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Combustion and Flame
Journal Prestige (SJR): 2.427
Citation Impact (citeScore): 5
Number of Followers: 135  
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ISSN (Print) 0010-2180
Published by Elsevier Homepage  [3157 journals]
  • Understanding the formation and growth of polycyclic aromatic hydrocarbons
           (PAHs) and young soot from n-dodecane in a sooting laminar coflow
           diffusion flame
    • Abstract: Publication date: April 2019Source: Combustion and Flame, Volume 202Author(s): Tirthankar Mitra, Tongfeng Zhang, Anton D. Sediako, Murray J. Thomson The mechanistic pathways of PAH formation and growth remain uncertain. In addition, our current understanding of the transformation of PAHs into young soot is limited. The PAHs participating in this transformation remain ambiguous. Simultaneous measurements of PAHs and particles are necessary to better understand how particulates are formed in flames. A comprehensive analysis has been performed on a n-dodecane doped methane coflow diffusion flame. PAHs have been analyzed with GC/MS while the particulates have been collected from the flame centreline for studying under a Transmission Electron Microscope (TEM). The soot measurements from a previous study have been used to complete the comprehensive analysis. The novelty of the current study lies in the fact that it provides quantitative information on the growth of PAHs ranging from naphthalene (A2) to pyrene and fluoranthene (together they are referred to as A4 in this study) and how these species participate in the formation, growth and aging of the young soot. The experimental results show that before the commencement of the young soot, the gaseous phase is dominated by species equal to and smaller than A4. PAHs larger than A4 were not detected. With the growth of the young soot, the mole fraction of A4 decreases. The results suggest that small and medium-sized PAHs (A4 and smaller) are responsible for the formation and growth of the young soot. As the young soot transform into mature solid soot, A4 increases abruptly. The study shows that PAH growth and soot maturity are not mutually exclusive. The target flame has also been simulated numerically to identify the problems associated with the state-of-the-art model. The numerical model with irreversible nucleation can not capture the formation of young soot even though it considers nucleation from A4. A comprehensive database of PAHs and soot has been created for future numerical model validations.
  • An experimental study of the detailed flame transport in a SI engine using
           simultaneous dual-plane OH-LIF and stereoscopic PIV
    • Abstract: Publication date: April 2019Source: Combustion and Flame, Volume 202Author(s): Brian Peterson, Elias Baum, Andreas Dreizler, Benjamin Böhm Understanding the detailed flame transport in IC engines is important to accurately predict ignition and combustion phasing, rate of heat release and assess engine performance. This is particularly important for RANS and LES engine simulations, which often struggle to accurately predict flame propagation and heat release without first adjusting model parameters. Detailed measurements of flame transport in technical systems are required to guide model development and validation.This work introduces an experimental dataset designed to study the detailed flame transport and flame/flow dynamics for spark-ignition engines. Simultaneous dual-plane OH-LIF and stereoscopic PIV are used to acquire 3D measurements of unburnt gas velocity (U⇀gas), flame displacement speed (SD) and overall flame front velocity (U⇀Flame) during the early flame development. Experiments are performed in an optical engine operating at 800 and 1500 RPM with premixed, stoichiometric isooctane-air mixtures. Analysis reveals several distinctive flame/flow configurations that yield a positive or negative flame displacement for which the flame progresses towards the reactants or products, respectively. For the operating conditions utilized, SD exhibits and inverse relationship with flame curvature and a strong correlation between negative SD and convex flame contours is observed. Trends are consistent with thermo-diffusive flames, but have not been quantified in context of IC engines. Flame wrinkling is more severe at the higher RPM, which results in a broader SD distribution towards higher positive and negative velocities. Spatially-resolved distributions of U⇀gas and SD are presented to describe in-cylinder locations where either convection or thermal diffusion is the dominating mechanism contributing to flame transport. Findings are discussed in relation to common engine flow features, including flame transport near solid surfaces. Findings provide a first insight into the detailed flame transport within a technically relevant environment and are designed to support the development and validation of engine simulations.
  • Understanding strong knocking mechanism through high-strength optical
           rapid compression machines
    • Abstract: Publication date: April 2019Source: Combustion and Flame, Volume 202Author(s): Jiaying Pan, Zhen Hu, Haiqiao Wei, Mingzhang Pan, Xingyu Liang, Gequn Shu, Lei Zhou Strong knocking combustion has become the greatest challenge for advanced internal combustion engines to pursue thermal efficiency limits at high power density conditions. Arising from enclosed space and extreme combustion situations, the fundamental mechanism for strong knocking combustion has still not been fully understood. In this study, synchronization measurement was performed through simultaneous pressure acquisition and high-speed direct photography, and knocking experiments were comparatively conducted under spark-ignition (SI) and compression-ignition (CI) conditions in a high-strength optical rapid compression machine (RCM) with flat piston design. Strong knocking phenomena were reproduced through varying initial thermodynamic conditions, and localized autoignition (AI) initiation and reaction wave evolutions were visualized, companied by synchronous pressure and temperature trajectories. The results show that compared with initial temperature, initial pressure and equivalence ratio exhibit greater influence on the variations of knocking severity. The weighting of different contributors can be further quantified by an effective energy density that shows positive but nonlinear correlations with knocking severity. However, the distinctions between CI and SI knocking characteristics at identical effective energy density also reflect the essential role of the interplay between primary flame propagation and end-gas AI progress. Visualized combustion images show that through improving end-gas thermodynamic state and reactivity sensitivity, the primary flame propagation can enhance localized AI initiation and secondary intensive AI evolutions, facilitating combustion mode transitions into developing detonation. The significant influence of primary flame propagation is diminished until ignition delay time becomes sufficiently short. Finally, with estimated thermal heterogeneities in flat-piston RCM configurations, the ignition modes of strong knocking cycles are quantified by a non-dimensional ignition regime diagram, and favorable scaling agreements with strong and mixed ignition regimes are observed.
  • Experimental and kinetic modeling investigation on anisole pyrolysis:
           Implications on phenoxy and cyclopentadienyl chemistry
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Wenhao Yuan, Tianyu Li, Yuyang Li, Meirong Zeng, Yan Zhang, Jiabiao Zou, Chuangchuang Cao, Wei Li, Jiuzhong Yang, Fei Qi In this work, the flow reactor pyrolysis of anisole was studied at pressures of 0.04 and 1 atm and temperatures from 850 to 1160 K. Comprehensive speciation was achieved using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A detailed kinetic model for anisole combustion was developed and validated against experimental results in the present work. Fuel decomposition and aromatics formation processes were investigated based on modeling analyses. The results show that the dominant decomposition pathway of anisole is the unimolecular OCH3 bond dissociation reaction at both pressures, while the role of bimolecular reactions becomes significant at 1 atm. At lower temperatures, phenoxy radical is mainly consumed via the reactions with methyl radical, producing methylcyclohexadienone. At higher temperatures, it is mainly consumed via the unimolecular decomposition reaction producing cyclopentadienyl. Cyclopentadienyl is responsible for the abundant production of aromatic products such as benzene, toluene, styrene and naphthalene. Furthermore, the bimolecular reactions of anisole also contribute to the formation of aromatic products at lower temperatures. Possible formation pathways of oxygenated aromatics such as benzofuran and dibenzofuran were also analyzed in this work. The present model was also validated against literature experimental data of anisole combustion, including global combustion parameters like ignition delay times and speciation profiles in flow reactor pyrolysis and jet stirred reactor pyrolysis and oxidation.
  • Flame propagation in nano-aluminum–water (nAl–H2O) mixtures: The role
           of thermal interface resistance
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Murali Gopal Muraleedharan, Umesh Unnikrishnan, Asegun Henry, Vigor Yang A detailed numerical analysis of flame propagation in nano-aluminum–water (nAl–H2O) mixture is performed. Emphasis is placed on investigating the role of particle thermal conductivity in the prediction of the burning properties of the mixture. Flame structure and burning characteristics are obtained by solving the energy equation using finite difference discretization and the Gauss–Seidel iteration method. Particle thermal conductivity is modeled using the temperature-dependent thermal conductivities of the aluminum core and oxide layer, as well as their interface resistance. The effective thermal conductivity of the mixture is modeled as a function of temperature, spatial coordinate, and local mixture composition, by means of the unified Maxwell–Eucken–Bruggeman model, accounting for random particle distribution and inter-particle interaction. Results indicate that the combined thermal resistance offered by the oxide layer and the interface constitute 95% of the total resistance of the particle. The calculated particle-size dependent linear burning rates show good agreement with experimental data, with only 5% error. Error in burning rate prediction increases, however, to 20% when interface resistance is excluded from the particle thermal conductivity model. It was also observed that burning rate varies as the inverse of particle size. Finally, an analysis of the sensitivity of burning rate to the individual components of the particle thermal conductivity model is also performed. Results suggest a 30% decrease in burning rate for two orders of magnitude reduction in both interface conductance and oxide thermal conductivity. The burning rate drops by only 15%, however, for a similar reduction in aluminum thermal conductivity. A heat conduction perspective on flame propagation in nanocomposites is presented, identifying the highest and the lowest conductive pathways for energy transport.
  • New form for reduced modeling of soot oxidation: Accounting for multi-site
           kinetics and surface reactivity
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Michael Frenklach New formulation is introduced to model surface oxidation of soot particles. In the new development, the surface is represented by an arbitrary number of reactive sites and their physically-founded transformations. The latter are combined and integrated with gas-phase and particle-dynamics models. The surface reaction model defines two state properties and establishes a structural relationship between them that guides evolution of the surface. This new model form for the surface-chemistry led to close reproduction of shock-initiated oxidation of soot: CO profiles in two experiments performed at substantially different temperatures, 1990 and 2780 K, as well as CO production rates over a wide range of temperatures, 1652–3130 K, all without employing the parameter-α empiricism of the previous model formulation.
  • Multi-stage oxidation of a CH2F2/air mixture examined by weak flames in a
           micro flow reactor with a controlled temperature profile
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Shintaro Takahashi, Hisashi Nakamura, Takuya Tezuka, Susumu Hasegawa, Kaoru Maruta Oxidation of CH2F2 (R32), a widely used refrigerant, was investigated experimentally and computationally using a micro flow reactor with a controlled temperature profile (MFR). Specifically, we examined weak flames of a stoichiometric CH2F2/air mixture in MFR observed at low flow velocities. In the case of the maximum wall temperature of 1300 K, weak flames showed two luminous regions at 1240 and 1290 K, implying two-stage oxidation. Computations also indicated two-stage heat release. Computational weak flame positions also showed good agreement with experimentally obtained results. To assess the validity of chemical kinetics in detail, species measurements of CH2F2, CO, CO2, H2O, HF, and CF2O for a stoichiometric CH2F2/air weak flame were conducted using Fourier Transform Infrared spectrometry (FTIR). The present species measurements elucidated the CH2F2/air weak flame structure at 800–1300 K and remaining intermediates of H2O and CF2O at the end of the reactor. Computational results also showed good agreements with measurement, indicating the validity of the computations using the chemical kinetics. For further analysis for CH2F2 weak flame at 1300 K, the reaction path and rate-of-production analyses revealed a major reaction at the first-stage heat release: the formation of HF and intermediates, and that at the second stage is the CO2 formation from CO. Computations also showed intermediates of H2O and CF2O remain at the end of the reactor. An additional numerical experiment for a CH2F2/air mixture at the maximum wall temperature of 2000 K, demonstrated that the reaction of H2O and CF2O proceeded slowly at temperatures higher than 1300 K, with complete oxidation of CH2F2/air mixture. In conclusion, results show that the overall reactivity of CH2F2 decreases because of the low reactivity intermediates such as CF2O and H2O if oxidation occurs in a non-adiabatic system such as MFR. Such specific characteristics of the CH2F2/air mixture are concealed in the common zero-dimensional adiabatic simulations. Results demonstrated that the CH2F2 reactivity depend strongly on the temperature history during oxidation.
  • Dynamic simulation of ignition, combustion, and extinguishment processes
           of HMX/GAP solid propellant in rocket motor using moving boundary approach
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Gun Hee Lee, Min Young Jung, Ji Chang Yoo, Byoung Sun Min, Hong Min Shim, Min Oh Solid propellant burning rate, gas phase temperature, and condensed phase thickness depend on combustion chamber pressure, laser intensity, and propellant compositions. The ignition and combustion of solid propellants occur in three phases namely solid phase, condensed phase, and gas phase. In this study, moving boundary modeling was applied to each of the phases by coordinate transformation. This research includes modeling and dynamic simulation of the ignition and combustion of HMX/GAP, a high-energy material in the ratio of 8:2, with the gas phase of the combustion model consisting of 50 species and 234 reactions. The mathematical modeling used mass, energy, and momentum conservation equations, as well as constitutive equations for the moving boundary. Parametric studies were run under different operating conditions, with an initial temperature of 300 K, a pressure of 10–100 atm and a laser intensity of 100 W/cm2. A burning rate of 2.2 cm/s and a gas phase temperature of 2700 K were obtained under an operating pressure of 100 atm. Extinguishment of the solid propellant was rigorously analyzed in terms of dynamic simulation with various depressurization rates during combustion. This was carried out by depressurizing the solid propellant from 70 atm to 40 atm. Important factors of the extinguishment were discussed based on the mathematical model and various depressurization rates with parametric studies. At a depressurization rate of −8000 atm/s, the solid propellant was fully extinguished. From this study, one can identify the phenomenon for the extinguishment of HMX/GAP propellant using fast depressurization with rigorous mathematical model used for ignition and combustion.
  • Effects of bluff bodies on the propagation behaviors of gaseous detonation
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Lu-Qing Wang, Hong-Hao Ma, Zhao-Wu Shen, Jun Pan In this study, we report an experimental investigation of detonation propagation in a tube filled with bluff bodies. Experiments were carried out in a 6 m long rectangular cross-section (112 mm  ×  107 mm) tube. Firstly, the effect of an array of cube bodies (60 mm  ×  60 mm  ×  60 mm) on the detonation propagation characteristics were studied. Hydrogen, ethylene and acetylene mixed with air and hydrogen-oxygen diluted with argon were used as the test mixtures. Evenly spaced photodiodes were mounted on the top wall to recorded the optical signals, from which the detonation velocity could be determined. Soot foils were adopted in hydrogen-oxygen-argon mixture to record the evolution of the cellular structure. The results show that the flame accelerates rapidly in the obstructed tube. The critical conditions for deflagration to detonation transition (DDT) are found to be consistent with L/λ > 7, where L is the modified characteristic geometrical size for the tube with repeated cubes and λ is the detonation cell size. The long soot foils indicate that the detonation modes are influenced significantly by the bluff bodies. Near the limits, only some traces of transverse waves and shock waves can be observed, making the distinction between choked flame and quasi-detonation blurred. Secondly, the effect of a single bluff body on the detonation diffraction was investigated. Stoichiometric hydrogen-oxygen and those diluted with argon at sub-atmospheric pressures were used as the test mixtures. In the vicinity of the bluff body, a soot foil was used to record the detonation cellular structure evolution, from which the effect of the geometry of the bluff body on the diffraction and re-initiation processes was studied. Three transition regimes are observed: (1) a detonation propagates continuously over the bluff body; (2) the cellular structure initially fails and then recover due to the two symmetrical diffraction waves collision or interaction between shock waves and the bottom wall; (3) a detonation quenches and then re-initiate by flame acceleration. The re-initiation distance was found to be dependent on mixture sensitivity (chemical length scale) and the geometry of the bluff bodies (physical length scale).
  • On the minimum ignition energy and its transition in the localised forced
           ignition of turbulent homogeneous mixtures
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Charles Turquand d’Auzay, Vassilios Papapostolou, Samer F. Ahmed, Nilanjan Chakraborty The minimum energy requirements for ensuring (i) just successful ignition and (ii) successful self-sustained flame propagation without the assistance of an external energy source following a successful ignition event have been analysed for the forced ignition of a homogeneous stoichiometric methane–air mixture under a wide range of turbulence intensities using three-dimensional Direct Numerical Simulation (DNS) data. It has been found that the minimum energy needed for successful ignition is also sufficient to ensure self-sustained flame propagation for small turbulence intensities. However, for large turbulence intensities, the minimum energy for ensuring self-sustained flame propagation can be considerably greater than the minimum energy needed just to successfully ignite the mixture. At low turbulence intensities, the thermal runaway has been obtained for the minimum ignition energy after the end of the energy deposition indicating an autoignition. For larger energy inputs and turbulence intensity, the thermal runaway was obtained during the energy deposition period. It has been found that the minimum energy requirements for ignition and self-sustained flame propagation increase with increasing turbulence intensity but a transition in this behaviour has been observed. There is a critical turbulence intensity such that the increase in the energy demand is significantly more rapid above the critical value than that for turbulence intensities smaller than the critical value. This has been found to be qualitatively consistent with previous experimental findings. The stochastic nature of the ignition event has been demonstrated by considering different realisations of statistically similar turbulent flow fields. The conditions giving rise to a successful ignition have been identified by a detailed analysis of the energy budget. A scaling analysis has been performed for the critical condition for ensuring self-sustained flame propagation and the insights gained from this analysis have been utilised to explain the physical mechanisms behind the transition of the minimum ignition energy.
  • Limit map of pulsating instability in hydrogen/air partially premixed
           counterflow flames
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Tianqi Li, Huiqiang Zhang, Fan Yang Pulsating instabilities in hydrogen/air double-flame partially premixed counterflow flames are numerically investigated with detailed chemistry and transport. The whole stability map on the equivalence ratio-strain rate plane is obtained. With the increase of the equivalence ratio or strain rate, there are two transitions for combustion patterns from stable to unstable, and to stable again. For the one transition presented at the smaller equivalence ratio and strain rate, it is similar to that in the pure premixed flame. After this transition, the flames do not extinguish with the increase of equivalence ratio or strain rate, but transit from unstable to stable. For the second transition appeared at the larger equivalence ratio and strain rate, the effects of equivalence ratio and strain rate on the pulsating instability are completely contrary to those in pure premixed flames. The effective activation energy of the premixed flame of partially premixed flame predicted by a new method is applied to calculate the Zeldovich number. Then the first transition is proved to satisfy Sivashinsky-like criterion, but the critical value is larger than that for pure premixed flame due to the heat transfer between premixed and non-premixed flames. It means that the pulsating instability is more difficult to happen in partially premixed flames. From the first transition to the second transition along the equivalence ratio or strain rate, though the premixed flame becomes weaker, the increase degree of heat transfer from non-premixed flame to premixed flame is much larger than the decrease degree of max heat release of the premixed flame due to decrease of distance and increase of temperature difference of two flames, which induces the premixed flame stable again. The second transition is therefore controlled by the heat transfer between two flames.
  • Comparison study of the ignition and combustion characteristics of
           directly-written Al/PVDF, Al/Viton and Al/THV composites
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Haiyang Wang, Miles Rehwoldt, Dylan J. Kline, Tao Wu, Peng Wang, Michael R. Zachariah The aluminum–fluorine reaction is attracting growing interest due to its higher density over aluminum–oxygen. Fluorine rich polymers are particularly interesting for their applications as an energetic binder in advanced additive manufacturing of energetic materials. In this paper, three soluble polymers of PVDF (59 wt% F), Viton (66 wt% F) and THV (73 wt% F) are incorporated with aluminum nanoparticles (Al NPs) and prepared as free-standing films using solvent-based direct writing. The three composite films are compared for their mechanical properties as well as the ignition and combustion performance. Tensile stress was found to order as Al/PVDF > Al/THV > Al/Viton while the elasticity of Al/Viton is much higher than the other two. The burn rate of different composite films increases with Al content, while the flame temperature peaks slightly fuel-rich. The Al/PVDF had the highest burn rate, however, the flame temperature ordered as Al/THV (∼2500 K) > Al/Viton (∼2000 K) > Al/PVDF (∼1500 K), consistent with fluorine content. With higher fluorine and lower hydrogen content, THV releases more CFx gas than HF, which generates higher temperature. However, HF which is predominantly produced from PVDF has the lowest ignition by far and may be responsible for its high flame speed.
  • An experimental study on the burning rates of n-heptane pool fires with
           various lip heights in cross flow
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Chen Kuang, Longhua Hu, Xiaolei Zhang, Yujie Lin, Larry W. Kostiuk This paper presents an experimental investigation into the burning rates of pool fires with different lip heights in cross flows for which data are not previously reported. Square pool fires with side lengths (D) of 5, 10, 15 and 20 cm and dimensionless lip heights (h⁎ = h/D) of 0.25, 0.5, 1.0, 1.5 and 2.0 were tested burning n-heptane using a system that maintains the fuel level. The mass burning rates were measured with cross flow air speeds of 0–∼3.0 m/s. Previous works have mostly been concerned with pool fires with similar lip heights (h⁎) in still air for relative smaller pools, or in cross flows with very small lip heights (i.e., h⁎ ≤ 0.1). It was found that the burning rate behavior differs with pool size and h⁎. For small pools (D = 5 and 10 cm), the burning rate increased monotonically with cross flow air speed when h⁎ was small, but first decreased then increased with large h⁎. For larger pools (D = 15 and 20 cm), the burning rate first increased, then decreased, followed by a final increasing trend with small h⁎; while for larger h⁎, the burning rate first decreased and then increased with increasing cross flow air speed. These different trends were discussed in terms of the considerable change in the distance from the flame to the fuel surface, which affected the conduction, convection and radiation feedbacks to the fuel. It was observed that the flame was “pushed down” into the fuel-containing vessels by relatively strong cross flows, which further changed the heat feedback. A burning rate correlation was proposed to characterize the combined effects of cross flow air speed and lip height.
  • A conceptual model of the flame stabilization mechanisms for a lifted
           Diesel-type flame based on direct numerical simulation and experiments
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Fabien Tagliante, Thierry Poinsot, Lyle M. Pickett, Perrine Pepiot, Louis-Marie Malbec, Gilles Bruneaux, Christian Angelberger This work presents an analysis of the stabilization of diffusion flames created by the injection of fuel into hot air, as found in Diesel engines. It is based on experimental observations and uses a dedicated Direct Numerical Simulation (DNS) approach to construct a numerical setup, which reproduces the ignition features obtained experimentally. The resulting DNS data are then used to classify and analyze the events that allow the flame to stabilize at a certain Lift-Off Length (LOL) from the fuel injector. Both DNS and experiments reveal that this stabilization is intermittent: flame elements first auto-ignite before being convected downstream until another sudden auto-ignition event occurs closer to the fuel injector. The flame topologies associated to such events are discussed in detail using the DNS results, and a conceptual model summarizing the observation made is proposed. Results show that the main flame stabilization mechanism is auto-ignition. However, multiple reaction zone topologies, such as triple flames, are also observed at the periphery of the fuel jet helping the flame to stabilize by filling high-temperature burnt gases reservoirs localized at the periphery, which trigger auto-ignitions.
  • Experimental and modeling study of the pyrolysis and oxidation of an
           iso-paraffinic alcohol-to-jet fuel
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Juan Guzman, Goutham Kukkadapu, Kenneth Brezinsky, Charles Westbrook Single pulse shock tube experiments were conducted to determine the pyrolytic and oxidative decomposition products of an alcohol-to-jet fuel (ATJ) composed primarily of about 90% iso-dodecane. Experiments were performed at 4 bar nominal pressure, 2 ms nominal reaction time, and temperatures ranging from 900 K to 1550 K. For oxidation, the equivalence ratio was 0.27. Gas chromatography (GC) was used to analyze the products of the chemical reactions and to determine their mole fractions. The experimental results were compared against mole fractions computed using a detailed iso-alkane kinetic model, developed for the present study. The model was able to accurately reproduce the experimental results of both ATJ pyrolysis and oxidation. The experimental data and the kinetic model developed in the current study shall aid in progress in understanding the oxidation of the alternative jet fuels.
  • Delineating and explaining the limits of self-sustained smouldering
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Marco A.B. Zanoni, José L. Torero, Jason I. Gerhard Self-sustained, forward smouldering combustion is both a major fire hazard that resists engineering control and an applied technology for destroying organic contaminants and wastes. In both contexts, success depends on understanding the threshold between self-sustaining and extinction conditions as well as the system's sensitivity to parameters that drive it in the desired direction. In this work, a previously validated one-dimensional numerical model was employed to simulate a wide range of bitumen-contaminated sand scenarios, quantifying the complex interplay between chemical reactions and heat transfer processes evolving in space and time during smouldering. It was confirmed that the traditional, local (smoulder front) energy balance becomes negative when the reaction is extinct. However, the work reveals that a global (bed) energy balance always becomes negative earlier, predicting extinction conditions despite active smouldering. Progress towards extinction looks similar for all cases, regardless if caused by low air flux, low fuel concentration, very low oxygen content, low fuel energy content, or high heat losses. Moreover, the cause is always similar: the energy gained by oxidation is exceeded by global heat losses, which are significant and neglected in the local energy analysis. Smouldering robustness was shown to be quantified by the degree to which the global energy balance exceeds zero. Robustness was promoted most effectively by increases in injected air flux, fuel concentration, and fuel energy content. These parameters also were the dominant influence on the peak temperature, since that was shown to be dependent on the local net energy rate. However, the front velocity (i.e., mass destruction rate) was shown to depend on the rate that energy convectively exits the front, which was controlled primarily by the air flux. Taken as a whole, these results provide a new way of understanding the balance of energy components that dictate the behavior of smouldering systems and provide novel insights into manipulating them, towards extinction or robust conditions, for a wide range of applications.
  • Modelling and numerical simulation of n-heptane pyrolysis coking
           characteristics in a millimetre-sized tube reactor
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Xiaohan Wang, Qianshi Song, Yong Wu, Xing Li, Tao Li, Xiaojun Zeng Based on the optimization of the recently proposed gas phase mechanism for n-heptane pyrolysis, mechanisms for polycyclic aromatic hydrocarbon (PAH) formation and a detailed wall surface reaction, a basic gaseous pyrolysis mechanism with 1127 steps and a simplified surface reaction mechanism with 34 steps were developed, respectively. After performing the detailed estimation and modelling procedures for the surface site density (SDEN) related to the particle diameter, particle density, C/H ratio and other parameters determined by the experimental tests and observations, numerical studies were integrated with the developed coke model to simulate the pyrolysis process and the coking properties in a millimetre-sized tube reactor on the Chemkin Pro software platform using a two-dimensional cylindrical shear flow (CSF) module. The prediction values of the pyrolysis product concentration and coking rate at different temperatures were compared with the experimental data, and agreement was obtained. Under pyrolysis conditions of an n-heptane inlet flow rate of 0.5 ml/min, an operating pressure of 1.0 MPa and a furnace temperature of 973–1073 K, the research results showed that the hydrogen abstraction reaction, as the first and key step of coke formation, is easily affected by gaseous diffusion. As the temperature and particle diameter increase, the inhibition of diffusion becomes stronger. Among all the considered radicals, the abstraction ability of the H radical is the strongest, and it is easily restricted by diffusion. The presence of abundant radicals such as CH3, C2H3, and C3H5 at low temperatures can compensate for this limitation. For the unsaturated species addition reactions, the calculation results showed that the addition of olefins, alkynes, diolefins and aromatic hydrocarbons occurs in sequence with an increase in residence time. With increasing temperature, the addition contributions of olefins such as C2H4 and C3H6 gradually weaken, while those of small molecule aromatic hydrocarbons such as benzene (A1) and styrene (A1C2H3) remarkably increase. Alkynes and diolefins, such as C2H2 and C3H4, have low concentrations under the experimental conditions, and their addition contributions are very low and can be ignored. Due to the addition of unsaturated species, the concentration of H2 in the gas phase is significantly improved. The modeling work can predict the coke formation characteristics and quantitatively evaluate the change in the coke formation rate, which is helpful for explaining the intrinsic thermal pyrolysis coking mechanism in a round tube, such as the regenerative cooling system of supersonic engines, among others.
  • On the influence of distance between two jets on flickering diffusion
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Liu Changchun, Liu Xinlei, Ge Hong, Deng Jun, Zhou Shasha, Wang Xueyao, Cheng Fangming The interactions of multiple flames are often encountered in real-world fire and industrial burners. The distance between two jets has a strong influence on the instability mode, oscillation frequency, and mean height of flickering diffusion flames, which is experimentally studied and analyzed in this paper. Five different types of instability modes are identified as the separation distance between two nozzles is increased. When the nozzle separation distance is smaller, the flame mode is similar to that of a single-nozzle flame. In this case, the flame can switch between the merged sinuous mode and the merged varicose mode due to external disturbances on the flame. As the nozzle separation distance increases, the probability of mode switching from merged varicose to merged sinuous decreases. As the nozzle separation distance increases further, the flame mode translates into symmetric sinuous mode, alternated sinuous or independent mode. In addition, the flame height and oscillation frequency of a dual-nozzle flame have their own characteristics, which are different from that of the single-nozzle flame. The increase of the nozzle separation distance, leads to a decrease and then an increase of the flame mean height. It is interesting that the flame mean height of the alternated sinuous flame is the lowest, even lower than that of the single-nozzle flame. The oscillation frequency of the symmetrical sinuous flame is lower than that of the single-nozzle flame, and the frequency of alternated sinuous flame is higher than that of the single-nozzle flame.
  • Investigation of OH* chemiluminescence and heat release in laminar
           methane–oxygen co-flow diffusion flames
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Lei He, Qinghua Guo, Yan Gong, Fuchen Wang, Guangsuo Yu OH* chemiluminescence is one of the major spontaneous emission in flames, and often applied in combustion diagnostics to indicate flame structure, strain rate, equivalence ratio, heat release rate, etc. In this work, OH* chemiluminescence in the laminar methane–oxygen co-flow diffusion flames was investigated. A high resolution ultra-violet imaging system was used to capture the OH* chemiluminescence images. Numerical simulations of the experimental cases were performed based on OH* chemiluminescence reaction mechanism. The numerical results show good agreement with the experimental measurements. It's found that there are two OH* distribution zones in laminar methane–oxygen co-flow diffusion flames. Analysis on the production pathway of OH* chemiluminescence shows that the reaction H + O + M = OH* + M (R1) is the major formation reaction of OH* chemiluminescence in laminar methane–oxygen diffusion flames. The increase of diluent addition in oxidizer will lead to the dominant OH* production pathway changing from the reaction R1 to the reaction CH + O2 = OH* + CO (R2). The OH* distribution characteristics under different global oxygen-fuel equivalence ratios indicate that OH* chemiluminescence can be employed as an appropriate indicator to characterize the combustion condition. Moreover, the correlation between integrated heat release rate and integrated OH* concentration is derived for the oxygen-deficient flames. The integrated heat release rate can be predicted in terms of integrated OH* concentration, methane flow rate and global oxygen-fuel equivalence ratio.
  • Experimental analysis of oscillatory premixed flames in a Hele-Shaw cell
           propagating towards a closed end
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Fernando Veiga-López, Daniel Martínez-Ruiz, Eduardo Fernández-Tarrazo, Mario Sánchez-Sanz An experimental study of methane, propane and dimethyl ether (DME) premixed flames propagating in a quasi-two-dimensional Hele-Shaw cell placed horizontally is presented in this paper. The flames are ignited at the open end of the combustion chamber and propagate towards the closed end. Our experiments revealed two distinct propagation regimes depending on the equivalence ratio of the mixture as a consequence of the coupling between the heat-release rate and the acoustic waves. The primary acoustic instability induces a small-amplitude, of around 8 mm, oscillatory motion across the chamber that is observed for lean propane, lean DME, and rich methane flames. Eventually, a secondary acoustic instability emerges for sufficiently rich (lean) propane and DME (methane) flames, inducing large-amplitude oscillations in the direction of propagation of the flame. The amplitude of these oscillations can be as large as 30 mm and drastically changes the outline of the flame. The front then forms pulsating finger-shaped structures that characterize the flame propagation under the secondary acoustic instability.The experimental setup allows the recording of the flame propagation from two different points of view. The top view is used to obtain accurate quantitative information about the flame propagation, while the lateral view offered a novel three dimensional perspective of the flame that gives relevant information on the transition between the two oscillatory regimes.The influence of the geometry of the Hele-Shaw cell and of the equivalence ratio on the transition between the two acoustic-instability regimes is analyzed. In particular, we find that the transition to the secondary instability occurs for values of the equivalence ratio ϕ above (below) a critical value ϕc for propane and DME (methane) flames. In all the tested fuels, the transition to the secondary instability emerges for values of the Markstein number M below a critical value Mc. The critical Markstein number varies with the gap size h formed by the two horizontal plates that bound the Hele-Shaw cell. As h is reduced, the critical Markstein number is shifted towards larger values.
  • Cascaded group-additivity ONIOM: A new method to approach CCSD(T)/CBS
           energies of large aliphatic hydrocarbons
    • Abstract: Publication date: March 2019Source: Combustion and Flame, Volume 201Author(s): Junjun Wu, Hongbo Ning, Liuhao Ma, Peng Zhang, Wei Ren We report a cascaded group-additivity (CGA) ONIOM method for high-level energy calculations of large aliphatic hydrocarbon molecules by combining the group additivity and two-layer ONIOM methods. This hybrid method is implemented by partitioning the target molecule into individual groups, which are cascaded via the overlapping between them. The energy of the entire molecule is first calculated at a low level of theory such as M06-2x/cc-pVTZ. Then all the groups and their overlappings are treated at the levels of CCSD(T)/CBS and M06-2x/cc-pVTZ to obtain their energy difference to be used as the energy correction. We selected small-to-middle size aliphatic hydrocarbons including 79 C4C8 molecules as the validation set to demonstrate the feasibility of the CGA-ONIOM method, followed by the calculations of 12 representative C10, C12 and C16 aliphatic hydrocarbons (including normal-, branched-, cyclo- and unsaturated categories). Our calculations agree well with the reference values available in the literature with the modest deviation around 1.0 kcal mol−1. Compared with the conventional CCSD(T)/CBS calculation of the whole molecule, the computational cost can be dramatically reduced by a factor of ∼102 for molecules with 10 carbons and ∼104 for molecules with 16 carbons. Considering its outstanding computational efficiency and accuracy, our proposed CGA-ONIOM method is promising for combustion chemistry studies of large fuel molecules at a high level of theory.
  • Corrigendum to ``The unimportance of the reaction H2 + N2O ⇆
           H2O + N2: A shock-tube study using H2O time histories and ignition
           delay times'' [Combustion and Flame 196 (2018) 478-486]
    • Abstract: Publication date: Available online 10 December 2018Source: Combustion and FlameAuthor(s): Clayton R. Mulvihill, Olivier Mathieu, Eric L. Petersen
  • Five kHz thermometry in turbulent spray flames using chirped-probe pulse
           femtosecond CARS, part I: Processing and interference analysis
    • Abstract: Publication date: Available online 22 November 2018Source: Combustion and FlameAuthor(s): Levi M. Thomas, Albyn Lowe, Aman Satija, Assaad R. Masri, Robert P. Lucht We have applied chirped-probe-pulse (CPP) femtosecond (fs) coherent anti-Stokes Raman scattering for 5 kHz temperature measurements in turbulent spray flames. The CPP fs CARS technique has previously been used to perform spectroscopic temperature measurements in highly turbulent laboratory burners with excellent accuracy, precision, temporal resolution, and spatial resolution. In this paper, ultrafast CARS measurements in spray flames are presented as part of a larger effort to provide spatially and temporally resolved temperature fields in harsh spray environments. The Sydney Needle Spray Burner (SYNSBURNTM) was used to stabilize turbulent spray flames of acetone and ethanol. The burner features a retractable fuel injector so that the droplet density at the nozzle exit could be systematically varied. Results from selected regions of the turbulent spray flames are discussed in detail to highlight the challenges of CPP fs CARS temperature measurements. Sources of spectral distortion due to interaction with droplets are discussed along with an uncertainty analysis. The passage of fuel through the probe volume caused varying levels of signal degradation and resulted in complete signal loss on approximately 10% of the laser shots for dense spray conditions. The interferences are attributed to two separate phenomena and are categorized based on the probable phase of the fuel – liquid or gas. Interference caused by liquid fuel was unavoidable in certain regions at certain operating conditions, but easily identified and removed. Interference from vapor fuel was more problematic as the nitrogen signal was only moderately corrupted in the high-frequency portion of the spectrum, and the temperature was generally biased to higher values. Rejecting individual signal spectra, based on a fitting error threshold, was shown to be effective in excluding shots with significant interference from fuel droplets, but shots with only minor interference require a more-advanced rejection criterion. Analysis of the temperature fields for a few selected conditions is presented showing trends with the atomization quality of the liquid fuel. Fourier analysis revealed hydrodynamic instabilities in the shear layer and relatively weak thermoacoustic instabilities in the reaction zone.
  • Five kHz thermometry in turbulent spray flames using chirped-probe-pulse
           femtosecond CARS, part II: Structure of reaction zones
    • Abstract: Publication date: Available online 7 November 2018Source: Combustion and FlameAuthor(s): Albyn Lowe, Levi M. Thomas, Aman Satija, Robert P. Lucht, Assaad R. Masri Temperature was measured in turbulent spray flames of ethanol and acetone stabilized on the piloted Sydney Needle Spray Burner (SYNSBURNTM) using single-laser-shot, chirped-probe-pulse femtosecond coherent anti-Stokes Raman spectroscopy (CPP-fs-CARS) with a repetition rate of 5 kHz. The burner features air-blast atomization of liquid injected from a needle that can be translated by a length Lr within a co-flowing air stream so that piloted spray flames ranging from dilute to dense can be studied. Part I of these investigations has reported on the CPP-fs-CARS technique and extensive details of data processing methodology. Part II is concerned with the structure of the reaction zones at different spray loadings and for different departures from blow-off. While not performed simultaneously, measurements of the size distribution of liquid fragments are also reported and discussed in conjunction with the measured temperature. Measured probability density functions of temperature show that for flames with the same liquid loading but different recess lengths, Lr, the near-field spray structure that forms upstream of x/D = 10 affects flame structure and stability further downstream. As the spray exiting the burner becomes denser, with a higher proportion of ligaments and ‘irregular’ shaped objects, the entrainment of hot pilot gases into the spray envelope is affected, hence changing the rates of vaporization and subsequent combustion. The reported results will also form a useful platform for validating sub-models of atomization and combustion in turbulent, dilute to dense spray flames.
School of Mathematical and Computer Sciences
Heriot-Watt University
Edinburgh, EH14 4AS, UK
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