jian Chen High-concentration alumina particle jet can erode the surface of the solid propellant inside a solid rocket motor under some special conditions such as flight acceleration. Because of lack of effective testing methods, knowledge of the combustion characteristics of solid propellants under dense-particle erosion conditions is very limited. In this study, the combustion characteristics of hydroxyl‑terminated polybutadiene and nitrate ester plasticized polyether propellants were investigated using real-time X-ray radiography technology and an overload simulation erosion motor. The high-temperature dense-particle jet can cause erosive burning of the solid propellants. No clear threshold velocity is found for the particle erosive burning, and the erosive burning rate increases with the particle velocity. The erosion of the dense-particle jet includes the heat flux increment effect and mechanical damage effect, and the particle erosive burning is mainly caused by the heat flux increment effect of the particles. Numerical simulation results show that the particles can impact the propellant surface at a very low speed and cause heat flux increments; this explains the lack of a clear threshold velocity for the particle erosive burning of solid propellants.
       
  • Vaporization model for arsenic during single-particle coal combustion:
           Model development
    • Abstract: Publication date: Available online 15 March 2019Source: Combustion and FlameAuthor(s): Huimin Liu, Chunbo Wang, Yue Zhang, Chan Zou, Edward Anthony The kinetic parameters for chemical reactions associated with the vaporization of arsenic species are rarely reported due to the difficulties in obtaining suitably purified arsenic compounds as well as the issues associated with the extreme toxicity of many arsenic species. Here, we used a single-particle coal combustion model combined with a vaporization yield model of arsenic fitted by experimental data, which was used to determine the activation energy and frequency factor of the oxidation/decomposition reactions of arsenic species in this work, namely: As-org, FeAsS, FeAsO4 and Ca3(AsO4)2. The combustion kinetics of volatile/char and arsenic thermodynamic properties were used to model the vaporization zone and intensity of emissions for arsenic compounds. The results show that the reaction kinetic parameters of these arsenic species could be determined within an order of magnitude despite the variation of compositions in the coal sample and temperature, and this approach provides a new method to determine the reaction kinetics of hazardous elements such as As. Combining the vaporization yield and reaction kinetics of arsenic species with the single-particle coal combustion model, a novel vaporization model of arsenic was developed. With this model, the temporal evolution of combustion parameters (temperature, conversion ratio of coal, particle porosity, flue gas concentration) as well as arsenic vaporization ratio and As2O3(g) concentration can be predicted at the microscopic level.
       
  • Predicting combustion characteristics in externally heated micro-tubes
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Ingmar Schoegl, Vinicius M. Sauer, Pawan Sharma This study employs a simplified analytical model to obtain qualitative predictions of transitions between non-adiabatic combustion regimes found in non-catalytic tubular micro-flow reactors, i.e. normal stable flames, flames with repetitive extinction and ignition (FREI), and weak flames that are stabilized by wall heat transfer. Assuming large activation energy for single-step kinetics, a comparison of exact solutions for constant and variable wall temperature profiles reveals that while a non-zero wall temperature gradient introduces a temperature lag, it has no impact on the structure of the flame solution itself. This result is significant as it establishes a link to quenching characteristics within heated micro-tubes, and further facilitates a simplified micro-flow reactor analysis. Predictions for sweeps of wall temperatures are combined to S-curves, which allow for a combustion regime classification for a given combination of micro-flow reactor geometry and mixture properties. A variation of parameters such as tube diameter or mixture stoichiometry yields predictions of shifts in regime transitions, thus providing insights on interactions between heat transfer and first order effects of chemical kinetics. A comparison to experimental data illustrates that the theoretical framework, – within inherent limitations, – captures qualitatively correct trends that reflect experimental observations, thus providing valuable insights for micro-flow reactor design and operation.
       
  • Laser-induced structural modifications of differently aged soot
           investigated by HRTEM
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Barbara Apicella, P. Pré, J.N. Rouzaud, J. Abrahamson, R.L. Vander Wal, A. Ciajolo, A. Tregrossi, C. Russo Laser heating was applied to soot probed in the first inception region (nascent soot) and downstream (mature soot) of premixed methane and ethylene flames, burning in similar temperature and equivalence ratio conditions. Different properties of nascent and mature soot were investigated by TEM/HRTEM and EELS along with elemental analysis, UV–visible and Raman spectroscopy. Structural modifications were followed by HRTEM imaging applied to the soot before and after laser heating. Laser heating produced varied modifications of the lamellae organization dependent upon the morphology of pristine soot, in turn dependent on soot aging and fuel type (ethylene and methane). Specifically, structures in form of void shells versus multifaceted rosette structures were observed in the case of young nascent and mature soot, respectively, showing a relationship with their different nanostructures, namely amorphous versus disordered turbostratic structures. Additionally, the differentiation between void shells or multifaceted rosette structures can be used as signature for the occurrence of laser heating effects, which have to be avoided when the laser is instead employed for diagnostic purposes.
       
  • Downstream radiative and convective heating from methane and propane fires
           with cross wind
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Xiaoyu Ju, Michael J. Gollner, Yiren Wang, Wei Tang, Kun Zhao, Xingyu Ren, Lizhong Yang Experiments were conducted to elucidate the radiative and convective heating occurring downstream of wind-driven fires produced by a gaseous burner. These flames model, at reduced scale, some of the dynamics observed in wind-driven fire spread through wildlands, buildings, mines or tunnels. Methane and propane were used to create fires ranging from 5 to 25 kW with ambient velocities ranging from 0.6 to 2.2 m/s. The total and incident radiative heat flux to a nearly-adiabatic downstream surface were measured by a water-cooled total heat flux gauge and a radiometer, respectively. The interaction between the buoyancy induced by the flame and momentum from the free stream was represented by a mixed-convection parameter, ξ=Grx2/Rex1n, where n = 3/2, 2 or 5/2. ξ was evaluated with two length scales in order to capture effects of both the boundary layer development length (x1) and heated distance downstream of the burner (x2). Results showed that the propane flame (high luminosity) exhibited slightly higher radiative heat fluxes than methane flames (low luminosity) under the same external conditions, while the convective heat flux followed an opposite trend. The downstream local radiative heat flux was quantified using a dimensionless flame thickness δx*, which showed a good relationship with ξ for n = 5/2 but not 3/2 or 2. The local convective heat transfer coefficient was expressed in the form of a local Nusselt number, Nux2Rex1−1/2, and correlated well as a piecewise function with ξ for n = 5/2. It was found that both δx* and Nux2Rex1−1/2 have a turning point at ξ ≈ 0.005, which was visually shown to denote the location where transition between an attachment and plume-like flame occurs. By separately describing both radiative and convective downstream heating, the mechanisms controlling heating which drives flame spread in wind-driven fires can be further understood.
       
  • Sparse, iterative simulation methods for one-dimensional laminar flames
    • Abstract: Publication date: June 2019Source: Combustion and Flame, Volume 204Author(s): Simon Lapointe, Russell A. Whitesides, Matthew J. McNenly Sparse, iterative simulation methods for one-dimensional laminar flames are proposed. The resulting steady and unsteady flame solvers exploit approximate Jacobians to greatly reduce the computational cost associated with matrix operations. The constant, non-unity Lewis number assumption is introduced to further reduce the computational cost. The solvers are also parallelized to reduce time-to-solution on distributed memory computer systems. Computed laminar flame speeds and species profiles for a range of chemical mechanisms (from 10 to 2878 species) are compared against a well-validated commercial code and are found to be consistent within solver tolerances. The computation times of both the unsteady and steady solutions increase only linearly with the number of species, which is a significant improvement over the quadratic or cubic scaling of existing steady-state flame solvers. For the largest mechanism tested, the steady-state flame solver is two orders of magnitude faster than commonly-used codes. The use of an approximate Jacobian is shown to reduce the rate of convergence for the steady-state solver, but does not significantly affect the domain of convergence. The steady-state solver with approximate Jacobian is thus well suited for computationally efficient laminar flame speed sweeps with large kinetic mechanisms.
       
  • Detailed kinetic modeling of dimethoxymethane. Part II: Experimental and
           theoretical study of the kinetics and reaction mechanism
    • Abstract: Publication date: Available online 7 March 2019Source: Combustion and FlameAuthor(s): Sascha Jacobs, Malte Döntgen, Awad B.S. Alquaity, Wassja A. Kopp, Leif C. Kröger, Ultan Burke, Heinz Pitsch, Kai Leonhard, Henry J. Curran, K. Alexander Heufer 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.
       
  • Corrigendum to ``The unimportance of the reaction H2 + N2O ⇆
           H2O + N2: A shock-tube study using H2O time histories and ignition
           delay times'' [Combustion and Flame 196 (2018) 478-486]
    • Abstract: Publication date: Available online 10 December 2018Source: Combustion and FlameAuthor(s): Clayton R. Mulvihill, Olivier Mathieu, Eric L. Petersen
       
 
 
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