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Combustion and Flame
Journal Prestige (SJR): 2.427
Citation Impact (citeScore): 5
Number of Followers: 119  
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ISSN (Print) 0010-2180
Published by Elsevier Homepage  [3162 journals]
  • Ozone assisted cool flame combustion of sub-millimeter sized n-alkane
           droplets at atmospheric and higher pressure
    • Abstract: Publication date: September 2018Source: Combustion and Flame, Volume 195Author(s): 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.
  • A dedication to Professor Chung King (Ed) Law
    • Abstract: Publication date: September 2018Source: Combustion and Flame, Volume 195Author(s): Suk Ho Chung, Vigor Yang, Richard A. Yetter
  • A diffusion-flame analog of forward smolder waves: (I) 1-D steady
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
    • 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
  • Experimental and computational investigation of partially-premixed
           methoxymethane flames
    • Abstract: Publication date: Available online 29 June 2018Source: Combustion and FlameAuthor(s): 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, and measurements are made for various selected values of ϕl, in the other set ϕl=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. 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
  • Investigating oxidation growth routes in the flame synthesis of
           tungsten-oxide nanowires from tungsten substrates
    • Abstract: Publication date: Available online 29 June 2018Source: Combustion and FlameAuthor(s): 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.
  • 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.
  • Dynamic adaptive combustion modeling of spray flames based on chemical
           explosive mode analysis
    • Abstract: Publication date: Available online 4 June 2018Source: Combustion and FlameAuthor(s): 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.
  • The effects of particle size and reducing-to-oxidizing environment on coal
           stream ignition
    • Abstract: Publication date: Available online 1 June 2018Source: Combustion and FlameAuthor(s): 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.
  • A detonation paradox: Why inviscid detonation simulations predict the
           incorrect trend for the role of instability in gaseous cellular
    • Abstract: Publication date: Available online 1 June 2018Source: Combustion and FlameAuthor(s): 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.
  • Ember: An open-source, transient solver for 1D reacting flow using large
           kinetic models, applied to strained extinction
    • Abstract: Publication date: Available online 24 May 2018Source: Combustion and FlameAuthor(s): 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 
  • Theory and modeling of relevance to prompt-NO formation at high pressure
    • Abstract: Publication date: Available online 21 May 2018Source: Combustion and FlameAuthor(s): 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).
  • A combined laser absorption and gas chromatography sampling diagnostic for
           speciation in a shock tube
    • Abstract: Publication date: Available online 18 May 2018Source: Combustion and FlameAuthor(s): 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.
  • A systematic approach to high-fidelity modeling and efficient simulation
           of supercritical fluid mixing and combustion
    • Abstract: Publication date: Available online 17 May 2018Source: Combustion and FlameAuthor(s): Xingjian Wang, Hongfa Huo, Umesh Unnikrishnan, Vigor Yang Advances in fluid-flow modeling and simulation techniques over the past two decades have improved understanding of the intricate flow physics and combustion dynamics in the supercritical regime. However, there remain many numerical issues to be addressed, including turbulence closure modeling, combustion modeling, and the evaluation of real-fluid thermodynamic and transport properties. The challenges can be broadly categorized into two areas: (1) achieving highly accurate simulation through inclusion of all the necessary physics and (2) developing a computationally efficient framework to achieve simulation results in a reasonable turnaround time. This paper investigates these challenges and presents a systematic approach to achieve high-fidelity and efficient simulation of supercritical fluid mixing and combustion using large-eddy simulation (LES) techniques. The unresolved subgrid-scale (SGS) term in the filtered equation of state (EOS), which is generally neglected for ideal gases, becomes significant for real fluids, especially in regions of strong property gradients at supercritical conditions. The relative error for the filtered density can reach up to 40%, and this uncertainty can propagate and contaminate calculations of the conservation equations. Two closure models for the SGS term in the EOS are proposed: a gradient-based and a mixing-based approach. Both approaches reduce the modeling error considerably. Flamelet-based combustion models are also examined at supercritical conditions. The probability density functions (PDFs) for mixture fraction and scalar dissipation rate are evaluated using a data-driven approach. The presumed beta-function distribution accurately describes the PDF of the mixture fraction at low mixture fraction variance, but deviates at high variance (> 0.01). The lognormal distribution can capture the shape of the extracted PDF of the scalar dissipation rate but underestimates the peak value. An alternative combustion model using finite-rate chemistry integrated with dynamic adaptive chemistry and correlated transport is developed, rendering a computationally efficient and affordable framework. The efficiency of evaluating real-fluid thermodynamic and transport properties, a computationally expensive procedure, is dramatically enhanced using tabulation and correlated dynamic evaluation techniques. Finally, suggestions are provided regarding opportunities for future research.
  • Tuning the morphological, ignition and combustion properties of
           micron-Al/CuO thermites through different synthesis approaches
    • Abstract: Publication date: Available online 16 May 2018Source: Combustion and FlameAuthor(s): 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.
  • Autoignited lifted flames of dimethyl ether in heated coflow air
    • Abstract: Publication date: Available online 16 May 2018Source: Combustion and FlameAuthor(s): 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.
  • A computational analysis of methanol autoignition enhancement by dimethyl
           ether addition in a counterflow mixing layer
    • Abstract: Publication date: Available online 16 May 2018Source: Combustion and FlameAuthor(s): 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.
  • Propagation and extinction of subatmospheric counterflow methane flames
    • Abstract: Publication date: Available online 27 April 2018Source: Combustion and FlameAuthor(s): 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.
  • Soot formation in counterflow non-premixed ethylene flames at elevated
    • Abstract: Publication date: Available online 21 April 2018Source: Combustion and FlameAuthor(s): 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.
  • Nonlinear development of hydrodynamically-unstable flames in
           three-dimensional laminar flows
    • Abstract: Publication date: Available online 16 April 2018Source: Combustion and FlameAuthor(s): 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.
  • Sooting limits of non-premixed counterflow ethylene/oxygen/inert flames
           using LII: Effects of flow strain rate and pressure (up to 30 atm)
    • Abstract: Publication date: Available online 14 April 2018Source: Combustion and FlameAuthor(s): 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.
  • A new chemical kinetic method of determining RON and MON values for single
           component and multicomponent mixtures of engine fuels
    • Abstract: Publication date: Available online 14 April 2018Source: Combustion and FlameAuthor(s): 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.
  • Low-temperature multistage warm diffusion flames
    • Abstract: Publication date: Available online 11 April 2018Source: Combustion and FlameAuthor(s): 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.
  • The impacts of three flamelet burning regimes in nonlinear combustion
    • Abstract: Publication date: Available online 10 April 2018Source: Combustion and FlameAuthor(s): 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.
  • Combustion of Mg and composite Mg·S powders in different oxidizers
    • Abstract: Publication date: Available online 10 April 2018Source: Combustion and FlameAuthor(s): 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.
  • Deflagration-to-detonation transition in an unconfined space
    • Abstract: Publication date: Available online 3 April 2018Source: Combustion and FlameAuthor(s): 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.
  • Direct numerical simulation of a temporally evolving air/n-dodecane jet at
           low-temperature diesel-relevant conditions
    • Abstract: Publication date: Available online 28 March 2018Source: Combustion and FlameAuthor(s): 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.
  • Critical kinetic uncertainties in modeling hydrogen/carbon monoxide,
           methane, methanol, formaldehyde, and ethylene combustion
    • Abstract: Publication date: Available online 2 March 2018Source: Combustion and FlameAuthor(s): 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.
  • Unsteady droplet combustion with fuel thermal expansion
    • Abstract: Publication date: Available online 2 March 2018Source: Combustion and FlameAuthor(s): 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.
  • Role of induced axial acoustics in transverse acoustic flame response
    • Abstract: Publication date: Available online 6 February 2018Source: Combustion and FlameAuthor(s): 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.
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