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Journal Cover
Combustion and Flame
Journal Prestige (SJR): 2.427
Citation Impact (citeScore): 5
Number of Followers: 123  
 
  Full-text available via subscription Subscription journal
ISSN (Print) 0010-2180
Published by Elsevier Homepage  [3162 journals]
  • Shock tube study of normal heptane first-stage ignition near 3.5 atm
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Matthew F. Campbell, Shengkai Wang, David F. Davidson, Ronald K. Hanson Shock tube ignition delay times and species time history measurements for Primary Reference Fuels (PRFs) such as normal heptane provide targets for the validation of combustion models, which in turn are used to develop more fuel-efficient engines that have smaller environmental footprints. However, a review of the literature has revealed that most of the shock tube ignition delay time and species measurement data for normal heptane have been obtained at elevated pressures, rather than at relatively low pressures where many other important experimental techniques such as jet-stirred reactors and flow reactors can provide corroborating results. One central problem preventing previous shock tube studies from examining first-stage ignition at these lower pressures was that ignition times were too long under these conditions to be measured within the available shock tube test times. To address this issue, recent advances in shock tube techniques for achieving long uniform test times have been applied in order to measure low-pressure first-stage ignition times of normal heptane together with normal heptane fuel time-history records at times up to about 30 ms. These measurements were performed in the Negative Temperature Coefficient (NTC) region (T=664−792 K) in lean mixtures (21%O2/Ar, equivalence ratio ϕ=0.5) at pressures of roughly P=3.5 atm using both the conventional and Constrained Reaction Volume (CRV) shock tube filling strategies. The data have been used to evaluate the performance of several combustion models, have been compared with other higher-pressure shock tube first-stage ignition times in n-heptane/20–21%O2 mixtures found in the literature, and have been fit using a two-zone Arrhenius model for first-stage ignition delay times. The results showed that some combustion models yield ignition delay time predictions that differ from the experimental results by as much as an order of magnitude, that under these conditions n-heptane first-stage ignition times scale by roughly P−0.71 but are insensitive to ϕ, and that the fraction of fuel remaining after first-stage ignition increases with increasing initial experimental temperature.
       
  • Mesoscopic simulation of nonequilibrium detonation with discrete Boltzmann
           method
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Chuandong Lin, Kai H. Luo Thanks to its mesoscopic nature, the recently developed discrete Boltzmann method (DBM) has the capability of providing deeper insight into nonequilibrium reactive flows accurately and efficiently. In this work, we employ the DBM to investigate the hydrodynamic and thermodynamic nonequilibrium (HTNE) effects around the detonation wave. The individual HTNE manifestations of the chemical reactant and product are probed, and the main features of their velocity distributions are analyzed. Both global and local HTNE effects of the chemical reactant and product increase approximately as a power of the chemical heat release that promotes the chemical reaction rate and sharpens the detonation front. With increasing relaxation time, the global HTNE effects of the chemical reactant and product are enhanced by power laws, while their local HTNE effects show changing trends. The physical gradients are smoothed and the nonequilibrium area is enlarged as the relaxation time increases. Finally, to estimate the relative height of detonation peak, we define the peak height as H(q)=(qmax−qs)/(qvon−qs), where qmax is the maximum of q around a detonation wave, qs is the CJ solution and qvon is the ZND solution at the von-Neumann-peak. With increasing relaxation time, the peak height decreases, because the nonequilibrium effects attenuate and widen the detonation wave. The peak height is an exponential function of the relaxation time.
       
  • Staggered swirler arrangement in two self-excited interacting swirl flames
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Taesong Lee, Jiho Lee, Junhyeong Park, Dongsik Han, Kyu Tae Kim Interference of acoustic and convective disturbances controls the development of self-excited combustion oscillations of a lean-premixed swirl-stabilized flame with a central bluffbody. How this interference mechanism influences the dynamics of multiple interacting flames in a multi-nozzle environment is currently unknown. Here we present observations of a multi-nozzle system's response to staggered swirler arrangements (ξsw, 1 ≠ ξsw, 2) as compared to non-staggered arrangements; the distance between the swirler and the flame is the dominant length scale of vortical disturbances. Our results demonstrate that a slight modification of the swirler arrangement in the streamwise direction – staggered or non-staggered – has a remarkable influence on the stability map of the whole combustion system. Phase-resolved flame imaging measurements indicate that under non-staggered conditions interacting swirl flames feature a coherent motion during a period of oscillation. By contrast, the staggered swirler combination creates significantly non-symmetric flame dynamics, disturbing the development of well-organized motion over the entire reaction zone. Flame surface modulations in the lateral direction are particularly pronounced due to the formation of non-symmetric convection delays of vortical disturbances between adjacent swirl nozzles. For a given swirler arrangement, the system's response to a wide range of combinations of mean nozzle velocities, including symmetric (u¯1=u¯2) and non-symmetric (u¯1≠u¯2) conditions, were explored to account for the simultaneous effects of the two convection parameters. Our data show that a major determinant of the onset of the instability is the combination of the Strouhal numbers, 〈St1, St2〉, which can be even or uneven depending on the manipulation of the convection time of each nozzle.
       
  • Identification of local extinction and prediction of reignition in a
           spark-ignited sparse spray flame using data mining
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Andrew P. Wandel Direct Numerical Simulations (DNS) of droplet fields which are ignited using a spark are investigated to deduce any behaviour that distinguishes between the cases where successful flame propagation occurs and where a flame ignites but subsequently extinguishes. At the instant the spark was deactivated, some of the studied cases displayed no local extinction, others showed some local extinction (one with reignition and the rest with global extinction) and the rest showed global extinction. The gaseous field at this instant was analysed using the data mining technique the Gaussian Mixture Model on each case separately; this method groups data points, enabling distinction between the various behaviours. The results from this analysis showed that in the case with local extinction–reignition, the regions of space near the flame kernel which produced local quenching were caused by evaporating droplets. These regions of local quenching were relatively small compared to the strong flame front surrounding them; the regions of local quenching were also relatively far from the centre of the flame kernel. In contrast, in cases with local then global extinction, the droplets created regions which were extensions of the relatively-small flame front, and these regions behaved in a similar manner to the flame propagation. As a consequence, these cases were unable to support a self-sustaining flame. Such distinctive behaviour promises opportunities to detect situations where global extinction is imminent and implement appropriate control strategies to prevent global extinction.
       
  • Quantification of the resonance stabilized C4H5 isomers and their reaction
           with acetylene
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Can Huang, Bin Yang, Feng Zhang, Guangjun Tian The resonance stabilized C4H5 radicals (CH3CCCH2, 2-C4H5; CH2CHCCH2, i-C4H5; CH3CHCCH, 12-C4H5) are among the most important precursors of benzene in hydrocarbon flames. Previous studies have revealed that i-C4H5 mainly reacts with acetylene to produce fulvene which promptly transforms to benzene, while the contribution from 2-C4H5 and 12-C4H5 remains unclear due to the obscure composition of these isomers in flames and the lack of accurate rate constants for related reactions. In the present work, we first calculated the cross sections of the resonance stabilized C4H5 radicals to quantify their composition in hydrocarbon flames (Hansen et al., 2006). The ratio of i-C4H5/(12-C4H5 + 2-C4H5) in the flame zone is deduced as 0.8-1.2 in fuel-rich allene, propyne, cyclopentene or benzene flames, and 2-C4H5 constitutes more than 70% in the sum of 2-C4H5 and 12-C4H5. We further studied the reaction kinetics of 2-/12-C4H5 with acetylene. Similar to i-C4H5, 2-/12-C4H5 tends to produce fulvene rather than directly form benzene when reacting with acetylene. However, the reaction rates of C2H2 + 2-/12-C4H5 are ∼one magnitude lower than that of i-C4H5 under combustion conditions. The role of the reaction between 2-/12-C4H5 and acetylene is controlled by the combined effect of concentration and reaction rates. By including above computational results into kinetic modeling, we finally conclude that although the concentration of 2-/12-C4H5 is comparable to that of i-C4H5, their contribution to the first aromatic ring in hydrocarbon flames from acetylene addition is limited. However, considering the noticeable concentration and the resonance stabilized structure, these species still have potential to generate aromatics.
       
  • Detonation initiation in pipes with a single obstacle for mixtures of
           hydrogen and oxygen-enriched air
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Sergio Bengoechea, Joshua A.T. Gray, Julius Reiss, Jonas P. Moeck, Oliver C. Paschereit, Jorn Sesterhenn This work presents an experimental and numerical study of a pulsed detonation combustion chamber. It consists of a pipe obstructed by one convergent–divergent nozzle, filled with a stoichiometric mixture of hydrogen and oxygen-enriched air. The proposed geometry is analysed with regard to its influence on the outset of detonation and its suitability for pulse detonation engines (PDEs). The study reveals the essential aspects for detonation initiation. The results of one of the configurations indicate a deterministic and reliable deflagration-to-detonation transition (DDT) with a short run-up distance, crucial for technical applications. The simulation reproduces the measurements in great detail and the origin of detonation is unequivocally identified.
       
  • Large deformation and gas retention during cookoff of a plastic bonded
           explosive (PBX 9407)
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Michael L. Hobbs, Michael J. Kaneshige, Cole D. Yarrington We have used several configurations of the Sandia Instrumented Thermal Ignition (SITI) experiment to develop a pressure-dependent, four-step ignition model for a plastic bonded explosive (PBX 9407) consisting of 94 wt.% RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), and a 6 wt.% VCTFE binder (vinyl chloride/chlorotrifluoroethylene copolymer). The four steps include desorption of water, decomposition of RDX to form equilibrium products, pressure-dependent decomposition of RDX forming equilibrium products, and decomposition of the binder to form hydrogen chloride and a nonvolatile residue (NVR). We address drying, binder decomposition, and decomposition of the RDX component from the pristine state through the melt and into ignition. We used Latin Hypercube Sampling (LHS) of the parameters to determine the sensitivity of the model to variation in the parameters. We also successfully validated the model using one-dimensional time-to-explosion (ODTX and P-ODTX) data from a different laboratory. Our SITI test matrix included 1) different densities ranging from 0.7 to 1.63 g/cm3, 2) free gas volumes ranging from 1.2 to 38 cm3, and 3) boundary temperatures ranging from 170 to 190 °C. We measured internal temperatures using embedded thermocouples at various radial locations as well as pressure using tubing that was connected from the free gas volume (ullage) to a pressure gauge. We also measured gas flow from our vented experiments. A borescope was included to obtain in situ video during some SITI experiments. We observed significant changes in the explosive volume prior to ignition. Our model, in conjunction with data observations, imply that internal accumulation of decomposition gases in high density PBX 9407 (90% of the theoretical maximum density) can contribute to significant strain whether or not the experiment is vented or sealed.
       
  • On imaging nascent soot by transmission electron microscopy
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Kevin Wan, Dongping Chen, Hai Wang High-resolution Transmission Electron Microscopy (HRTEM) imaging of nascent soot was carried out with an emphasis in demonstrating the annealing of soot samples under continuous irradiation of the high-energy electron beam. Images were taken for several soot samples over the duration of 16 min in 2 min time intervals to reveal the crystallization process. Fringe properties, including fringe length, tortuosity, and spacing were analyzed over the duration of the imaging. The sensitivity of the fringe properties to the apparent changes in the nanostructures imaged was examined. The difficulties in quantifying soot composition and structures are further illustrated by analyzing simulated TEM images of molecular-dynamics generated particles. Together, the results highlight some of the challenges in using HRTEM to obtain unambiguous structural properties for nascent soot particles.
       
  • An analysis of the ignition limits of premixed hydrogen/oxygen by heated
           nitrogen in counterflow
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Shangpeng Li, Wenkai Liang, Qiang Yao, Chung K. Law The non-monotonic ignition response of counterflowing premixed hydrogen/oxygen mixtures with nitrogen dilution versus heated nitrogen is studied numerically and theoretically. It is shown that the three ignition limits can be theoretically obtained by considering only the linear system involving at most only one radical in each reaction, while the influences of the nonlinear reactions, each involving two radicals, together with thermal feedback, introduce higher-order corrections, particularly for the third ignition limit. It is also demonstrated that the high diffusivity of H2 promotes ignition at the third limit. On the other hand, the high diffusivity of the H atom suppresses ignition at the first limit, while the assumption of unity Lewis number for H yields remarkably good results for the other two limits. Furthermore, by solving the time evolution of the crucial H and HO2 radicals, simplified formulations of the three individual limits and the two quadratic double limits are obtained analytically, in analogy with results for the homogeneous explosion problem.
       
  • Differential diffusion effect on the stabilization characteristics of
           autoignited laminar lifted methane/hydrogen jet flames in heated coflow
           air
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Ki Sung Jung, Seung Ook Kim, Tianfeng Lu, Suk Ho Chung, Bok Jik Lee, Chun Sang Yoo The characteristics of autoignited laminar lifted methane/hydrogen jet flames in heated coflow air are numerically investigated using laminarSMOKE code with a 57-species detailed methane/air chemical kinetic mechanism. Detailed numerical simulations are performed for various fuel jet velocities, U0, with different hydrogen ratio of the fuel jet, RH, and the inlet temperature, T0. Based on the flame characteristics, the autoignited laminar lifted jet flames can be categorized into three regimes of combustion mode: the tribrachial edge flame regime, the Moderate or Intense Low-oxygen Dilution (MILD) combustion regime, and the transition regime in between. Under relatively low temperature and high hydrogen ratio (LTHH) conditions, an unusual decreasing liftoff height, HL, behavior with increasing U0 is observed, qualitatively similar to those of previous experimental observations. From additional simulations with modified hydrogen mass diffusivity, it is substantiated that the unusual decreasing HL behavior is primarily attributed to the high diffusive nature of hydrogen molecules. The species transport budget, autoignition index, and displacement speed analyses verify that the autoignited lifted jet flames are stabilized by autoignition-assisted flame propagation or autoignition depending on the combustion regime. Chemical explosive mode analysis (CEMA) identifies important variables and reaction steps for the MILD combustion and tribrachial edge flame regimes.
       
  • Hetero-/homogeneous chemistry interactions and flame formation during
           methane catalytic partial oxidation in rhodium-coated channels
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Behrooz O. Arani, John Mantzaras, Christos E. Frouzakis, Konstantinos Boulouchos The hetero-/homogeneous chemistry interactions during the catalytic partial oxidation (CPO) of methane were investigated numerically in rhodium-coated cylindrical channels using axisymmetric simulations with detailed catalytic and gas-phase chemistries, conjugate heat transfer in the solid wall and detailed transport. Simulated conditions spanned pressures 1–25 bar, methane-to-air equivalence ratios 2.5–4.0, inlet temperatures 300–900 K and channel diameters 0.5–2.0 mm. The formation of vigorous flames in the oxidation zone of the CPO reactor was promoted as the pressure, inlet temperature and channel diameter increased. The catalytic pathway induced a strong radial stratification of the reactant and temperature distributions over the homogeneous combustion zones. This in turn resulted in flames spatially confined to the channel core, such that the catalytic wall temperature was only modestly affected by the flames (∼25 K wall temperature rise due to the flame presence), a result highly desirable for the reactor thermal management and for the catalyst thermal stability. Even when strong flames were formed, combined hetero-/homogeneous combustion persisted over the entire axial extent of the flames. The deficient oxygen reactant leaked through the flame zones and was subsequently converted catalytically on the channel walls, with the oxygen leakage increasing as the channel diameter, pressure, and inlet temperature decreased. Extensive parametric simulations delineated the regimes of operating conditions and geometrical parameters (pressure, inlet temperature, equivalence ratio and channel diameter) for which gas-phase combustion could not be ignored during methane CPO over rhodium. It was shown that for practical power generation systems (pressures and inlet temperatures above 15 bar and 600 K, respectively) gaseous chemistry could not be neglected and offered the benefit of reducing the extent of the oxidation zone and hence the overall reactor length.
       
  • Mechanism of cellulose fast pyrolysis: The role of characteristic chain
           ends and dehydrated units
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Qiang Lu, Bin Hu, Zhen-xi Zhang, Yu-ting Wu, Min-shu Cui, Ding-jia Liu, Chang-qing Dong, Yong-ping Yang Understanding the fundamental reactions and mechanisms during biomass fast pyrolysis is essential for the development of efficient pyrolysis techniques. In this work, quantum chemistry calculation, kinetic analysis and fast pyrolysis experiment were combined to reveal the cellulose pyrolysis mechanism. During cellulose pyrolysis, the indigenous interior units, reducing end (RE end) and non-reducing end (NR end) initially form various characteristic chain ends and dehydrated units which then evolve into different pyrolytic products. As the rising of the degree of polymerization (DP), reactions occurring at the interior unit and NR end are more competitive than those taking place at the RE end, resulting in distinct pyrolytic product distribution for cellulose and glucose-based carbohydrates. The reactions occurring at the three indigenous units of cellulose chain all favor the formation of levoglucosan-terminated end (LG end) and/or NR end, which then generate levoglucosan (LG). The acyclic d-glucose end (AG end), which mainly derives from the RE end, is essential for the formation of 1,6-anhydro-β-d-glucofuranose (AGF), 1,4:3,6-dianhydro-α-d-glucopyranose (DGP), furfural (FF), 5-hydroxymethyl furfural (5-HMF) and hydroxyacetaldehyde (HAA). Compared with the chain ends, the dehydrated units are not feasible to be generated, and their decomposition favors the formation of HAA.
       
  • Dynamic responses of counterflow nonpremixed flames to AC electric field
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Dae Geun Park, Suk Ho Chung, Min Suk Cha Although ionic wind has been observed to play important roles in the effects of electric fields on flames, there is a lack of systematic quantification of ionic wind that allows interpretation of a flame's responses to electric fields. Here, we report on various responses of nonpremixed flames, such as the flame's dynamic responses and the generation of bidirectional ionic wind, in relation to the applied voltage and frequency of an alternating current (AC) in a counterflow burner. We find that although the Lorentz force acting on charged molecules initiates related effects, each effect is both complex and different. When the applied voltage is in the sub-saturated regime (small) as determined by the voltage-current behavior, flame movements and flow motion are minimally affected. However, when the applied voltage is in the saturated regime (large), flame oscillation occurs and a bidirectional ionic wind is generated that creates double-stagnation planes. The flame's oscillatory motion could be categorized in the transport-limited regime and in the oscillatory decaying regime, suggesting a strong dependence of the motion on the configuration of the burner. We also observed bidirectional ionic wind in visibly stable flames at higher AC frequencies. We present detailed explanations for flame behaviors, electric currents, and flow characteristics under various experimental conditions.
       
  • Soot evolution and flame response to acoustic forcing of laminar
           non-premixed jet flames at varying amplitudes
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Kae Ken Foo, Zhiwei Sun, Paul R. Medwell, Zeyad T. Alwahabi, Graham J. Nathan, Bassam B. Dally New details regarding the soot evolution and its controlling parameters in steady and forced flames have been studied using high spatial resolution laser diagnostic techniques. Steady laminar non-premixed ethylene/nitrogen flames with three different diameters burners were acoustically forced using a loudspeaker. 10-Hz-sinusoidal signals of different amplitudes were transmitted to the loudspeaker to drive the flames. The results reveal that the spatial correlation between the soot field and the temperature profile is influenced by the burner diameter and forcing conditions. The soot field in steady laminar flames is confined to a relatively narrow temperature range, 1500–2000 K. In contradiction, the soot field in forced flames spread across a wider range of temperature, 1400–2100 K. Furthermore, the spatial correlation between the normalised soot concentration and primary particle size can be described with an exponential function. While it is observed that the exponential coefficients vary with burner diameter and forcing conditions, further study is necessary for a better understanding. In general, laminar flames forced at a lower amplitude (α=25%) tend to produce less soot than moderately forced (α=50%) flames. Further increasing the forcing amplitude to α=75% does not increase the soot production in laminar flames; conversely, lower peak and volume-integrated soot volume fraction are observed in the strongly forced flame (α=75%) as relative to the moderately forced counterpart. These findings shed new light on the seemingly contradictory results published in the literature regarding the effect of the forcing intensity on the soot production.
       
  • Direct simulation Monte Carlo modeling of
           H2–O2 deflagration waves
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Israel B. Sebastião, Li Qiao, Alina Alexeenko Combustion at extreme conditions such as high-speed and microscale involve nonequilibrium transport and chemical reactions that require atomistic treatment of molecular processes. We present a framework for applying the direct simulation Monte Carlo method (DSMC) to model combustion at the molecular scale. We show that the standard DSMC approach employing Total Collision Energy (TCE) chemistry and Larsen–Borgnakke (LB) energy exchange models is not applicable for combustion simulations which are dominated by exchange and recombination reactions. A methodology for modifying the TCE-LB approach is developed to ensure detailed balance and relaxation towards thermal equilibrium regardless of the internal energy relaxation rates. A simplified 6-species and 7-reversible reaction mechanism with rates modified to account for discrete vibrational levels in DSMC is used for the benchmark flame study. The laminar flame structure of H2–O2 premixed systems and the corresponding deflagration wave speeds by DSMC are compared with PREMIX results. The DSMC simulations based on the extended TCE-LB framework correctly reproduces the 1-D flame structure and its propagation speed is consistent with continuum modeling predictions for the same reaction mechanism and similar flow conditions. The DSMC approach presents opportunities to study combustion phenomena at the molecular scale including state-to-state processes in conditions far from thermal equilibrium for improved combustion diagnostics and control.
       
  • DFT study of the catalytic effect of Na on the gasification of
           carbon--CO2
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Deng Zhao, Hui Liu, Chenglin Sun, Lianfei Xu, Qingxi Cao A key question in the study of the CO2-gasification process is the mechanism of the Na-containing catalytically active center. In this study, density functional theory calculation was carried out to examine the catalytic role of Na in two graphite models (armchair-edge and zigzag-edge). Two Na-containing groups (CNa, CONa) were considered in each model. The activation energies for all reaction pathways indicated that Na showed catalytic activity in the dissociative adsorption of CO2 in both of the graphite models. However, in the subsequent desorption of CO, which is the rate-determining step, only the armchair-edge model indicated catalysis by Na. The molecular structures and reduced density gradient analysis showed that, during CO2 dissociative adsorption, the Na atom promoted breakage of the CO bond and weakened the conjugate structure at the aromatic nucleus to release CO.
       
  • Effect of dimethyl ether (DME) addition on sooting limits in counterflow
           diffusion flames of ethylene at elevated pressures
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Zepeng Li, Hafiz M.F. Amin, Peng Liu, Yu Wang, Suk H. Chung, William L. Roberts The effects of dimethyl ether (DME) addition to ethylene fuel on sooting tendencies with varying pressure were investigated in counterflow diffusion flames by using a laser scattering technique. Sooting limit maps were determined in the fuel (XF) and oxygen (XO) mole fraction plane, separating sooting and non-sooting regions. The results showed that when DME is mixed to ethylene, the sooting region was appreciably shrank, especially in the cases of soot formation/oxidation (SFO) flames as compared with the cases of soot formation (SF) flames. This indicated an inhibiting role of DME on sooting. An interesting observation was that the critical XO required for sooting initially decreased and then increased with the DME mixing ratio to ethylene β for the cases of SF flames, exhibiting a non-monotonic behavior. This implied a promoting role of DME on sooting when small amount of DME is mixed to ethylene. As the pressure increased, the sooting region generally expanded. Specifically, the range of β in promoting soot formation extended with pressure. This implies that a strategy in reducing soot by adding DME to ethylene at high pressures required a large amount of DME addition. To interpret the observed phenomena, kinetic simulations including reaction pathway and sensitivity analyses were conducted with the opposed-flow flames model using the KAUST-Aramco PAH Mech. The results showed that the thermal effect of DME addition on sooting tendency monotonically decreases with β. The chemical effect was found to be the main contributor to the DME addition effect on sooting tendency, resulting in the non-monotonic sooting limt behavior. The pathway analysis showed the role of methyl radicals generated from DME promoted incipient benzene ring formtion when small amount of DME was added, which can be attributed to the soot promoting role of DME addition for small β.
       
  • Automated chemical kinetic mechanism simplification with minimal user
           expertise
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Riccardo Malpica Galassi, Pietro P. Ciottoli, S. Mani Sarathy, Hong G. Im, Samuel Paolucci, Mauro Valorani An improved algorithm to generate skeletal mechanisms from the original detailed chemical kinetic mechanisms is proposed. The new algorithm builds on the computational singular perturbation (CSP) framework, by adding an additional layer of automation based on the tangential stretching rate (TSR) and the species’ participation index to TSR. The main advantage of the new approach is that it does not require the specification of a set of target species. Instead, the target species set is dynamic and automatically identified through the simplification algorithm, which defines the system’s state variables that the skeletal mechanism is required to accurately predict. In this way, the new procedure pursues an optimum set of target species that leads the algorithm to include a minimal number of species/reactions that ensures the replication of global observables, such as ignition delay time, without any a-priori knowledge of the chemical pathways. The capabilities and performance of the new simplification algorithm are demonstrated in a test problem employing the 397-species detailed mechanism for C0-C2 species, augmented with polycyclic aromatic hydrocarbon (PAH) soot precursor species. The results are compared with those obtained from the standard CSP-based algorithm in terms of the ignition delay times, main species evolution and their equilibrium state. Subsequent examples also demonstrate the capabilities of the TSR-based algorithm to generate additional sub-mechanisms that, combined with the core mechanism, allow to extend its range of operating conditions and target variables.
       
  • An experimental and chemical kinetic modeling study of 1,3-butadiene
           combustion: Ignition delay time and laminar flame speed measurements
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Chong-Wen Zhou, Yang Li, Ultan Burke, Colin Banyon, Kieran P. Somers, Shuiting Ding, Saadat Khan, Joshua W. Hargis, Travis Sikes, Olivier Mathieu, Eric L. Petersen, Mohammed AlAbbad, Aamir Farooq, Youshun Pan, Yingjia Zhang, Zuohua Huang, Joseph Lopez, Zachary Loparo, Subith S. Vasu, Henry J. Curran Ignition delay times for 1,3-butadiene oxidation were measured in five different shock tubes and in a rapid compression machine (RCM) at thermodynamic conditions relevant to practical combustors. The ignition delay times were measured at equivalence ratios of 0.5, 1.0, and 2.0 in ‘air’ at pressures of 10, 20 and 40 atm in both the shock tubes and in the RCM. Additional measurements were made at equivalence ratios of 0.3, 0.5, 1.0 and 2.0 in argon, at pressures of 1, 2 and 4 atm in a number of different shock tubes. Laminar flame speeds were measured at unburnt temperatures of 295 K, 359 K and 399 K at atmospheric pressure in the equivalence ratio range of 0.6–1.7, and at a pressure of 5 atm at equivalence ratios in the range 0.6–1.4. These experimental data were then used as validation targets for a newly developed detailed chemical kinetic mechanism for 1,3-butadiene oxidation.A detailed chemical kinetic mechanism (AramcoMech 3.0) has been developed to describe the combustion of 1,3-butadiene and is validated by a comparison of simulation results to the new experimental measurements. Important reaction classes highlighted via sensitivity analyses at different temperatures include: (a) ȮH radical addition to the double bonds on 1,3-butadiene and their subsequent reactions. The branching ratio for addition to the terminal and central double bonds is important in determining the reactivity at low-temperatures. The alcohol-alkene radical adducts that are subsequently formed can either react with HȮ2 radicals in the case of the resonantly stabilized radicals or O2 for other radicals. (b) HȮ2 radical addition to the double bonds in 1,3-butadiene and their subsequent reactions. This reaction class is very important in determining the fuel reactivity at low and intermediate temperatures (600–900 K). Four possible addition reactions have been considered. (c) 3Ö atom addition to the double bonds in 1,3-butadiene is very important in determining fuel reactivity at intermediate to high temperatures (> 800 K). In this reaction class, the formation of two stable molecules, namely CH2O + allene, inhibits reactivity whereas the formation of two radicals, namely Ċ2H3 and ĊH2CHO, promotes reactivity. (d) Ḣ atom addition to the double bonds in 1,3-butadiene is very important in the prediction of laminar flame speeds. The formation of ethylene and a vinyl radical promotes reactivity and it is competitive with H-atom abstraction by Ḣ atoms from 1,3-butadiene to form the resonantly stabilized Ċ4H5-i radical and H2 which inhibits reactivity. Ab initio chemical kinetics calculations were carried out to determine the thermochemistry properties and rate constants for some of the important species and reactions involved in the model development. The present model is a decent first model that captures most of the high-temperature IDTs and flame speeds quite well, but there is room for considerable improvement especially for the lower temperature chemistry before a robust model is developed.
       
  • Ignition of hydrogen/air mixtures by a heated kernel: Role of Soret
           diffusion
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Wenkai Liang, Chung K. Law, Zheng Chen Effects of Soret diffusion on the ignition of hydrogen/air mixtures by a heated kernel, and the structure and dynamics of the embryonic flame that is subsequently formed, were investigated numerically with detailed chemistry and transport. Results show that Soret diffusion leads to larger (smaller) minimum ignition energy (MIE) for relatively rich (lean) mixtures, that this effect is mainly engendered by the Soret diffusion of H2 while that of the H radical is almost negligible, and that Soret diffusion also leads to an increase (decrease) of the Markstein length for rich (lean) mixtures. Satisfactory agreement with literature experimental data on the MIE is shown, especially for the critical states near lean and rich flammability limits. Evolvement of the flame structure shows that before the self-sustained flame is formed, the high temperature gradient associated with the ignition kernel has driven the H2 in the mixture towards the ignition kernel and formed a locally high H2 concentration region, which consequently renders lean (rich) mixtures easier (harder) to ignite. It is further shown that Soret diffusion of both H and H2 affect the propagation dynamics of the stretched spherical flame that is subsequently formed, from its embryonic state until that of free propagation, in that Soret diffusion of H2 is the dominant mode at small flame radius with the large strain rate, while that of H is the dominant mode at large flame radius with the small strain rate similar to that of the unstretched adiabatic planar flame.
       
  • Influence of chemical kinetics on detonation initiating by temperature
           gradients in methane/air
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Cheng Wang, Chengeng Qian, JianNan Liu, Mikhail A. Liberman Different simplified and detailed chemical models and their impact on simulations of combustion regimes initiating by the initial temperature gradient in methane/air mixtures are studied. The limits of the regimes of reaction wave propagation depend upon the spontaneous wave speed and the characteristic velocities of the problem. The present study mainly focus to identify conditions required for the development a detonation and to compare the difference between simplified chemical models and detailed chemistry. It is shown that a widely used simplified chemical schemes, such as one-step, two-step and other simplified models, do not reproduce correctly the ignition process in methane/air mixtures. The ignition delay times calculated using simplified models are in orders of magnitude shorter than the ignition delay times calculated using detailed chemical models and measured experimentally. This results in considerably different times when the exothermic reaction affects significantly the ignition, evolution, and coupling of the spontaneous reaction wave and pressure waves. We show that the temperature gradient capable to trigger detonation calculated using detailed chemical models is much shallower (the size of the hot spot is much larger) than that, predicted by simulations with simplified chemical models. These findings suggest that the scenario leading to the deflagration to detonation transition (DDT) may depend greatly on the chemical model used in simulations and that the Zel'dovich gradient mechanism is not necessary a universal mechanism triggering DDT. The obtained results indicate that the conclusions derived from the simulations of DDT with simplified chemical models should be viewed with great caution.
       
  • Dynamic adaptive acceleration of chemical kinetics with consistent error
           control
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Wenwen Xie, Zhen Lu, Zhuyin Ren, Graham M. Goldin The incorporation of detailed chemistry in combustion simulations is challenging due to the large number of chemical species and the wide range of chemical timescales. The performance of acceleration methods such as tabulation/retrieval strategies may deteriorate dramatically when large variation in the accessed composition space is present. In this study, a dynamic adaptive acceleration method (DAAM) is proposed, in which in situ adaptive tabulation (ISAT) or dynamic adaptive chemistry (DAC) is dynamically selected for chemistry integration based on the encountered composition inhomogeneity. The principle component analysis (PCA) of instantaneous representative compositions is employed to identify a low-dimensional subspace, in which the composition inhomogeneity of the computational cells is quantified through reconstructing the histogram of composition. ISAT is invoked for cells being in composition regions with high cell/particle numbers to avoid unnecessary tabulations and DAC is employed for the remaining ones by invoking on-the-fly reduction to generate small skeletal mechanisms for local thermo-chemical conditions and therefore accelerates the chemistry integration. A heuristic approach that dynamically adjusts the DAC reduction threshold based on the user-defined ISAT error tolerance has been proposed for DAAM, which enables a single, intuitive error control parameter for the combined use of these two methods and more importantly enables rigorous local error control. DAAM have been demonstrated in internal combustion engine (ICE) model simulations and premixed charge compression ignition (PCCI) engine simulations of n-heptane/air mixture, respectively. DAAM can improve the acceleration performance up to 50% compared to standalone ISAT while maintaining the same level of accuracy in temperature and species. It also shows advantage in speedup performance over the fixed ISAT-DAC method at the same level of accuracy.
       
  • Soot formation in diluted laminar ethene, propene and 1-butene diffusion
           flames at elevated pressures
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Elizabeth A. Griffin, Ömer L. Gülder Soot formation characteristics of ethene, propene, and 1-butene, the most abundant unsaturated intermediates in thermal decomposition of paraffinic hydrocarbons, were investigated in laminar diffusion flames stabilized on a co-flow burner installed in a high-pressure combustion chamber with optical access. All three olefins were diluted with nitrogen to produce sooting but non-smoking diffusion flames at desired pressures. Pressure range was 1–2.5 bar with 1-butene, and 1–8 bar with propene and ethene. Upper pressure limits of 1-butene and propene were established by their respective vapour pressure characteristics. The spectral soot emission technique, in which radiation emitted by the soot within the flame was collected as line-of-sight intensity and spectrally resolved over the range 690–945 nm, was used to measure radially-resolved temperature and soot volume fraction. The carbon mass flow rates of the three fuels were kept constant at 0.505 mg/s to facilitate direct comparison among the fuels at elevated pressures. With the same dilution level, the sooting propensity increased from ethene to 1-butene as expected; however, the pressure sensitivity of propene and 1-butene differed significantly from that of ethene. Soot yields in both propene and 1-butene flames showed a much weaker dependence on pressure than the soot in ethene flames. In the decomposition of propene and 1-butene, allyl radical and 1,3-butadiene are known to form in critical quantities leading to formation of higher molecular growth species specifically six-membered ring aromatics, and presence of these simple aromatics is argued to play a role in lowering the pressure sensitivity of the soot in C3 and C4 olefin flames.
       
  • Experimental and numerical study of OH* chemiluminescence in hydrogen
           diffusion flames
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Mengmeng Zhao, David Buttsworth, Rishabh Choudhury A co-flow burner was designed to generate axisymmetric diffusion flames for the application of line-of-sight optical diagnostics to hydrogen flames. Chemiluminescence images of OH* from laminar hydrogen diffusion flames, with and without co-flowing air, were recorded using an intensified camera system with a narrow-band filter at approximately 310 nm. The spectra of OH* chemiluminescence was acquired by a separate optical system. Local concentrations of the radiating radical OH* were determined using the inverse Abel transformation and calibration against a light source of known radiance. The uncertainty of the OH* concentration measurements is analysed to be −22% to +12% in the current experimental configuration. Numerical reconstruction of the physical flames was performed using a two dimensional axisymmetric flow model coupled with a detailed H2/O2 oxidation chemistry mechanism and an OH* chemiluminescent sub-scheme which includes options to use 6 different rate coefficients recommended in the literature for the OH* formation reaction H+O+M⇌OH*+M (R1). The numerical simulations using the rate coefficient of 1.5 × 1013exp(−5.98kcalmol−1/RT)cm6mol−2s−1 for R1 demonstrate the best agreement with the measured OH* chemiluminescence.
       
  • Revealing the doping effects of C2H6O isomers on a benzene flame: An
           experimental and modeling study
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Long Zhao, Wenyu Sun, Jiuzhong Yang, Bin Yang Chemical structures of six laminar premixed ethanol- and dimethyl ether (DME)-doped benzene flames were investigated at the pressure of 30 Torr with the carbon/oxygen (C/O) ratio maintained at 0.7. Dozens of flame species including some reactive radicals and aromatics were identified and quantified with the technique of synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A kinetic model was constructed by combining our previous benzene model with sub-mechanisms for ethanol and DME. The model was tested with the current speciation measurements, showing satisfactory predictive performances. Based on the rate of production (ROP) analyses, the pathways for fuel decompositions and the aromatics growth were revealed. Compared to the neat benzene flame, the higher concentrations of monocyclic aromatic hydrocarbons (MAHs) in doped flames are due to the higher production of C1 and C2 hydrocarbons from the consumptions of oxygenated additives. While the formation of polycyclic aromatic hydrocarbons (PAHs) is inhibited, which results from the reduced formation of phenyl, C3 and C5 species.
       
  • Scaling nonreactive cross flow over a heated plate to simulate forest
           fires
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Nikolay Gustenyov, Nelson K Akafuah, Ahmad Salaimeh, Mark Finney, Sara McAllister, Kozo Saito The paper reports visualization of the flow of smoke over a flat surface inside of a low-speed wind tunnel. A heating plate flush mounted on the wind tunnel floor simulated a spreading line fire that produces uniform heat flux under constant wind speed condition. A paper-thin cloth was soaked with commercially available Vaseline and placed on top of the heating plate; when it is heated, it produced thick white smoke, ideal for flow visualization. Two sides and top of the wind tunnel were made of a transparent acrylic sheet that enabled LED and laser sheet light illumination of the flow. A still camera with a full-frame CMOS sensor was used to record time-series images of illuminated smoke flow patterns from different angles. From these images, the following four flow structures were identified: organized horizontal vortex flows, weak vortex flow interactions, strong vortex flow interactions (also described as the ‘transition regime’), and, turbulent flows. Previously developed scaling laws on forest fires were applied to find similarity in flow structures created by the current small-scale convective heat-transfer experiments and the USDA's mid-scale wind tunnel fire experiments.
       
  • Effect of CO2 on the characteristics of soot derived from coal
           rapid pyrolysis
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Qinghua Chang, Rui Gao, Hongjun Li, Guangsuo Yu, Fuchen Wang The present work aims to investigate the effects of a CO2-rich atmosphere on the characteristics of coal-derived soot. The rapid pyrolysis of Shenfu bituminous coal was conducted in a Drop tube furnace (DTF) in N2 and CO2 atmospheres with a wall temperature of 1073–1473 K and residence time below 700 ms. The yields and microstructure characteristics of N2-soot and CO2-soot were analysed by using a series of techniques (elemental analysis, HRTEM, Raman, XRD, FT-IR and thermogravimetry techniques). CO2 enhanced the soot formation and proceeded the dehydrogenation. CO2 improved the order of internal carbon lattices, enhanced the lateral extension of carbon nanostructures, decreased the interplanar spacing of the graphene layers and also promoted the stack of polyaromatic layers. The variations of the SOLO, DUO, TRIO and QUARTO structures were also analysed, and the contribution of CO2 was found to reduce the defects of the basic structure units (BSU). The defects of soot were an important indication of the initial gasification reactivity. Overall, CO2-soot was more mature and low active than N2-soot.
       
  • Effect of aluminum micro- and nanoparticles on ignition and combustion
           properties of energetic composites for interfacial bonding of metallic
           substrates
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Kyung Ju Kim, Myung Hoon Cho, Soo Hyung Kim In this study, the effect of micro- and nanoscale energetic materials in the formulation of aluminum microparticles (Al MPs)/aluminum nanoparticles (Al NPs)/iron oxide nanoparticles (Fe2O3 NPs) as a heat energy source for melting solder microparticles (SAC 305 MPs) on the interfacial bonding properties of Cu metallic substrates, is investigated. The optimized mixing ratio is Al MP:Al NP:Fe2O3 NP = 30:30:40 wt%, which generates a maximum total exothermic energy of ∼2.0 kJ g−1. The presence of Al NPs is essential to make stable ignition and initiation of Al MPs, which enable to attain relatively long combustion duration. The use of highly reactive Al NPs/Fe2O3 NPs can improve the aluminothermic reaction, while the addition of Al MPs to the Al NPs/ Fe2O3 NPs is also required to maintain their high thermal energy for a longer duration. An energetic material (EM) layer composed of Al MP/Al NP/Fe2O3 NP composites is employed as a heat source between solder material (SM) layers composed of SAC305 MPs. The SM/EM/SM multilayer pellets are assembled and ignited between interfacial Cu substrates for bonding. Thus, interfacial bonding between the Cu substrates is successfully achieved, and the resulting maximum mechanical strength for the bonded Cu substrates using the SM/EM/SM multilayer pellets increases by ∼40% compared to that when using a pure SM layer pellet. Hence, EM layers can act as both an effective heat energy generation source and a mechanical reinforcing medium, while the interfacial bonding process using SM/EM/SM multilayer pellets demonstrated herein provides an easy and versatile means of welding and jointing for industrial applications.
       
  • Experimental and soot modeling studies of ethylene counterflow diffusion
           flames: Non-monotonic influence of the oxidizer composition on soot
           formation
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Lei Xu, Fuwu Yan, Mengxiang Zhou, Yu Wang, Suk Ho Chung Previous soot studies in counterflow diffusion flames revealed that the sooting limit curve in regions with large oxygen mole fractions (XO) exhibited marked bending behaviors that indicated a non-monotonic variation of sooting tendency with oxygen concentration. The underlying mechanisms of this bending behavior remained unclear. In this regard, the present study systematically investigated the effect of oxygen mole fraction in the oxidizer stream on the sooting characteristics of ethylene counterflow diffusion flames. We used the near-infrared light extinction technique to measure soot volume fractions of two types of flames that significantly differed in sooting structures: soot formation oxidation (SFO) flames with a fixed fuel mole fraction (XF) of 0.28 and varying XO between 0.5 and 1.0 and soot formation (SF) flames with pure ethylene (XF = 1.0) in the fuel stream and varied XO from 0.25 to 0.3. We also conducted detailed soot modeling studies by combining the gas-phase chemistry with a sectional soot model that accounted for soot inception, surface growth, particle coagulation as well as soot oxidation. Our experimental and modeling results demonstrated the non-monotonic relationship between soot volume fractions and XO in SFO flames. A detailed analysis of the evolutionary process of soot formation revealed that the suppression of soot inception and the enhancement of the soot oxidation process with increasing XO led to a reduction of soot volume fraction. On the contrary, the surface growth rates increased with XO, resulting in an increase in soot mass concentration. These competing effects led to the non-monotonic variation of soot volume fractions with XO in SFO flames. On the other hand, in SF flames both the inception and surface growth of soot increased as XO increased, resulting in the observed monotonic relationship between soot volume fraction and XO. We also analyzed the soot zone structures and made comparisons between SFO and SF flames.
       
  • Effect of volatile–char interactions on PM10 emission during the
           combustion of biosolid chars under air and oxyfuel conditions
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Xujun Chen, Sui Boon Liaw, Hongwei Wu This study reports the significant effect of volatile–char interactions on the emission of particulate matter (PM) during the combustion of biosolid chars in drop-tube furnace at 1300 °C under air and oxyfuel conditions. Slow and fast heating chars were prepared from biosolid pyrolysis and then interacted with the volatiles produced in situ from the pyrolysis of polyethylene (PE) and double acid-washed biosolid (DAWB) in a novel two-stage quartz reactor at 1000 °C (limited by the operating temperature of quartz). The results clearly show that under the experimental conditions, the interactions between chars and small non-oxygenated reactive species in both volatiles substantially decrease the yield of PM with aerodynamic diameter 
       
  • Mass interminglement and hypergolic ignition of TMEDA and WFNA droplets by
           off-center collision
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Dawei Zhang, Chengming He, Peng Zhang, Chenglong Tang Binary collision between a smaller N, N, N′, N′- tetramethylethylenediamine (TMEDA) droplet and a larger white fuming nitric acid (WFNA) droplet was investigated experimentally and computationally for understanding the influence of off-center collision on the hypergolic ignitability of the system, which is controlled by the mass interminglement and mixing subsequent to the droplet coalescence. The ignition delay time was experimentally found to non-monotonically vary with the impact parameter, which measures the degree of off-center collisions. This phenomenon was hypothetically attributed to the non-monotonicity of mass interminglement of colliding droplets with increasing the impact parameter—the increased droplet stretching by slightly off-center collision promotes mass interminglement, but the stretching separation by significantly off-center collision reduces mass interminglement. This hypothesis was computationally verified by a validated volume-of-fluid (VOF) simulation of a simplified problem, in which the transport phenomena and chemical reactions are neglected and the controlling physics of droplet mass interminglement is emphasized. Furthermore, a parametric study for wide ranges of controlling non-dimensional parameters, such as the collision Weber number of 20–140 and the droplet size ratio of 1.3–2.0, further confirms that the non-monotonicity of ignition delay time with the impact parameter is a general characteristic of the present hypergolic system.
       
  • On the application of tabulated dynamic adaptive chemistry in
           ethylene-fueled supersonic combustion
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Kun Wu, Francesco Contino, Wei Yao, Xuejun Fan The demands for extending the limiting operation conditions and enhancing the combustion efficiency of scramjets raise new challenges to the research of reliable robust and controllable flame stabilization in supersonic flows. In the present study, Large Eddy Simulation of flame stabilization in a realistic supersonic combustor, employing the tabulation of dynamic adaptive chemistry (TDAC) method were conducted, in comparisons with other relevant chemistry treatment methods, i.e., dynamic adaptive chemistry (DAC), global skeletal mechanism, and detailed mechanism. The wall pressures, the pseudo one-dimensional metrics, combustor global performance and flame structures are all well reproduced by the DAC/TDAC methods compared with the experimental measurements and the benchmark predictions by the detailed mechanism, while the global skeletal mechanism fails to predict the flame stabilization characteristics. The reason for the discrepancy induced by the skeletal mechanism in the flame stabilization simulation was further illustrated through reaction path analyses. Regarding the computational efficiency, the DAC method shows high efficiency for complex reaction systems, with an almost linear increasing speedup factor with the increase of species number. The TDAC method almost doubly further improves the DAC efficiency. The DAC/TDAC methods show great potential of alleviating the huge computational cost while improving the chemistry fidelity for supersonic combustion especially for flame stabilization modeling.
       
  • Electric field in Ns pulse and AC electric discharges in a hydrogen
           diffusion flame
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Marien Simeni Simeni, Yong Tang, Yi-Chen Hung, Zakari Eckert, Kraig Frederickson, Igor V. Adamovich Time-resolved electric field is measured in ns pulse and AC sine wave dielectric barrier discharges sustained in an atmospheric pressure hydrogen diffusion flame, using picosecond second harmonic generation. Individual electric field vector components are isolated by measuring the second harmonic signals with different polarizations. Electric field measurements in a ns pulse discharge are self-calibrating, since the field follows the applied voltage until breakdown. Electric field is measured in a ns pulse discharge sustained both in the hydrogen flow below the flame and in the reaction zone of the flame. Peak electric field in the reaction zone is lower compared to that in the near-room temperature hydrogen flow, due to a significantly lower number density. In hydrogen, most of the energy is coupled to the plasma at the reduced electric field of E/N ≈ 50–100 Td. In both cases, the electric field decreases to near zero after breakdown, due to plasma self-shielding. The time scale for the electric field reduction in the plasma is relatively long, several tens of ns, indicating that it may be controlled by a relatively slow propagation of the ionization wave over the dielectric surfaces. In the AC discharge, the electric field is put on the absolute scale by measuring a Laplacian electric field between two parallel cylinder electrodes. The measurement results demonstrate that a strong electric field in the plasma-enhanced flame is produced during the entire AC voltage period, without correlation with the random micro-discharges detected in the plasma images. The measurement results indicate consistently higher peak electric field during the negative AC half-period, as well as a significant electric field offset. Both the asymmetry and the offset of the electric field are likely responsible for the ion wind resulting in the flame distortion. The results suggest that at the present conditions the ion wind is dominated by the transport of negative ions generated in the ambient air plasma near the flame. The results demonstrate a significant potential of ps second harmonic generation diagnostics for non-intrusive measurements of the electric field in atmospheric pressure flames enhanced by electric discharge plasmas.
       
  • Aims and Scope/Editorial Board
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s):
       
  • Detailed kinetic model for ammonium dinitramide decomposition
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Yu-ichiro Izato, Atsumi Miyake Ammonium dinitramide (ADN; [NH4]+[N(NO2)2]−) is the most promising oxidizer for use with future green solid and liquid propellants for spacecraft applications. To allow the effective development and use of ADN-based propellants, it is important to understand ADN reaction mechanisms. This work presents a detailed chemical kinetics model for the liquid phase reactions of ADN based on quantum chemical calculations. The thermal corrections, entropies, and heat capacities of chemical species were calculated from the partition function using statistical machinery based on the G4 level of theory. Rate coefficients were also determined to allow the application of transition state theory and variational transition state theory to reactions identified in our previous study. The new model employed herein simulates the thermal decomposition of ADN under specific heating conditions and successfully predicts heats of reaction and the gases that result from decomposition under those conditions. The thermal behavior predicted from the new model was an excellent match with the experimental behavior observed from thermal analysis using differential scanning calorimetry and Raman spectroscopy. The new kinetic model reveals the mechanism for the decomposition of ADN.
       
  • Visualization of detonation propagation in a round tube equipped with
           repeating orifice plates
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Georgina Rainsford, Deepinder Jot Singh Aulakh, Gaby Ciccarelli Self-luminous, high-speed photography was used to visualize fast-flame and detonation propagation through a transparent round tube equipped with repeating orifice plates, in stoichiometric hydrogen-oxygen mixtures at initial pressures up to 60 kPa. Experiments were conducted in a 1.55 m, 7.6 cm inner-diameter plastic tube filled with equally spaced 5.33 cm and 3.81 cm orifice plates (50% and 75% area blockage ratio, respectively). The unprecedented visualization of quasi-detonation propagation in a round tube was used to identify the propagation mechanisms. For both sets of orifice plates, fast-flames were observed below a critical initial pressure. Fast-flame propagation involved the interaction of an uncoupled shock wave and flame with the orifice plates. Detonation propagation involved repeated detonation failure and initiation along the channel length; the limits measured in the 50% and 75% blockage ratio (BR) orifice plates were 7 kPa and 40 kPa, respectively. The orifice diameter-to-detonation cell size ratio (d/λ) corresponding to these limits are 1.4 and 14, respectively. It is proposed that the significant variance in the d/λ at the two limits is attributable to the difference in the detonation propagation mechanism. For the 50% BR orifice plates, near the limit, detonation initiation occurred on the tube wall between orifice plates following reflection of the lead shock wave. Whereas, for the 75% BR orifice plates, detonation initiation at the tube wall was not possible for initial pressures up to 40 kPa. This is the result of a weaker shock wave at the time of reflection due primarily to the larger distance from the orifice edge to the tube wall. Steady propagation of a curved detonation wave was observed for the 50% BR orifice plates for an initial pressure of 50 kPa (d/λ = 25), or greater; a similar propagation was not observed in the 75% BR orifice plates at initial pressures up to 60 kPa (d/λ = 27). Numerical simulations carried out using a single-step reaction model demonstrated the key processes involved in detonation initiation at the tube wall and the orifice plate but could not predict quantitatively the critical initial pressure required for detonation propagation measured in the experiments.
       
  • Effect of paraffin wax on combustion properties and surface protection of
           Al/CuO-based nanoenergetic composite pellets
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Kyung Joo Kim, Myung Hoon Cho, Ji Hoon Kim, Soo Hyung Kim We systematically investigated the effect of a polymer binder on various combustion properties and surface protection of nanoenergetic composite pellets containing Al and CuO nanoparticles (NPs) as the fuel and oxidizer, respectively. Al/CuO NP-based composite pellets were then fabricated by a pelletization process and the effect of paraffin wax (PW) binder concentration was investigated. The burn rate decreased with increasing PW content as the binder thermochemically interfered with the aluminothermic reaction between Al and CuO. However, the presence of a critical amount of PW (
       
  • Soot formation in shock-wave-induced pyrolysis of acetylene and benzene
           with H2, O2, and CH4 addition
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Alexander Drakon, Alexander Eremin, Ekaterina Mikheyeva, Bo Shu, Mustapha Fikri, Christof Schulz Experiments on the pyrolysis of C2H2/Ar and C6H6/Ar mixtures with addition of H2, O2, and CH4 have been carried out behind reflected shock waves at temperatures ranging from 1400 to 2600 K. Soot formation was measured by laser extinction at 633 nm. Time-resolved temperature measurements were performed via two-color CO absorption on the P(8) and R(21) lines at 2111.54 and 2191.50 cm-1 using quantum-cascade lasers. For this purpose, 0.5–0.8% CO was added to the gas mixtures. The measured temperature dependence of soot formation in experiments with added O2, and CH4 was corrected for the temperature effect caused by the thermochemistry of either endothermic pyrolysis or exothermic oxidation or reactions that cause time-dependent deviation from the initial frozen-shock temperatures. In all mixtures, the addition of H2 resulted in a noticeable decrease of the soot yield. A considerable increase in the soot yield was found with addition of methane to acetylene mixtures. In contrast, in benzene mixtures, the addition of methane caused a decrease of the soot yield. The qualitative analysis of the kinetics of the gas-phase stage of the pyrolysis reactions elucidated the influence of all investigated additives on the change in the key routes of initial stages of PAH and soot formation. We observed that the addition of H2 to acetylene inhibits the initial stages of the pyrolysis reaction, while the addition of CH4 and O2 opens up new ways for the formation of benzene and phenyl and following growth of pyrene. In contrast to that, in benzene all the additives studied lead to the suppression of the kinetics pathways for the formation of pyrene and the subsequent growth of soot.
       
  • ReaxFF simulations of petroleum coke sulfur removal mechanisms during
           pyrolysis and combustion
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Qifan Zhong, Qiuyun Mao, Jin Xiao, Adri C.T. van Duin, Jonathan P. Mathews Green petroleum coke (petcoke) is used as a feedstock for raw carbon material or as a fuel. Petcoke with high sulfur (S) content (>4 wt%) is typically restricted to fuel use unless extensive S removal is successful. Here, the S removal mechanisms during both pyrolysis and combustion were explored using the Reactive Force Field (ReaxFF) MD approach. A structural representation (C1648H772O59N24S47) of a green Qingdao petcoke was generated coupling high-resolution transmission electron microscopy lattice fringe image analysis and analytical data. This structure was consistent with elemental, aromaticity (FT-IR), the pair correlation function (XRD), and functional group (S, O, and N from XPS) data. The ReaxFF pyrolysis simulation produced gas and tar yields of 44.7 and 11.0 wt% at 3000 K after 250 ps of simulation. The combustion simulation on the same initial structure was performed in an O2 environment. During the pyrolysis simulation, the first-step for S-removal was thiophenic sulfur conversion to C1–4S (mostly C2S), COS, or CNS. The heteroatom pyrolysis overlapped, for this structure, at these conditions. However, for the combustion simulation earlier conversion of thiophenic sulfur to COS was observed. No NS containing structures occurred in this O-rich environment, as pyrrolic and pyridinic N quickly oxidized into CON or NO compounds. The S transformation during combustion can be summarized by COS → CO2S → CO3S → CO4S. The H atoms reacted with S-containing gases like COS/C2S/CNS producing HS and H2S rather than with the coke-S.
       
  • Experimental and numerical studies on detonation reflections over
           cylindrical convex surfaces
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Jian Li, Jianguo Ning The detonation reflection over a cylindrical convex surface was investigated experimentally and numerically by focusing on the length-scale effect on the reflection process, such as the triple-point trajectory and the critical wedge angle at which a transition occurs from regular reflection to Mach reflection. The results show that the critical wall angle plots exhibit significant scatter because of the cellular properties of the detonation front. If the transverse spacing is large as compared to the radius of curvature, the scatter range extends. If the transverse spacing is small as compared to the radius of curvature, the scattering is dramatically reduced. The critical wall angle is found to mainly depend on the scaled length i.e., the radius of curvature (R) over the cell size λ (or the reaction zone thickness Δ). Moreover, the critical wall angle increases with the decrease in the detonation thickness or with the increase in the radius. As R/λ increases to approximately ten, the critical wall angle approaches a value calculated using the non-reactive two-shock theory for pseudo-steady flows. The numerical results reveal that the transition to Mach reflection occurs earlier in the case of a ZND detonation than in the case of an inert shock wave because of the higher sound speed due to the release of chemical heat.
       
  • High-pressure 1D fuel/air-ratio measurements with LIBS
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Yue Wu, Mark Gragston, Zhili Zhang, Paul S. Hsu, Naibo Jiang, Anil K. Patnaik, Sukesh Roy, James R. Gord Quantitative, one-dimensional (1D), single-laser-shot, fuel–air ratio (FAR) measurements in both laminar and turbulent methane–air flames were conducted using time-gated nanosecond-laser-induced breakdown spectroscopy (ns-LIBS) line imaging. In the laminar methane–air flames at a pressure of 1–11 bar, hydrogen (Hα) and nitrogen (NII) atomic emission lines at 568 and 656 nm, respectively, were selected to establish a correlation between the line intensities and the local FAR. The spatial calibration profiles of the N/H ratios in the flames at various pressures were obtained in one dimension. The effects of the laser energy and pressure on the stability and precision of the 1D FAR measurements were investigated. It was observed that the N/H correlation is significantly reduced at ∼11 bar, which sets the limits of the 1D LIBS-based FAR measurements. Single-laser-shot 1D FAR measurements were conducted in a turbulent flame at atmospheric pressure, and multiline LIBS was performed to extend the measurement area of interest. Spatially and spectrally resolved line LIBS can provide the local FAR with a spatial resolution of ∼0.1 mm. These results hold promise for the utilization of ns-LIBS for spatially resolved 1D FAR measurements in turbulent flames at elevated pressures.
       
  • Using SIMD and SIMT vectorization to evaluate sparse chemical kinetic
           Jacobian matrices and thermochemical source terms
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Nicholas J. Curtis, Kyle E. Niemeyer, Chih-Jen Sung Accurately predicting key combustion phenomena in reactive-flow simulations, e.g., lean blow-out, extinction/ignition limits and pollutant formation, necessitates the use of detailed chemical kinetics. The large size and high levels of numerical stiffness typically present in chemical kinetic models relevant to transportation/power-generation applications make the efficient evaluation/factorization of the chemical kinetic Jacobian and thermochemical source-terms critical to the performance of reactive-flow codes. Here we investigate the performance of vectorized evaluation of constant-pressure/volume thermochemical source-term and sparse/dense chemical kinetic Jacobians using single-instruction, multiple-data (SIMD) and single-instruction, multiple thread (SIMT) paradigms. These are implemented in pyJac, an open-source, reproducible code generation platform. Selected chemical kinetic models covering the range of sizes typically used in reactive-flow simulations were used for demonstration. A new formulation of the chemical kinetic governing equations was derived and verified, resulting in Jacobian sparsities of 28.6–92.0% for the tested models. Speedups of 3.40–4.08 ×  were found for shallow-vectorized OpenCL source-rate evaluation compared with a parallel OpenMP code on an avx2 central processing unit (CPU), increasing to 6.63–9.44 ×  and 3.03–4.23 ×  for sparse and dense chemical kinetic Jacobian evaluation, respectively. Furthermore, the effect of data-ordering was investigated and a storage pattern specifically formulated for vectorized evaluation was proposed; as well, the effect of the constant pressure/volume assumptions and varying vector widths were studied on source-term evaluation performance. Speedups reached up to 17.60 ×  and 45.13 ×  for dense and sparse evaluation on the GPU, and up to 55.11 ×  and 245.63 ×  on the CPU over a first-order finite-difference Jacobian approach. Further, dense Jacobian evaluation was up to 19.56 ×  and 2.84 ×  times faster than a previous version of pyJac on a CPU and GPU, respectively. Finally, future directions for vectorized chemical kinetic evaluation and sparse linear-algebra techniques were discussed.
       
  • A generalized flamelet tabulation method for partially premixed combustion
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Xu Wen, Xue-Song Bai, Kun Luo, Haiou Wang, Yujuan Luo, Jianren Fan A flamelet tabulation method for partially premixed flames is proposed, in which partially premixed flamelets are incorporated as the archetypal flamelet elements. This method considers triple flame structures with both the partial premixing of fuel in the oxidizer side and the partial premixing of oxidizer in the fuel side, by replacing the pure-air and pure-fuel in the counterflow diffusion flame with a range of fuel-lean and -rich mixtures, respectively. The thermo-chemical quantities in the partially premixed flamelet are stored in a four-dimensional flamelet library as a function of the mixture fraction Z, describing the mixing process, the reaction progress variable YPV, describing the progress of reactions, and the trajectory variables YF and YO, characterizing the partial premixings of fuel and oxidizer, respectively. The performance of the proposed partially premixed flamelet tabulation (PPFT) method is evaluated through both a priori and a posteriori tests on laminar tribrachial flames with different mixture fraction gradients. The PPFT results are compared with those from a premixed flamelet tabulation (PFT) method and a diffusion flamelet tabulation (DFT) method. It is found that the combustion-mode-sensitive species such as CO and H2 can be accurately predicted by the PPFT method for both the low and high mixture fraction gradient flame cases, which cannot be well predicted by the PFT and DFT methods.
       
  • Enhanced ignition of milled boron-polytetrafluoroethylene mixtures
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Trevor D. Hedman, Andrew R. Demko, Joseph Kalman The combustion and physical properties of boron-polytetrafluoroethylene (PTFE) mixtures were modified by ball milling. Examination of the milled material through optical microscopy, scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) reveal that the milled mixtures are more intimately mixed and arranged into larger aggregate particles. Differential scanning calorimetry indicates the appearance of an endothermic reaction, brought on by milling. Fourier transform infrared spectroscopy provides evidence that the milling process enhances the chemical reaction of boron and PTFE. The milled boron-PTFE mixtures are demonstrated to be more reactive than those mixed by hand, despite containing larger particle sizes. Laser-ignition studies of the materials show that milling boron-PTFE mixtures results in the ignition delay times being reduced by a factor of 2. The milled mixtures were able to sustain combustion in air and emitted a strong BO2 signal while simple physical mixtures do not. Enhanced reactivity of the milled materials is attributed to a combination of decreased diffusion lengths and disruption of the boron oxide shell during the milling process.
       
  • An experimental and modeling study of dimethyl ether/methanol blends
           autoignition at low temperature
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Hongfu Wang, Ruozhou Fang, Bryan W. Weber, Chih-Jen Sung New rapid compression machine (RCM) ignition delay data for dimethyl ether (DME), methanol (MeOH), and their blends are acquired at engine-relevant conditions (T = 600 K–890 K, P = 15 bar and 30 bar, and equivalence ratios of ϕ = 0.5, 1.0, and 2.0 in synthetic dry air). The data are then used to validate a detailed DME/MeOH model in conjunction with literature RCM and shock tube data for DME and MeOH. This detailed DME/MeOH model, constructed by systematically merging literature models for the combustion of the individual fuel constituents, is capable of accurately predicting the experimental ignition delay data at a wide range of temperatures and pressures. The experiments and simulations both show a non-linear promoting effect of DME addition on MeOH autoignition. Additional analyses are performed using the merged DME/MeOH model to gain deeper insight into the binary fuel blend autoignition, especially the promoting effect of DME on MeOH. It is found that the unimolecular decomposition of HO2CH2OCHO plays an essential role in low temperature DME/MeOH blend autoignition. The accumulation of HO2CH2OCHO before the first-stage ignition and later quick consumption not only triggers the first-stage ignition, but also causes the non-linear promoting effect by accumulating to higher levels at higher DME blending ratios. These analyses suggest the rate parameters of HO2CH2OCHO unimolecular decomposition are critical to accurately predict the first-stage and overall ignition delay times as well as the first-stage heat release profile for low temperature DME/MeOH oxidation.
       
  • Modelling spark-plug discharge in dry air
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Lucas W.S. Crispim, Patricia H. Hallak, Mikhail S. Benilov, Maikel Y. Ballester This work presents a novel numerical strategy for studying the electric discharge produced by a vehicular spark plug in dry air. For such a task an axial symmetric 2D domain is used. The starting gas mixture is formed by molecular nitrogen and oxygen (8:2 ratio). The mathematical model considers heat and species diffusion and convection jointly with a discrete sub-model for energy transfer in electronic, atomic and molecular collisions. Chemical reactions between species are also included. Solutions of source terms is accomplished in the frame of ZDPlaskin, a zero-dimensional plasma modelling tool. The used plasmo-chemical kinetics model includes 53 species and 430 processes. Experimental properties from an actual spark plug discharge are introduced to the simulation. Spatio-temporal evolution of species concentrations are obtained within this model. Gas temperature evolution and species distribution is discussed and compare with available values in literature.Graphical abstractGraphical abstract for this article
       
  • Effects of jet in crossflow on flame acceleration and deflagration to
           detonation transition in methane–oxygen mixture
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Han Peng, Yue Huang, Ralf Deiterding, Zhenye Luan, Fei Xing, Yancheng You The fluidic jet turbulator has been a novel perturbation generator in the pulse-detonation engines research field for the past few years. In this paper, an experiment is performed to study the deflagration to detonation transition (DDT) process in a detonation chamber with a reactive transverse methane–oxygen mixture jet in crossflow (JICF). The jet injection arrangement is fundamentally investigated, including single jet and various double jets patterns. Corresponding two-dimensional direct numerical simulations with a multistep chemical kinetics mechanism are employed for analyzing details in the flow field, and the interaction between the vortex and flame temporal evolution is characterized. Both the experiments and simulations demonstrate that the JICF can distinctly accelerate flame propagation and shorten the DDT time and distance. The vortex stream induced by the jet distorts and wrinkles the flame front resulting in local flame acceleration. Moreover, the double jet patterns enhance flame acceleration more than the single jet injection because of the intrinsic counter-rotating vortex pairs and enhanced turbulence intensity.
       
  • Study on the ignition mechanism of Ni-coated aluminum particles in air
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Sangmin Kim, Jihwan Lim, Sanghyup Lee, Jaechul Jeong, Woongsup Yoon Since aluminum responds to various oxidizers and has a high energy density, there are high expectations for its usefulness as a fuel. However, it is covered with an aluminum oxide film, which has a high melting point, and thus, its ignition is difficult. One method suggested to solve this problem is nickel coating; however, in contrast to the extensive amount of research conducted on the overall phenomenon of aluminum combustion, research regarding Ni-coated aluminum is still in nascent stages. This study was carried out to further elucidate the ignition mechanism; thus, millimeter-sized (∼2.38 mm) aluminum particles were used to observe the surface where ignition occurs in air. The spatial and temporal resolutions were heightened by prolonging the heating period. The aluminum particles were nickel coated using electro/electroless methods, and surface analysis by SEM, thermal analysis by TGA/DSC, and species analysis by XRD and EDS were carried out. In addition, two-wavelength pyrometry was used to measure the ignition temperature. The results show that regardless of the nickel content in the coating of the aluminum particles, the ignition temperature was approximately 2400 K, similar to the melting point of aluminum oxide. The thermodynamic and thermophysical characteristics of nickel, aluminum, aluminum oxide, and nickel (II) oxide, and the surface/cross-sectional analysis, thermal and species analysis, and high-speed cinematography of the quenched samples provided a detailed explanation of the ignition process. Through this ignition mechanism, the emitted spectrum of AlO (as an intermediate combustion material) was traced to explain the decrease in ignition delay with increase in nickel content.
       
  • Laminar combustion regimes for hybrid mixtures of coal dust with methane
           gas below the gas lower flammability limit
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Chris T. Cloney, Robert C. Ripley, Michael J. Pegg, Paul R. Amyotte Understanding flame propagation in dust clouds and hybrid mixtures requires knowledge of the fundamental combustion processes and their coupling interaction. The objective of this work is to use computational fluid dynamics to classify laminar flame structure in hybrid mixtures where the initial gas concentration is below the lower flammability limit. Particular focus is given to the role of reaction chemistry and overall equivalence ratio on flame structure and burning velocity. Through this study, five flame regimes were determined: fuel-lean flames (Type I), volatile-lean flames (Type II), volatile-rich flames (Type III), transition flames (Type IV), and kinetic-limited flames (Type V). Gas-phase chemistry was found to play a critical role in burning velocity for Type III, IV, and V flames. Burning velocities at hybrid volatile component equivalence ratios less than 0.9, were found to be less sensitive to reaction kinetics. Further research using this model will focus on initial gas concentrations above the lower flammability limit, exploring the flammability limits of hybrid mixtures, and extending the results to turbulent flames in system geometries relevant to industrial safety.
       
  • Impact of engine operating cycle, biodiesel blends and fuel impurities on
           soot production and soot characteristics
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Julie Schobing, Valerie Tschamber, Alain Brillard, Gontrand Leyssens, Eduard Iojoiu, Vincent Lauga The impact of engine operating cycle, Biodiesel blends and fuel impurities on soot production and soot properties are evaluated in the present work. To this end, soot were produced on engine test bench and then collected inside a Diesel Particulate Filter (DPF). Two engine cycles (a Natural Loading and an Accelerated Loading) were tested. A standard Euro VI fuel blended with 7% of Biodiesel (B7) and a pure Biofuel (B100 RME EN 14214) were used. This latter was additivated with potassium and phosphorus at a low (B100+) or at a high (B100++) concentration. Soot characterization through elemental analyses, nitrogen adsorption, Raman spectroscopy, TGA and TPO experiments show that the engine operating cycle impact the soot reactivity through modifications of their texture and structure. Test bench experiments also show that increasing Biodiesel blend from B7 to B100+ divides by five the soot production. Moreover, soot obtained with B100+ are more reactive because of higher oxygen and ash content. When the inorganic content of the fuel is increased, few effects on the soot production are observed but the soot reactivity is significantly increased. In fact, analyses highlight that impurities present in the fuel are retrieved inside the soot composition and then catalyze their oxidation. K has a beneficial effect on both passive and active regenerations. On the contrary, P inhibits the active regeneration but has a significant catalytic impact on the CNO2H2O reaction. Finally, a numerical simulation allows to extract the kinetic constants of real B7- and B100+-soot, whose values confirm the differences of the soot reactivity.
       
  • Fuel particle shape effects in the packed bed combustion of wood
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Élizabeth Trudel, William L.H. Hallett, Evan Wiens, Jeremiah D. O'Neil, Marina K. Busigin, Dana Berdusco Experiments on the overfeed packed bed combustion and gasification of seven different shapes of parallelepipedal wood particles are presented. Attention is focussed on the part of the bed in which char conversion occurs; results for the pyrolysis zone at the top of the bed are not included. It is shown that fuel particle shape can affect conversion through the sphericity of the particle, through the orientation of the wood grain in the particle, and through the overlap of particles in the bed. These effects were incorporated into an existing numerical model of packed bed combustion and gasification. Particle sphericities for input to the model were determined directly from particle geometry, and particle overlap factors were estimated photographically. Comparison of predicted gas analyses and temperatures in the bed with experimental values then allowed the effect of the orientation of the original wood grain on char conversion to be estimated, with the conclusion that the rate of carbon conversion by the CO2 reduction reaction is faster by a factor of about 5 on surfaces normal to the wood fibres compared to the rate on surfaces parallel to the fibres. The carbon oxidation reaction at the bottom of the bed, on the other hand, is controlled by external gas phase diffusion and is not affected by the fibre orientation.
       
  • A Physics-based approach to modeling real-fuel combustion chemistry
           – III. Reaction kinetic model of JP10
    • Abstract: Publication date: Available online 18 September 2018Source: Combustion and FlameAuthor(s): Yujie Tao, Rui Xu, Kun Wang, Jiankun Shao, Sarah E. Johnson, Ashkan Movaghar, Xu Han, Ji-Woong Park, Tianfeng Lu, Kenneth Brezinsky, Fokion N. Egolfopoulos, David F. Davidson, Ronald K. Hanson, Craig T. Bowman, Hai Wang The Hybrid Chemistry (HyChem) approach has been proposed previously for combustion chemistry modeling of real, liquid fuels of a distillate origin. In this work, the applicability of the HyChem approach is tested for single-component fuels using JP10 as the model fuel. The method remains the same: an experimentally constrained, lumped single-fuel model describing the kinetics of fuel pyrolysis is combined with a detailed foundational fuel chemistry model. Due to the multi-ring molecular structure of JP10, the pyrolysis products were found to be somewhat different from those of conventional jet fuels. The lumped reactions were therefore modified to accommodate the fuel-specific pyrolysis products. The resulting model shows generally good agreement with experimental data, which suggests that the HyChem approach is also applicable for developing combustion reaction kinetic models for single-component fuels.
       
  • Kinetic modeling for unimolecular β-scission of the methoxymethyl radical
           from quantum chemical and RRKM analyses
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Jing Gao, Yulei Guan, Junpeng Lou, Haixia Ma, Jirong Song Unimolecular β-scission of the methoxymethyl (CH3OCH2) radical has been considered to be the crucial chain-propagating step in both oxidation and pyrolysis of dimethyl ether. The present work employs hybrid density functionals M06-2X, BB1K, B3LYP, and MPW1K with the MG3S basis set as well as double-hybrid density functional B2PLYP and Moller-Plesset perturbation theory MP2 with the TZVP basis set to study the detailed mechanism of unimolecular decomposition of CH3OCH2. Energies of all stationary points are refined with the CCSD(T), QCISD(T), CBS-QB3, and G4 calculations. The minimum energy path was computed at the CCSD(T)/aug-cc-PVTZ//M062X/MG3S level. Kinetic calculations are performed by means of high-pressure multi-structural canonical variational transition state (MS-CVT) theory and pressure-dependent Rice–Ramsperger–Kassel–Marcus (RRKM) theory to clarify the available experimental observations and previous theoretical results. A kinetic model for the low and the high-pressure limiting, and falloff region was extracted. For high pressure limit, k∞ = 2.08 × 1012 (T/300)1.002 exp(–11097.64/T) s−1 at temperatures of 200–2600 K based on the MS-CVT/SCT method. Furthermore, the intermediate falloff curve was found to be best represented by k/k∞=[x/(1+x)]Fcent1/[1+(a+logx)2/(N±ΔN)2] with x = k0/k∞, a = 0.263, N = 1.208, ΔN = 0.096, (+ΔN for (a + logx)  0), and Fcent(DME) = 0.348 independent of temperature. The low and high pressure limiting rate constants have been extracted by extrapolation of the fall-off curves: k0 = [DME] 2.49 × 1016 (T/300)0.053 exp(–9067.58/T) cm3 mol–1 s–1 and k∞ = 1.88 × 1012 (T/300)1.05 exp(–11061.79/T) s−1 at temperatures of 450–800 K, which agree well with the reported experimental low and high pressure limit results.
       
  • Pressure-gradient tailoring effects on the turbulent flame-vortex dynamics
           of bluff-body premixed flames
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Marissa K. Geikie, Kareem A. Ahmed This paper explores the effects of pressure-gradient tailoring on the turbulent flame and vorticity generation mechanisms of premixed flames. A turbulent premixed flame stabilized by a bluff-body in a high-speed combustor is used for the investigation. The combustor pressure gradient is altered using a variable-geometry test section. The turbulent flame-flow field is measured and characterized using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. A Lagrangian tracking technique is applied to analyze the details of the flame-vortex interactions from the experimental data. Lagrangian fluid elements are tracked as they evolve across the flame. The vorticity mechanisms are decomposed along the Lagrangian trajectories to determine their relative balance under various pressure gradient conditions. It is demonstrated that the induced pressure-gradient affects the relative magnitudes of combustion-generated dilatation and baroclinic torque, as well as the vortex stretching. An increase in the magnitudes of the vorticity mechanisms is shown with the largest gain in baroclinicity for the augmented pressure gradient relative to the attenuated.
       
  • Experimental study of the stabilization mechanism of a lifted Diesel-type
           flame using combined optical diagnostics and laser-induced plasma ignition
           
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Fabien Tagliante, Louis-Marie Malbec, Gilles Bruneaux, Lyle M. Pickett, Christian Angelberger The understanding of the stabilization process of Diesel spray flames is a key challenge because of its effect on pollutant emissions. In particular, the close relationship between lift-off length and soot production is now well established. However, different stabilization mechanisms have been proposed and are still under debate. The objective of this paper is to provide an experimental contribution to the investigation of these governing mechanisms. Combustion of a Diesel spray issued from a single-hole nozzle (90 µm orifice, ECN spray A injector) was studied in a constant-volume precombustion vessel using a combination of optical diagnostic techniques. Simultaneous high frame rate (6kfps) schlieren, 355 LIF (excitation at 355 nm and maximum collection at 430 nm) and high-temperature chemiluminescence (collection from 400 nm to 490 nm) or OH* chemiluminescence (collection at 310 nm and frame rate at 60kfps) are respectively used to follow the evolution of the gaseous jet envelope, formaldehyde location and lift-off position. Additional experiments are performed where the ignition of the mixture is forced at a location upstream of the natural lift off position by laser-induced plasma ignition (at 1064 nm). The evolution of the lift-off position until its return to the natural steady-state position is then studied for different ambient temperatures (800 K to 850 K), densities (11 kg/m3 to 14.8 kg/m3) and rail pressures (100 MPa to 150 MPa) using the same set of optical diagnostics. The analysis of the evolution of the lift off position without laser ignition reveals two main types of behaviors: sudden jumps in the upstream direction and more progressive displacement towards the downstream direction. While the former is attributed to auto-ignition events, the latter is studied through the forced laser ignition results. It is found that the location of formaldehyde greatly impacts the return velocity of the lift-off position: if laser ignition occurs upstream of the zone where formaldehyde is naturally present, the lift-off position convects rapidly until it reaches the region where formaldehyde is present and then returns more slowly towards its natural position, suggesting that cool-flame products greatly assist lift-off stabilization. The average return velocity in this second stage depends on the operating conditions.
       
  • Supercritical combustion of gas-centered liquid-swirl coaxial injectors
           for staged-combustion engines
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Xingjian Wang, Liwei Zhang, Yixing Li, Shiang-Ting Yeh, Vigor Yang The combustion characteristics of gas-centered, liquid-swirl coaxial injectors typically used in oxidizer-rich staged combustion cycle engines are numerically investigated at supercritical conditions. Turbulence closure is achieved using large-eddy-simulation techniques, and turbulence/chemistry interaction is modeled by a steady laminar flamelet approach. Gaseous oxygen (GOX) at 687.7 K is injected into the center post while kerosene at 492.2 K is delivered tangentially into the outer coaxial annulus. The operating pressure is 25.3 MPa. Detailed flow structures and flame dynamics are explored. The entire flowfield can be divided into four regimes: propellant injection, flame initialization, flame development, and intensive combustion. The diffusion-dominated flame is anchored in the wake of the GOX post and further enhanced in the downstream taper region. The surface of the coaxial annulus and taper is covered by fuel-rich mixtures and thus protected from thermal flux in the flame zone. Effects of the recess length (from the end of GOX post to the entrance of taper region) on the flow and flame evolution are investigated in depth. The efficiency of propellant mixing and subsequent combustion is found to increase with increasing recess length. The kerosene film is nearly depleted before the exit of the recess region for cases with long recess length, and the flame spreads upwards in the taper region for cases with reduced recess length due to insufficient mixing between GOX and kerosene. In a fully recessed injector without fuel shielding, the injected kerosene behaves like a liquid jet in a crossflow. Two recirculating zones containing fuel-rich mixtures are formed between the injection slit and the headend. A broad flame region is established at the exit of the recess region. In a non-recessed injector, the occurrence of combustion is delayed to the taper region. The flame resides along the taper surface and the injector faceplate, with most of GOX convecting downstream unburned. Results obtained from the present study can also be used to characterize combustion responses to local flow oscillations.
       
  • Downward flame spread along a single pine needle: Numerical modelling
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Kambam Naresh, Amit Kumar, Oleg Korobeinichev, Andrey Shmakov, Ksenia Osipova In this work downward flame spread over single pine needle of Pinus Sibirica is studied. Pine needles are thin cellulosic charring combustible forest fuel elements. Idealising pine needles to thin cylinders, a 2D axisymmetric numerical model is developed accounting for char formation and char oxidation to investigate the important mechanisms which control the downward spread of flame over a pine needle in normal gravity, atmospheric condition and at various opposed flow conditions. Simultaneous formation of char and pyrolysate during the pyrolysis process was found to significantly reduce the flame spread rate over thin fuel. Presence of char resulted in change in distribution of fuel vapour mass flux above the fuel surface which led to decrease in forward heat feedback to the fuel and hence the flame spread rate. This mechanism is different from char acting as a thermal barrier to heat transfer from the flame in case of thick fuel. Char oxidation had no influence on flame spread rate as char oxidation was found to occur only after passage of flame with the availability of surrounding oxygen diffusing through the hot plume of combustion products. Char oxidation was primarily controlled by oxygen diffusion rate to the charred fuel surface. The flame spread data for quiescent flame spread, and the blow off opposed flow velocity was used to calibrate gas phase kinetics and pyrolysis kinetics. The model predicted flame spread rate variation with opposed flow velocity quite well. The predicted spatial distribution of temperature and species concentration also compared very well with the experimentally determined flame structure.
       
  • L-shaped+wall+in+a+crossflow&rft.title=Combustion+and+Flame&rft.issn=0010-2180&rft.date=&rft.volume=">Fire whirls behind an L-shaped wall in a
           crossflow
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Tomomi Sasaki, Moe Igari, Kazunori Kuwana This paper studies fire whirls formed behind an L-shaped wall in a crossflow. Wind-tunnel experiments at various crossflow velocities were conducted, and it was found that there was a narrow range of crossflow velocity that led to the formation of an intense and stable fire whirl, i.e., the existence of a critical wind velocity. Scaling analysis and computational fluid dynamics (CFD) calculations of different scales suggested that the Froude number is the governing parameter of the phenomenon; the critical wind velocity is therefore roughly proportional to the square root of the fire size. Particle image velocimetry (PIV) measurements showed that the rotational velocity component was reduced near the bottom floor, which then induced a fast radial inflow toward the axis in the vicinity of the floor. This radial inflow pushes the flame toward the fuel surface, enhancing heat transfer between flame and fuel and thereby leading to the formation of an intense fire whirl. The inflow velocity was much slower when the crossflow velocity was outside the critical range. Finally, it was demonstrated that the formation of an intense fire whirl could be prevented by blocking the near-floor flow using an obstacle.
       
  • The evolution of autoignition kernels in turbulent flames of dimethyl
           ether
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Andrew R.W. Macfarlane, Matthew Dunn, Mrinal Juddoo, Assaad Masri Simultaneous planar laser-induced fluorescence (PLIF) imaging of CH2O and OH was performed at a repetition rate of 10.kHz, jointly with chemiluminescence to explore autoigniting dimethyl ether (DME) flames in a hot vitiated coflow burner. The focus of the study is the imaging of the flame stabilization region and the temporal evolution of ignition kernels upstream of the flame base. Results detail the evolution of kernels throughout their formation, growth and final merging with the flame base. The ignition events were explored for a range of different fuel premixing and dilution ratios over two coflow temperatures which result in different lift-off heights. Images of CH2O and OH over the entire flame length show that not only is the lift-off height much higher at low coflow temperatures, but that the fluctuations are more intense and the region of kernel formation is larger both radially and axially. In these autoignition stabilized flames, increased premixing leads to the lift-off height and location of the maximum kernel formation rate being further downstream. Transient 1-D simulations of hot coflow products opposed against jet fuel mixtures identify that the overlap of CH2O and OH PLIF signals are a reliable marker of heat release in autoignition kernels. Measurements indicate that for the high coflow temperature cases, on average, the heat release of individual kernels is low, despite the high total kernel formation rate. This can be correlated to the slow growth rate and elongated aspect ratio of the kernels. For low coflow temperature cases, kernels are growing faster and have high heat release rates with near unity aspect ratios.
       
  • Premixed flame stability and transition to detonation in a supersonic
           combustor
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Gabriel B. Goodwin, Elaine S. Oran Simulations of a supersonic, reacting, premixed flow in a channel were performed to investigate the effect of flow speed on ignition, flame stability, and transition to detonation. The configuration studied was a rectangular channel with a supersonic inflow of stoichiometric ethylene–oxygen, a transmissive outflow boundary, and no-slip adiabatic walls. The compressible reactive Navier–Stokes equations were solved by a high-order numerical algorithm on an adapting mesh for inflow Mach numbers, M∞, of 3 to 10. For M∞= 3, the fuel-oxidizer mixture does not reach a sufficient temperature for autoignition. Boundary layers that form on the top and bottom walls deflect the incoming flow, resulting in the formation of an oblique shock train. For M∞ ≥  5, the fuel-oxidizer mixture ignites in the boundary layers and the flame front expands into the channel. The flame front becomes unstable and turbulent with time due to a Rayleigh–Taylor (RT) instability at the interface between the low-density burned gas and high-density unburned gas. Detonation is initiated in several locations at the flame front and in the unburned gas through an energy-focusing mechanism. As M∞ increases, the time scales for growth of the RT instability at the flame front and eventual detonation increase significantly. Despite the difference in time scales, the flame evolution process is qualitatively independent of M∞: ignition in the boundary layer, laminar flame expansion, growth of an RT instability at the flame front, turbulent flame expansion, and deflagration-to-detonation transition.
       
  • Boron ignition and combustion with doped δ-Bi2O3: Bond energy/oxygen
           vacancy relationships
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Xizheng Wang, Tao Wu, Haiyang Wang, Jeffery B. DeLisio, Yong Yang, Michael R. Zachariah The purpose of this paper is to extract a clearer relationship between atomic properties of the oxidizer, and ignition temperature and combustion kinetics. Pure Bi2O3 and a series of Y3+ and W6+ doped Bi2O3 nanoparticles with the same crystal structure and morphology were synthesized via aerosol spray pyrolysis and used as oxidizers in boron-based thermites. This enabled us to vary bond energy and oxygen vacancy concentration systematically. The ignition temperatures and the reactivities of different B/Bi2O3 thermites were measured by rapid heating (> 105 K/s) temperature-jump/time-of-flight mass spectroscopy and a confined pressure cell, respectively. With pure Bi2O3, the boron could be ignited at a temperature as low as 520 °C. In-situ high heating rate TEM was used to observe the reaction before/after heating. We find very clear relationships that higher oxygen vacancy concentration and smaller metal–oxygen bond energy lead to lower ignition temperature and higher combustion reactivity.
       
  • Preparation and combustion of laminated iodine containing
           aluminum/polyvinylidene fluoride composites
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Haiyang Wang, Scott Holdren, Michael R. Zachariah Energetic materials with a high iodine content and tunable reactivity are desirable for application as a biocidal agent. In this paper, aluminum/polyvinylidene fluoride (Al/PVDF) composites with different iodine content were prepared by an electrospray deposition method. Most of the iodine in the films are found to be fixed by PVDF and aluminum, which is released at 250 °C and 450 °C respectively. The heat release and burning rate of the iodine-containing films decreases with the increase of iodine content. With an iodine content of ≥40 wt.%, the film did not propagate. However, when fabricated in a laminate structure the threshold for iodine loading to sustain propagation increased to 67 wt.%. Evaluation of several multi-layered structured films indicated that an optimum single layer thickness of ∼25 µm produced the fastest reaction velocity, with loadings of up to 67 wt.% iodine. The thermal decomposition and oxidation of the laminated Al/PVDF films are also investigated. It appears thus that iodine which acts as a reaction retardant can be loaded in higher concentrations if it is physically separated from the primary energetic. In so doing, the primary energetic can maintain a continuous ignition threshold to propagate and enable the heat released from reaction to evolve gas phase iodine.
       
  • Cubane decomposition pathways – A comprehensive study
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Bimal Shyamala, Sohan Lal, Arindrajit Chowdhury, Irishi N.N. Namboothiri, Neeraj Kumbhakarna This work focuses on the development of a detailed chemical kinetics mechanism for the decomposition of the high energy density compound cubane. Quantum mechanics based ab initio calculations have been carried out to elucidate the various chemical pathways that lead to the formation of previously known product species from cubane. Optimised structures of ground states and transition states appearing in the chemical reaction scheme were obtained by using various levels of theory. Minimum energy paths were also traced for each elementary reaction. The mechanism thus obtained, along with the computed rate parameters and thermodynamic data, was used in a flow reactor model to simulate a flow reactor experiment that was carried out previously by others. Comparison of the simulation and experimental results validated the formulated reaction mechanism and provided valuable insights into the chemical behaviour of cubane.
       
  • Hydrogen shift isomerizations in the kinetics of the second oxidation
           mechanism of alkane combustion. Reactions of the hydroperoxypentylperoxy
           OOQOOH radical
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Lili Xing, Junwei Lucas Bao, Zhandong Wang, Xuetao Wang, Donald G. Truhlar Hydroperoxyalkylperoxy species are important intermediates that are generated during the autoignition of transport fuels. In combustion, the fate of hydroperoxyalkylperoxy is important for the performance of advanced combustion engines, especially for autoignition. A key fate of the hydroperoxyalkylperoxy is a 1,5 H-shift, for which kinetics data are experimentally unavailable. In the present work, we study 1-hydroperoxypentan-3-yl)dioxidanyl (CH3CH2CH(OO)CH2CH2OOH) as a model compound to clarify the kinetics of 1,5 H-shift of hydroperoxyalkylperoxy species, in particular α-H isomerization and alternative competitive pathways. With a combination of electronic structure calculations, we determine previously missing thermochemical data, and with multipath variational transition state theory (MP-VTST), a multidimensional tunneling (MT) approximation, multiple-structure anharmonicity, and torsional potential anharmonicity, we obtained much more accurate rate constants than the ones that can computed by conventional single-structure harmonic transition state theory (TST) and than the empirically estimated rate constants that are currently used in combustion modeling. The roles of various factors in determining the rates are elucidated. The pressure-dependent rate constants for these competitive reactions are computed using system-specific quantum RRK theory. The calculated temperature range is 298–1500 K, and the pressure range is 0.01–100 atm. The accurate thermodynamic and kinetics data determined in this work are indispensable in the detailed understanding and prediction of ignition properties of hydrocarbons and alternative fuels.
       
  • Numerical study of a micro flow reactor at engine pressures: Flames with
           repetitive extinction and ignition and simulations with a reduced chemical
           model
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Simon Lapointe, Clara L. Druzgalski, Matthew J. McNenly Combustion in a narrow channel with an imposed temperature gradient is studied numerically at elevated pressure with engine-relevant fuels. The focus is placed on unsteady flames with repetitive extinction and ignition (FREI) to determine the potential of this regime for fuel testing and calibration of reduced chemical mechanisms. First, it is shown that the FREI regime does occur at elevated pressures for sufficiently small tube diameters. The sensitivity of the extinction and ignition temperatures to low-temperature chemistry is found to be significantly enhanced at 25 bar compared to atmospheric conditions. The ignition and extinction temperatures differ by up to 100 K between PRF mixtures with varying octane numbers. Ternary mixtures of iso-octane/n-heptane/toluene and iso-octane/n-heptane/ethanol at similar research and motor octane numbers are also studied. Second, the potential of using data from the micro flow reactor to infer reaction rates is assessed. A reduced chemical mechanism combining a small fuel-dependent submechanism with a detailed fuel-independent submechanism for the core species chemistry is used for that purpose. Only the most sensitive fuel-dependent reactions are inferred. the calibrated reduced model is compared to a detailed model and good agreement in ignition delay times and laminar flame speeds is observed. This illustrates the potential of a micro flow reactor with a controlled temperature profile to test fuels and infer kinetic data.
       
  • Studies of low temperature oxidation of n-pentane with nitric oxide
           addition in a jet stirred reactor
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Hao Zhao, Lingnan Wu, Charles Patrick, Zunhua Zhang, Yacine Rezgui, Xueliang Yang, Gerard Wysocki, Yiguang Ju The low temperature oxidation of n-pentane with nitric oxide (NO) addition has been investigated at 500–800 K in an atmospheric jet stirred reactor (JSR). The molar fraction of NO in the mixture is varied between 0 to 1070 ppm to study its chemical sensitization effect on low temperature oxidation of both fuel lean and rich n-pentane/oxygen mixtures. N-pentane, O2, CO, CO2, CH2O, C2H4, CH3CHO, NO, and NO2 are quantified simultaneously, in-situ by using an electron impact molecular beam mass spectrometer (MBMS), a micro-gas chromatograph (µ-GC), and a sensitive mid-IR dual-modulation faraday rotation spectrometer (DM-FRS). The experimental results reveal that NO addition delays the onset temperature of low temperature oxidation of n-pentane between 550–650 K, but reduces the negative temperature coefficient (NTC) behavior in the NTC region (650–750 K) and dramatically shifts the onset of high temperature fuel oxidation to an intermediate temperature (750–800 K). A recently developed n-pentane/NOx model by using Reaction Mechanism Generation (RMG) and a new n-pentane/NOx model in the present work were used to predict the experimental results. The results show that the three distinct temperature-dependent characteristics of NO sensitized n-pentane oxidation are captured appropriately by these two models at both fuel rich and lean conditions, while the onset temperature of low temperature oxidation is not accurately predicted by these two models. It shows that the RMG model has a better prediction of the onset delay of n-pentane oxidation than Zhao's model, while Zhao's model performs better at NTC and intermediate temperature regions. Besides RO2 + NO, additional fuel/NOx reaction pathway, like R + NO2, RO + NO, and RO + NO2, and the interconversion reactions among NO, NO2, and HONO may need to be further studied.
       
  • A physics-based approach to modeling real-fuel combustion chemistry –
           IV. HyChem modeling of combustion kinetics of a bio-derived jet fuel and
           its blends with a conventional Jet A
    • Abstract: Publication date: Available online 7 August 2018Source: Combustion and FlameAuthor(s): Kun Wang, Rui Xu, Tom Parise, Jiankun Shao, Ashkan Movaghar, Dong Joon Lee, Ji-Woong Park, Yang Gao, Tianfeng Lu, Fokion N. Egolfopoulos, David F. Davidson, Ronald K. Hanson, Craig T. Bowman, Hai Wang A Hybrid Chemistry (HyChem) approach has been recently developed for the modeling of real fuels; it incorporates a basic understanding about the combustion chemistry of multicomponent liquid fuels that overcomes some of the limitations of the conventional surrogate fuel approach. The present work extends this approach to modeling the combustion behaviors of a two-component bio-derived jet fuel (Gevo, designated as C1) and its blending with a conventional, petroleum-derived jet fuel (Jet A, designated as A2). The stringent tests and agreement between the HyChem models and experimental measurements for the combustion chemistry, including ignition delay and laminar flame speed, of C1 highlight the validity as well as potential wider applications of the HyChem concept in studying combustion chemistry of complex liquid hydrocarbon fuels. Another aspect of the present study aims at answering a central question of whether the HyChem models for neat fuels can be simply combined to model the combustion behaviors of fuel blends. The pyrolysis and oxidation of several blends of A2 and C1 were investigated. Flow reactor experiments were carried out at pressure of 1 atm, temperature of 1030 K, with equivalence ratios of 1.0 and 2.0. Shock tube measurements were performed for the blended fuel pyrolysis at 1 atm from 1025 to 1325 K. Ignition delay times were also measured using a shock-tube. Good agreement between measurements and model predictions was found showing that formation of the products as well as combustion properties of the blended fuels were predicted by a simple combination of the HyChem models for the two individual fuels, thus demonstrating that the HyChem models for two jet fuels of very different compositions are “additive.”
       
  • A novel method for trigger location control of the oblique detonation wave
           by a modified wedge
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Qiongyao Qin, Xiaobing Zhang Reducing the scale of the oblique detonation wave engine is of great importance as the aircrafts are getting smaller and smaller. A key factor that determines the scale of the oblique detonation wave engine is the trigger location of the detonation wave. Motivated by a cavity stabilized micro combustion phenomenon, a novel wedge with a step added on the surface is proposed to control the trigger location of the oblique detonation wave. A numerical model based on two-dimensional compressible multi-species Euler equations is established to simulate the shock induced combustion phenomenon induced by the wedge. Detailed reaction kinetics mechanism is taken into consideration. An AUSM + scheme (Advection Upstream Splitting Method) is adopted to solve the model. Eleven cases considering different step locations, different Mach numbers of the incoming flow and different rear wedge angles are simulated. It is found that the novel wedge is capable to control the trigger location of the oblique detonation wave through a compression–expansion–compression process. The trigger location control can be accomplished through variations of the step location and the rear wedge angle. The trigger location is always following the step with a constant distance from the step as the step moves along the wedge surface. The trigger location moves towards the step as the rear wedge angle increases.
       
  • Experimental, numerical and theoretical analyses of the ignition of
           thermally thick PMMA by periodic irradiation
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Jun Fang, Ya-Ru Meng, Jing-Wu Wang, Lu-Yao Zhao, Xuan-Ze He, Jie Ji, Yong-Ming Zhang In this work, the pyrolysis and ignition of thermally thick poly (methylmethacrylate) material with low periodic on-off irradiation was investigated, the solid and gas absorption was ignored, an ignition time formula with periodic heating was established based on the deduced ignition time model. The results show that the surface and in-depth sample temperatures as well as the mass flux all increase during the periodic ‘on’ cycle prior to ignition, at the moment there is a small luminous sustained flame, followed by flame spreading. For the surface temperature, the fluctuation magnitude increases with increasing cycle time ∝τ. The in-depth temperature decay relating to the distance and cycle as ∝exp(−x/τ). The surface and in-depth temperatures, mass flux oscillates due to the periodic on-off irradiation with a time delay, which increases with increasing cycle and in-depth distance as ∝τx. The cycle has slight influence upon the surface temperature and mass flux at the moment of ignition, where the ignition temperature maintains at about 340 °C, while the critical mass flux is in a range of 1–1.4 g/m2s, which are both independent of the external heat flux. The linear relationship of successive peak surface temperature with heat flux via time (Ts*−T0q˙″e)2∝t in the periodic on-off heating is retained. The theoretical predictions of the periodic ignition times derived in this study are in good agreement with the experimental measurements. Finally, compared with constant heat flux, the periodic heating delays the ignition, but with increasing cycle time, the ignition time is seen to decrease, which is primarily attributed to increases in the time-averaged irradiative heat flux. The classical model over-predicts the ignition time, the prediction error is expected to increase for long time ignition with low thermal inertia, big perturbation heat flux and long cycle time.
       
  • Ignition delay times measurement and kinetic modeling studies of
           1-heptene, 2-heptene and n-heptane at low to intermediate temperatures by
           using a rapid compression machine
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Yingtao Wu, Yang Liu, Chenglong Tang, Zuohua Huang In this work, we have firstly investigated the auto-ignition behaviors of 1-heptene, 2-heptene and n-heptane in the low to intermediate temperature range (650–950 K) over various equivalence ratios at 15 and 23 bar using a rapid compression machine. Results show that n-heptane exhibits the expected negative temperature coefficient (NTC) behavior and shows the shortest IDTs among the three fuels, while the NTC behavior for 1-heptene and 2-heptene is moderated and quasi-Arrhenius temperature dependence of the 1st stage IDTs is observed at all test conditions. As the temperature increased over 900 K, the IDTs of the three fuels begin to be consistent indicating a moderated effect of the unsaturated bond. In the NTC temperature region, 1-heptene shows higher reactivity than 2-heptene, while opposite relative reactivity is observed in the temperature beyond the NTC region. The IDT data of 1-heptene, 2-heptene and n-heptane were then used to validate several kinetic models. Results show that the performance of the n-heptane models is generally good, while all the models underestimate the low temperature reactivity of 1-heptene. Finally, a model refinement has been made and the prediction shows better agreement with the present measured IDT as well as the experimental pressure evolution trace in literature.
       
  • An experimental and kinetic study of propanal oxidation
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Gianluca Capriolo, Vladimir A. Alekseev, Alexander A. Konnov Propanal is a critical stable intermediate derived from the oxidation of 1-propanol, a promising alcohol fuel additive. To deepen the knowledge and accurately describe propanal combustion characteristics, new burning velocity measurements at different temperatures were carried out and a new detailed kinetic mechanism for propanal was proposed. Experiments were performed using the heat flux method and compared with literature data. Important discrepancies were noted between the new and available data, and possible reasons were suggested. Flow rate sensitivity analysis highlighted that, as expected, the important reactions influencing the propanal oxidation in flames are pertinent to H2 and CO sub-mechanism. Current mechanism is based on the most recent Konnov model, extended to include propanal chemistry subset. Rate constant parameters were selected based on careful evaluation of experimental and theoretical data available in literature. Model validation included assessment against a large set of combustion experiments obtained at different regimes, i.e. flames, shock tubes, and well stirred reactor, as well as comparison with the semi-detailed (lumped) kinetic mechanism for hydrocarbon and oxygenated fuels from Politecnico di Milano, detailed kinetic model from Veloo et al. and low temperature oxidation of aldehydes kinetic model of Pelucchi et al. The proposed model reproduced experimental burning velocities, ignition delay times, flame structure and JSR data with an overall good fidelity, while it reproduces only qualitatively the species distribution of propanal pyrolysis.
       
  • Aluminum–nickel combustion for joining lunar regolith ceramic tiles
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Robert E. Ferguson, Evgeny Shafirovich Combustion-based methods are attractive for space manufacturing because the use of chemical energy stored in reactants dramatically decreases the required external energy input. Recently, a sintering technique has been developed for converting lunar/Martian regolith into ceramic tiles, but it is unclear how to build a reliable launch/landing pad from these tiles with small amounts of energy and materials. Here we explored the feasibility of joining the regolith tiles using self-propagating high-temperature reactions between two metal powders. Combustion of an aluminum/nickel mixture placed in a gap between two tiles, made of JSC-1A lunar regolith simulant, was studied in an argon environment at 1 kPa pressure. Stable propagation of the combustion front was observed over the tested range of distances between the tiles, 2–8 mm. The front velocity increases with increasing the distance between the tiles. Joining of the tiles was achieved in several experiments and improvement with increasing the tile thickness was observed. Thermophysical properties of the tiles, the reactive mixture, and the reaction product were determined using differential scanning calorimetry and laser flash analysis. A model for steady propagation of the combustion wave over a condensed substance layer placed between two inert media was applied for analysis of the investigated system. Testing the model has resulted in reasonable agreement between the experimental and modeling dependencies. Both experimental and modeling results indicate a narrow quenching distance in the investigated system, which implies that a small amount of the reactive mixture would be required for sintering regolith tiles on the Moon.
       
  • The inhibiting effect of NO addition on dimethyl ether high-pressure
           oxidation
    • Abstract: Publication date: November 2018Source: Combustion and Flame, Volume 197Author(s): Lorena Marrodán, Álvaro J. Arnal, Ángela Millera, Rafael Bilbao, María U. Alzueta The high-pressure dimethyl ether (DME, CH3OCH3) oxidation has been investigated in a plug flow reactor in the 450–1050 K temperature range. Different pressures (20, 40 and 60 bar), air excess ratios (λ = 0.7, 1 and 35), and the absence/presence of NO have been tested, for the first time under these conditions. An early reactivity of DME and a negative temperature coefficient (NTC) zone have been observed under the studied conditions, although under very oxidizing conditions (λ = 35), NTC zone is almost imperceptible because DME is completely consumed at lower temperatures. A chemical kinetic mechanism has been used to describe the DME high-pressure oxidation, with a good agreement with the experimental trends observed. In general, modeling calculations with the present mechanism have been successfully compared with experimental data from literature. The presence of NO has an inhibiting effect on DME high-pressure consumption at low-temperatures because of: (i) the competition between CH3OCH2+O2⇌CH3OCH2O2 and CH3OCH2+NO2⇌CH3OCH2O+NO reactions, and (ii) the participation of NO in CH3OCH2O2+NO⇌CH3OCH2O+NO2 reaction, preventing CH3OCH2O2 radicals continue reacting through a complex mechanism, which includes a second O2 addition and several isomerizations and decompositions, during which highly reactive OH radicals are generated. Consequently, NO and NO2 are interchanged in a cycle but never consumed.
       
 
 
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