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  • A Physics-based approach to modeling real-fuel combustion chemistry – III. Reaction kinetic model of JP10
    Combust. Flame (IF 4.494) Pub Date : 2018-09-18
    Yujie Tao, Rui Xu, Kun Wang, Jiankun Shao, Sarah E. Johnson, Ashkan Movaghar, Xu Han, Ji-Woong Park, Tianfeng Lu, Kenneth Brezinsky, Fokion N. Egolfopoulos, David F. Davidson, Ronald K. Hanson, Craig T. Bowman, Hai Wang

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

    更新日期:2018-09-18
  • 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
    Combust. Flame (IF 4.494) Pub Date : 2018-08-07
    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.”

    更新日期:2018-08-08
  • A diffusion-flame analog of forward smolder waves: (II) stability analysis
    Combust. Flame (IF 4.494) Pub Date : 2018-07-28
    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.

    更新日期:2018-07-29
  • Investigating oxidation growth routes in the flame synthesis of tungsten-oxide nanowires from tungsten substrates
    Combust. Flame (IF 4.494) Pub Date : 2018-06-29
    Zhizhong Dong, Cassandra D'Esposito, Bernard H. Kear, Stephen D. Tse

    Tungsten-oxide nanowires are synthesized directly from the surface of tungsten substrate probes inserted into counter-flow diffusion-flames to correlate as-formed morphologies with local conditions because of the quasi-one-dimensionality of the flow field. Computational simulations aid in designing the flame structure for the experiments with respect to relevant chemical species and temperature. The tungsten substrates are inserted into the flame structure on either the air side or fuel side of the flame reaction zone, permitting evaluation of the roles of H2O (or CO2) versus O2, which serve as reactant species in the growth of the resulting tungsten-oxide nanostructures. Furthermore, methane flames are compared with hydrogen flames, which only have H2O (and no CO2) as product species. The temperature profiles of the methane and hydrogen flames are purposefully matched to compare the effect of chemical species produced by the flame which serve as reactants for nanostructure growth. Single-crystalline, well-vertically-aligned, and dense WO2.9 nanowires (diameters of 20–50 nm, lengths of > 10 µm, and coverage density of 109–1010 cm−2) are obtained at a gas-phase temperature of 1720 K on the air-side of the methane flame. Comparisons among the probed locations and flame species indicate that the CO2 route is a heterogeneous one that helps in seeding the growth of nanowires at the nucleation stage, with subsequent vapor–solid growth occurring from other routes. Probing on the fuel side of the hydrogen flame isolates the H2O route and confirms that it is able to produce tungsten-oxide nanowires, albeit at a very reduced rate and yield. Moreover, given the thermodynamic unfavorability of H2O reaction with W to form gaseous W/O species, a self-photocatalytic mechanism is proposed where H2O decomposes to reactive OH on the surface of WOx, facilitating production of volatile W/O species for continued growth by the vapor–solid mechanism for the tungsten-oxide nanowires. The effect of gas-phase temperatures of 1280, 1500, and 1720 K are examined, with increasing temperatures corresponding to higher yield density because of increased nucleation and augmented formation of volatile W/O compounds.

    更新日期:2018-06-30
  • Experimental and computational investigation of partially-premixed methoxymethane flames
    Combust. Flame (IF 4.494) Pub Date : 2018-06-29
    Yuanjie Jiang, Ryan Gehmlich, Thomas Knoblinger, Kalyanasundaram Seshadri

    Experimental and computational studies are carried out to elucidate the structure and extinction of laminar partially-premixed flames employing the counterflow configuration. The fuel is methoxymethane (DME). The formulation considers two laminar streams that flow toward a stagnation plane. One stream called the fuel-rich stream is made up of DME (CH3OCH3), and nitrogen (N2) with small amounts of oxygen (O2) and the other stream called the fuel-lean stream is made up of O2, and N2 with small amounts of CH3OCH3. The level of partial premixing is characterized by the equivalence ratio defined as the ratio of the mass of methoxymethane to the mass of oxygen normalized by the corresponding stoichiometric value of this ratio. The equivalence ratio of the fuel-rich stream is ϕr and that of the fuel-lean stream is ϕl. Previous studies have established that the scalar dissipation rate at extinction depends on the stoichiometric mixture fraction, ξst, and the adiabatic flame temperature, Tst. To clarify the chemical influences of partial premixing on extinction, studies are carried at fixed values of ξst and Tst and for various values of ϕl and ϕr . Use of this procedure separates the chemical influences from thermal effects. A previously developed Burke–Schumann (flame-sheet) formulation is employed to estimate the boundary values of the mass fractions of the reactants. Two sets of experiments are conducted, in one set ϕr−1=0, ϕ r − 1 = 0 , and measurements are made for various selected values of ϕl , in the other set ϕl=0 ϕ l = 0 and measurements are made for various selected values of ϕr. The computations are carried out using the San Diego mechanism that was recently updated to include kinetic steps describing combustion of methoxymethane. For DME addition to the fuel-lean stream, experiments and predictions show that the value of the strain rate at extinction, increases with increasing ϕl. For O2 addition to the fuel-rich stream, experiments and predictions show very little changes in the values of the strain rate at extinction with increasing ϕr−1 ϕ r − 1 . The key observation is that addition of DME to the fuel-lean stream enhances the overall reactivity while addition of oxygen to the fuel-rich stream has little influence on the overall reactivity

    更新日期:2018-06-30
  • Decomposition and isomerization of 1-pentanol radicals and the pyrolysis of 1-pentanol
    Combust. Flame (IF 4.494) Pub Date : 2018-06-23
    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.

    更新日期:2018-06-25
  • Dynamic adaptive combustion modeling of spray flames based on chemical explosive mode analysis
    Combust. Flame (IF 4.494) Pub Date : 2018-06-04
    Chao Xu, Muhsin M. Ameen, Sibendu Som, Jacqueline H. Chen, Zhuyin Ren, Tianfeng Lu

    A dynamic adaptive combustion modeling framework based on chemical explosive mode analysis (CEMA) is proposed to account for different flame features such as local auto-ignition, premixed and non-premixed flamelets in diesel spray flames. The proposed modeling strategy is achieved by assigning zone-dependent combustion models on-the-fly to different flame zones segmented using a CEMA-based approach. An approximate CEMA formulation is developed to approximate the eigenvalue of the chemical explosive mode with high computational efficiency in three-dimensional (3-D) turbulent flame simulations. The utility of the CEMA-based criterion for dynamic flame segmentation is first demonstrated using CEMA-based adaptive chemistry by applying different reduced chemistry to different flame zones. The capability of the dynamic adaptive combustion modeling strategy is then demonstrated in large eddy simulations (LES) of turbulent lifted n-dodecane spray flames. Specifically, inert mixing is used for chemically inactive zones, and the well-mixed combustion model with finite rate chemistry is applied in the pre-ignition zone to capture the two-stage ignition as well as premixed reaction fronts. Adaptive mesh refinement (AMR) is further adopted near the premixed reaction fronts to capture the local flame structure and flame propagation speed. For the post-ignition zone, a recently developed tabulated flamelet model (TFM) is applied and compared with the flamelet progress variable (FPV) method. It is shown that CEMA-based adaptive chemistry induces small errors to the statistically-averaged flame structures, as CEMA is an effective and robust approach for on-the-fly flame segmentation. It is further seen that the CEMA-based adaptive modeling strategy more accurately predicts the ignition delay time and flame lift-off length compared with the low-cost flamelet models such as TFM and FPV, while the computational cost is substantially lower compared with the well-mixed combustion model using finite rate chemistry.

    更新日期:2018-06-04
  • A diffusion-flame analog of forward smolder waves: (I) 1-D steady structures
    Combust. Flame (IF 4.494) Pub Date : 2018-01-24
    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.

    更新日期:2018-06-03
  • Ozone assisted cool flame combustion of sub-millimeter sized n-alkane droplets at atmospheric and higher pressure
    Combust. Flame (IF 4.494) Pub Date : 2018-02-06
    Fahd E. Alam, Sang Hee Won, Frederick L. Dryer, Tanvir I. Farouk

    Cool flame combustion of individual and isolated sub-millimeter sized n-heptane (n-C7H16) and n-decane (n-C10H22) droplets are computationally investigated for atmospheric and higher operating pressure (25 atm) conditions with varying levels of ozone (O3) mole fractions in the surroundings. A sphero-symmetric, one-dimensional, transient, droplet combustion model is utilized, employing reduced versions of detailed chemical kinetic models for the fuel species and an appended ozone reaction subset. Comprehensive parametric computations show that the regime of the cool flame burning mode and the transition from cool to hot flames are sensitive to the changes of O3 loading, pressure, diluent variation, the strength of initiation source, and the influence of fuel vapor pressure at the ambient condition. For both fuels and over a range of O3 concentrations in the ambient, sustained cool flame burning can be directly produced, even for sub-millimeter sized droplets. Over some range of O3 concentrations, operating pressure, and drop diameter, a self-sustaining, continuous cool flame burn can be produced without incurring a hot flame transition. For sufficiently high O3 concentrations, combustion initiation is always followed by a hot flame transition. Fuel volatility is also shown to be important for initiation and transition to cool flame and hot flame initiation. For fuels having a flash point lower than the ambient temperature (e.g. n-heptane), atomic O radicals formed by O3 decomposition react with the partially premixed, flammable gas phase near the droplet surface, leading to OH radicals, water production, and heat that auto-thermally accelerates the combustion initiation process. For fuels with flashpoints higher than the ambient temperature (e.g. n-decane), the reaction progress is limited by the local fuel vapor concentration and the necessity to heat the droplet surface to sufficiently high temperatures to produce locally flammable conditions. As a result, the initial transient for establishing either cool flame or hot flame transition is significantly longer for high flash point fuels. The transition of locally partially premixed reaction to diffusive burning conditions is more evident for high flash point conditions.

    更新日期:2018-06-03
  • Role of induced axial acoustics in transverse acoustic flame response
    Combust. Flame (IF 4.494) Pub Date : 2018-02-06
    Travis Smith, Benjamin Emerson, William Proscia, Tim Lieuwen

    This paper addresses the mechanisms through which transverse acoustic oscillations excite unsteady heat release. Forced and self-excited transverse acoustic instability studies to date have strong coupling between the transverse and axial acoustic fields near the flame. This is significant, as studies suggest that it is not the transverse disturbances themselves, but rather the induced axial acoustic disturbances, that control the bulk of the heat release response. This paper presents results from an experiment that controls the relative amplitudes of transverse and axial disturbances and measures the flow field and heat release response for an acoustically compact, swirling flame. 5 kHz, simultaneous sPIV and OH-PLIF measured the flow field and flame edge, and OH* chemiluminescence measured the relative heat release. Experiments performed with essentially the same transverse acoustic wave field, but with and without axial acoustics, show that significant heat release oscillations are only excited in the former case. The results show that the axial disturbances are the dominant cause of the heat release oscillations. These observations support the theory that the key role of the transverse motions is to act as the “clock” for the instability, setting the frequency of the oscillations while having a negligible direct effect on the actual heat release fluctuations. They also show that transverse instabilities can be damped by either actively canceling the induced axial acoustics in the nozzle (rather than the much larger energy transverse combustor disturbances), or by passively tuning the nozzle impedance to drive an axial acoustic velocity node at the nozzle outlet.

    更新日期:2018-06-03
  • Unsteady droplet combustion with fuel thermal expansion
    Combust. Flame (IF 4.494) Pub Date : 2018-03-02
    Vedha Nayagam, Daniel L. Dietrich, Forman A. Williams

    Millimeter-size fuel droplets burning in microgravity show substantial thermal expansion at earlier times in their burning history. Here, we develop a simple model that accounts for thermal expansion of the liquid fuel and compare it against experimental measurements. The results show that excellent agreement with measured droplet-diameter histories throughout the hot-flame period of combustion is obtained when the effect of thermal expansion is included.

    更新日期:2018-06-03
  • Critical kinetic uncertainties in modeling hydrogen/carbon monoxide, methane, methanol, formaldehyde, and ethylene combustion
    Combust. Flame (IF 4.494) Pub Date : 2018-03-02
    Yujie Tao, Gregory P. Smith, Hai Wang

    In view of the critical role of the underlying uncertainties of the reaction model in future progress of combustion chemistry modeling, Foundational Fuel Chemistry Model 1.0 (FFCM-1) was developed with uncertainty minimization against available fundamental combustion data of H2, H2/CO, CH4, CH2O, and C2H6. As a critical feature, FFCM-1 not only reconciles a large body of fundamental combustion data, it also has rigorously evaluated uncertainties for the rate coefficients, the combustion experimental targets used for model optimization and uncertainty minimization, and most importantly, an optimized reaction model with quantified uncertainties. In the present work, the remaining kinetic uncertainties of FFCM-1 are examined using a perfectly stirred reactor (PSR) as the relevant model platform for which reliable experiments under the conditions tested are unavailable. The key questions to address include the level of improvement from model optimization in the prediction uncertainties of PSR residence times at extinction and ignition and the rate coefficients of reactions that must be improved in order to reduce the prediction uncertainties. Computational tests are made for H2/CO, CH2O, CH4, CH3OH and C2H4–air mixtures over the pressure range of 10–100 atm and PSR inlet temperatures that would yield residence times comparable to the time scales typical of fuel combustion in practical combustors. The results show that although model optimization reduces the prediction uncertainties of residence time at extinction and ignition, the remaining uncertainties remain rather large. Key reactions for which reduced rate uncertainties would greatly improve the reaction model quality and accuracy have been identified and discussed in detail.

    更新日期:2018-06-03
  • Detailed SGS atomization model and its implementation to two-phase flow LES
    Combust. Flame (IF 4.494) Pub Date : 2018-03-02
    Akira Umemura, Junji Shinjo

    A novel turbulent atomization model, which is physically closed itself and free of case-by-case parameter tuning using experimental data, has been formulated and demonstrated in the framework of turbulent spray combustion large-eddy simulation (LES). Based on our accumulated research findings that elementary droplet/ligament generation is a deterministic phenomenon, not something random as considered in the conventional understanding, the model describes two dominant modes of turbulent atomization, i.e. the turbulent resonant mode and the Rayleigh–Taylor (RT) mode, in a physically straightforward manner. Extending the baseline theory proposed in Umemura (2016), to a hybrid turbulent spray LES formulation which includes both an Eulerian liquid jet core and Lagrangian droplets, the subgrid-scale (SGS) atomization characteristics are completely detailed in this study. Using the LES-resolved turbulent Weber and Bond numbers on the liquid core surface, the atomization mode and the SGS atomization characteristics such as droplet size, number, ejection velocity and core regression velocity are all identified locally, and the information is transferred back to the LES code as input information. Test cases of Diesel fuel jets demonstrate that the present formulation well reproduces the turbulent spray behavior. Thanks to the obtained detailed data, the spray formation process can be tracked both temporally and spatially, from the initial head formation with edge atomization to the later core atomization and spray spreading. It is essentially featured that the present turbulent atomization model works well without ambiguous user input, contrary to the conventional way of spray simulation. This is a significant breakthrough to urge paradigm shift in spray simulation, from unclosed/unpredictable to closed/predictable, which enables drastic improvement in the accuracy of spray simulation and may exert a large impact on both research studies and industrial applications.

    更新日期:2018-06-03
  • Direct numerical simulation of a temporally evolving air/n-dodecane jet at low-temperature diesel-relevant conditions
    Combust. Flame (IF 4.494) Pub Date : 2018-03-28
    Giulio Borghesi, Alexander Krisman, Tianfeng Lu, Jacqueline H. Chen

    We present a direct numerical simulation of a temporal jet between n-dodecane and diluted air undergoing spontaneous ignition at conditions relevant to low-temperature diesel combustion. The jet thermochemical conditions were selected to result in two-stage ignition. Reaction rates were computed using a 35-species reduced mechanism which included both the low- and high-temperature reaction pathways. The aim of this study is to elucidate the mechanisms by which low-temperature reactions promote high-temperature ignition under turbulent, non-premixed conditions. We show that low-temperature heat release in slightly rich fuel regions initiates multiple cool flame kernels that propagate towards very rich fuel regions through a reaction-diffusion mechanism. Although low-temperature ignition is delayed by imperfect mixing, the propagation speed of the cool flames is high: as a consequence, high-temperature reactions in fuel-rich regions become active early during the ignition transient. Because of this early start, high-temperature ignition, which occurs in fuel-rich regions, is faster than homogeneous ignition. Following ignition, the high-temperature kernels expand and engulf the stoichiometric mixture-fraction iso-surface which in turn establish edge flames which propagate along the iso-surface. The present results indicate the preponderance of flame folding of existing burning surfaces, and that ignition due to edge-flame propagation is of lesser importance.. Finally, a combustion mode analysis that extends an earlier classification [1] is proposed to conceptualize the multi-stage and multi-mode nature of diesel combustion and to provide a framework for reasoning about the effects of different ambient conditions on diesel combustion.

    更新日期:2018-06-03
  • The effects of particle size and reducing-to-oxidizing environment on coal stream ignition
    Combust. Flame (IF 4.494) Pub Date : 2018-06-01
    Adewale Adeosun, Zhenghang Xiao, Zhiwei Yang, Qiang Yao, Richard L. Axelbaum

    Coal particles experience a transition from a reducing to oxidizing environment in the near-burner region of pulverized coal (pc) boilers. For the first time, we report a fundamental study of ignition of a coal-particle stream experiencing a flame environment that transitions from a reducing to an oxidizing environment (termed reducing-to-oxidizing environment). High-speed videography is used to observe the particles in situ, and scanning electron microscopy is used to characterize the sampled particles. The effects of particle size on ignition are presented for four size bins (63–74 µm, 75–89 µm, 90–124 µm and 125–149 µm) for PRB subbituminous coal at two nominal gas temperatures (1300 K and 1800 K). An oxidizing environment with 20% molar oxygen composition is used as base-case. In contradistinction to single particle studies where particles are reported to ignite heterogeneously at higher temperatures, this study shows that coal streams ignite homogeneously, irrespective of particle size, in the oxidizing environment. By changing nominal gas temperature from 1300 K to 1800 K, ignition time decreases, on average, by a factor of five for each of the particle size bins. For both gas temperatures, the trend in ignition delays as particle size changes is non-monotonic. However, at 1800 K nominal gas temperature, ignition delays are independent of particle size in the reducing-to-oxidizing environment and ignition delays are doubled on average when compared to those in the oxidizing environment. It is more noticeable at the lower gas temperature of 1300 K that homogeneous ignition of coal streams is oxygen-dependent below 90 µm particle size and temperature-dependent above 90 µm. In general, ignition delay is determined by volatile release rate (controlled by the particle temperature) and the local oxygen concentration. Micrographs of particles also confirm that ignition and char burnout times are longer in the reducing-to-oxidizing environments than those in the oxidizing environments.

    更新日期:2018-06-02
  • A detonation paradox: Why inviscid detonation simulations predict the incorrect trend for the role of instability in gaseous cellular detonations?
    Combust. Flame (IF 4.494) Pub Date : 2018-06-01
    Matei I. Radulescu

    Experiments conducted over the past several decades have shown that the cellular structure of detonations is responsible for enhancing the detonability of gaseous detonations in the presence of losses, as compared with that predicted by the classical Zel’dovich-Von Neuman-Döring model for detonations, which neglects the time varying cellular structure of the front. Paradoxically, numerical studies conducted over the past decade have revealed that the propagation of inviscid detonations was hampered if the detonation was allowed to have a cellular structure, the effect increasing with the cellular irregularity. This apparent paradox is discussed in relation to the burning mechanism of unstable cellular detonations established experimentally, which shows that diffusive effects control the reaction of approximately half of the gases passing across the detonation front in unstable cellular detonations.

    更新日期:2018-06-02
  • Ember: An open-source, transient solver for 1D reacting flow using large kinetic models, applied to strained extinction
    Combust. Flame (IF 4.494) Pub Date : 2018-05-24
    Alan E. Long, Raymond L. Speth, William H. Green

    Simulation of quasi one-dimensional reacting flow is a standard in many combustion studies. Here Ember, a new open-source code for efficiently performing these calculations using large, detailed chemical kinetic models is presented. Ember outperforms other standard software, such as Chemkin, in computation time by leveraging rebalanced Strang operator splitting which does not suffer the steady-state inaccuracies of most splitting methods. The splitting approach and implementation used in Ember are described. Ember is validated for computation of flame extinction through imposed strain, extinction strain rate (ESR), and shown to be capable of modeling three typical experimental strained flame configurations: premixed twin flames, premixed single flames opposing inert, and diffusion flames. As further demonstration, Ember is used to investigate Lewis number effects on ESR using a detailed chemical kinetic model with 500 species for simulation of strained extinction of lean (Le > 1) and rich (Le < 1) propane/air flames. Primary trends predicted by Law (2006) using asymptotic theories of strained flames are accurately reproduced with the large, detailed chemical kinetic model. However, the complicated chemistry introduces some subtle phenomena not seen with single-step models. The Ember software is open-source and freely available to any user online.

    更新日期:2018-05-25
  • Theory and modeling of relevance to prompt-NO formation at high pressure
    Combust. Flame (IF 4.494) Pub Date : 2018-05-21
    Stephen J. Klippenstein, Mark Pfeifle, Ahren W. Jasper, Peter Glarborg

    An improved understanding of NOx formation at high pressures would be of considerable utility to efforts to develop advanced combustion devices. A combination of theoretical and modeling studies are implemented in an effort to improve the accuracy of models for the prompt NO process, which is the dominant source of NO under many conditions, and to improve our understanding of the role of this process at high pressures. The theoretical effort implements state-of-the-art treatments of NCN thermochemistry, the interrelated CH + N2 and NCN + H kinetics, and the kinetics of the NCN + OH reaction. For both reaction systems, we implement high level ab initio transition state theory based master equation simulations paying particular attention to the role of stabilization processes. For the NCN + H kinetics we include a treatment of inter-system crossing. The modeling effort focuses on exploring the role of pressure and prompt NO for premixed laminar flames at pressures ranging from 1 to 15 atm, via a comparison with the available experimental data. Additional simulations at higher pressures further explore the mechanistic changes at the pressures of relevance to applied combustion devices (e.g., 100 atm).

    更新日期:2018-05-21
  • A combined laser absorption and gas chromatography sampling diagnostic for speciation in a shock tube
    Combust. Flame (IF 4.494) Pub Date : 2018-05-18
    Alison M. Ferris, David F. Davidson, Ronald K. Hanson

    The first implementation of a combined laser absorption diagnostic/gas chromatography (GC) sampling system for the measurement of combustion-relevant species in a conventional shock tube configuration is reported, with ethylene pyrolysis as an example application. A heated, endwall sampling system is used to extract a post-shock sample for GC analysis. Analysis of the gas sample yields a measurement of the ultimate mole fraction values of multiple species (currently ethylene, acetylene, hydrogen, and methane) at the end of the reflected shock test time. A 10.532-µm laser absorption diagnostic is simultaneously used to measure time-resolved ethylene. A method to accurately model sampled speciation results using published kinetic models is discussed. A method for extending laser measurements into the expansion fan region for direct comparison with sampled GC results has also been developed. The combined optical and sampled-gas measurement techniques were used to study ethylene pyrolysis (1.0% mole fraction ethylene/argon) at approximately 5 atm, over a range of temperatures (1200–2000 K). The ethylene mole fraction measurements obtained using both techniques show close agreement.

    更新日期:2018-05-18
  • A computational analysis of methanol autoignition enhancement by dimethyl ether addition in a counterflow mixing layer
    Combust. Flame (IF 4.494) Pub Date : 2018-05-16
    Wonsik Song, Efstathios-Al. Tingas, Hong G. Im

    To provide fundamental insights into the ignition enhancement of methanol (MeOH) by the addition of the more reactive dimethyl ether (DME), computational parametric studies were conducted in a one-dimensional counterflow fuel versus air mixing layer configuration with the incorporation of detailed chemistry and transport. Various computational analysis tools based on the computational singular perturbation (CSP) framework were employed for detailed identifications of complex chemical pathways. CSP tools were also used to develop a 43-species skeletal mechanism for efficient computation of ignition of methanol-DME blends at engine conditions. The overarching practical question was the extent to which the addition of DME improves the ignitability of the methanol. As a baseline analysis, the results of a uniform temperature condition at 850 K showed that the low temperature chemistry associated with the DME fuel was highly effective in promoting autoignition. The increase in the oxidizer side temperature was found to diminish the ignition enhancement by DME blending, as the overall reactivity increases and the dominant chemical pathways become shifted towards the high temperature reactions. Finally, the strain rate effect on the ignition delay time was found to be significant for the pure methanol case, and then the effect diminishes as the amount of DME addition increases. This behavior was explained by examining the spatial locations of the ignition kernels and the Damköhler number history for different strain rate conditions.

    更新日期:2018-05-17
  • Autoignited lifted flames of dimethyl ether in heated coflow air
    Combust. Flame (IF 4.494) Pub Date : 2018-05-16
    Saeed M. Al-Noman, Byung Chul Choi, Suk Ho Chung

    Autoignited lifted flames of dimethyl ether (DME) in laminar nonpremixed jets with high-temperature coflow air have been studied experimentally. When the initial temperature was elevated to over 860 K, an autoignition occurred without requiring an external ignition source. A planar laser-induced fluorescence (PLIF) technique for formaldehyde (CH2O) visualized qualitatively the zone of low temperature kinetics in a premixed flame. Two flame configurations were investigated; (1) autoignited lifted flames with tribrachial edge having three distinct branches of a lean and a rich premixed flame wings with a trailing diffusion flame and (2) autoignited lifted flames with mild combustion when the fuel was highly diluted. For the autoignited tribrachial edge flames at critical autoignition conditions, exhibiting repetitive extinction and re-ignition phenomena near a blowout condition, the characteristic flow time (liftoff height scaled with jet velocity) was correlated with the square of the ignition delay time of the stoichiometric mixture. The liftoff heights were also correlated as a function of jet velocity times the square of ignition delay time. Formaldehydes were observed between the fuel nozzle and the lifted flame edge, emphasizing a low-temperature kinetics for autoignited lifted flames, while for a non-autoignited lifted flame, formaldehydes were observed near a thin luminous flame zone. For the autoignited lifted flames with mild combustion, especially at a high temperature, a unique non-monotonic liftoff height behavior was observed; decreasing and then increasing liftoff height with jet velocity. This behavior was similar to the binary mixture fuels of CH4/H2 and CO/H2 observed previously. A transient homogeneous autoignition analysis suggested that such decreasing behavior with jet velocity can be attributed to partial oxidation characteristics of DME in producing appreciable amounts of CH4/CO/H2 ahead of the edge flame region.

    更新日期:2018-05-17
  • Tuning the morphological, ignition and combustion properties of micron-Al/CuO thermites through different synthesis approaches
    Combust. Flame (IF 4.494) Pub Date : 2018-05-16
    Sili Deng, Yue Jiang, Sidi Huang, Xinjian Shi, Jiheng Zhao, Xiaolin Zheng

    Aluminum (Al)-based thermite, due to its high energy density and low cost, has found wide applications in aerospace propulsion, explosion, pyrotechnics, thermal batteries, and power generations. Though significant efforts have been devoted to improving the ignition and combustion performance of Al-based thermites by using nano-Al, micron-Al (m-Al) remains of practical importance over nano-Al due to its lower cost and smaller dead mass. For m-Al based thermite, the main approach to improve its ignition and combustion performance is to bring Al and metal oxide as close as possible to facilitate the oxidizer diffusion process. Herein, we demonstrated two simple synthesis methods, i.e., the precipitation (PC) method and displacement (DP) method, to prepare m-Al/CuO thermites with the intention to bring Al and CuO to shorter diffusion distance and achieve better dispersion. The PC-thermites have flocculent nanostructured CuO closely attached to the surface of m-Al, and the DP-thermites have a dense shell of CuO coated on the surface of m-Al. Both PC- and DP-thermites have reduced agglomeration and diffusion distance over the traditional mechanically mixed (MM)-thermites that have randomly distributed and agglomerated CuO and m-Al. Consequently, both PC- and DP-thermites exhibit shorter ignition delay time, lower reaction onset temperatures, higher heat release, larger pressure rise, and extended reactivity limits than MM-thermites. Particularly, PC-thermites, due to their flocculent structures, exhibit the shortest ignition delay time, lowest reaction onset temperature, and highest amount of heat release. Moreover, the superior ignition and combustion performance of PC- and DP-thermites is more pronounced under high heating rates over low heating rates. Similar PC and DP methods are applicable to prepare diverse thermites with reduced diffusion distance and improved dispersion to improve their ignition and combustion properties.

    更新日期:2018-05-16
  • Propagation and extinction of subatmospheric counterflow methane flames
    Combust. Flame (IF 4.494) Pub Date : 2018-04-27
    Robert R. Burrell, Dong J. Lee, Fokion N. Egolfopoulos

    Measurements of flame propagation velocities and extinction states in counterflow provide a valuable source of flame data that contain information about fundamental combustion physics. The approach to properly account for stretch effects in counterflow flame measurements through non-intrusive laser-based local velocity characterization was advanced in the mid-80s by Law and coworkers at atmospheric conditions with simple fuels. Subsequently, several research groups have extended the measurements to elevated pressures and complex fuels. However, counterflow flame data at subatmospheric pressures are limited. In the present study, a method is introduced for measuring laminar flame speeds and extinction strain rates in subatmospheric counterflow flames. A numerical study was performed to assess the dynamics of tracer particles used to facilitate measurements. It was found that the particle phase dynamics used in particle velocimetry measurements are not always representative of the underlying gas phase motion due to thermophoresis and insufficient drag, especially at low pressures. A numerical scheme was implemented whereby the computed particle phases were used for proper comparison with measurements and, based on the computed results, to infer the corresponding values of the gas phase. The method was applied to premixed methane/air and non-premixed methane–nitrogen/oxygen flames at pressures as low as 0.1 atm. Complimentary flame structure simulations were carried out which show that the kinetics of formyl radical prompt dissociation strongly impact the computed subatmospheric flames and may influence the validation of unimolecular and bimolecular reactions rate constants when tested against laminar flame data.

    更新日期:2018-04-27
  • Soot formation in counterflow non-premixed ethylene flames at elevated pressures
    Combust. Flame (IF 4.494) Pub Date : 2018-04-21
    Xin Xue, Pradeep Singh, Chih-Jen Sung

    Quantitative soot volume fraction measurements were conducted in a counterflow non-premixed flame configuration using ethylene/nitrogen as the fuel stream, oxygen/nitrogen as the oxidizer stream, and a pressure range of 1–8 atm. The laser-induced incandescence technique, calibrated using the light extinction method, was used to measure the soot volume fraction distributions. The variations of soot formation along the centerline of the counterflow flame with pressure were compared by keeping the density-weighted strain rate constant. Maintaining a constant density-weighted strain rate allows the overall flame thickness, as well as the reactant mass fluxes entering the flame, to remain unchanged for all pressures. As such, the effect of pressure on soot chemistry can be isolated from the effect of convective-diffusive transport. Based on the measured soot volume profiles, the soot layer thickness variation with pressure was determined. It was found that when keeping the density-weighted strain rate constant, the soot layer thickness remains similar over the pressure range investigated. However, the soot layer thickness was seen to decrease with increasing pressure when holding the strain rate fixed. In addition, the effects of fuel mole fraction and oxygen mole fraction on soot formation were investigated. Furthermore, the pressure scaling factors of soot formation under varying mixture conditions were deduced from experimental measurements. A literature gas-phase reaction mechanism including polycyclic aromatic hydrocarbon (PAH) chemistry up to pyrene was also used to simulate the experimental counterflow flames. The pressure effect on PAH formation was presented and discussed.

    更新日期:2018-04-25
  • A physics-based approach to modeling real-fuel combustion chemistry - I. Evidence from experiments, and thermodynamic, chemical kinetic and statistical considerations
    Combust. Flame (IF 4.494) Pub Date : 2018-04-21
    Hai Wang, Rui Xu, Kun Wang, Craig T. Bowman, Ronald K. Hanson, David F. Davidson, Kenneth Brezinsky, Fokion N. Egolfopoulos

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

    更新日期:2018-04-25
  • Nonlinear development of hydrodynamically-unstable flames in three-dimensional laminar flows
    Combust. Flame (IF 4.494) Pub Date : 2018-04-16
    Advitya Patyal, Moshe Matalon

    The hydrodynamic instability, which results from the large density variations between the fresh mixture and the hot combustion products, was discovered by Darrieus and Landau over seventy years ago, and has been named after its inventors. The instability, which prevents flames from being too flat, was thought to lead immediately to turbulent flames. Recent studies, initiated by weakly nonlinear analyses and extended by two-dimensional simulations suggest that this is not the case. It was established that the flame beyond the onset of instability, develops into a cusp-like structure pointing towards the burned gas region that propagates at a speed substantially larger than the laminar flame speed. In this work, we present for the first time a systematic study of the bifurcation phenomena in the more realistic three-dimensional flow. The computations are carried out within the context of the hydrodynamic theory where the flame is treated as a surface of density discontinuity separating burned gas from the fresh mixture, and propagates at a speed that depends on the local curvature and hydrodynamic strain rate. A low Mach-number Navier–Stokes solver modified by an appropriate source term is used to determine the flow field that results from the gas expansion and the flame is tracked using a level-set methodology with a surface parameterization method employed to accurately capture the local velocity and stretch rate. The numerical scheme is shown to recover the known exact solutions predicted in the weak gas expansion limit and corroborates the bifurcation results from linear stability analysis. The new conformations that evolve beyond the instability threshold have sharp crest pointing towards the burned gas with ridges along the troughs, and propagate nearly 40% faster than planar flames. Indeed, the appearance of sharp folds and creases, which are some manifestations of the Darrieus–Landau instability, have been observed on the surface of premixed flames in various laminar and turbulent settings.

    更新日期:2018-04-25
  • A new chemical kinetic method of determining Ron and Mon values for single component and multicomponent mixtures of engine fuels
    Combust. Flame (IF 4.494) Pub Date : 2018-04-14
    C.K. Westbrook, M. Sjöberg, N.P. Cernansky

    A new method of using chemical kinetic reaction modeling to predict the Research Octane Number (RON) and Motor Octane Number (MON) of single component fuels and fuel mixtures is described and illustrated via comparisons between computed and experimental values obtained using the well-established ASTM test procedures in a Cooperative Fuels Research (CFR) engine. Comparisons include predictions of RON and MON for a large variety of neat fuels, studies determining the RON and MON of mixtures of primary reference fuels (PRF) and toluene, and studies of RON and MON for mixtures of single-component and multiple-component gasoline surrogate mixtures with ethanol. Advantages in costs, time, and experimental complexity of the kinetic modeling approach compared to the existing engine test procedures are discussed.

    更新日期:2018-04-25
  • Sooting limits of non-premixed counterflow ethylene/oxygen/inert flames using LII: Effects of flow strain rate and pressure (up to 30 atm)
    Combust. Flame (IF 4.494) Pub Date : 2018-04-14
    Brendyn G. Sarnacki, Harsha K. Chelliah

    An absolute irradiance-calibrated Laser Induced Incandescence (LII) technique and a standard particle image velocimetry (PIV) technique were utilized to collect quantitative data on soot volume fraction and corresponding flow strain rates of diluted ethylene-air non-premixed counterflow flames. Pressures up to 30 atm were explored with increasing dilution with nitrogen or helium to minimize flow strain limits at which incipient soot was detected and to maintain the flame in laminar mode. For weakly strained flames considered, the species and velocity boundary conditions were used to predict the gas-phase flame structure (e.g., temperature and major species). The predicted gas properties, together with soot particle temperature decay rate measured by two-color pyrometry were used in the LII heat transfer model to extract the effective soot particle size and particle number density. Estimates of global activation energy of incipient soot yield with pressure indicated a sudden change around a pressure of 20 atm, which may be attributed to a shift in soot nucleation and growth pathways.

    更新日期:2018-04-25
  • Low-temperature multistage warm diffusion flames
    Combust. Flame (IF 4.494) Pub Date : 2018-04-11
    Omar R. Yehia, Christopher B. Reuter, Yiguang Ju

    We report on experimental evidence of the existence of a new self-sustaining low-temperature multistage warm diffusion flame, existing between the cool flame and hot flame, at atmospheric pressure in the counterflow geometry. The structure of multistage warm diffusion flames was examined by using thermometry, laser-induced fluorescence, and chemiluminescence measurements. It was found that the warm diffusion flame has a two-staged double flame structure, with a leading diffusion cool flame stage on the fuel side and a second intermediate stage on the oxidizer side, with strong heat release in the second stage that can be comparable to that of the first stage. The results demonstrate that the spatially-distinct multistage character is due to the low-temperature fuel reactivity that allows for the production of reactive intermediates in a leading cool flame. These intermediates are then oxidized, on the oxidizer side, in a second stage via intermediate-temperature chemistry. In the case of dibutyl ether, the low-temperature peroxy branching pathway supports the first cool flame oxidation stage and produces intermediates such as alkyl and carbonyl radicals. The alkyl and carbonyl radicals then react with the hydroperoxyl radical and molecular oxygen to form the second oxidation stage. A detailed analysis revealed that ozone addition in the oxidizer promotes the second stage oxidation by increasing both the radical pool population and the flame temperature, but does not fundamentally change the multistage flame structure. Furthermore, the analysis revealed that with the increase of fuel concentration, a single-stage cool flame can ignite to a warm flame or a hot flame. Moreover, a warm flame can extinguish into a cool flame or ignite to a hot flame when the fuel concentration is substantially reduced or increased, respectively. Finally, under certain conditions, a hot flame can extinguish directly into either a warm flame or a cool flame. Hence, the results suggest that the multistage warm flame can act as a critical bridge between cool flames and hot flames and that it is a fundamental burning mode characteristic of low-temperature non-premixed combustion. The multistage warm diffusion flame is particularly relevant to combustion in highly turbulent flow fields and in microgravity environments, owing to the possibility of long residence times.

    更新日期:2018-04-11
  • A physics-based approach to modeling real-fuel combustion chemistry – II. Reaction kinetic models of jet and rocket fuels
    Combust. Flame (IF 4.494) Pub Date : 2018-04-10
    Rui Xu, Kun Wang, Sayak Banerjee, Jiankun Shao, Tom Parise, Yangye Zhu, Shengkai Wang, Ashkan Movaghar, Dong Joon Lee, Runhua Zhao, Xu Han, Yang Gao, Tianfeng Lu, Kenneth Brezinsky, Fokion N. Egolfopoulos, David F. Davidson, Ronald K. Hanson, Craig T. Bowman, Hai Wang

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

    更新日期:2018-04-11
  • The impacts of three flamelet burning regimes in nonlinear combustion dynamics
    Combust. Flame (IF 4.494) Pub Date : 2018-04-10
    Tuan M. Nguyen, William A. Sirignano

    Axisymmetric simulations of a liquid rocket engine are performed using a delayed detached-eddy-simulation (DDES) turbulence model with the Compressible Flamelet Progress Variable (CFPV) combustion model. Three different pressure instability domains are simulated: completely unstable, semi-stable, and fully stable. The different instability domains are found by varying the combustion chamber and oxidizer post length. Laminar flamelet solutions with a detailed chemical mechanism are examined. The β probability density function (PDF) for the mixture fraction and Dirac δ PDF for both the pressure and the progress variable are used. A coupling mechanism between the volumetric Heat Release Rate (HRR) and the pressure in an unstable cycle is demonstrated. Local extinction and reignition are investigated for all the instability domains using the full S-curve approach. A monotonic decrease in the amount of local extinctions and reignitions occurs when pressure oscillation amplitude becomes smaller. The flame index is used to distinguish between the premixed and non-premixed burning mode in different stability domains. An additional simulation of the unstable pressure oscillation case using only the stable flamelet burning branch of the S-curve is performed. Better agreement with experiments in terms of pressure oscillation amplitude is found when the full S-curve is used.

    更新日期:2018-04-10
  • Combustion of Mg and composite Mg·S powders in different oxidizers
    Combust. Flame (IF 4.494) Pub Date : 2018-04-10
    Xinhang Liu, Mirko Schoenitz, Edward L. Dreizin

    Micron-sized, spherical magnesium powders were ignited by a CO2 laser beam and by injecting them in the products of air–C2H2 and air–H2 flames. The same experiments were performed with composite Mg·S powders prepared by mechanical milling magnesium and elemental sulfur powders. The non-spherical Mg powder used to prepare composites was also explored in selected combustion tests. Flow conditions were varied in experiments performed in air with all materials. The combustion products were collected for particles burning in air; the products were studied using electron microscopy. Optical emission produced by burning particles was recorded using filtered photomultipliers. The emission pulses were processed to recover the particle burn times and their temperatures. Fine Mg particles burn in air very rapidly, with the burn times under 1 ms for particles finer than ca. 10 µm. The apparent trend describing burn time as a function of the particle size for such particles is t∼d0.5. The particles burn without generating a detectable standoff flame zone or producing smoke; combustion products are particles of MgO with dimensions comparable to those of the starting Mg powder particles. Both the particles burn times and their measured flame temperatures decrease slightly when particles are carried by faster air flows. The present experimental results is interpreted qualitatively assuming that the reaction occurs at or very near the boiling Mg surface and its rate is affected by both surface kinetics and the inward diffusion of oxygen. It is further proposed that the fine, solid MgO particles form either directly on surface of Mg droplet or in its immediate vicinity. Deposition of MgO crystals on liquid Mg causes little change in the particle burn rate. Combustion of Mg in air–C2H2 and air–H2 flames occurs much slower than in air. Combustion of composite Mg·S particles follows a two-step process. In the first step, sulfur is evaporated. When the particles are heated by a CO2 laser beam, rapid evaporation of sulfur leads to a sudden change in the particle velocity. Once sulfur is removed, the particles burn similarly to the pure Mg.

    更新日期:2018-04-10
  • Deflagration-to-detonation transition in an unconfined space
    Combust. Flame (IF 4.494) Pub Date : 2018-04-03
    Andrey Koksharov, Viatcheslav Bykov, Leonid Kagan, Gregory Sivashinsky

    Whereas deflagration-to-detonation transition in confined systems is a matter of common knowledge, feasibility of the transition in unconfined space is still a matter of controversy. With a freely expanding self-accelerating spherical flame as an example, it is shown that deflagration-to-detonation transition in unconfined gaseous systems is indeed possible provided the flame is large enough. The transition is caused by positive feedback between the accelerating flame and the flame-driven pressure buildup, which results in the thermal runaway when the flame speed reaches a critical level.

    更新日期:2018-04-04
  • An experimental and modeling study on the low temperature oxidation of surrogate for JP-8 part II: Comparison between neat 1,3,5-trimethylbenzene and its mixture with n-decane
    Combust. Flame (IF 4.494) Pub Date : 2018-03-10
    Bing-Yin Wang, Yue-Xi Liu, Jun-Jie Weng, Guan-Fu Pan, Zhen-Yu Tian

    The low temperature oxidation of neat 1,3,5-trimethylbenzene (T135MB) and n-decane/T135MB mixture as a surrogate for JP-8 has been investigated in a jet-stirred reactor over the temperature range of 500–1100 K at atmospheric pressure under fuel-rich condition with residence time from 2.33 to 1.06 s. Mole fraction profiles of 29 intermediates including light hydrocarbons, oxygenated and aromatic compounds were identified by gas chromatographic techniques. In general, the concentrations of intermediates tend to increase progressively with temperature from 925 K in neat T135MB oxidation, while these species exhibit bimodal distributions from 550 K in the oxidation of n-decane/T135MB mixture (surrogate). By considering the calculated rate constants of T135MB and analogous coupling reactions between T135MB and n-decane, a detailed kinetic mechanism involving 910 species and 5329 reactions was established with a reasonable agreement with the experimental results. The low temperature chemistry of T135MB and surrogate was analyzed including the NTC behavior below 800 K. The oxidation process of T135MB is occurring mainly by H-abstraction with OH radical and subsequent reactions. The difference is that in the NTC region H-abstraction by OH radicals is the major consumption pathway for T135MB in the surrogate. But for neat T135MB, the dominant channels change to the H-abstractions by H-atoms, OH and CH3 radicals. Addition of n-decane can promote the oxidation of T135MB by providing OH radicals in the low temperature oxidation of surrogate fuel. Moreover, the model was also validated against the experimental data on n-decane and JP-8 combustion, including species profiles in low temperature jet-stirred reactor oxidation and high temperature flow reactor pyrolysis as well as ignition delay times. These extended results yielded overall satisfactory agreement, and will benefit for further application of practical JP-8 fuels, particularly for their combustion properties at wide temperature range.

    更新日期:2018-03-11
  • An experimental and modeling study on the low temperature oxidation of surrogate for JP-8 part I: Neat 1,3,5-trimethylbenzene
    Combust. Flame (IF 4.494) Pub Date : 2018-03-02
    Bing-Yin Wang, Dan Yu, Guan-Fu Pan, Yue-Xi Liu, Jun-Jie Weng, Zhen-Yu Tian

    This work describes the experimental and modeling study of low temperature oxidation of 1,3,5-trimethylbenzene (T135MB) in a jet-stirred reactor over the temperature range of 700–1100 K at atmospheric pressure under fuel-lean and stoichiometric conditions. 9 C0 single bond C5 hydrocarbons, 6 oxygenated products and 6 aromatic compounds were identified and quantified using GC and GC-MS. A detailed kinetic based on T135MB model of Diévart et al. was proposed to simulate the low-temperature experimental results in the present work. Rate constants of T135MB decomposition and metatheses reactions were calculated with CBS-QB3 method implemented in Gaussian 09. The performance of proposed mechanism in reproducing the experimental data is reasonably good. Reaction flux analysis shows that dominant consumption channels for T135MB oxidation are H-abstraction reactions to form 3,5-dimethylbenzyl radicals, while reactions with O/OH radicals to generate 1,3,5-trimethylphenoxyl/1,3,5-trimethylphenyl and ipso-addition to form m-xylene play minor roles. Sensitivity analysis reveals that H-abstraction from side methyl groups of T135MB by OH radical is the most inhibiting reaction oxidation at Φ = 1.0, while it is a promoting reaction at Φ = 0.4. Moreover, current model were validated against experimental results on T135MB oxidation in flow reactor from Diévart et al. as well as global combustion property ignition delay times from Rao et al. and Diévart et al. with reasonable predictions.

    更新日期:2018-03-02
  • Dielectric-barrier-discharge plasma-assisted hydrogen diffusion flame. Part 1: Temperature, oxygen, and fuel measurements by one-dimensional fs/ps rotational CARS imaging
    Combust. Flame (IF 4.494) Pub Date : 2018-02-21
    Jonathan E. Retter, Gregory S. Elliott, Sean P. Kearney

    One-dimensional hybrid fs/ps CARS imaging provides single-laser-shot measurements of temperature, oxygen, and hydrogen in a plasma-assisted hydrogen diffusion flame. The coaxial dielectric-barrier-discharge burner collapses the Re ∼50 hydrogen diffusion flame to within ∼5 mm of the burner surface at an applied AC potential of 8.75 kV at 18 kHz, coinciding nicely with the full spatial extent of the 1D CARS measurements. Translating the burner through the measurement volume allowed for measurements at numerous radial locations in increments of 1 mm with a resolution of 140 µm × 30 µm × 600 µm, sufficient to resolve spatial gradients in this unsteady flame. Longer probe delays, required for improved dynamic range in regions of high temperature fluctuations, proved difficult to model as a result of a nontrivial decay in the O2 Raman coherence arising from complexities associated with the triplet ground electronic state of the O2 molecule. Oxygen linewidths were treated empirically using the observed O2 coherence decay in spectra acquired from the product gases of lean, near-adiabatic H2/air flames stabilized on a Hencken flat-flame burner. While still leading to errors up to 10% at worst, the empirically determined Raman linewidth factors eliminated any systematic error in the O2/N2 measurements with probe delay. Temperature measurements in the Hencken Burner flames proved to be insensitive to probe pulse delay, providing robust thermometry. Demonstration of this technique in both the canonical Hencken burner flames and a new DBD burner validates its effectiveness in producing multiple spatially resolved measurements in combustion environments. Measurements in the DBD burner revealed an unsteady, counterflow flattened flame structure near the fuel orifice which became unsteady as the reaction zone curves towards the surface for larger radial positions. Fluctuations in the fuel concentration were largest at the source, as the large, plasma-generated, unsteady external toroidal vortex that dominates the transport in this flame provides enhanced ventilation of the flame surface in close proximity to the fuel tube.

    更新日期:2018-02-21
  • Dielectric-barrier-discharge plasma-assisted hydrogen diffusion flame. Part 2: Modeling and comparison with experiments
    Combust. Flame (IF 4.494) Pub Date : 2018-01-31
    Luca Massa, Jonathan E. Retter, Gregory S. Elliott, Jonathan B. Freund

    Recent visual, PIV, and fs-CARS measurements show the multiple fundamental changes that occur when a hydrogen stand-burner flame is actuated by a dielectric-barrier-discharge (DBD). These are far more significant than in corresponding actuation without combustion. The un-actuated flame is conical and flickers at ∼ 10 Hz. As the DBD voltage is increased, the oscillations decrease then cease, the flame flattens and light emission significantly increase. We develop a simulation model that reproduces and allows us to analyze the mechanisms that underlie these changes. The main mechanisms are body forces due to charge sheaths, with radicals produced by plasma excitation playing a secondary role for the present conditions. The basic model introduced for charge density is based on a Poisson–Boltzmann description, with charge assumed to be in equilibrium with the potential. However, in this limit a constant-property plasma produces only an irrotational force, which we show fails to reproduce the observed vortex-ring. To correct this, we introduce a non-constant property, linearized Poisson–Boltzmann model, which thus includes charge-induced vorticity via the misalignment of the gradient of the screening length and the electric field. This reproduces the observations. For weak actuation, the vortex ring remains within the stoichiometric surface. This flame widens some, but flickers at the same frequency with reduced amplitude. Above 8 kV, a sudden increase in plasma extent occurs due to electrical breakdown on the outer dielectric surface facilitated by the flame lying now close to the actuator surface. This supports a broader plasma and consequently a broader vortex ring. The vortex ring moves outside of the stoichiometric surface, leading to a reduction of the flame surface, which is now ventilated by the air entrained by the vortex. This brings the flickering conical flame into a state that resembles a counter-flow flat flame.

    更新日期:2018-02-02
  • Experimental study of corner fires—Part I: Inert panel tests
    Combust. Flame (IF 4.494) Pub Date : 2017-11-06
    Davood Zeinali, Steven Verstockt, Tarek Beji, Georgios Maragkos, Joris Degroote, Bart Merci

    Corner fires are known to spread more intensely in comparison with single wall fires. In view of the challenges associated with prediction of such fire behavior, the fire growth in a corner configuration of Medium Density Fiberboard (MDF) panels is investigated to provide a set of experimental data, performing Single Burning Item (SBI) tests. First, though, test results with inert calcium silicate panels are discussed for three values of HRR (10, 30 and 55 kW), allowing to address the main physics involved. The experimental data for 30 kW, the default SBI HRR, is used for detailed discussion of the observations. The SBI testing methodology, materials, and set-up are described. The results of total Heat Release Rates (HRR) and Smoke Production Rates (SPR), as well as the panel temperatures and total heat fluxes at several characteristic locations are analyzed. Moreover, the puffing frequency of the corner fire is characterized thanks to Video Fire Analysis (VFA) of the experimental footage. Additionally, flame heights are discussed, including the concept of mirroring. A new correlation for mean flame height is introduced, using the hypotenuse of the triangle as characteristic length for entrainment of air into the fire plume, and expressing that the flame height increases proportional to the square root of the fire heat release rate. The 30 kW propane burner of the standard SBI test is shown to feature a mean flame height of nearly 0.9 m and a puffing frequency of 2 ± 0.3 Hz, and an average total heat flux exceeding 44 kW/m2 near the burner early on in the test. The completeness of the dataset is expected to be useful for testing and development of CFD codes for corner fire scenarios.

    更新日期:2017-12-14
  • Experimental study of corner fires—Part II: Flame spread over MDF panels
    Combust. Flame (IF 4.494) Pub Date : 2017-11-04
    Davood Zeinali, Steven Verstockt, Tarek Beji, Georgios Maragkos, Joris Degroote, Bart Merci

    Having explained the characteristics of a corner fire in the configuration of Single Burning Item (SBI) test in Part I [1], the results of three flame spread experiments conducted with Medium Density Fiberboard (MDF) panels are discussed. The fire growth, in terms of flame heights and spread, is examined from two different angles visually and through Video Fire Analysis (VFA) with a flame detection algorithm. Total Heat Release Rates (HRR) and Smoke Production Rates (SPR), as well as total heat fluxes at several characteristic locations, are presented. Moreover, temperature evolutions are discussed for multiple locations and through the thickness of the panels. Also the backside temperatures, important as boundary condition for numerical simulations, are reported. The corner fire tests with MDF panels yield an average peak HRR of 151 kW and an average total heat flux exceeding 60 kW/m2 close to the burner, with average flame heights surpassing 1.5 m in about 60 s.

    更新日期:2017-12-14
  • Premixed flames subjected to extreme levels of turbulence part I: Flame structure and a new measured regime diagram
    Combust. Flame (IF 4.494) Pub Date : 2017-09-29
    Aaron W. Skiba, Timothy M. Wabel, Campbell D. Carter, Stephen D. Hammack, Jacob E. Temme, James F. Driscoll

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

    更新日期:2017-12-14
  • Effect of fuel composition on soot and aromatic species distributions in laminar, co-flow flames. Part 2. Partially-premixed fuel
    Combust. Flame (IF 4.494) Pub Date : 2017-09-11
    Anandkumar Makwana, Yefu Wang, Suresh Iyer, Milton Linevsky, Robert J. Santoro, Thomas A. Litzinger, Jacqueline O'Connor

    The goal of this work is to understand the effect of fuel molecular structure on soot precursors and soot in an axisymmetric, co-flow, laminar flame configuration at atmospheric pressure with partially-premixed fuel jets. Five fuels with varying molecular structure are investigated: methylcyclohexane/n-dodecane mixture, n-heptane/n-dodecane mixture, iso-octane/n-dodecane mixture, m-xylene/n-dodecane mixture and pure n-dodecane. The flames investigated have jet equivalence ratios of 24 and 6. The total carbon flow rate and carbon fraction of the two components are kept constant to facilitate comparison among fuels. A laser-induced fluorescence technique is used to obtain spatially-resolved polycyclic aromatic hydrocarbons, soot precursors, in the jet flames. The polycyclic aromatic hydrocarbons are identified into two classes: single/two ring aromatics (small) and aromatics having three to five rings (large). A laser-induced incandescence and laser extinction technique are applied to obtain two-dimensional soot volume fraction for all the flames. The experimental results indicate that the level of soot is highest for the m-xylene/n-dodecane fuel at approximately three times the peak soot levels in the paraffinic fuels. The data show the effects of premixing on the spatial distribution of aromatic species and soot, including the shift from a soot field that peaks near the flame front to one that has maximum soot volume fractions near the centerline in m-xylene/n-dodecane flame. The iso-octane/n-dodecane and methylcyclohexane/n-dodecane fuels show a similar transition of soot field that has an annular peak in the non-premixed flame to a more uniform soot field under premixed conditions. Normalizing the maximum soot volume fraction by the maximum for the n-dodecane base fuel, the data shows that, within measurement uncertainty, the effect of fuel structure on maximum soot volume fraction is independent of the equivalence ratio of the fuel jet.

    更新日期:2017-12-14
  • Effect of fuel composition on soot and aromatic species distributions in laminar, co-flow flames. Part 1. Non-premixed fuel
    Combust. Flame (IF 4.494) Pub Date : 2017-09-09
    Yefu Wang, Anandkumar Makwana, Suresh Iyer, Milton Linevsky, Robert J. Santoro, Thomas A. Litzinger, Jacqueline O’Connor

    The work described in this paper is part of Department of Defense-Industry-University collaboration to develop fundamental understanding of the effects of alternative fuels on emissions from military gas turbine engines. The study was conducted in an axisymmetric, co-flow, laminar flame configuration at atmospheric pressure to determine the effects of fuel structure on aromatic species and soot. Five fuels with varying molecular structure were investigated: pure n-dodecane and four binary mixtures: m-xylene/n-dodecane, n-heptane/n-dodecane, iso-octane/n-dodecane, and methylcyclohexane/n-dodecane. Flames with non-premixed and partially-mixed fuel jets were studied. This paper describes the results of flames with non-premixed fuel jets; a companion paper describes the results for the partially pre-mixed fuel jets. In all of the experiments, the total carbon flow rate and the fraction of carbon from the two components in the binary mixtures were kept constant to facilitate comparison among the fuels. A laser-induced fluorescence technique was used to obtain spatially-resolved information on the aromatic species. Signals from aromatic species were collected in two wavelength ranges corresponding to species with one or two rings and three–five rings. Laser-induced incandescence and laser extinction were applied to obtain two-dimensional soot volume fraction for all flames. The flames of the paraffin fuels are non-smoking and have similar spatial distributions of aromatic species and soot as well as maximum soot volume fractions. The flame from the m-xylene fuel is smoking and has spatial distributions of aromatic species and soot that are distinctly different than for the paraffin fuels. Maximum soot volume fraction produced by the m-xylene fuel is approximately three times that of the paraffin fuels.

    更新日期:2017-12-14
  • Detailed kinetic modeling of dimethoxymethane. Part I: Ab initio thermochemistry and kinetics predictions for key reactions
    Combust. Flame (IF 4.494) Pub Date : 2017-09-06
    Wassja A. Kopp, Leif C. Kröger, Malte Döntgen, Sascha Jacobs, Ultan Burke, Henry J. Curran, Karl Alexander Heufer, Kai Leonhard

    Despite the great interest in oxygenated methyl ethers as diesel fuel additives and as fuels themselves, the influence of their methylenedioxy group(s) (O–CH2–O) has never been quantified using ab initio methods. In this study we elucidate the kinetics and thermochemistry of dimethoxymethane using high-level ab initio (CCSD(T)/aug-cc-pV(D+T)Z//B2PLYPD3BJ/6-311++g(d,p)) and statistical mechanics methods. We model torsional modes as hindered rotors which has a large influence on the description of the thermal behavior. Rate constants for hydrogen abstraction by Ḣ and ĊH3 are computed and show that abstraction from the methylenedioxy group is favored over abstraction from the terminal methyl groups. β-scission and isomerization of the radicals are computed using master equations. The effect of rovibrationally excited radicals from preceding hydrogen abstraction reactions on subsequent hot β-scission is computed and has large influence on the decomposition of the formed dimethylether radical. The quantification of the effect of the dominant methylenedioxy group using ab initio methods can guide modeling of oxygenated methyl ethers that contain that group several times.

    更新日期:2017-12-14
Some contents have been Reproduced with permission of the American Chemical Society.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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