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Combustion and Flame
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ISSN (Print) 0010-2180
Published by Elsevier Homepage  [3159 journals]
  • 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. ZachariahAbstractThe 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. ZachariahAbstractEnergetic 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.
       
  • A diffusion-flame analog of forward smolder waves: (II) stability analysis
    • Abstract: Publication date: Available online 28 July 2018Source: Combustion and FlameAuthor(s): Zhanbin Lu We proceed to examine the stability of the adiabatic and non-adiabatic structures of forward smolder waves elaborated in Part (I) of this series. The dispersion relation for adiabatic forward smolder waves with a reaction trailing structure turns out to take a form similar to that for premixed flames, thereby strengthening the analogy of the reaction trailing structure with the premixed flame regime of diffusion flames. According to the dispersion relation, corresponding to each Damköhler number there exists a marginal oxygen Lewis number, below which cellular instability occurs. In particular, similar to the Burke–Schumann limit of diffusion flames, the stoichiometric limit at infinite Damköhler number is unconditionally stable. Such unconditional stability is found to further extend to the entire Damköhler number range for adiabatic forward smolder waves with a reaction leading structure. Linear stability analysis of non-adiabatic forward smolder waves indicates that, for both reaction trailing and reaction leading structures, the low smolder temperature (or high reactant leakage) solution branch is physically unrealistic, whereas on the high smolder temperature (or low reactant leakage) branch different kinds of instabilities may develop near the quenching limit. Under a fixed Damköhler number, the range of the heat loss coefficient corresponding to these instabilities shows a trend to grow with decreasing oxygen Lewis number. 2-D time-dependent numerical simulations of unstable non-adiabatic forward smolder waves confirm that fingering or cellular instability occurs exclusively for the reaction trailing structure, whereas traveling wave instability prevails for the reaction leading structure. A comparison is made between the current stability analysis results of non-adiabatic forward smolder waves and results from a concurrent flame spread experiment. Agreement is achieved not only on the existence of reaction front instabilities near the quenching limit, but also on the conditions determining the type of these instabilities.
       
  • 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.
       
  • Optical and laser diagnostic investigation of flame stabilization in a
           novel, ultra-lean, non-premixed model GT burner
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Rajesh Sadanandan, Aritra Chakraborty, Vinoth K. Arumugam, Satyanarayanan R. Chakravarthy The flame dynamics and stabilization mechanism in a novel, ultra-lean, non-premixed, model GT burner is experimentally investigated with optical and laser diagnostics. The burner operating with methane as fuel exhibits a convergent-divergent flow field and is capable of stabilizing a non-sooty flame at ultra-low global equivalence ratio (ϕglob) down to 0.1. At 1.0 > ϕglob > 0.6, the flame stabilizes in the diverging section where the flow field is characterized predominantly by the swirl. The flame stabilizes at locations of low velocity and low turbulence where the mean flow strain rates are found to be well below the extinctions strain rates for turbulent non-premixed flames. With decreasing ϕglob, the flame is located progressively near the burner lip, where a large recirculation zone of the central bluff body in the burner exists. The transition from swirl stabilized to bluffbody stabilized region occurs between 0.6 > ϕglob > 0.4 and bi-stability of the flame with low-frequency oscillation between the two stabilization locations was observed. The upstream marching of the flame, despite a decreasing ϕglob, and a corresponding decrease in heat input, is explained in terms of the interaction of the flame front with the evolution of the scalar profile upstream, in presence of the recirculating hot gases. The results support flame stabilization theories based on partial premixing and flow modification through heat release upstream of the flame. For the leaner conditions, 0.4 > ϕglob > 0.1, the flame is stabilizing in a region characterized by bluffbody induced toroidal flow structures of high strain rates, where the presence of recirculating hot burned gases aid in flame stabilization by the ignition of flammable mixture.
       
  • Global sensitivity analysis of n-butanol reaction kinetics using rate
           rules
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Mireille Hantouche, S. Mani Sarathy, Omar M. Knio We investigate the sensitivity of the ignition delay time, τign, of n-butanol to uncertainties in the rate-rule kinetic rate parameters, at various initial temperatures (600–1000 K), pressures (10–80 bar) and equivalence ratios (0.5–2.0). We start by considering a 30-dimensional stochastic germ in which each random variable is associated with one reaction class, and build a surrogate model for the ignition delay time using polynomial chaos expansions. The adaptive pseudo-spectral projection technique is used for this purpose. The surrogate model is used to estimate first-order and total-order sensitivity indices characterizing the dependence of the ignition delay time on the uncertain inputs. Results indicate that τign is mostly sensitive to variations in four dominant reaction classes, namely, H–atom abstraction from the fuel (reaction class 2), addition of O2 to the fuel radicals (reaction class 11), fuel radical isomerization including Waddington type reactions (reaction class 15), and concerted elimination reactions (reaction class 16). We consequently focus our attention on these four reaction classes, and consider variations within corresponding subrules. We explore two approaches to define the subrules of reaction class 2, one based on the radical abstracting from the fuel resulting in eleven subrules and another based on the abstraction site resulting in five subrules. Hence, we investigate the sensitivity of τign due to variability in the rate parameters of 26 and 20 subrules of the resulting models. In particular, the simulations indicate that in reaction class 2 H–atom abstraction by HO2 dominates the variability in τign at all initial conditions considered. Analysis of this finding reveals that correlations inherent in the rate rule construction plays an important role in the resulting sensitivity predictions, and suggests a hierarchical approach to the calibration of elementary reaction rates.
       
  • Structure and behavior of water-laden CH4/air counterflow
           diffusion flames
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Rosa E. Padilla, D. Escofet-Martin, T. Pham, William J. Pitz, D. Dunn-Rankin A counterflow configuration was used to measure thermal and species structure in water-vapor diluted nonpremixed methane–air flames. The motivation is to understand the chemical and thermal effects that water has when it is introduced as a diluent into the fuel side. This work is relevant to combustion processes where water is incorporated naturally in the fuel; e.g., methane hydrates, and when water is added intentionally for emission reduction such as in flares and H2O/fuel emulsions combustion. Experimental data are compared to 1-D computations. The agreement is generally very good, but the one dimensional counterflow diffusion model overpredicts flame temperature and major radical, OH, concentration very near extinction in highly diluted H2O–methane/air diffusion flames. Changes in flame position, flame width, and peak temperature with the addition of water were measured. Flame temperatures were measured with thin filament pyrometry. OH-PLIF is used to characterize the flame reaction zone with water dilution; the OH distribution, flame position and thickness from the OH-PLIF images were measured. The results show that the OH intensity and reaction zone thickness decreases with the increase in water. Predictions and experiments demonstrate that water mainly acts thermally to lower the flame temperature until extinction. The OH maximum intensity shifts towards the air side of the counterflow burner with water addition. OH is also measured with CO2 dilution of the fuel stream, and the results are compared with H2O addition, including comparisons with the OH molar peak predictions obtained using the GRI 3.0 mechanism and the CHEMKIN Pro one dimensional counterflow model. The study indicates that water’s chemical effects are to change the production and depletion of OH, H and O radicals, especially near extinction. Chemical kinetics simulation of the flame demonstrates good agreement in OH and flame temperatures over a wide range of dilution away from extinction, particularly for CO2. An over prediction of the water carrying capacity near extinction is found for highly water-diluted flames.
       
  • Low-frequency combustion instabilities of an airblast swirl injector in a
           liquid-fuel combustor
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Byeonguk Ahn, Jeongjae Lee, Seungchai Jung, Kyu Tae KimABSTRACTLow-frequency combustion dynamics of a prefilming airblast injector were experimentally investigated in a laboratory-scale liquid-fuel combustor operated with Jet A-1 fuel and air at atmospheric pressure and elevated temperatures. Our measurements reveal that multiple modes – ranging from 45 to 292 Hz – can be excited in the non-premixed combustion system, including the Helmholtz, longitudinal, and hydrodynamic instabilities. The system's propensity for mode selection depends strongly on combustor length and pilot equivalence ratio. A strong Helmholtz mode with a peak-to-peak amplitude of ∼14 kPa occurs, provided that the combustor length is relatively short and the pilot equivalence ratio is high. A high-amplitude, intermittent burst at approximately 12 Hz always develops when the system is on the verge of transition to the Helmholtz instability. With a longer combustor length, on the other hand, the system transitions to the ¾-wave longitudinal mode via a discontinuous mode-hopping process. The system undergoes well-defined, limit cycle oscillations with an extremely large pressure amplitude of ∼30 kPa, but the global OH* fluctuation amplitude is limited to merely 8.3%. Phase-resolved flame imaging measurements demonstrate that a ring-like partially-premixed flame emerges periodically in the dump region, when the non-premixed flame wrapping around the central recirculation zone is lifted off the fuel nozzle. The mutual interaction between the partially-premixed and non-premixed reaction zones is the dominant mechanism for intense sound generation. Under certain conditions, superposition of the Helmholtz and longitudinal modes occurs, leading to nonlinear interactions manifested by additional spectral peaks at the sum and difference of the two frequencies. In contrast to the other two cases, this superimposed tone is not characterized by limit cycle behavior, but by a noise-driven unsteadiness.
       
  • Characterization of ozone-enhanced propane cool flames at sub-atmospheric
           pressures
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Mohammadhadi Hajilou, Erica Belmont A recently developed experimental setup for the study of cool flames was employed to investigate low temperature combustion of propane. Cool flames were stabilized though ozone activation using a laminar flat flame Hencken burner at sub-atmospheric pressures. Propane cool flame stability was observed to be highly sensitive to pressure, and a pressure stability map is presented for a range of equivalence ratios (ϕ) from 0.17 to 1.0 in 0.17 increments and a range of reactant flow rates. Based on the stability map, a propane cool flame of ϕ = 0.17 at 17.3 kPa was chosen for further study. Flame lift-off height above the burner surface was measured and two operating regimes of burner-stabilized and freely propagating flames were observed. The intersection of these two regimes was used to estimate the cool flame propagation speed. Temperature measurements of the cool flame were taken and used in fixed-temperature numerical flame simulations. Additionally, flame temperatures and propagation speeds are presented for all equivalence ratios and pressures where stable, freely propagating flames were observed in the stability map. In order to enable non-fixed temperature, freely propagating cool flame simulations to be performed, reduction of a chemical kinetics model using the Directed Relation Graph method (DRG) in conjunction with reaction rate sensitivity analysis was performed. Experimentally determined propagation speed, temperature and species concentrations at partial equilibrium were compared to numerical simulations with reasonable agreement and results are discussed.
       
  • Insight into cooling agent addition on combustion activity and mechanism
           of catalyzed 5AT-Sr(NO3)2 based propellant
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Dan Zhang, Lin Jiang, Song Lu, He-Ping Zhang Thermal decomposition characteristics and combustion behaviors of 5-amino-1H-tetrazole-strontium nitrate (5AT-Sr(NO3)2) propellant are of great essence in determining fire-fighting efficiency of the novel Halon alternative technology-solid propellant gas generators (SPGGs). This study will seek to find the impacts on combustion mechanism of adding two kinds of coolants, aluminum hydroxide (Al(OH)3) and calcium carbonate (CaCO3) into 5AT-Sr(NO3)2 based propellant with the employments of experimental measurement and theoretical analysis. Results show that apart from three similar reactions occurring around the close temperature ranges for three different propellants, an endothermic reaction stemming from the breakdown of Al(OH)3 forms when adding Al(OH)3, and two extra exothermic reactions representing the reaction between CaCO3 and HCN, and the generation of isocyanato exist with CaCO3 addition. Especially, according to traditional Kissinger method calculation, great variations can be seen in the activation energy except for the 5AT decomposition reaction. With the presence of Al(OH)3 or CaCO3, the propellant's theoretical outlet temperature has been reduced by ∼300 °C, while experimental combustion temperature at different sampling points are lowered approximately by 342–417 °C. With the employment of closed-bomb test, it is discovered that the burning rate in the relatively-low pressure zone is mastered by condensed-phase thermal decomposition reaction of 5AT, whereas that in the relatively-high pressure area is dominant by the redox reaction. Unlike Al(OH)3, addition of CaCO3 could reduce the pressure exponent effectively. The specific shapes of burning surface are found to determine the flame shapes for three 5AT-Sr(NO3)2 based propellants, which turn out be parallel, divergent, and converging profile, respectively. Besides, a gas flow model for 5AT-Sr(NO3)2 based propellant is proposed in this study, which can well illustrate the burning rate differences from the perspective of porous structure for propellant particles. Results of this study have implications concerning designs for 5AT-Sr(NO3)2-based propellants widely used in SPGGs apparatus, which can provide a potential approach for making a novel 5AT-Sr(NO3)2-based propellant with low combustion temperature as well as low burning rate pressure exponent.
       
  • Reactive nanoenergetic graphene aerogel synthesized by one-step chemical
           reduction
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Anqi Wang, Sangho Bok, Rajagopalan Thiruvengadathan, Keshab Gangopadhyay, Jacob A. McFarland, Matthew R. Maschmann, Shubhra Gangopadhyay Adoption of nanoenergetic materials into large-scale applications is hindered by problems associated with scalability, particle aggregation, stability, and electrostatic discharge (ESD) sensitivity. We report a macroscale energetic graphene aerogel that simultaneously overcomes each of these problems while increasing the energy production and flame speed with respect to neat nanothermite sample. The aerogel is comprised of reduced graphene oxide (RGO), aluminum (Al) nanoparticles, and bismuth oxide (Bi2O3) nanoparticles. Synthesis of the aerogel requires chemical reduction and gelling that preserves the reactivity of embedded fuel and oxide nanoparticles. A new gelation process is adopted in which ethylenediamine was added to a propylene carbonate dispersion to gel and reduce RGO while retaining material reactivity. The energetic aerogel enhances the heat of reaction to 967 J/g, which is 36% higher than that from loose Al/Bi2O3 powder. A combustion speed of 960 ± 190 m/s under open ambient is measured for RGO/Al/Bi2O3 gel, which is the highest value reported for an Al/Bi2O3 system. Further, the underlying reduced graphene oxide scaffold reduces ESD sensitivity of the aerogel by three orders of magnitude.
       
  • Genesis and evolution of premixed flames in turbulence
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Himanshu L. Dave, Abinesh Mohan, Swetaprovo Chaudhuri Flames interacting with turbulence are continuously generated and annihilated by stretching and folding processes over a range of length-scales and time-scales. In this paper, we address: from where and how do the complex topology and physico-chemical state of a fully developed turbulent premixed flame generate and evolve in time by analyzing the motion of flame particles. Flame particles are points that co-move with reactive isoscalar surfaces which are representative of turbulent premixed flames. Direct Numerical Simulation (DNS) of H2-air turbulent premixed flame with detailed chemistry is combined with a computational methodology called the Backward Flame Particle Tracking (BFPT) algorithm. Uniform distribution of flame particles that entirely span isotherms at time tf is tracked backwards to an earlier time ti (ti 
       
  • Quantum chemical and kinetics calculations for the NO reduction with
           char(N): Influence of the carbon monoxide
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Anyao Jiao, Hai Zhang, Jiaxun Liu, Xiumin Jiang The effect of CO during the NO heterogeneous reduction on the carbonaceous surface has been studied using density functional theory (DFT) at the M06-2X/6-311G(d) level of theory. The zigzag model with pyridinic-N embedded in the aromatic ring clusters was selected to model the nitrogen-containing char (char(N)). The rate coefficients for the key reaction steps were computed by the canonical variational transition-state theory (CVT), providing a deeper understanding of the NO reduction process. The calculations show that the most stable intermediates for NO chemisorption involve interactions with the previously adsorbed CO moiety. Schematic energy profiles for each favorable adsorption route were obtained in order to elucidate the mechanisms for N2, CO2 and N2O releases. In the presence of CO, the emission of CO2 in the NO reduction is available at low temperatures, indicating that CO enhances CO2 release. The promoting action of additional CO is mainly reflected in a reduction of the energy barrier with its concomitant increasing of reaction rate. The different results drawn from CVT provide direct insight into the limitation of the conventional transition state theory (TST), which deserves more attention in further theoretical research.
       
  • Thermodynamic structure of supercritical LOX–GH2 diffusion flames
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Daniel T. Banuti, Peter C. Ma, Jean-Pierre Hickey, Matthias Ihme In this study, we evaluate the thermodynamic structure of laminar hydrogen/oxygen flames at supercritical pressures using 1D flame calculations and large-eddy simulation (LES) results. We find that the real fluid mixing behavior differs between inert (cold flow) and reactive (hot flow) conditions. Specifically, we show that combustion under transcritical conditions is not dominated by large-scale homogeneous real-fluid mixing: similar to subcritical atomization, the supercritical pure oxygen stream undergoes a distinct transition from liquid-like to gas-like conditions; significant mixing and combustion occurs primarily after this transition under ideal gas conditions. The joint study of 1D flame computations and LES demonstrates that real-fluid behavior is chiefly confined to the bulk LOX stream; real fluid mixing occurs but in a thin layer surrounding the LOX core, characterized by water mass fractions limited to 3%. A parameter study of 1D flame solutions shows that this structure holds for a wide range of relevant injection temperatures and chamber pressures. To analyze the mixing-induced shift of the local fluid critical point, we introduce a state-space representation of the flame trajectories in the reduced temperature and reduced pressure plane which allows for a direct assessment of the local thermodynamic state. In the flame, water increases the local mixture critical pressures, so that subcritical conditions are reached. This view of limited mixing under supercritical conditions may yield more efficient models and an improved understanding of the disintegration modes of supercritical flows.
       
  • Gas phase combustion in the vicinity of a biomass particle during
           devolatilization – Model development and experimental verification
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Hesameddin Fatehi, Florian M. Schmidt, Xue-Song Bai A numerical and experimental study on the devolatilization of a large biomass particle is carried out to quantify the effect of homogeneous volatile combustion on the conversion of the particle and on the temperature and species distribution at the particle vicinity. A global chemical kinetic mechanism and a detailed reaction mechanism are considered in a one dimensional numerical model that takes into account preferential diffusivity and a detailed composition of tar species. An adaptive moving mesh is employed to capture the changes in the domain due to particle shrinkage. The effect of gas phase reactions on pyrolysis time, temperature and species distribution close to the particle is studied and compared to experiments. Online in situ measurements of average H2O mole fraction and gas temperature above a softwood pellet are conducted in a reactor using tunable diode laser absorption spectroscopy (TDLAS) while recording the particle mass loss. The results show that the volatile combustion plays an important role in the prediction of biomass conversion during the devolatilization stage. It is shown that the global reaction mechanism predicts a thin flame front in the vicinity of the particle deviating from the measured temperature and H2O distribution over different heights above the particle. A better agreement between numerical and experimental results is obtained using the detailed reaction mechanism, which predicts a wider reaction zone.
       
  • A diffusion-flame analog of forward smolder waves: (I) 1-D steady
           structures
    • Abstract: Publication date: Available online 24 January 2018Source: Combustion and FlameAuthor(s): Zhanbin Lu A solid fuel may be viewed as a special kind of gas of vanishing molecular mobility. Accordingly, a forward smolder wave may be regarded as a special kind of diffusion flame with fuel Lewis number tending to infinity. Such a perspective is explored in this study to examine the structural characteristics of steady planar forward smolder waves, with particular emphasis placed on the heat loss effects. The problem is formulated by employing a diffusive-thermal model, in which the complex smolder reactions are modeled by a one-step exothermic char oxidation reaction. For both adiabatic and non-adiabatic cases, the reaction layer is analyzed by using the activation energy asymptotic method, which ends up with jump conditions connecting quantities across the reaction front. The asymptotic results indicate that adiabatic forward smolder waves do not have a blowoff limit in the small Damköhler number limit, whereas a quenching limit develops when heat loss effects are incorporated. For non-adiabatic forward smolder waves with a reaction trailing structure, the leakage of oxygen through the reaction layer vanishes to leading order, so the reaction zone is described by a structure that is essentially analogous to the premixed flame regime of diffusion flames. By contrast, in the presence of heat loss the reaction leading structure is characterized by O(1) leakage of both reactants, so the analogy is with the partial burning regime of diffusion flames. The description of these two distinct structures, however, can be unified through a common dimensionless parameter m, which is defined as the fraction of heat conducted to the fresh solid fuel side among the total amount of heat generated in the reaction zone.
       
  • Ignition delay times of decalin over low-to-intermediate temperature
           ranges: Rapid compression machine measurement and modeling study
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Liang Yu, Zhiyong Wu, Yue Qiu, Yong Qian, Yebing Mao, Xingcai Lu Autoignition characteristics of decalin, a bicyclic alkane, were investigated in a heated rapid compression machine (RCM) over a wide range of conditions. Ignition delay time (IDT) was measured at compressed pressures of 10, 15 and 20 bar, for equivalence ratios of 0.5, 1.0, 1.5 and 2.0, and at temperatures in the range 631−930 K. Negative temperature coefficient (NTC) behavior of decalin ignition delay time was observed within the temperatures of 750−860 K, in which the ignition delay time increases with rising temperature. The dependence of ignition delay time on compressed pressure, equivalence ratio, and oxygen concentration was systemically studied. A reasonable modification was made to a literature mechanism. The simulation results using the tuned mechanism are found to well capture the dependence of the measured ignition delay time on temperautre, pressure, equivalence ratio, and oxygen concentration over the entire temperature range. Correlation formulas of the simulated and measured ignition delay times were proposed to quantitatively reveal the ignition delay time dependence and to evaluate the mechanism performance. A reaction pathway analysis was carried out at low temperature (700 K), NTC temperature (850 K), and high temperature (1000 K), respectively, to identify the dominant reaction pathways consuming decalin and intermediate species. A sensitivity analysis was also performed at different temperatures and equivalence ratios to find out the important reactions that promote and inhibit decalin autoignition.
       
  • Synchrotron-based measurement of aluminum agglomerates at motor conditions
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Joseph Kalman, Andrew R. Demko, Bino Varghese, Katarzyna E. Matusik, Alan L. Kastengren Solid rocket propellant combustion is hindered by agglomeration of aluminum particles on its burning surface and determining the particle size has been a problem for half a century. The actual size of the agglomerates at motor pressures is unknown due to the opacity of the combustion plume, particularly at the elevated pressures seen in operational rocket motors. Sampling techniques can provide data at elevated pressure but may be biased due to the sampling method and do not provide information on the dynamics of agglomerate formation. This study uses time-resolved synchrotron x-ray imaging (with both absorption and phase contrast) to view aluminum agglomerate formation in situ at relevant rocket pressures. We have for the first time observed agglomerate formation at motor-relevant pressures in real time with unprecedented fidelity, providing critical data for understanding the combustion of aluminized solid rocket propellants.
       
  • Multiple mapping conditioning coupled with an artificially thickened flame
           model for turbulent premixed combustion
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Carmen Straub, Andreas Kronenburg, Oliver T. Stein, Guido Kuenne, Johannes Janicka, Robert S. Barlow, Dirk Geyer A hybrid Euler/Lagrange approach is introduced for the simulation of turbulent stratified flames. Large eddy simulations (LES) are used for the simulation of the flow field while artificial thickening of the flame provides sufficient resolution for the computation of the evolution of the filtered reaction progress variable. This model is complemented by a sparse Lagrangian particle method that provides instantaneous and local solutions of the species composition and can account for deviations from the flamelet-structure due to turbulence. The combined approach provides a model applicable to different premixed flame regimes including the corrugated and thickened flame regimes. The particle mixing model is based on a multiple mapping conditioning (MMC) approach that conditions mixing on a reference field (the reaction progress variable). Thus, the model ensures localness of mixing in composition space and prevents unphysical mixing of unburnt fluid with burnt fluid across the flame front. The MMC-LES results show good agreement with experimental data, and flamelet-like structures as well as deviations thereof can be predicted. The results are rather insensitive towards the MMC specific modelling parameters but the modelling of the mixing time scale needs to be adapted to achieve consistency between the flame propagation speed predicted by the artificially thickened flame model and the flame dynamics predicted by MMC.
       
  • Formation of ultra-lean comet-like flame in swirling hydrogen–air
           flow
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Akane Uemichi, Kento Kouzaki, Kazunori Warabi, Kohei Shimamura, Makihito Nishioka In this study, a hydrogen–air premixed flame in a partially tapered swirl burner in which a stable counterflow of unburned and burned gases is expected to be formed, was investigated. The experimental results indicate the formation of almost steady flames at equivalence ratios of as lean as 0.084, and the resulting ultra-lean flames in the swirling flow had a comet shape. Furthermore, the flame was numerically reproduced, and the mechanisms behind the phenomenon were identified by checking the balance among the chemical enthalpy through diffusion, heat flux by conduction, and transport of these parameters by convection. It was determined that the region around the tip of the flame head was almost dominated only by diffusion and heat conduction similar to a flame ball, but its formation mechanism was found to be essentially different from that of a flame ball because the comet-like flame can be numerically reproduced even without a radiative heat loss, in contrast to a flame ball.
       
  • Self-similar scaling of pressurised sooting methane/air coflow flames at
           constant Reynolds and Grashof numbers
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Fabrizio Bisetti, Ahmed Abdelgadir, Scott A. Steinmetz, Antonio Attili, William L. Roberts Coflow diffusion flames are a canonical laboratory-scale flame configuration, which is routinely employed in fundamental combustion studies on flame stabilization, chemical kinetics, and pollutants’ emissions. In particular, pressurized coflow flames are used to study the effect of pressure on soot formation. In this work, we explore the opportunity to scale sooting coflow flames at constant Reynolds and Grashof numbers as pressure increases. This is achieved by decreasing the bulk velocity and the diameter of the fuel nozzle with increasing pressure. Despite some minor departures from the ideal scaling due to the effect of radiative heat losses from soot, the coflow flames are shown to be self-similar to a very good approximation. By keeping the Reynolds and Grashof numbers constant, one obtains a set of pressurized flames, which have self-similar nondimensional flow fields. Self-similarity is observed experimentally via direct photography and documented thoroughly by direct numerical simulation of steady axisymmetric coflow flames of methane and air at pressures from 1 to 12 atm. Although the study does not include data on soot yields, the implications for soot formation are explored with emphasis on the field of scalar dissipation rate and on the residence time, temperature, and mixture fraction experienced by a parcel of fluid moving along the centerline and along a streamline on the flame’s wing. We explain how the proposed approach to scaling pressurized flames facilitates the analysis of the effect of pressure on soot formation.
       
  • Instantaneous 3D flame imaging by background-oriented schlieren tomography
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Samuel J. Grauer, Andreas Unterberger, Andreas Rittler, Kyle J. Daun, Andreas M. Kempf, Khadijeh Mohri We apply background-oriented schlieren (BOS) imaging with computed tomography to reconstruct the instantaneous refractive index field of a turbulent flame in 3D. In BOS tomography, a network of cameras are focused through a variable index medium (such as a flame) onto a background of patterned images. BOS data consist of pixel-wise “deflections” between a reference and distorted image, caused by variations in the refractive index along the path between the camera and background. Multiple simultaneous BOS images, each from a unique perspective, are combined with a tomography algorithm to reconstruct the refractive index distribution (or optical density) in the probe volume. This quantity identifies the edges of the wrinkled turbulent flame surface. This is the first application of BOS imaging to flame tomography, setting the stage for low-cost 3D flame thermometry. Tomography is carried out within the Bayesian framework, using Tikhonov and total variation (TV) priors. The TV prior is more compatible with the abrupt spatial variation in the refractive index field caused by the flame front. We demonstrate the suitability of TV regularization using a proof-of-concept simulation of BOS tomography on an LES phantom. The technique was then used to reconstruct the instantaneous 3D refractive index field of an unsteady natural gas/air flame from a Bunsen burner using a 23-camera setup. Our results show how BOS tomography can capture and visualize 3D features of a flame and provide benchmark data for simulations.
       
  • Modeling soot formation from solid complex fuels
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Alexander J. Josephson, Rod R. Linn, David O. Lignell A detailed model is proposed for predicting soot formation from complex solid fuels. The proposed model resolves two particle size distributions, one for soot precursors and another for soot particles. The precursor size distribution is represented with a sectional approach while the soot particle-size distribution is represented with the method of moments and an interpolative closure method is used to resolve fractional methods. Based on established mechanisms, this model includes submodels for precursor coagulation, growth, and consumption, as well as soot nucleation, surface growth, agglomeration, and consumption. The model is validated with comparisons to experimental data for two systems: coal combustion over a laminar flat-flame burner and biomass gasification. Results are presented for soot yield for three coals at three temperatures each, and for soot yield from three types of biomass at two temperatures each. These results represent a wide range of fuels and varying combustion environments, demonstrating the broad applicability of the model.
       
  • Investigation of wall chemical effect using PLIF measurement of OH radical
           generated by pulsed electric discharge
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Yong Fan, Weirong Lin, Sui Wan, Yuji Suzuki Near-wall distribution of OH radical concentration provides a measure of the wall chemical effect near a solid surface. However, it is not a straightforward process to isolate the wall chemical effect from the effect of gas-phase reactions because the OH radical distribution can be influenced by both the wall chemical effect and the effect of heat and radicals released by gas-phase reactions. In the present study, instead of the flame, OH radicals generated by a pulsed electric discharge is used in order to eliminate the interference from the gas-phase reaction. OH field with comparable concentration to a methane/air premixed flame has been achieved by tuning the input electric power of pulsed electric discharge. The wall chemical effect is investigated by comparing OH distributions near the quartz wall and the quartz wall with 100-nm-thick alumina coating, while the wall thermal boundary condition is kept identical. High-resolution near-wall measurement of OH distribution was carried out by microscopic planar laser-induced fluorescence (PLIF), and the result was analyzed with the aid of numerical simulations with a surface reaction mechanism. It is found that the initial sticking coefficients estimated on the quartz/alumina surfaces are almost the same with the results in our previous methane/air flame experiment (Saiki and Suzuki, 2013). In the present OH field with electric discharge, it is easier to investigate the radical quenching effect, as the wall chemical effect on OH is decoupled from the gas-phase reaction. The chemical action defined as the wall-normal OH concentration gradient divided by the local OH concentration, which is an index of the wall chemical effect, increases with increasing wall temperature in the OH field generated by pulsed electric discharge.
       
  • A Lattice-Boltzmann model for low-Mach reactive flows
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Yongliang Feng, Muhammad Tayyab, Pierre Boivin A new Lattice-Boltzmann model for low-Mach reactive flows is presented. Based on standard lattices, the model is easy to implement, and is the first, to the authors’ knowledge, to pass the classical freely propagating flame test case as well as the counterflow diffusion flame, with strains up to extinction. For this presentation, simplified transport properties are considered, each species being assigned a separate Lewis number. In addition, the gas mixture is assumed to be calorically perfect. Comparisons with reference solutions show excellent agreement for mass fraction profiles, flame speed in premixed mixtures, as well as maximum temperature dependence with strain rate in counterflow diffusion flames.
       
  • Predicting the consumption speed of a premixed flame subjected to unsteady
           stretch rates
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Meysam Sahafzadeh, Seth B. Dworkin, Larry W. Kostiuk The stretched laminar flame model provides a convenient approach to embed realistic chemical kinetics when simulating turbulent premixed flames. When positive-only periodic strain rates are applied to a laminar flame there is a notable phase lag and diminished amplitude in heat release rate. Similar results have being observed with respect to the other component of stretch rate, namely the unsteady motion of a curved flame when the stretch rates are periodic about zero. Both cases showed that the heat release rate or consumption speed of these laminar-premixed flames vary significantly from the quasi-steady flamelet model. Deviation from quasi-steady behavior increases as the unsteady flow time scale approaches the chemical time scale that is set by the stoichiometry. A challenge remains in how to use such results predictively for local and instantaneous consumption speed for small segments of turbulent flames where their unsteady stretch history is not periodic.This paper uses a frequency response analysis as a characterization tool to simplify the complex non-linear behavior of premixed methane air flames for equivalence ratios from 1.0 down to 0.7, and frequencies from quasi-steady up to 2000 Hz using flame transfer functions. Various linear and nonlinear models were used to identify appropriate flame transfer functions for low and higher frequency regimes, as well as extend the predictive capabilities of these models. Linear models were only able to accurately predict the flame behavior below a threshold of when the fluid and chemistry time scales are the same order of magnitude. Other proposed transfer functions were tested against arbitrary multi-frequency stretch inputs and were shown to be effective over the full range of frequencies.
       
  • The formation of (Al2O3)n clusters as a probable mechanism of aluminum
           oxide nucleation during the combustion of aluminized fuels: Numerical
           analysis
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Alexander M. Savel'ev, Alexander M. Starik The model of formation and growth of stoichiometric (Al2O3)n clusters during the combustion of aluminized fuels has been developed. In this model, the thermodynamic properties of large clusters (n = 5–75) have been determined by matching the thermodynamic properties of small clusters n = 2–4, calculated earlier by quantum-chemical methods, with similar characteristics of liquid droplets. The developed model was used for a numerical simulation of the formation of (Al2O3)n clusters during the combustion a single aluminum particle with a diameter of 200 μm in O2/Ar atmosphere. It has been shown that, during the first 12 ms after the aluminum particle ignition, the rapid growth of clusters occurs. The mass of clusters in the combustion zone is comparable to the mass of aluminum oxide. The modeling results indicate that the growth of (Al2O3)n clusters can be the most probable mechanism of the formation of condensation nuclei of alumina.
       
  • Comprehensive Hg/Br reaction chemistry over Fe2O3 surface during coal
           combustion
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Yingju Yang, Jing Liu, Feng Liu, Zhen Wang, Junyan Ding A combination of experiments, density functional theory (DFT) and kinetic calculations was used to systematically understand the detailed chemistry of heterogeneous mercury reaction with HBr over Fe2O3 surface. Fe2O3 shows catalytic activity for mercury reaction with HBr. The chemisorption mechanism is responsible for the adsorption of mercury species (Hg0, HgBr and HgBr2) on Fe2O3 surface. Heterogeneous mercury reaction with HBr over Fe2O3 surface follows Langmuir–Hinshelwood mechanism in which adsorbed Hg0 reacts with active surface bromine species derived from HBr decomposition. On the basis of the experimental and DFT calculation results, a new comprehensive heterogeneous reaction kinetic model was established to describe the detailed reaction process of Hg/Br over Fe2O3 surface. This heterogeneous model includes 17 elementary reactions governing the elimination and formation of mercury species on Fe2O3 surface. This kinetic model was validated against the experimental data. The model predictions were found to be in good agreement with the experimental data. X-ray photoelectron spectroscopy (XPS) results, DFT calculations and sensitivity analysis indicate that the dominant reaction pathway of Hg/Br over Fe2O3 surface is a four-step process Hg0 → Hg(s) → HgBr(s) → HgBr2(s) → HgBr2, in which gaseous Hg0 is first adsorbed on Fe2O3 surface and subsequently reacts with brominated iron site to form HgBr(s), HgBr(s) can be further converted to HgBr2(s) and released into flue gas. The proposed dimensionless temperature coefficient can be used to better understand the temperature-dependent relationship between heterogeneous Hg/Br chemistry and mercury transformation.
       
  • Computational acceleration of multi-dimensional reactive flow modelling
           using diesel/biodiesel/jet-fuel surrogate mechanisms via a clustered
           dynamic adaptive chemistry method
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Dezhi Zhou, Kun Lin Tay, Han Li, Wenming Yang This study proposes a clustered dynamic adaptive chemistry (CDAC) method, which uses an iterative K-means algorithm to partition the computational cells into different groups according to their similarity in terms of the temperature and significant species compositions. Taking advantage of the clustered cells, the averaged thermo-chemical properties of the cells in the respective clusters are then used to identify the adaptive dynamic reduced chemistry through the method of direct relation graph with error propagation (DRGEP). Moreover, the integration of the chemical source term, which commonly dominates the computational effort in reactive flow simulations, is now performed using the dynamic adaptive reduced chemistry at the cluster level instead of the cell level. With this CDAC method, the on-the-fly DRGEP process as well as the chemistry integration only needs to be conducted at the cluster level, dramatically reducing the unnecessary repeated computation for similar computational cells. In addition, the adaptive dynamic reduced chemistry further accelerates the chemistry integration process due to less ordinary differential equations (ODEs) to be solved for each cluster. This newly proposed CDAC method was tested in multi-dimensional homogeneous charged compression ignition engine (HCCI), direct injection compression ignition (DICI) engine and constant volume chamber combustion (CVCC) fueled with diesel, biodiesel and kerosene through the use of their respective surrogate fuel mechanisms under different operating conditions. The performance of CDAC in terms of accuracy and efficiency were extensively analyzed and discussed using different user-defined parameters, mechanisms with different surrogate components and number of species as well as different meshes with different number of grid cells. Based on this analysis, the error tolerances in DRGEP and the error tolerances of the temperature and significant species’ mass fraction are recommended as: εd  ≤  0.001, εT  ≤  20 K and εY  ≤  0.01 to achieve less than 0.1% integral error. With these recommended user-defined tolerances, it can be observed that the current CDAC method is able to accurately predict the in-cylinder pressure as well as the species profile in HCCI, DICI and the flame lift-off length in CVCC when compared with the full chemistry calculations as well as the experimental results. Moreover, with the recommended user defined error tolerances and a chemical kinetic model of 112 species, this CDAC method is capable of achieving a computational time speed-up factor of more than 3 compared to the conventional DAC and of almost 5 compared to the full chemistry. Finally, the CDAC method coupled CFD was applied to simulate a diesel DICI engine under three different engine speeds with a detailed primary reference fuel (PRF) mechanism of more than 1000 species. The 3-D simulation could be finished in acceptable CPU time while being able to well capture the experimental in-cylinder pressure and heat release rate for all the three engine speeds.
       
  • State space parameterization of explosive eigenvalues during autoignition
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Michael A. Hansen, Elizabeth Armstrong, James C. Sutherland Explosive modes such as ignition and extinction are characterized by an eigenvalue of the chemical Jacobian matrix with positive real part, representing the transient instability of chain-branching chemistry and thermal feedback. Formation and eigen-decomposition of the Jacobian matrix are expensive operations whose cost increases cubically with chemical mechanism size. As an alternative to directly computing the eigenvalues of the Jacobian, we explore principal component analysis (PCA) along with nonlinear regression as a methodology to parameterize the eigenvalues by state variables (or linear combinations thereof). We evaluate this modeling strategy using homogeneous autoignition data on two different applications: pseudotransient continuation (Ψtc)-based ODE solvers and chemical explosive mode analysis (CEMA). Results indicate that the PCA-based parameterization of the eigenvalues appears feasible for Ψtc solvers in autoignition calculations over a range of temperatures and pressures. Our results also show that eigenvalue models are capable of tracking sharp discontinuities (such as ignition or extinction) in the eigenvalue for computational flame diagnostics such as CEMA.
       
  • A further experimental and modeling study of acetaldehyde combustion
           kinetics
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Tao Tao, Shiqing Kang, Wenyu Sun, Jiaxing Wang, Handong Liao, Kai Moshammer, Nils Hansen, Chung K. Law, Bin Yang Acetaldehyde is an important intermediate and a toxic emission in the combustion of fuels, especially for biofuels. To better understand its combustion characteristics, a detailed chemical kinetic model describing the oxidation of acetaldehyde has been developed and comprehensively validated against various types of literature data including laminar flame speeds, oxidation and pyrolysis in shock tubes, chemical structure of premixed flames, and low-temperature oxidation in jet-stirred reactors. To extend the validation range, the chemical structure of a counterflow flame fueled by acetaldehyde at 600 Torr has been measured using vacuum ultra-violet photoionization molecular-beam mass spectrometry. In addition, ignition delay times at 10 atm and 700-1100 K were measured in a rapid compression machine, and a negative temperature coefficient (NTC) behavior was observed. The present kinetic model well reproduces the results of various acetaldehyde combustion experiments covering wide ranges of temperatures (300–2300 K) and pressures (0.02–10 atm), and explains well the observed NTC behavior based on the competition between multiple oxidation pathways for the methyl radicals and their self-recombination forming ethane, a relatively stable species at temperatures below 1000 K.
       
  • Flame attachment and kinetics studies of laminar coflow CO/H2 diffusion
           flames burning in O2/H2O
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Huanhuan Xu, Fengshan Liu, Shaozeng Sun, Yijun Zhao, Shun Meng, Lei Chen, Longfei Chen In this study, experimental and numerical investigations were conducted to study the attachment and oxidation process of laminar CO/H2 diffusion flames burning in coflow O2/H2O at 1 atm with an inlet temperature of 400 K for both the fuel and oxidizer streams. The effects of fuel composition were investigated by considering a wide range of CO/H2 mole ratio from 95%CO–5%H2 to 5%CO–95%H2 and also pure H2. The oxidizer has a fixed composition of 75%H2O–25%O2. The measured flame heights determined by OH*-chemiluminescence images were used to validate the flame model adopted in this work. Through numerical simulations using a two-dimensional flame code with the preheating effect, detailed reaction mechanism, and detailed thermal and transport properties, the details of flame attachment and flame structure were obtained and analysed. Although both CO and H2 diffuse over the burner rim and move upstream into the oxidizer stream, the attachment point of a H2-rich syngas flame is further upstream below the burner exit than that of a CO-rich flame. This is attributed to the high reactivity of H2 through reaction OH + H2 = H + H2O and the high diffusivity of H2. Reaction pathways for syngas burning in the oxidizer of O2/H2O based on a detailed kinetics analysis were revealed, not only inside the fuel tube and above the fuel exit, but also near the flame sheet and in the flame attachment zone. Significant consumption of H2O was observed in the flame core due to the reverse reaction of OH + H2 = H + H2O which shifts to proceed forward outside the flame in the radial direction also at higher streamwise locations if H2 in the fuel flow is rich, oxidizing unburned H2 to H2O.
       
  • Detonation diffraction in a circular arc geometry of the insensitive high
           explosive PBX 9502
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Mark Short, Carlos Chiquete, John B. Bdzil, James J. Quirk We describe the details of an unconfined insensitive high explosive (PBX 9502) circular arc section experiment, in which, after a transient period, a detonation sweeps around the arc with constant angular speed. The arc section is sufficiently wide that the flow along the centerline of the arc section remains two-dimensional. Data includes time-of-arrival diagnostics of the detonation along the centerline inner and outer arc surfaces, which is used to obtain the angular speed of the steadily rotating detonation. We also obtain the lead shock shape of the detonation as it sweeps around the arc. Reactive burn model simulations of the PBX 9502 arc experiment are then conducted to establish the structure of the detonation driving zone, i.e. the region enclosed between the detonation shock and flow sonic locus (in the frame of the steady rotating detonation). It is only the energy released in this zone which determines the speed at which the steady detonation sweeps around the arc. We show that the sonic flow locus of the detonation driving zone largely lies at the end of, or within, the fast reaction stage of the PBX 9502 detonation, with the largest section of the detonation driving zone lying close to the inner arc surface. We also demonstrate that the reactive burn model provides a good prediction of both the angular speed of the detonation wave and the curved detonation front shape.
       
  • A comprehensively validated compact mechanism for dimethyl ether
           oxidation: an experimental and computational study
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Rohit S. Khare, Senthil K. Parimalanathan, Vasudevan Raghavan, Krithika Narayanaswamy Dimethyl ether (DME) is regarded as one of the most promising alternatives to fossil fuels used in compression ignition engines. In order to critically evaluate its overall combustion behaviour via numerical simulations, an accurate as well as compact kinetic mechanism to describe its oxidation is most essential. In the present study, a short kinetic mechanism consisting of 23 species and 89 reactions is proposed to describe the oxidation of DME. This is based on the detailed San Diego mechanism. The short mechanism accurately reproduces the available experimental data for ignition delays, laminar flame speeds, and species profiles in flow reactors as well as jet-stirred reactors. To assess the validity of this reaction mechanism in non-premixed systems, extinction strain rates of DME–air mixtures, which are not available in the literature, are measured in a counter-flow diffusion flame burner as a part of the present work. The 23 species reaction mechanism is also able to predict the experimental data for extinction within the uncertainty limits. This mechanism is further reduced by introducing quasi-steady state assumptions for six intermediate species to finally obtain a 14-step global kinetic scheme. A code is developed in MATLAB to obtain these 14 global steps and their corresponding rate expressions in terms of the individual reaction rates. The 14-step mechanism performs as good as the 23 species mechanism for all the experimental data sets tested for.
       
  • Metal catalyzed preparation of carbon nanomaterials by
           hydrogen–oxygen detonation method
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Tiejun Zhao, Xiaojie Li, Honghao Yan A hydrogen–oxygen gas detonation was direct initiated by using a 20 J electronic spark, and the pressure and temperature of which were measured by a pressure sensor and high-speed camera, respectively. The results showed the mixed gas was direct initiated in the propagation of detonation wave. The carbon nanomaterials were prepared by decomposition of ferrocene and cobalt (III) acetylacetonate (Co(acac)3), a the samples were characterized by X-ray diffractometer, transmission electron microscope, engergy dispersive X-ray detector and Raman spectrometer. The results indicated that carbon-encapsulated metal nanoparticles were fabricated by using ferrocene, ferrocene–Co(acac)3 as a precursor, and the core–shell nanostructures were carbon-encapsulated Fe/Fe3C nanoparticles (Fe@C) and carbon-encapsulated Co nanoparticles (Co@C). However, the Fe–Co alloy was absent in sample from ferrocene–Co(acac)3. It is interesting that the sample from Co(acac)3 were Co@C and multi-walled carbon nanotubes (MWCNTs), and the crystallization degrees of the carbon and Co nanoparticles in the MWCNTs were higher than that of in carbon-encapsulated metal nanoparticles, however, the degree of graphitization of the powders was low. The physical properties of precursors, hydrogen content and rapid reaction were the main factors which contributed to the different morphologies and the absence of Fe–Co alloy.
       
  • Laminar burning velocities of methylcyclohexane + air flames at room
           and elevated temperatures: A comparative study
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Vladimir A. Alekseev, Sergey S. Matveev, Ivan V. Chechet, Sergey G. Matveev, Alexander A. Konnov Laminar burning velocities of methylcyclohexane + air flames were determined using the heat flux method at atmospheric pressure and initial temperatures of 298–400 K. The measurements were performed on two experimental setups at Lund University and Samara National Research University. Our results obtained at the same initial temperatures are in good agreement. Consistency of the measurements performed at different temperatures was tested employing analysis of the temperature dependence of the burning velocities. This analysis revealed increased scatter in the burning velocity data at some equivalence ratios which may be attributed to the differences in the design of the burners used. New measurements were also compared to available literature data. Reasonably good agreement with the data of Kumar and Sung (2010) was observed at 400 K, with significantly higher burning velocities at the maximum at 353 K as compared to other studies from the literature. Predictions of two detailed reaction mechanisms developed for jet fuels – PoliMi and JetSurF 2.0 were compared with the present generally consistent measurements. The two kinetic models disagreed with each other, with the experimental data being located in between the model predictions. Sensitivity analysis revealed that behavior of the models is largely defined by C0–C2 chemistry. Comparison of the model predictions with the burning velocities of ethylene and methane showed the same trends in over- and under-predictions as for methylcyclohexane + air flames.
       
  • The influence of sample thickness on the combustion of Al:Zr and Al-8Mg:Zr
           nanolaminate foils
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Kyle R. Overdeep, Travis A. Schmauss, Atman Panigrahi, Timothy P. Weihs Al:Zr and Al-8Mg:Zr nanocomposite foils do not combust completely in air because the penetration of oxygen and nitrogen into the foils can become limited as the product phases grow. The heat produced during the combustion of these foils could feasibly depend upon the volume fraction of the surface oxide layer that forms and therefore the initial foil thickness as well. To test this, Al:Zr and Al-8Mg:Zr foils of various thicknesses (9–61 µm) were fabricated by Physical Vapor Deposition and their heats of combustion were measured using bomb calorimetry in 1 atm of air. We found that combustion efficiency decreased significantly for Al:Zr foils as thickness increased, but Al-8Mg:Zr foils had a nearly constant combustion efficiency for the range of thicknesses studied. SEM-EDS measurements across the foil cross-sections showed that for Al:Zr foils, a distinct oxide layer formed on the external surfaces and there were low levels of oxygen and nitrogen toward their centers. For Al-8Mg:Zr foils though, there was minimal dependence between heat output and foil thickness, the surface oxide layer was more diffuse, and the oxygen and nitrogen contents were higher throughout the foil. We propose that the addition of magnesium improves heat generation by increasing the rates of oxygen and nitrogen diffusion and thus enabling the formation of solid solutions that are richer in oxygen and nitrogen throughout the bulk of the foils.
       
  • Low-temperature chemistry in n-heptane/air premixed turbulent
           flames
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Bruno Savard, Haiou Wang, Andrzej Teodorczyk, Evatt R. Hawkes The effects of low-temperature chemistry (LTC) on n-heptane/air premixed turbulent flames in the thin reaction zones regime are investigated using direct numerical simulations (DNS) with reduced multi-step chemistry (129-species, 1234-reaction mechanism reduced from CaltechMech). An initial mixture of n-heptane/air at an equivalence ratio of 0.7, unburnt temperature of 650 K, and atmospheric pressure, which is in the negative temperature coefficient (NTC) region, is considered. The focus is put on three separate aspects: 1) LTC in turbulent hot flames propagating in this unburnt (fresh) mixture, 2) turbulent hot flames (with LTC) propagating in a mixture that has undergone first-stage ignition, and 3) turbulent cool flames. These types of flames can all be encountered in modern gasoline compression ignition and diesel engines for example. For the first aspect, it is found that LTC has negligible effect for the conditions considered. For the second aspect, at constant Karlovitz number, the increase in turbulent flame speed (relative to that of turbulent hot flames propagating in the unburnt mixture) due to partial ignition of the reactants is attributed to the increase in laminar flame speed, as opposed to turbulence–LTC interaction. Furthermore, the reaction zone is affected by turbulence in the same way as hot flames propagating in an unburnt mixture. For the third aspect, the first DNS of turbulent cool premixed n-heptane/air flames are presented. Under the current conditions, the initial laminar cool flames are strongly affected by auto-ignition, which is expected to occur under engine conditions, and has an ignition front structure. As the turbulent flames develop, turbulent diffusion becomes sufficiently large to initiate self-propagation of the cool flames. The flames are observed to propagate upstream steadily until they reach the inlet. The steady-state turbulent flames are found to have a highly distributed reaction zone. Nevertheless, their reaction zone structure is found to approach that of the reference (self-propagating) laminar flame (which is significantly different than that of the initial ignition fronts). In addition, this strong turbulence does not affect the global chemical pathways compared to those in the reference laminar flame. Finally, their normalized turbulent flame speed is comparable to that of hot flames at similar Karlovitz numbers.
       
  • Oxidation of 2-methylfuran and 2-methylfuran/n-heptane blends: An
           experimental and modeling study
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Rupali Tripathi, Ultan Burke, Ajoy K. Ramalingam, Changyoul Lee, Alexander C. Davis, Liming Cai, Hatem Selim, Ravi X. Fernandes, K. Alexander Heufer, S. Mani Sarathy, Heinz Pitsch There have been significant advances in understanding ignition behavior of oxygenated biofuels (mainly alcohols) and their blends with conventional fuel components. However, the oxidation behavior of lignocellulosic derived furanic compounds blended with hydrocarbons has received little attention. The present work is an experimental and numerical investigation of 2-methylfuran (2-MF) combustion and its blend with n-heptane. These results are compared with pure n-heptane results to better understand 2-MF reactivity. Ignition delay times of pure 2-MF and the 2-MF/n-heptane (50/50 2-MF/n-heptane molar %) blend in air were measured in three different facilities; a rapid compression machine and two different shock tubes. Experiments were performed in the temperature range of 861–913 K at a pressure of 20 bar for stoichiometric pure 2-MF. The ignition delay times of 2-MF/n-heptane blends were measured in the temperature range of 672–1207 K, at pressures of 10 and 20 bar, and at equivalence ratios of 0.5, 1.0, and 1.5. A comprehensive chemical kinetic model containing low- to high-temperature chemistry of 2-MF and n-heptane was formulated based on a combination of available 2-MF and n-heptane mechanisms and available theoretical studies on 2-MF form literature. The developed detailed kinetic model was validated against the ignition delay data measured in this work as well as against high-temperature shock tube ignition delay, flame speed, and flame species data from literature to ensure the competence of the model. The proposed mechanism predicts the measured and literature data to a reasonable extent. To elucidate fuel specific oxidation pathways, reaction path analyses were performed at various conditions. Furthermore, sensitivity analyses on the ignition delay times were conducted and the dominant reaction pathways in the oxidation of pure and binary mixtures at high, intermediate, and low temperatures were identified. It is found that the competition between n-heptane and 2-MF for ȮH radicals inhibits the consumption of n-heptane and promotes the consumption of 2-MF. This work provides the first insight into the global low-temperature oxidation behavior of a second generation furanic blended with a hydrocarbon.
       
  • A theoretical kinetics study on low-temperature reactions of methyl
           acetate radicals with molecular oxygen
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Qinghui Meng, Xudong Zhao, Lidong Zhang, Peng Zhang, Liusi Sheng Theoratical studies on the chemistry of methyl acetate radicals with molecular oxygen was conducted to get further understanding of biodiesel combustion. Reactions of the first oxygen addition to methyl acetate radicals has been investigated by high level quantum chemical methods, and rate constants were computed by using microcanonical variational transition state theory coupled with Rice–Ramsberger–Kassel–Marcus/Master-Equation theory. The calculated rate constants agree reasonably well with both theoretical and experimental results of chain-like alkoxy radicals. We considered each step in the oxidation process as a class of reaction, including all the possible reactions taking place, only the formation and re-dissociation of initial adducts are critical for the low temperature combustion of methyl acetate. The current study is an extension of kinetic data for such chain propagation reactions for methyl acetate oxidation in a wider pressure and temperature range, which can be used for the modeling study of low temperature oxidation of methyl esters.
       
  • Ignition and combustion of a single aluminum particle in hot gas flow
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Yunchao Feng, Zhixun Xia, Liya Huang, Likun Ma For simulating the aggregated aluminum bulks on the burning surface of solid propellants, large aluminum particles (40–160 µm) are used in this work. The isolated aluminum particles are ignited in hot oxidizing gas. Based on the bright-spot diameter profiles and the known respective reaction mechanisms, the total ignition and combustion process of aluminum particle can be divided into three stages, namely, pre-heating, ignition and combustion. The initial and bright-spot diameters of the aluminum particle are measured directly from the images by using the in-house automated data processing routines. Ignition delay time, ti, and combustion time, tc, are also obtained by post-processing the sequential images and can be associated with the particle diameters, D, in the form of ti = aD + b and tc = αD, respectively. The changing trends of ignition delay time and combustion time with the effective oxidizer mole fraction in the range of 22.8%–49.1% are distinctly different. The oxidizing environments with a high effective oxidizer mole fraction can result in short combustion time but long ignition delay time. For small particles (40–110 µm), the environmental effective oxidizer mole fraction exerts a limited effect on the sum of ignition delay time and combustion time, which indicates total time. By considering the effects of particle sizes and effective oxidizer mole fractions of environments, the percentages of ignition delay time in the total time are analyzed. These results suggest that with the goal of decreasing the total time, suitable methods can be employed for different conditions. Furthermore, we observe and discuss the phenomenon of aluminum particle microexplosion in an environment with high effective oxidizer mole fraction, which decreases particle combustion time by a large margin.
       
  • The growth of AlN dendritic crystals with uniform morphology by an
           aluminum microdroplet localization approach
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Hayk H. Nersisyan, Seong Hun Lee, Bung Uk Yoo, Jong Hyeon Lee We developed an attractive combustion approach for synthesizing uniformly shaped AlN dendritic crystals by combustion of Al + 0.1AlF3 + kAl2O3 powder mixtures in a nitrogen atmosphere. The combustion temperature measured for various k values was between 1650 and 1750 °C and the micro-droplets of Al formed in the beginning stages of the process were enveloped by the solid layers of Al2O3, and the subsequent multipoint nucleation and crystallization produced morphologically and size uniform dendritic crystals. We proposed a theoretical model for calculating the thickness and the number of Al2O3 layers around of Al microdroplets at known concentration of Al2O3. Depending on the concentration of Al2O3, these structures were simple stars with six points and stellar dendrites with multiple petals.
       
  • Experimental investigation on detonation dynamics through a reactivity
           sink
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Stéphane Boulal, Pierre Vidal, Ratiba Zitoun, Takuya Matsumoto, Akiko Matsuo This article reports on an experimental investigation into dynamical behaviours of detonation in non-uniform mixtures, generated from stoichiometric propane–oxygen, oxygen and ethane, with initial temperature and pressure 290 K and 20 kPa, respectively. Composition gradients are parallel to the direction of detonation propagation, with an equivalence ratio (ER) that first decreases from lean values and then increases to rich ones. Composition distributions are characterized according to the depth of the ER sink. Gradients are generated in a 50 × 50-mm2-square cross-section and a 665-mm total length chamber. The mixture components are injected separately in the pre-evacuated chamber in their order of decreasing density through porous plates at the chamber top-end to ensure planar filling of the chamber. Non-uniform distributions are then precisely controlled as a function of time by means of optical oxygen sensors. A Chapman–Jouguet (CJ) detonation is transmitted at the chamber bottom-end from a 3.6-m-long driver tube. Fast pressure transducers, sooted plates and Schlieren visualizations coupled with high-speed cameras are used to characterize the longitudinal velocity, cellular structure and transmission, failure and re-initiation mechanisms of the detonation front. Shallowest ER sinks produce the supercritical transmission mode of the CJ detonation with continuous adaptation of velocity and multicellular structure to local composition. Deepest sinks lead to the subcritical behaviour characterized by sudden detonation failure from shock-flame decoupling when ER decreases, and without detonation re-initiation when ER increases again. Intermediate sink depths generate critical behaviour with detonation re-initiat ion at chamber walls from expanding combustion kernels and reflected transverse Mach waves and then from SWACER retro-active mechanism. An elaboration of the failure criterion used in a previous study is found to well predict conditions for shock-flame decoupling.
       
  • Counterflow flame experiments and chemical kinetic modeling of dimethyl
           ether/methane mixtures
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Christopher B. Reuter, Rui Zhang, Omar R. Yehia, Yacine Rezgui, Yiguang Ju As advanced engines become more controlled by the fuel reactivity, it is important to have a complete understanding of combustion chemistry of fuel blends at both high and low temperatures. While the high-temperature chemistry coupling with transport and heat release can be examined through the use of flame experiments, low-temperature chemistry has been traditionally limited to homogeneous reactor experiments at fixed temperatures, which leaves the heat release rate unconstrained. In this study, the kinetic coupling between dimethyl ether and methane is examined by studying hot flames, cool flames, and ozone-assisted cool flames in a counterflow burner. At fixed fuel mass fraction, it is found that methane addition to dimethyl ether raises the hot flame extinction limit but lowers the cool flame extinction limit. Ozone addition to cool flames is seen to lead to a substantial increase in the extinction limit, but it also produces a decrease in sensitivity of the extinction limit to the fuel mass fraction.The cool flame extinction measurements are then used to examine the uncertainties of reactions contributing significantly to the low-temperature heat release. The measurements indicate that the original kinetic model significantly overpredicts the cool flame extinction limits. However, by targeting the H-abstraction reaction of dimethyl ether by OH, among other reactions, an updated chemical kinetic model for dimethyl ether/methane mixtures is developed and validated. This study shows the value of the ozone-assisted counterflow cool flame platform in examining the key low-temperature reactions contributing to the heat release rate in cool flames.
       
  • Extension of a wide-range three-step hydrogen mechanism to syngas
    • Abstract: Publication date: October 2018Source: Combustion and Flame, Volume 196Author(s): Pierre Boivin, Forman A. Williams
       
  • Decomposition and isomerization of 1-pentanol radicals and the pyrolysis
           of 1-pentanol
    • Abstract: Publication date: Available online 23 June 2018Source: Combustion and FlameAuthor(s): Ruben Van de Vijver, Kevin M. Van Geem, Guy B. Marin, Judit Zádor Stable species and saddle points on the C5H11O potential energy surface relevant for 1-pentanol pyrolysis and combustion have been determined starting from the terminal adduct of the OH + 1-pentene reaction. A large number of stationary points were explored automatically with the KinBot software at the M06-2X/6-311++G(d,p) level. The kinetically relevant stationary points have been further characterized using UCCSD(T)-F12a/cc-pVTZ-F12//M06-2X/6-311++G(d,p) quantum chemistry calculations. The entrance channel consists of a barrierless outer transition state leading into a van der Waals well followed by a submerged saddle point, overall described with an effective two-transition-state model. The master equation has been solved to obtain pressure- and temperature-dependent rate coefficients for all reactions on the potential energy surface in the 300–2500 K temperature range and 0.01–100 atm pressure range. The newly obtained rate coefficients have been implemented in a kinetic model for the thermal decomposition of 1-pentanol diluted in a nitrogen stream. We measured the conversion of major species using gas chromatography with a flame ionization detector, and two-dimensional gas chromatography with time-of-flight mass spectrometric and flame ionization detectors in the effluent of a flow reactor at 0.17 MPa between 913 and 1023 K. Comparison of the simulated versus the experimental data acquired in this work shows that the reactions found by KinBot, for which earlier only poor estimates existed, are of significant importance to correctly describe conversion and product selectivities. It proves to be possible to generate adequate chemical models automatically provided that the underlying high-level ab initio data is computationally affordable.
       
 
 
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