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Combustion and Flame
Journal Prestige (SJR): 2.427
Citation Impact (citeScore): 5
Number of Followers: 127  
 
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ISSN (Print) 0010-2180
Published by Elsevier Homepage  [3162 journals]
  • Flame spread: Effects of microgravity and scale
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): David L. Urban, Paul Ferkul, Sandra Olson, Gary A. Ruff, John Easton, James S. T'ien, Ya-Ting T. Liao, Chengyao Li, Carlos Fernandez-Pello, Jose L. Torero, Guillaume Legros, Christian Eigenbrod, Nickolay Smirnov, Osamu Fujita, Sébastien Rouvreau, Balazs Toth, Grunde Jomaas For the first time, a large-scale flame spread experiment was conducted inside an orbiting spacecraft to study the effects of microgravity and scale and to address the uncertainty regarding how flames spread when there is no gravity and if the sample size and the experimental duration are, respectively, large enough and long enough to allow for unrestricted growth. Differences between flame spread in purely buoyant and purely forced flows are presented. Prior to these experiments, only samples of small size in small confined volumes had been tested in space. Therefore the first and third flights in the experimental series, called “Saffire,” studied large-scale flame spread over a 94 cm long by 40.6 cm wide cotton-fiberglass fabric. The second flight examined an array of nine smaller samples of various materials each measuring 29 cm long by 5 cm wide. Among them were two of the same cotton-fiberglass fabric used in the large-scale tests and a thick, flat PMMA sample (1-cm thick). The forced airflow was 20–25 cm/s, which is typical of air circulation speeds in a spacecraft. The experiments took place aboard the Cygnus vehicle, a large unmanned resupply spacecraft to the International Space Station (ISS). The experiments were carried out in orbit before the Cygnus vehicle, reloaded with ISS trash, re-entered the Earth's atmosphere and perished. The downloaded test data show that a concurrent (downstream) spreading flame over thin fabrics in microgravity reaches a steady spread rate and a limiting length. The flame over the thick PMMA sample approaches a non-growing, steady state in the 15 min burning duration and has a limiting pyrolysis length. In contrast, upward (concurrent) flame spread at normal gravity on Earth is usually found to be accelerating so that the flame size grows with time. The existence of a flame size limit has important considerations for spacecraft fire safety as it can be used to establish the heat release rate in the vehicle. The findings and the scientific explanations of this series of innovative, novel and unique experiments are presented, analyzed and discussed.
       
  • The effects of cross-flow fuel injection on the reacting jet in vitiated
           cross-flow
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Matthew D. Pinchak, Vincent G. Shaw, Ephraim J. Gutmark The effects of cross-flow fuel injection on a slotted jet flame consisting of an ethylene-air premixture are investigated experimentally. Cross-flow conditions of 900 K and 100 m/s were chosen to closely simulate the environment of a secondary combustor in a staged combustion system. It was found that increasing the cross-flow equivalence ratio (Φ∞) requires a consequent reduction in the jet equivalence ratio (Φj) for jet flame stabilization in order to avoid the formation of locally rich mixtures beyond the flammability limits of the flame. Stable flames were achieved for a low cross-flow equivalence ratio of Φ∞ = 0.4 across a range of jet equivalence ratio values and momentum flux ratios, demonstrating the ability of the transverse jet to extend the flammability limits of the cross-flow mixture. Significantly, as Φ∞ is increased beyond a certain point, no ethylene is required to be present in the jet mixture, and a jet consisting only of air is able to stabilize the flame. Due to the fluidic nature of the flame stabilization mechanism of these flames, they are dubbed fluidically stabilized flames (FSF). OH* chemiluminescence and high-speed particle image velocimetry were utilized to gain deeper understanding of the flame behavior and flow field features of the FSF. In contrast to bluff-body stabilized flames, it was found that the FSF provides increased control of the flame shape, with increasing flame width and penetration for higher jet momentum flux ratios (J). The FSF was also demonstrated to be a highly dynamical phenomenon, characterized by a dominant peak frequency that is dependent on both Φj and Φ∞. Proper orthogonal decomposition of the time-resolved velocity fields shows that heat release affects the dynamics of the FSF in a similar manner to the reacting jet in cross-flow (RJICF), as demonstrated in a previous study. Finally, the flame behavior was found to be highly dependent on the cross-flow fueling mechanism, with coherent flame oscillations present when the fuel injection point is closely-coupled to the flame stabilization location.
       
  • Effects of fuel structure on structural characteristics of soot aggregates
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Quanxi Tang, Mengda Wang, Xiaoqing You Two critical structural characteristics of soot aggregates, i.e., the primary particle size and the spatial arrangement of primary particles in aggregates, were investigated for laminar premixed burner-stabilized stagnation propane flames with and without toluene addition under similar temperature profiles. To link the structural characteristics of soot aggregates with the dynamic processes of soot formation, the particle mass and mobility size were measured by using a centrifugal particle mass analyzer and a scanning mobility particle sizer respectively, which enabled us to obtain the particle size distributions and the spatially fractal feature or mass-mobility exponents. The results show that for pure propane flames, the size growth is relatively rapid as indicated from the particle size distributions, and soot aggregates are highly branched with bigger primary particles. By contrast, for toluene-added propane flames, soot nucleation is faster, while the size growth of primary particles is suppressed, resulting in more compact arrangement of smaller primary particles in the soot aggregates. For both types of flames, with the increase of flame temperature, the process of soot inception is accelerated, but the production of soot is suppressed due to the thermodynamic reversibility of soot precursors at higher temperatures.
       
  • Effects of hydrogen addition on non-premixed ignition of iso-octane by hot
           air in a diffusion layer
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Zisen Li, Xiaolong Gou, Zheng Chen Hydrogen addition is widely used to improve the combustion performance of single-component fuel. In this study, the effects of hydrogen addition on non-premixed ignition of iso-octane by hot air in a diffusion layer were examined and interpreted numerically. Detailed chemistry and transport were considered in simulation. The non-premixed ignition delay times at different hydrogen blending levels were obtained and analyzed. It was found that hydrogen addition greatly reduces the ignition delay. This is mainly due to the fact that the preferential mass diffusion of hydrogen over iso-octane significantly increases the local hydrogen blending level at the ignition kernel. Besides, for the non-premixed ignition process, two modes of reaction front propagation were identified through the analysis based on Damköhler number and consumption speeds. One is the reaction-driven mode characterized by local or sequential homogeneous autoignition; and the other is the diffusion-driven mode, which depends on the balance of mass diffusion, heat transfer and chemical reaction. These two modes lead to different ignition behaviors. For pure iso-octane with low mass diffusivity, ignition is mainly caused by local homogeneous reaction occurring at the most reactive position. With the increase of diffusion layer thickness, the local temperature at the most reactive position increases and therefore the non-premixed ignition delay time of pure iso-octane decreases. However, when hydrogen with high mass diffusivity is added into iso-octane, the non-premixed ignition is controlled by fuel diffusion. With the increase of diffusion layer thickness, the concentration gradient becomes smaller and thereby less hydrogen diffuses into the ignition kernel. Consequently, unlike pure iso-octane, the non-premixed ignition delay time of hydrogen/iso-octane blends increases with the diffusion layer thickness.
       
  • Buoyancy effects on concurrent flame spread over thick PMMA
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Maria Thomsen, Carlos Fernandez-Pello, Gary A. Ruff, David L. Urban The flammability of combustible materials in a spacecraft is important for fire safety applications because the conditions in spacecraft environments differ from those on earth. Experimental testing in space is difficult and expensive. However, reducing buoyancy by decreasing ambient pressure is a possible approach to simulate on-earth the burning behavior inside spacecraft environments. The objective of this work is to determine that possibility by studying the effect of pressure on concurrent flame spread, and by comparison with microgravity data, observe up to what point low-pressure can be used to replicate flame spread characteristics observed in microgravity. Specifically, this work studies the effect of pressure and microgravity on upward/concurrent flame spread over 10 mm thick polymethyl methacrylate (PMMA) slabs. Experiments in normal gravity were conducted over pressures ranging between 100 and 40 kPa and a forced flow velocity of 200 mm/s. Microgravity experiments were conducted during NASA's Spacecraft Fire Experiment (Saffire II), on board the Cygnus spacecraft at 100 kPa with an air flow velocity of 200 mm/s. Results show that reductions of pressure slow down the flame spread over the PMMA surface approaching that in microgravity. The data is correlated in terms of a non-dimensional mixed convection analysis that describes the convective heat transferred from the flame to the solid, and the primary mechanism controlling the spread of the flame. The extrapolation of the correlation to low pressures predicts well the flame spread rate obtained in microgravity in the Saffire II experiments. Similar results were obtained by the authors with similar experiments with a thin composite cotton/fiberglass fabric (published elsewhere). Both results suggest that reduced pressure can be used to approximately replicate flame behavior of untested gravity conditions for the burning of thick and thin solids. This work could provide guidance for potential ground-based testing for fire safety design in spacecraft and space habitats.
       
  • The role of strain rate, local extinction, and hydrodynamic instability on
           transition between attached and lifted swirl flames
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Qiang An, Adam M. Steinberg The coupling between fluid strain rate, local flame extinction, hydrodynamic instability, and flame lift-off was studied in premixed swirl flames using multi-kHz repetition-rate OH* chemiluminescence (CL), OH planar laser induced fluorescence (PLIF), and stereoscopic particle image velocimetry (S-PIV). Over 50 different combinations of fuel composition (CH4/H2 ratio), equivalence ratio, and reactant preheat temperature were studied, allowing systematic variation of the reactant-to-product density ratio, laminar flame speed, and Lewis number. Depending on the test conditions, the flame could either be stably attached to the nozzle, stably lifted, or intermittently transitioning between attached and lifted states. Transition between stabilization states was linked with the transition between convective instability and absolute instability at the flame base; formation of an m=1 (m denotes the azimuthal wavenumber) globally unstable wave was associated with the lifted flame, which was manifested by a helical precessing vortex core (PVC). The minimum bulk velocity at which the flame was stably lifted was linearly correlated with the laminar flame extinction strain rate, while none of the other commonly reported key parameters governing hydrodynamic instability was able to collapse the data alone. Hence, lift-off was associated with a relatively constant Damköhler number based on the bulk fluid strain rate and extinction strain rate. The roles of local strain and extinction on the transition process were further elucidated by the cases with intermittent lift-off/reattachment. The probability of the flame being in the lifted state was roughly linearly correlated with the degree of local extinction at the flame base while the flame was still in the attached state. Moreover, this probability also was linearly related to the ratio of fluid-dynamic strain rate to extinction strain rate, but not the fluid-dynamic strain rate itself. These results demonstrate the importance of predicting extinction and hydrodynamic stability for predicting the attachment state of swirl flames.
       
  • Enhancement of the transition to detonation of a turbulent hydrogen–air
           flame by nanosecond repetitively pulsed plasma discharges
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Joshua A.T. Gray, Deanna A. Lacoste This work provides proof of concept for the use of nanosecond repetitively pulsed (NRP) plasma discharges to accelerate a propagating turbulent flame, resulting in enhanced deflagration-to-detonation transition and significant reduction in run-up length. The investigations are conducted on a stoichiometric hydrogen-air mixture at near ambient conditions. The effect of plasma actuation on the flame velocity is investigated using time-of-flight measurements of the propagating flame and detonation wave. The flame velocity shortly after the application of the NRP plasma discharges is more than double that obtained in cases in which no plasma is applied. High-speed imaging of OH* chemiluminescence in the electrode area confirms this result and provides insight about the mechanisms of plasma action. While the volumetric energy deposited during plasma actuation is sufficiently low as to not ignite the combustible mixture prior the arrival of the flame, the chemical and thermal enhancement of the gas is efficient enough to significantly accelerate the transition to detonation. The decrease in the run-up length to transition to detonation is obtained for a plasma power of less than 0.14% of the thermal power of the flame. This result indicates that low-energy active devices using NRP discharges might be suitable for replacing passive devices such as orifice plates or Shchelkin spirals.
       
  • Temperature measurements under diesel engine conditions using laser
           induced grating spectroscopy
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): F. Förster, C. Crua, M. Davy, P. Ewart Crank angle-resolved temperatures have been measured using laser induced grating spectroscopy (LIGS) in a motored reciprocating compression machine to simulate diesel engine operating conditions. A portable LIGS system based on a pulsed Nd:YAG laser, fundamental emission at 1064 nm and the fourth harmonic at 266 nm, was used with a c.w. diode-pumped solid state laser as probe at 660 nm. Laser induced thermal grating scattering (LITGS) using resonant absorption by 1-methylnaphthalene, as a substitute fuel, of the 266 nm pump-radiation was used for temperature measurements during non-combusting cycles. Laser induced electrostrictive grating scattering (LIEGS) using 1064 nm pump-radiation was used to measure temperatures in both combusting and non-combusting cycles with good agreement with the results of LITGS measurements which had a single-shot precision of ± 15 K and standard error of ± 1.5 K. The accuracy was estimated to be ± 3 K based on the uncertainty involved in the modified equation of state used in the derivation from the LIGS measurements of sound speed in the gas. Differences in the in-cylinder bulk gas temperature between combusting and non-combusting cycles were unambiguously resolved and temperatures of 2300 ± 100 K, typical of flames, were recorded in individual cycles. The results confirm the potential for LIGS-based thermometry for high-precision thermometry of combustion under compression-ignition conditions.
       
  • Testing the validity of a mechanism describing the oxidation of binary
           n-heptane/toluene mixtures at engine operating conditions
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Zisis Malliotakis, Colin Banyon, Kuiwen Zhang, Scott Wagnon, Jose Juan Rodriguez Henriquez, George Vourliotakis, Christos Keramiotis, Maria Founti, Fabian Mauss, William J. Pitz, Henry Curran The aim of this work is to evaluate the influence of the n-heptane/toluene ratio on the reactivity of binary toluene reference fuels (TRFs), through a combined experimental and numerical work. Novel experimental ignition delay time (IDT) data of three binary TRFs of varying n-heptane/toluene ratios have been obtained in a high-pressure shock tube and in a rapid compression machine at conditions relevant to novel engine operation. Measurements have been performed at two pressures (10 and 30 bar), and at three fuel/air equivalence ratios (0.5, 1.0 and 2.0) for TRF mixtures of 50%, 75% and 90% by volume toluene concentration, over the temperature range of 650–1450 K. It was found that, increasing the n-heptane content, led to an increase in reactivity and shorter measured IDTs. Reduced sensitivity to the equivalence ratio was observed at high temperatures, especially for high toluene content mixtures. A well validated detailed kinetic mechanism for TRF oxidation was utilized to provide further insight into the experimental evidence. The mechanism, which has recently been updated, was also assessed in terms of its validity, contributing thus to its continuous development. Reaction path analysis was performed to delineate critical aspects of toluene oxidation under the considered conditions. Further, sensitivity analysis highlighted the interactions between the chemistry of the two TRF components, revealing toluene's character as a reactivity inhibitor mainly through the consumption of ȮH radicals.
       
  • Effect of H2O and CO2 on propane, propene, and isopropanol oxidation at
           elevated pressures
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Oxana N. Fedyaeva, Denis O. Artamonov, Anatoly A. Vostrikov The article discusses research results on oxidation features of high-density fuel-enriched C3Н8/О2, C3Н6/О2, and C3Н7OH/О2 mixtures (ρfuel = 0.22–0.25 mol/dm3, ρO2 = 0.76–0.88 mol/dm3) diluted with argon, carbon dioxide, or water vapor (from 59 to 72% mol.) at the uniform heating (1 K/min) of tubular reactor to 640 K. Proceeding from the time dependences of the reaction mixtures temperature, it was revealed that the self-ignition temperature of fuels in the Ar medium increases in the following sequence: C3H6 
       
  • Study on the flame development patterns and flame speeds from homogeneous
           charge to stratified charge by fueling n-heptane in an optical engine
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Zunqing Zheng, Xinghui Fang, Haifeng Liu, Chao Geng, Zhi Yang, Lei Feng, Yu Wang, Mingfa Yao The operation range of some new compression ignition (CI) combustion modes was extended compared to that of HCCI because of fuel stratification. Few researches tried to analyze the mechanism by comparison of flame development patterns and flame speed under different stratified conditions. In this work, high-speed imaging of natural flame luminosity was used to study the combustion process from homogeneous charge to stratified charge with a higher frame rate. Different stratification conditions were formed by adjusting the injection timings. Results show that the proportion of flame propagation increases and combustion reaction rate decreases as fuel distribution in cylinder changes from homogeneous charge to stratified charge. Flame propagation of auto-ignition kernel exists although the combustion is dominated by multipoint auto-ignition in HCCI combustion. The flame spreading speed is much higher than the flame speed because the fictitious reaction front is shorter than the actual reaction front for flame spreading speed calculation. Four principles are proposed for equivalent radius method to get more reasonable results of flame speed. For conditions that do not satisfy these principles, effective front method can be used. Flame speed in CI combustion modes is in the range of 10–50 m/s depending on different stratification conditions in current study. There is a good correlation between the flame speed and the peak heat release rate as SOI is retarded, i.e., high flame speed corresponds to high peak heat release rate. It can be concluded that controlling fuel stratification is an effective method to regulate the ratio between auto-ignition and flame propagation and achieve effective control on combustion reaction rate.
       
  • The evolution of soot morphology and nanostructure along axial direction
           in diesel spray jet flames
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Hao Jiang, Tie Li, Yifeng Wang, Pengfei He, Bin Wang This study aims at providing new insights into the soot formation and oxidation processes in diesel spray jet flames under the diesel-like conditions. Soot particles are sampled within a reacting diesel jet spray flame in a constant-volume combustion chamber under diesel-like high-temperature and pressure ambient conditions. The experiments are conducted with an injector hole of 0.28 mm and injection mass of 30 mg per shot. Soot particles at different locations along the axis of the diesel jet flame are sampled by a thermophoretic probe, and the variation of soot morphology and nanostructure of the sampled soot particles along the axial location in the flame is analyzed by using the transmission electron microscopy (TEM). At upstream of the jet flame, large “liquid-like” particles are observed in the TEM images. As these particles are delivered to the downstream, (1) there are many immature primary particles formed on the surface of large “liquid-like” particles, and (2) they are carbonized into the larger soot aggregates with the matured structure, and then at last, (3) the larger soot aggregates gradually break up into the smaller particles with relatively compact structure. Both the diameter of primary particles and radius of gyration of the soot aggregates decrease as the distance between the injector nozzle tip and the center of TEM grid is increased, while the opposite trend is found in the behavior of the fractal dimensions of soot aggregates. The high resolution TEM images show that the soot primary particles exhibit the turbostratic state at the upstream of the jet flame, while more matured structures with the graphitic crystallite in outer shell and several fine particles in the inner core are observed as the sampling distance from orifice is increased. The average fringe length increases while the average tortuosity and separation decrease with the distance from the nozzle. These results suggest that the mechanism of mature soot aggregates generated directly from the large “liquid-like” particles should be one of the ways of soot particle formation along the axis of diesel jet flames.
       
  • Viewing internal bubbling and microexplosions in combusting metal
           particles via x-ray phase contrast imaging
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Elliot R. Wainwright, Shashank V. Lakshman, Andrew F.T. Leong, Alex H. Kinsey, John D. Gibbins, Shane Q. Arlington, Tao Sun, Kamel Fezzaa, Todd C. Hufnagel, Timothy P. Weihs The combustion performance of metal particles for explosives, propellants, pyrotechnics, and bio-agent defeat can be optimized by controlling the chemistry, size, and coatings of the particles. Many pure and composite metal fuels used in these applications have been observed to microexplode during combustion. This causes particles to fragment, revealing fresh surfaces, which may enhance burn rates and efficiencies. Despite these potential benefits, few have attempted to control the bubbling and the microexplosion of metal powders as they combust in molten states. Here, we choose Al:Zr composite powders as a representative system to study bubbling and microexplosions and to identify active mechanisms. Using synchrotron x-rays and phase contrast imaging, we observe molten metal particles of various sizes as they burn in air at temperatures ranging from 2700 to 3500 K. We characterize bubble nucleation and growth in the interior of the particles during the combustion process, and we identify heterogeneous nucleation, slow growth and coalescence of bubbles, as well as rapid bubble growth leading to microexplosions. Bubble growth rates and Laplace pressures are calculated, and we find that during slow expansion growth rates are similar to classical predictions of bubble growth in superheated liquids. For rapid expansions we find that a critical growth rate of ∼0.5 m s−1, independent of the initial particle size, is necessary to enable microexplosions. We calculate the rate of gas generation that is needed to enable this growth and we conclude that nitrogen is the gas most likely driving rapid bubble growth and fragmentation. These results provide the first in situ observations of the mechanisms that control bubbling and microexplosions during the combustion of metal particles.
       
  • A hybrid flamelet finite-rate chemistry approach for efficient LES with a
           transported FDF
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Martin Rieth, Jyh-Yuan Chen, Suresh Menon, Andreas M. Kempf A hybrid method combining flamelet tabulation with transported filtered density function (FDF) finite rate chemistry has been developed and applied to large eddy simulation (LES) of the Sydney/Sandia piloted turbulent flame with inhomogeneous inlets. Aiming to improve the efficiency while maintaining accuracy, the hybrid method applies the computationally expensive Lagrangian particles representing FDF transport and direct chemistry only in selected, dynamically varying locations. The rest of the domain is treated with flamelet chemistry based on the Eulerian fields and a presumed top-hat PDF closure. The method relies on consistency between Eulerian and Lagrangian fields through robust, accurate coupling and consistent modeling. The performance of the hybrid model is verified by an extensive comparison against the experiment and the ‘pure’ models, i.e., the (a) flamelet LES with presumed FDF, (b) flamelet LES with transported FDF, and (c) direct chemistry LES with transported FDF. Finite rate chemistry is found to improve species predictions over flamelet chemistry and the hybrid method is found to reproduce these improvements by using particles with finite rate chemistry only at locations where the flamelet is not sufficient, promising a reduced computational cost.
       
  • Mesoscale burner array performance analysis
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Rajavasanth Rajasegar, Jeongan Choi, Brendan McGann, Anna Oldani, Tonghun Lee, Stephen D. Hammack, Campbell D. Carter, Jihyung Yoo Combustion characteristics of a mesoscale burner array have been studied using several diagnostic and analysis techniques. The array was specifically configured to enhance overall combustion stability, particularly under lean operating conditions, by promoting flame to flame interactions between neighboring elements. The 4 × 4 burner array demonstrated stable operations up to 3 kW and is designed to flexibly accommodate wide range of combustion power outputs by scaling the element dimensions or array size. Flame stabilizing mechanisms were experimentally examined using OH, CH, and CH2O planar laser induced fluorescence (PLIF) of premixed CH4 and air flames at operating equivalence ratios between 0.7 and 1.2. A quantitative measure of flame stability was obtained through dynamic mode decomposition (DMD) analysis of high speed OH-PLIF images. Lean blow off limits and emissions were also characterized across a wider range of equivalence ratios to better understand mesoscale burner array combustion characteristics. Lastly, combustion experiments using liquid fuel, pentane (C5H12), were also carried out. Marked improvement in combustion stability was observed compared to a single swirl-stabilized flame of similar power output. Results indicate mesoscale burner arrays can potentially serve as flexible and scalable next generation propulsion and power generation systems.
       
  • Surrogate formulation methodology for biodiesel based on chemical
           deconstruction in consideration of molecular structure and engine
           combustion factors
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Ang Li, Lei Zhu, Yebing Mao, Jiaqi Zhai, Dong Han, Xingcai Lu, Zhen Huang. A novel methodology based on chemical deconstruction for the formulation of surrogate fuels for biodiesels was proposed, developed, and validated. Surrogates were formulated using sub-surrogates of methyl palmitate, stearate, oleate, linoleate, and linolenate, which are the major components of biodiesel. Each methyl ester sub-surrogate was constituted by a binary or ternary fuel mixture. To formulate the sub-surrogates, key physical and chemical parameters based on engine spray and combustion were considered with preferential weight functions. To calculate the optimal surrogates, novel weighted Euclidean distance and analytic hierarchy process algorithms were used to accurately determine the components and their proportions. Next, surrogates for biodiesels were assembled from the sub-surrogates according to the component ratios of the target fuels. Using this methodology, surrogates for biodiesels could be formulated regardless of feedstock origin and production region. For example, the surrogate fuel for soybean biodiesel consisted of 62.9% methyl decanoate, 15.0% n-hexadecane, 9.4% methyl trans-3-hexenoate, and 12.7% 1, 4-hexadiene in mole fractions. The process of experimental validation was divided into two steps: first, verifying the sub-surrogates for methyl palmitate, methyl oleate, and methyl linoleate; and second, comparing the quaternary surrogates with real soybean biodiesel. Point-to-point experiments were conducted on three platforms: a heated rapid compression machine, a laminar flow reactor, and an ignition quality tester. More specifically, the rapid compression machine experiments compared the autoignition characteristics, especially the low-temperature reactivity, of the surrogates and the target fuels under constant-volume adiabatic conditions. Laminar flow reactor was used to compare the low-to-intermediate oxidation properties of the surrogates with those of the target fuels, especially the early formation of olefins and carbon dioxide, which is a key characteristic of the combustion of biodiesel. Ignition quality tester was used to ensure that the surrogates had similar ignition and combustion delay times under engine-like conditions. The results of all of these experimental validations showed that the ignition and oxidation properties of the surrogates were consistent with those of their target fuels. In addition, a kinetic model of the quaternary surrogates was proposed and further validated in a laminar flow reactor. In sum, quaternary surrogate fuels were developed using a method based on chemical deconstruction, which was shown through validation experiments to be highly accurately and reliably reproduce the combustion properties of soybean biodiesel. Furthermore, chemical deconstruction method has great potential in surrogate modeling for various biodiesels.
       
  • Large-eddy simulation of dual-fuel ignition: Diesel spray injection into a
           lean methane-air mixture
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Heikki Kahila, Armin Wehrfritz, Ossi Kaario, Ville Vuorinen In the present study, large-eddy simulation (LES) together with a finite-rate chemistry model is utilized for the investigation of a dual-fuel (DF) ignition process where a diesel surrogate (n-dodecane) spray ignites a lean methane-air mixture in engine relevant conditions. The spray setup corresponds to the Engine Combustion Network (ECN) Spray A configuration enabling an extensive validation of the present numerical models in terms of liquid and vapor penetration, mixture distribution, ignition delay time (IDT) and spatial formaldehyde concentration. The suitability of two n-dodecane mechanisms (54 and 96 species) to cover dual-fuel chemical kinetics is investigated by comparing the predicted homogeneous IDTs and laminar flame speeds to reference values in single-fuel methane-air mixtures. LES of an n-dodecane spray in DF conditions is carried out and compared against the baseline ECN Spray A results. The main results of the study are: (1) ambient methane impacts the ignition chemistry throughout the oxidation process. In particular, the activation of the low-temperature chemistry is delayed by a factor of 2.6 with both mechanisms, whereas the high-temperature chemistry is delayed by a factor of 1.6–2.4, depending on the mechanism. (2) The ignition process starts from the spray tip. (3) There exists a characteristic induction time in the order of 0.1 ms between the start of the first high-temperature reactions and the time when maximum methane consumption rate is achieved. (4) The high-temperature ignition process begins near the most reactive mixture fraction conditions. (5) The role of low-temperature reactions is of particular importance for initiation of the production of intermediate species and heat, required in methane oxidation and (6) both applied mechanisms yield qualitatively the same features (1)–(5) in the DF configuration.
       
  • The effect of butanol isomers on the formation of carbon particulate
           matter in fuel-rich premixed ethylene flames
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Carmela Russo, Andrea D'Anna, Anna Ciajolo, Mariano Sirignano The effect of the butanol isomers on carbon particulate matter formation was studied by substituting up to 20% of the total carbon of ethylene, fed to premixed flames with different equivalence ratios, with the four butanol isomers. Soot and condensed-phase nanostructures were tracked by means of particle size distribution (PSD) measurements and laser induced emission spectroscopy, namely fluorescence and incandescence. Butanol isomers, especially t-butanol, significantly reduced the total amount and the size of the soot particles, whereas a negligible effect was detected on condensed-phase nanostructures. PSDs were measured along with the aromaticity and functionalities of the carbon particulate matter thermophoretically sampled in the highest equivalence ratio condition. No significant differences were found among the different butanol isomers neither in the soot aggregate size, as measured by size exclusion chromatography, nor in the aromaticity, as evaluated by Raman and UV–vis spectroscopy, of the particulate matter. Conversely, FTIR analysis showed that carbon particulate matter produced from 1-butanol and t-butanol-doped flames contained larger amounts of oxygen in form of C = O, C–O–C and OH functionalities. However, most of the differences in the oxygen functionalities disappeared after dichloromethane (DCM) treatment, suggesting that these oxygenated moieties belong to the condensed-phase nanostructures, soluble in DCM, rather than to soot particles.
       
  • Flow transportation inside shale rocks at low-temperature combustion
           condition: A simple scaling law
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Wei Chen, Yafeng Lei, Xiukun Hua The gas flow inside the shale undergoes a complicated process from continuum-flow to Knudsen diffusion during combustion. In this study, a shale sample obtained from Shanxi province with burial depth of 643 m was combusted at a constant temperature to study the transient internal pressure variation. In the beginning, a cylinder-shaped shale was combusted above an electric heater at a constant temperature 450 °C. The transient internal pressures and surface temperatures of the shale sample were recorded every 15 s. With the thermal wave propagating from the electric heater to the top of shale, the shale surface temperature gradually increased. The pressure inside the sample was quickly built up because of the thermal cracking of kerogen. It reached a peak value of 12,000 Pa. Afterwards, the pressure gradually declined with the improvement of the permeability of the shale. By the observations during the experiment, a combustion model based on the scaling power law was used to simulate the transient internal pressure changes of a cylinder shaped shale sample during the combustion process. The simulated results showed a good agreement with experimental data. Based on the pressure variation data, it was found that not only the original trapped/storage gas (free state gas, adsorbed state gas, and dissolved state gas) but also the thermal cracking hydrocarbon gases produced from kerogen could be extracted from tight shale formation for gas recovery. Thus, combustion can further improve the production rate of gas from shale reservoirs.
       
  • Oxidation of ethanol and hydrocarbon mixtures in a pressurised flow
           reactor
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Hao Yuan, Zhewen Lu, Zhongyuan Chen, Yi Yang, Michael J. Brear, James E. Anderson, Thomas Leone This paper presents a study of the oxidation of iso-octane, ethanol, toluene and their mixtures in a pressurised turbulent flow reactor operating at 900–930 K, 10 bar, and an equivalence ratio of 0.058. A large set of fuels is investigated, including neat iso-octane, ethanol, toluene, their binary mixtures, gasoline reference fuels (PRF91 and TRF91), and their mixtures with ethanol. The resulting species are measured along the length of the reactor and simulated using existing kinetic models from the literature.The existing models are found to reproduce measurements of the major oxidation products of iso-octane, ethanol, their binary mixtures, as well as that of PRF91 and PRF91/ethanol mixtures. However, significant differences are observed between the measurement and simulation of neat toluene. Adjustment is then made to the rate constants of key reactions in the toluene model. The adjusted model, whilst more accurately reproducing neat toluene oxidation, does not significantly improve the modelling of toluene containing mixtures. This suggests that further investigations should focus on the oxidation of neat toluene, as well as the chemical interactions of toluene containing mixtures.
       
  • Effects of carbon dioxide addition to fuel on soot evolution in ethylene
           and propane diffusion flames
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Jian Wu, Linghong Chen, Per-Erik Bengtsson, Jianwu Zhou, Jianfu Zhang, Xuecheng Wu, Kefa Cen The influence of carbon dioxide addition to the fuel on soot evolution in ethylene and propane diffusion flames was studied by optical diagnostics. The mole fraction of CO2 addition ranged from 0 to 0.5, while the flow rate of the fuel gas was kept constant for these two sets of flames. Spatial distributions of polycyclic aromatic hydrocarbons (PAHs), temperature, as well as volume fraction, primary particle size and number density of soot were observed by the methods of laser-induced fluorescence (LIF), ratio pyrometry and laser-induced incandescence (LII), respectively. It was found that the flame height decreased for ethylene flames while it was nearly constant for propane flames with increasing addition of CO2. The measurements showed a temperature reduction in the lower part but an increase in the upper part in the ethylene-based flames. By contrast, a slight temperature decrease was observed in overall propane-based flames with the addition of CO2. Similar suppression effects were observed in the total soot/PAHs loading, percentage of carbon conversion to soot, and the total number of primary soot particles regardless of the fuel type. Comparison between the total loading of soot and PAHs indicated that addition of CO2 inhibited the conversion of PAHs to soot. The results also showed that the addition of CO2 in the fuel had a small effect on the specific growth rate of soot regardless of the fuel type. Relative changes of particle surface area could reasonably well explain the shift in the peak volume fraction from the wings to the centerline with the addition of CO2 to the ethylene flames.
       
  • T-jump pyrolysis and combustion of diisopropyl methylphosphonate
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Bing Yuan, Hergen Eilers We monitor the T-jump pyrolysis and combustion reactions of diisopropyl methylphosphonate (DIMP) via FTIR and kinetic FTIR. DIMP is heated at 10 °C/s, ∼1800 °C/s, and ∼18,000 °C/s to temperatures between 500 °C and 1200 °C in nitrogen, air, and an oxygen-rich environment. We observe propene, isopropyl methylphosphonate (IMP), and methylphosphonic acid (MPA) as final decomposition products. In addition, we identify 2-propanol in the early phase of the reaction. The results indicate that DIMP has two possible decomposition pathways: (1) A two-step decomposition process in which DIMP first transfers an H atom from the OCH(CH3)2 group to the double-bonded oxygen through a six-membered ring transition state and forms IMP and propene. Subsequently, IMP decomposes into either propene and MPA or 2-propanol and methyl(oxo)-phosphoniumolate (MOPO). (2) DIMP transfers an H atom from the OCH(CH3)2 group to the O atom of the other OCH(CH3)2 group through a different six-membered ring and simultaneously forms MOPO, propene, and 2-propanol. Our theoretical calculation indicates that the energy barriers for the two reaction pathways are 36.5 kcal/mol (pathway 1, 1st step), 49.9 kcal/mol (pathway 1, 2nd step), and 51.2 kcal/mol (pathway 2), respectively. When using a heating rate of 10 °C/s, pathway (1) is the major reaction in the system; using a heating rate of ∼18,000 °C/s, pathway (2) is the most probable reaction; and when the heating rate is ∼1800 °C/s, both pathways are observed during the early phase of the reaction. When heated in air and an oxygen-rich environment, we observe the same decomposition products plus CO and CO2.
       
  • The site effect on PAHs formation in HACA-based mass growth process
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Peng Liu, Zepeng Li, Anthony Bennett, He Lin, S. Mani Sarathy, William L. Roberts Hydrogen-abstraction/acetylene-addition (HACA) pathway has long been postulated as the dominant pathway for the formation of polycyclic aromatic hydrocarbons (PAHs) and the surface growth of soot. In this study, the site effect on PAH formation following HACA pathway is systematically investigated using density functional theory, transition state theory and premixed flame kinetic modeling. The entire reaction network includes 186 elementary reactions, starting from benzene to pyrene. Analysis of the potential energy surface and kinetic parameters show that H abstraction and C2H2 addition reactions are greatly sensitive to the site position (ortho-, meta- and para-position) relative to the existing C2H chain and surface site type (zig-zag, free-edge and armchair). Specifically, H abstraction and C2H2 addition reactions on the ortho-position and armchair surface site are kinetically unsupported due to the relatively high energy barrier and orientation hindrance effect compared with other site options. Therefore, the formation of a new benzene ring by the addition of the second C2H2 molecule on the ortho-position (e.g., 1-ethynylnaphthalene + C2H2→phenanthrene) or the first C2H2 molecule on the armchair surface site (e.g., phenanthrene + C2H2→pyrene) is unlikely, as demonstrated by PAH simulations in a premixed C2H4/O2/N2 sooting flame. The yield distribution of various reaction products has been investigated using a 0-D reactor, where the combustion conditions are taken from experimental data. The results show that the dominant products are di-substituted PAHs in benzene-naphthalene reaction system and PAHs with 5-membered ring structures in larger PAHs reaction systems. The existence of abundant PAHs with 5-membered rings contributes to clarifying the PAHs signal detected using laser induced fluorescence technology. Additionally, the observed sequence of mass peaks at intervals of mass number 26 in C2H2/C2H4 pyrolysis is reasonably explained by the HACA pathway with considering site effect.
       
  • Vortex formation mechanism within fuel streams in laminar nonpremixed jet
           flames
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Min Suk Cha, Jin Woo Son, Sung Hwan Yoon, Hung Truyen Luong, Deanna A. Lacoste, Chae Hoon Sohn A vortical structure occurring at the fuel stream in laminar nonpremixed jet flames was recently found and shown to have both a fluid-dynamic impact on the flow field and a possible influence on the flame stability and soot formation. We designed a systematic experiment and numerical simulation to investigate the physical mechanisms of this recirculation phenomenon in a coflow system. We hypothesized that a negative buoyancy, caused by the fuel jet being heavier than the ambient air, may play a significant role in the recirculation. Therefore, we experimentally varied the density of the fuel jet using a binary mixture of methane and n-butane, and tested the density of the coflow oxidizer by replacing nitrogen with carbon dioxide. Several fuel jet velocities, flame temperatures, and nozzle diameters were also studied to thoroughly investigate all parameters that might possibly affect the recirculation. As a result, we found that our modified Richardson number, which is based on the cold density difference between the fuel and the coflow, the flame length, and the jet momentum flux, explained the physical mechanism of the recirculation well, with Ri ∼60 being the critical value for formation of the recirculation. The negative buoyancy was the primary driving force behind the recirculation, while the jet momentum mitigated its formation.
       
  • Large eddy simulation/probability density function simulations of the
           Cambridge turbulent stratified flame series
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Hasret Turkeri, Xinyu Zhao, Stephen B. Pope, Metin Muradoglu The LES/PDF methodology is applied to the Cambridge/Sandia turbulent stratified flame series. The methane chemistry is represented by the 16-species reduced ARM1 mechanism, and the in situ adaptive tabulation method is adopted to accelerate the chemistry calculations. Differential diffusion effects are taken into account. The simulations are performed for premixed (SwB1), and moderately and highly stratified (SwB5 and SwB9, respectively) cases under non-swirling conditions. The results from LES/PDF simulations are compared with the experimental measurements and with previous calculations. The calculated length of the recirculation zone, the mean and r.m.s. profiles of velocity, temperature, equivalence ratio and mass fractions of species are in very good agreement with the measurements. In the stratified cases, the CO profiles are underestimated within the recirculation zone, close to the bluff body. Scatter plots of species mole fractions and temperature are presented and compared with the experimental data. Conditional means of species mass fractions demonstrate overall good consistency with the measurements. A parametric study is then performed to examine the effect of differential diffusion and the effect of the parameter controlling the scalar mixing rate. It is found that differential diffusion has a negligible effect on the mean and r.m.s. results, whereas, the mixing rate parameter has a considerable effect on the flow structure. Finally, the effect of stratification is investigated and characterized by scatter plots of OH mass fraction and heat release rate (HRR) in the equivalence ratio space.
       
  • Microwave plasma enhancement of multiphase flames: On-demand control of
           solid propellant burning rate
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Stuart J. Barkley, Keke Zhu, Joel E. Lynch, James B. Michael, Travis R. Sippel This effort explores the microwave-supported plasma enhancement of an aluminized, ammonium perchlorate composite solid propellant flame. The technique is enabled through novel alkali metal doping, which provides increased levels of ionization and allows efficient microwave energy deposition to the flame structure and subsequent perturbation of the steady-state propellant burning rate. Three potential modes of energy deposition are identified in composite propellants: (1) plasma-enhancement promoted by the presence of sodium, (2) strong absorption by high temperature metallic oxide products, and (3) direct absorption of energy by the propellant burning surface condensed phase. Equilibrium calculations are used to identify propellant compositions with high free-electron populations, and solution of the Boltzmann equation using the BOLSIG + code is used to show the significant effects of sodium doping on microwave absorption of the flame. Experimental emission spectroscopy and imaging of the microwave-enhanced propellant flame structure show evidence of these mechanisms. Propellant burning rates can be increased by up to 60% through the application of 1 kW, continuous 2.46 GHz microwave radiation. Propellant doping with 3.5% by weight of sodium nitrate shows significant improvement in the effectiveness of microwave application in modifying the propellant burning rate. This technique provides high levels of dynamic propellant combustion control from a solid-state system. The on-demand control of energetic material burning rates through low level doping and microwave irradiation is a promising technique which could lead to the development of a new class of ‘smart’ dynamically controllable energetic materials.
       
  • Flame speed characteristics of turbulent expanding flames in a rectangular
           channel
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Dan Fries, Bradley A. Ochs, Abhishek Saha, Devesh Ranjan, Suresh Menon We present results from studies of freely expanding flames in a unique small-scale convective facility for different free-stream initial conditions, characterized by Taylor-Reynolds numbers in the range of Reλg=179−395 (using the streamwise RMS velocity component and the lateral Taylor-microscale) and an inertial subrange over two decades of wavenumbers. The isotropic, decaying turbulence is generated by an active vane grid. Adding natural gas far upstream, the premixed flow is ignited using Laser Induced Breakdown (LIB) ignition. The evolution of the resulting spherically expanding flames is investigated using qualitative OH-Planar Laser Induced Fluorescence (PLIF). It is shown that trends of flame speeds derived from mean flame radius growth agree well with results from different experimental setups, using a recently developed scaling based on a spectral closure of a thin flame model (Chaudhuri et al., Phys. Rev. E (2011)). Reliable computation of the flame surface density and turbulent flame brush thickness is enabled by the large number of ensembles that can be collected in this type of facility. Trends of these instantaneous statistical quantities are presented and used to further assess the results of time-dependent mean quantities.
       
  • Characterisation of flame surface annihilation events in self excited
           interacting flames
    • Abstract: Publication date: January 2019Source: Combustion and Flame, Volume 199Author(s): Nicholas A. Worth, James R. Dawson This paper investigates the non-linear flame dynamics of two interacting, premixed, V-shaped flames by characterising the two-dimensional topology of flame annihilation events when the separation distance, S, between them is reduced and large-scale flame merging occurs. The equivalence ratio was varied to promote self-excited oscillations, with the oscillation frequency, heat release phase, and stability limits shown to be dependent on S. High-speed OH-PLIF measurements show that these changes are correlated with the break-up of the shear layers into structures that lead to large-scale flame annihilation events. In isolated flames the shear layers break-up independently, but as S is reduced the shear layers combine leading to large-scale flame merging resulting in the roll up of a single large-scale vortex structure altering the flame annihilation events compared with the case of isolated flames. A flame front event tracking algorithm is developed to characterise the two-dimensional topology and identify the number and spatial location of flame front annihilation events, which shows a strong correlation between these events and the fluctuating heat release rate. Compared with stable flames for the same S, it is found that self-excited instabilities do not significantly increase the number of annihilation events but rather affects their spatial distribution and phase within the oscillation cycle. It is also shown that flame merging significantly increases the probability of flame front annihilation events which alters the phase of the fluctuating heat release rate.
       
  • Aims and Scope/Editorial Board
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s):
       
  • 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.
       
  • Improvement and validation of a detailed reaction mechanism for thermal
           decomposition of RDX in liquid phase
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Mayank Khichar, Lalit Patidar, Stefan T. Thynell The objective of this work is to validate an expanded version of a recently developed reaction mechanism describing liquid-phase decomposition of RDX. The validation involves a comparison of experimental results obtained from confined rapid thermolysis at various set temperatures. In the experiments, the decomposition occurs in the liquid phase, which results in evolution of species into the gas phase. The spectral transmittances of the gas-phase species are measured using FTIR spectroscopy, and these spectra are processed to obtain the temporal behavior of the evolved species using the HITRAN data base. A species conservation model was developed to simulate the confined rapid thermolysis experiments. The model incorporates the detailed liquid-phase reaction mechanism. The rate parameters in the reaction mechanism were optimized by comparing the experimental and computational results. With the optimized parameters, the computational model reproduces the experimentally observed trends with reasonable accuracy. Some of the deviations can be explained by experimental uncertainty. Based on the use of the computational model, initiation of decomposition occurs by HONO elimination. The subsequent decomposition occurs via the pathway starting with HONO addition and followed by ring opening. The detailed reaction mechanism containing 321 species and 500 elementary reactions was reduced to 53 species and 56 reactions using a sensitivity analysis.
       
  • Dynamics and kinematics of the reactive scalar gradient in weakly
           turbulent premixed flames
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Song Zhao, Aimad Er-raiy, Zakaria Bouali, Arnaud Mura In turbulent flames, chemical species mixing rates are controlled to a large extent by the geometric alignment of the species composition gradients with the local velocity gradients. This alignment indeed characterizes the turbulence–scalar interaction (TSI), which is one of the leading-order source terms in the scalar dissipation rate (SDR) evolution. In situations featuring density variations, which may be relevant either to (i) non-reactive but multiphase or compressible flows, or to (ii) reactive flows such as those considered herein, it should be acknowledged that there is still some controversy about the role of dilatational effects and its connection with the rotation of the strain-rate tensor principal axes. These two issues are analyzed by considering direct numerical simulation databases of flame kernel growth in homogeneous isotropic turbulence (HIT). Two distinct conditions of turbulence–combustion interaction (TCI) are considered: the first is a quasi-laminar reference case while the second displays some local thickening of the preheating zone. Special emphasis is placed on the kinematics of the scalar gradient orientation, which is studied through the direct inspection of the reactive scalar gradient orientation budget. To the best of the authors’ knowledge, this is first time that such a budget is scrutinized in premixed combustion conditions. The analysis shows that, for the flow conditions that are presently investigated, the role associated to the rotation of the strain-rate eigenframe is of paramount importance: it is the leading-order term in the scalar gradient orientation budget. This study also confirms that, in the vicinity of the flame, the resulting evolution opposes the rise of the reactive scalar gradient norm (i.e., the rise of the SDR) but the corresponding effects are lessened as the normalized root mean square (RMS) of the velocity fluctuations is increased.
       
  • On the Effective Density of Soot Particles in Premixed Ethylene Flames
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Mengda Wang, Quanxi Tang, Junyu Mei, Xiaoqing You In this work, the effective density of soot particles was studied in burner-stabilized-stagnation premixed ethylene flames, at an equivalent ratio of 2.0 and over the maximum flame temperature of 1747 K 
       
  • How to ensure the interpretability of experimental data in Rapid
           Compression Machines' A method to validate piston crevice designs
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Nicolas Bourgeois, Hervé Jeanmart, Grégoire Winckelmans, Olivier Lamberts, Francesco Contino In most Rapid Compression Machine (RCM) operations, it is sought to have a homogeneous temperature field inside the core region of the reaction chamber so that the adiabatic core assumption may be appropriately applied. Pistons with crevice on their side have shown to be effective in producing a uniform temperature field at the end of the compression. Although the efficiency of the crevice to produce a homogeneous environment is highly dependent on the operational conditions (pressure, temperature, gases composition, compression time, etc.), crevice designs are seldom adapted to the intended experiments. This is problematic as there are potentially many experiments which do not respect the conditions to obtain a uniform temperature field required to correctly interpret the data. Actually, no method other than CFD simulations has been proposed to this day to validate a specific crevice design, which is in practice most often an obstacle to the systematic validation of crevices. The model proposed in this study aims to fill this gap in allowing to check if a crevice design yields a homogeneous temperature field in the considered operational conditions. It is observed that as soon as a critical mass is transported from the chamber to the crevice, the crevice fulfills its role and guarantees the temperature homogeneity of the core region. The two key findings of the present work are therefore to be able to predict this critical mass for any possible configuration, and to propose an easy method to assess whether a specific crevice volume ensures this critical mass to be effectively sucked from the chamber. The whole process proposed in this study could then be applied as a certification of crevice design prior to any RCM experimental campaign.
       
  • Effects of fuel-bound methyl groups and fuel flow rate in the diffusion
           flames of aromatic fuels on the formation of volatile PAHs
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Gerardo D.J. Guerrero Peña, Vinu Pillai, Abhijeet Raj, Joaquin L. Brito Aromatic hydrocarbons constitute a significant fraction of fossil-derived transportation fuels. They are also used as additives to suppress autoignition and to increase fuel energy density. However, they produce harmful polycyclic aromatic hydrocarbons (PAHs) and soot during combustion. In this work, the effects of methyl group(s) and the flow rate of aromatic fuels (benzene, toluene, and m-xylene) on the concentration of volatile PAHs bound to soot formed in diffusion flames are studied. Soxhlet extraction using dichloromethane as the solvent is used to extract PAHs from soot particles collected from the flames of aromatic fuels. The extracts are analyzed using a gas chromatograph coupled to a mass spectrometer. Among the sixteen priority PAHs, fifteen of them are detected on soot particles along with benzo(e)pyrene and other organic compounds such as oxygenated compounds and mono-aromatic hydrocarbons. The PAHs containing two to three aromatic rings were found to be in higher concentrations than those with four or more rings. The total amount of PAHs collected at different fuel flow rates from the three aromatic fuels increased marginally in the following order: benzene 
       
  • Shock tube study of normal heptane first-stage ignition near 3.5 atm
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Matthew F. Campbell, Shengkai Wang, David F. Davidson, Ronald K. Hanson Shock tube ignition delay times and species time history measurements for Primary Reference Fuels (PRFs) such as normal heptane provide targets for the validation of combustion models, which in turn are used to develop more fuel-efficient engines that have smaller environmental footprints. However, a review of the literature has revealed that most of the shock tube ignition delay time and species measurement data for normal heptane have been obtained at elevated pressures, rather than at relatively low pressures where many other important experimental techniques such as jet-stirred reactors and flow reactors can provide corroborating results. One central problem preventing previous shock tube studies from examining first-stage ignition at these lower pressures was that ignition times were too long under these conditions to be measured within the available shock tube test times. To address this issue, recent advances in shock tube techniques for achieving long uniform test times have been applied in order to measure low-pressure first-stage ignition times of normal heptane together with normal heptane fuel time-history records at times up to about 30 ms. These measurements were performed in the Negative Temperature Coefficient (NTC) region (T=664−792 K) in lean mixtures (21%O2/Ar, equivalence ratio ϕ=0.5) at pressures of roughly P=3.5 atm using both the conventional and Constrained Reaction Volume (CRV) shock tube filling strategies. The data have been used to evaluate the performance of several combustion models, have been compared with other higher-pressure shock tube first-stage ignition times in n-heptane/20–21%O2 mixtures found in the literature, and have been fit using a two-zone Arrhenius model for first-stage ignition delay times. The results showed that some combustion models yield ignition delay time predictions that differ from the experimental results by as much as an order of magnitude, that under these conditions n-heptane first-stage ignition times scale by roughly P−0.71 but are insensitive to ϕ, and that the fraction of fuel remaining after first-stage ignition increases with increasing initial experimental temperature.
       
  • Mesoscopic simulation of nonequilibrium detonation with discrete Boltzmann
           method
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Chuandong Lin, Kai H. Luo Thanks to its mesoscopic nature, the recently developed discrete Boltzmann method (DBM) has the capability of providing deeper insight into nonequilibrium reactive flows accurately and efficiently. In this work, we employ the DBM to investigate the hydrodynamic and thermodynamic nonequilibrium (HTNE) effects around the detonation wave. The individual HTNE manifestations of the chemical reactant and product are probed, and the main features of their velocity distributions are analyzed. Both global and local HTNE effects of the chemical reactant and product increase approximately as a power of the chemical heat release that promotes the chemical reaction rate and sharpens the detonation front. With increasing relaxation time, the global HTNE effects of the chemical reactant and product are enhanced by power laws, while their local HTNE effects show changing trends. The physical gradients are smoothed and the nonequilibrium area is enlarged as the relaxation time increases. Finally, to estimate the relative height of detonation peak, we define the peak height as H(q)=(qmax−qs)/(qvon−qs), where qmax is the maximum of q around a detonation wave, qs is the CJ solution and qvon is the ZND solution at the von-Neumann-peak. With increasing relaxation time, the peak height decreases, because the nonequilibrium effects attenuate and widen the detonation wave. The peak height is an exponential function of the relaxation time.
       
  • Staggered swirler arrangement in two self-excited interacting swirl flames
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Taesong Lee, Jiho Lee, Junhyeong Park, Dongsik Han, Kyu Tae Kim Interference of acoustic and convective disturbances controls the development of self-excited combustion oscillations of a lean-premixed swirl-stabilized flame with a central bluffbody. How this interference mechanism influences the dynamics of multiple interacting flames in a multi-nozzle environment is currently unknown. Here we present observations of a multi-nozzle system's response to staggered swirler arrangements (ξsw, 1 ≠ ξsw, 2) as compared to non-staggered arrangements; the distance between the swirler and the flame is the dominant length scale of vortical disturbances. Our results demonstrate that a slight modification of the swirler arrangement in the streamwise direction – staggered or non-staggered – has a remarkable influence on the stability map of the whole combustion system. Phase-resolved flame imaging measurements indicate that under non-staggered conditions interacting swirl flames feature a coherent motion during a period of oscillation. By contrast, the staggered swirler combination creates significantly non-symmetric flame dynamics, disturbing the development of well-organized motion over the entire reaction zone. Flame surface modulations in the lateral direction are particularly pronounced due to the formation of non-symmetric convection delays of vortical disturbances between adjacent swirl nozzles. For a given swirler arrangement, the system's response to a wide range of combinations of mean nozzle velocities, including symmetric (u¯1=u¯2) and non-symmetric (u¯1≠u¯2) conditions, were explored to account for the simultaneous effects of the two convection parameters. Our data show that a major determinant of the onset of the instability is the combination of the Strouhal numbers, 〈St1, St2〉, which can be even or uneven depending on the manipulation of the convection time of each nozzle.
       
  • Identification of local extinction and prediction of reignition in a
           spark-ignited sparse spray flame using data mining
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Andrew P. Wandel Direct Numerical Simulations (DNS) of droplet fields which are ignited using a spark are investigated to deduce any behaviour that distinguishes between the cases where successful flame propagation occurs and where a flame ignites but subsequently extinguishes. At the instant the spark was deactivated, some of the studied cases displayed no local extinction, others showed some local extinction (one with reignition and the rest with global extinction) and the rest showed global extinction. The gaseous field at this instant was analysed using the data mining technique the Gaussian Mixture Model on each case separately; this method groups data points, enabling distinction between the various behaviours. The results from this analysis showed that in the case with local extinction–reignition, the regions of space near the flame kernel which produced local quenching were caused by evaporating droplets. These regions of local quenching were relatively small compared to the strong flame front surrounding them; the regions of local quenching were also relatively far from the centre of the flame kernel. In contrast, in cases with local then global extinction, the droplets created regions which were extensions of the relatively-small flame front, and these regions behaved in a similar manner to the flame propagation. As a consequence, these cases were unable to support a self-sustaining flame. Such distinctive behaviour promises opportunities to detect situations where global extinction is imminent and implement appropriate control strategies to prevent global extinction.
       
  • Quantification of the resonance stabilized C4H5 isomers and their reaction
           with acetylene
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Can Huang, Bin Yang, Feng Zhang, Guangjun Tian The resonance stabilized C4H5 radicals (CH3CCCH2, 2-C4H5; CH2CHCCH2, i-C4H5; CH3CHCCH, 12-C4H5) are among the most important precursors of benzene in hydrocarbon flames. Previous studies have revealed that i-C4H5 mainly reacts with acetylene to produce fulvene which promptly transforms to benzene, while the contribution from 2-C4H5 and 12-C4H5 remains unclear due to the obscure composition of these isomers in flames and the lack of accurate rate constants for related reactions. In the present work, we first calculated the cross sections of the resonance stabilized C4H5 radicals to quantify their composition in hydrocarbon flames (Hansen et al., 2006). The ratio of i-C4H5/(12-C4H5 + 2-C4H5) in the flame zone is deduced as 0.8-1.2 in fuel-rich allene, propyne, cyclopentene or benzene flames, and 2-C4H5 constitutes more than 70% in the sum of 2-C4H5 and 12-C4H5. We further studied the reaction kinetics of 2-/12-C4H5 with acetylene. Similar to i-C4H5, 2-/12-C4H5 tends to produce fulvene rather than directly form benzene when reacting with acetylene. However, the reaction rates of C2H2 + 2-/12-C4H5 are ∼one magnitude lower than that of i-C4H5 under combustion conditions. The role of the reaction between 2-/12-C4H5 and acetylene is controlled by the combined effect of concentration and reaction rates. By including above computational results into kinetic modeling, we finally conclude that although the concentration of 2-/12-C4H5 is comparable to that of i-C4H5, their contribution to the first aromatic ring in hydrocarbon flames from acetylene addition is limited. However, considering the noticeable concentration and the resonance stabilized structure, these species still have potential to generate aromatics.
       
  • Detonation initiation in pipes with a single obstacle for mixtures of
           hydrogen and oxygen-enriched air
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Sergio Bengoechea, Joshua A.T. Gray, Julius Reiss, Jonas P. Moeck, Oliver C. Paschereit, Jorn Sesterhenn This work presents an experimental and numerical study of a pulsed detonation combustion chamber. It consists of a pipe obstructed by one convergent–divergent nozzle, filled with a stoichiometric mixture of hydrogen and oxygen-enriched air. The proposed geometry is analysed with regard to its influence on the outset of detonation and its suitability for pulse detonation engines (PDEs). The study reveals the essential aspects for detonation initiation. The results of one of the configurations indicate a deterministic and reliable deflagration-to-detonation transition (DDT) with a short run-up distance, crucial for technical applications. The simulation reproduces the measurements in great detail and the origin of detonation is unequivocally identified.
       
  • Large deformation and gas retention during cookoff of a plastic bonded
           explosive (PBX 9407)
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Michael L. Hobbs, Michael J. Kaneshige, Cole D. Yarrington We have used several configurations of the Sandia Instrumented Thermal Ignition (SITI) experiment to develop a pressure-dependent, four-step ignition model for a plastic bonded explosive (PBX 9407) consisting of 94 wt.% RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), and a 6 wt.% VCTFE binder (vinyl chloride/chlorotrifluoroethylene copolymer). The four steps include desorption of water, decomposition of RDX to form equilibrium products, pressure-dependent decomposition of RDX forming equilibrium products, and decomposition of the binder to form hydrogen chloride and a nonvolatile residue (NVR). We address drying, binder decomposition, and decomposition of the RDX component from the pristine state through the melt and into ignition. We used Latin Hypercube Sampling (LHS) of the parameters to determine the sensitivity of the model to variation in the parameters. We also successfully validated the model using one-dimensional time-to-explosion (ODTX and P-ODTX) data from a different laboratory. Our SITI test matrix included 1) different densities ranging from 0.7 to 1.63 g/cm3, 2) free gas volumes ranging from 1.2 to 38 cm3, and 3) boundary temperatures ranging from 170 to 190 °C. We measured internal temperatures using embedded thermocouples at various radial locations as well as pressure using tubing that was connected from the free gas volume (ullage) to a pressure gauge. We also measured gas flow from our vented experiments. A borescope was included to obtain in situ video during some SITI experiments. We observed significant changes in the explosive volume prior to ignition. Our model, in conjunction with data observations, imply that internal accumulation of decomposition gases in high density PBX 9407 (90% of the theoretical maximum density) can contribute to significant strain whether or not the experiment is vented or sealed.
       
  • On imaging nascent soot by transmission electron microscopy
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Kevin Wan, Dongping Chen, Hai Wang High-resolution Transmission Electron Microscopy (HRTEM) imaging of nascent soot was carried out with an emphasis in demonstrating the annealing of soot samples under continuous irradiation of the high-energy electron beam. Images were taken for several soot samples over the duration of 16 min in 2 min time intervals to reveal the crystallization process. Fringe properties, including fringe length, tortuosity, and spacing were analyzed over the duration of the imaging. The sensitivity of the fringe properties to the apparent changes in the nanostructures imaged was examined. The difficulties in quantifying soot composition and structures are further illustrated by analyzing simulated TEM images of molecular-dynamics generated particles. Together, the results highlight some of the challenges in using HRTEM to obtain unambiguous structural properties for nascent soot particles.
       
  • An analysis of the ignition limits of premixed hydrogen/oxygen by heated
           nitrogen in counterflow
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Shangpeng Li, Wenkai Liang, Qiang Yao, Chung K. Law The non-monotonic ignition response of counterflowing premixed hydrogen/oxygen mixtures with nitrogen dilution versus heated nitrogen is studied numerically and theoretically. It is shown that the three ignition limits can be theoretically obtained by considering only the linear system involving at most only one radical in each reaction, while the influences of the nonlinear reactions, each involving two radicals, together with thermal feedback, introduce higher-order corrections, particularly for the third ignition limit. It is also demonstrated that the high diffusivity of H2 promotes ignition at the third limit. On the other hand, the high diffusivity of the H atom suppresses ignition at the first limit, while the assumption of unity Lewis number for H yields remarkably good results for the other two limits. Furthermore, by solving the time evolution of the crucial H and HO2 radicals, simplified formulations of the three individual limits and the two quadratic double limits are obtained analytically, in analogy with results for the homogeneous explosion problem.
       
  • Differential diffusion effect on the stabilization characteristics of
           autoignited laminar lifted methane/hydrogen jet flames in heated coflow
           air
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Ki Sung Jung, Seung Ook Kim, Tianfeng Lu, Suk Ho Chung, Bok Jik Lee, Chun Sang Yoo The characteristics of autoignited laminar lifted methane/hydrogen jet flames in heated coflow air are numerically investigated using laminarSMOKE code with a 57-species detailed methane/air chemical kinetic mechanism. Detailed numerical simulations are performed for various fuel jet velocities, U0, with different hydrogen ratio of the fuel jet, RH, and the inlet temperature, T0. Based on the flame characteristics, the autoignited laminar lifted jet flames can be categorized into three regimes of combustion mode: the tribrachial edge flame regime, the Moderate or Intense Low-oxygen Dilution (MILD) combustion regime, and the transition regime in between. Under relatively low temperature and high hydrogen ratio (LTHH) conditions, an unusual decreasing liftoff height, HL, behavior with increasing U0 is observed, qualitatively similar to those of previous experimental observations. From additional simulations with modified hydrogen mass diffusivity, it is substantiated that the unusual decreasing HL behavior is primarily attributed to the high diffusive nature of hydrogen molecules. The species transport budget, autoignition index, and displacement speed analyses verify that the autoignited lifted jet flames are stabilized by autoignition-assisted flame propagation or autoignition depending on the combustion regime. Chemical explosive mode analysis (CEMA) identifies important variables and reaction steps for the MILD combustion and tribrachial edge flame regimes.
       
  • Hetero-/homogeneous chemistry interactions and flame formation during
           methane catalytic partial oxidation in rhodium-coated channels
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Behrooz O. Arani, John Mantzaras, Christos E. Frouzakis, Konstantinos Boulouchos The hetero-/homogeneous chemistry interactions during the catalytic partial oxidation (CPO) of methane were investigated numerically in rhodium-coated cylindrical channels using axisymmetric simulations with detailed catalytic and gas-phase chemistries, conjugate heat transfer in the solid wall and detailed transport. Simulated conditions spanned pressures 1–25 bar, methane-to-air equivalence ratios 2.5–4.0, inlet temperatures 300–900 K and channel diameters 0.5–2.0 mm. The formation of vigorous flames in the oxidation zone of the CPO reactor was promoted as the pressure, inlet temperature and channel diameter increased. The catalytic pathway induced a strong radial stratification of the reactant and temperature distributions over the homogeneous combustion zones. This in turn resulted in flames spatially confined to the channel core, such that the catalytic wall temperature was only modestly affected by the flames (∼25 K wall temperature rise due to the flame presence), a result highly desirable for the reactor thermal management and for the catalyst thermal stability. Even when strong flames were formed, combined hetero-/homogeneous combustion persisted over the entire axial extent of the flames. The deficient oxygen reactant leaked through the flame zones and was subsequently converted catalytically on the channel walls, with the oxygen leakage increasing as the channel diameter, pressure, and inlet temperature decreased. Extensive parametric simulations delineated the regimes of operating conditions and geometrical parameters (pressure, inlet temperature, equivalence ratio and channel diameter) for which gas-phase combustion could not be ignored during methane CPO over rhodium. It was shown that for practical power generation systems (pressures and inlet temperatures above 15 bar and 600 K, respectively) gaseous chemistry could not be neglected and offered the benefit of reducing the extent of the oxidation zone and hence the overall reactor length.
       
  • Mechanism of cellulose fast pyrolysis: The role of characteristic chain
           ends and dehydrated units
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Qiang Lu, Bin Hu, Zhen-xi Zhang, Yu-ting Wu, Min-shu Cui, Ding-jia Liu, Chang-qing Dong, Yong-ping Yang Understanding the fundamental reactions and mechanisms during biomass fast pyrolysis is essential for the development of efficient pyrolysis techniques. In this work, quantum chemistry calculation, kinetic analysis and fast pyrolysis experiment were combined to reveal the cellulose pyrolysis mechanism. During cellulose pyrolysis, the indigenous interior units, reducing end (RE end) and non-reducing end (NR end) initially form various characteristic chain ends and dehydrated units which then evolve into different pyrolytic products. As the rising of the degree of polymerization (DP), reactions occurring at the interior unit and NR end are more competitive than those taking place at the RE end, resulting in distinct pyrolytic product distribution for cellulose and glucose-based carbohydrates. The reactions occurring at the three indigenous units of cellulose chain all favor the formation of levoglucosan-terminated end (LG end) and/or NR end, which then generate levoglucosan (LG). The acyclic d-glucose end (AG end), which mainly derives from the RE end, is essential for the formation of 1,6-anhydro-β-d-glucofuranose (AGF), 1,4:3,6-dianhydro-α-d-glucopyranose (DGP), furfural (FF), 5-hydroxymethyl furfural (5-HMF) and hydroxyacetaldehyde (HAA). Compared with the chain ends, the dehydrated units are not feasible to be generated, and their decomposition favors the formation of HAA.
       
  • Dynamic responses of counterflow nonpremixed flames to AC electric field
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Dae Geun Park, Suk Ho Chung, Min Suk Cha Although ionic wind has been observed to play important roles in the effects of electric fields on flames, there is a lack of systematic quantification of ionic wind that allows interpretation of a flame's responses to electric fields. Here, we report on various responses of nonpremixed flames, such as the flame's dynamic responses and the generation of bidirectional ionic wind, in relation to the applied voltage and frequency of an alternating current (AC) in a counterflow burner. We find that although the Lorentz force acting on charged molecules initiates related effects, each effect is both complex and different. When the applied voltage is in the sub-saturated regime (small) as determined by the voltage-current behavior, flame movements and flow motion are minimally affected. However, when the applied voltage is in the saturated regime (large), flame oscillation occurs and a bidirectional ionic wind is generated that creates double-stagnation planes. The flame's oscillatory motion could be categorized in the transport-limited regime and in the oscillatory decaying regime, suggesting a strong dependence of the motion on the configuration of the burner. We also observed bidirectional ionic wind in visibly stable flames at higher AC frequencies. We present detailed explanations for flame behaviors, electric currents, and flow characteristics under various experimental conditions.
       
  • Soot evolution and flame response to acoustic forcing of laminar
           non-premixed jet flames at varying amplitudes
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Kae Ken Foo, Zhiwei Sun, Paul R. Medwell, Zeyad T. Alwahabi, Graham J. Nathan, Bassam B. Dally New details regarding the soot evolution and its controlling parameters in steady and forced flames have been studied using high spatial resolution laser diagnostic techniques. Steady laminar non-premixed ethylene/nitrogen flames with three different diameters burners were acoustically forced using a loudspeaker. 10-Hz-sinusoidal signals of different amplitudes were transmitted to the loudspeaker to drive the flames. The results reveal that the spatial correlation between the soot field and the temperature profile is influenced by the burner diameter and forcing conditions. The soot field in steady laminar flames is confined to a relatively narrow temperature range, 1500–2000 K. In contradiction, the soot field in forced flames spread across a wider range of temperature, 1400–2100 K. Furthermore, the spatial correlation between the normalised soot concentration and primary particle size can be described with an exponential function. While it is observed that the exponential coefficients vary with burner diameter and forcing conditions, further study is necessary for a better understanding. In general, laminar flames forced at a lower amplitude (α=25%) tend to produce less soot than moderately forced (α=50%) flames. Further increasing the forcing amplitude to α=75% does not increase the soot production in laminar flames; conversely, lower peak and volume-integrated soot volume fraction are observed in the strongly forced flame (α=75%) as relative to the moderately forced counterpart. These findings shed new light on the seemingly contradictory results published in the literature regarding the effect of the forcing intensity on the soot production.
       
  • Direct simulation Monte Carlo modeling of
           H2–O2 deflagration waves
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Israel B. Sebastião, Li Qiao, Alina Alexeenko Combustion at extreme conditions such as high-speed and microscale involve nonequilibrium transport and chemical reactions that require atomistic treatment of molecular processes. We present a framework for applying the direct simulation Monte Carlo method (DSMC) to model combustion at the molecular scale. We show that the standard DSMC approach employing Total Collision Energy (TCE) chemistry and Larsen–Borgnakke (LB) energy exchange models is not applicable for combustion simulations which are dominated by exchange and recombination reactions. A methodology for modifying the TCE-LB approach is developed to ensure detailed balance and relaxation towards thermal equilibrium regardless of the internal energy relaxation rates. A simplified 6-species and 7-reversible reaction mechanism with rates modified to account for discrete vibrational levels in DSMC is used for the benchmark flame study. The laminar flame structure of H2–O2 premixed systems and the corresponding deflagration wave speeds by DSMC are compared with PREMIX results. The DSMC simulations based on the extended TCE-LB framework correctly reproduces the 1-D flame structure and its propagation speed is consistent with continuum modeling predictions for the same reaction mechanism and similar flow conditions. The DSMC approach presents opportunities to study combustion phenomena at the molecular scale including state-to-state processes in conditions far from thermal equilibrium for improved combustion diagnostics and control.
       
  • Detailed kinetic model for ammonium dinitramide decomposition
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Yu-ichiro Izato, Atsumi Miyake Ammonium dinitramide (ADN; [NH4]+[N(NO2)2]−) is the most promising oxidizer for use with future green solid and liquid propellants for spacecraft applications. To allow the effective development and use of ADN-based propellants, it is important to understand ADN reaction mechanisms. This work presents a detailed chemical kinetics model for the liquid phase reactions of ADN based on quantum chemical calculations. The thermal corrections, entropies, and heat capacities of chemical species were calculated from the partition function using statistical machinery based on the G4 level of theory. Rate coefficients were also determined to allow the application of transition state theory and variational transition state theory to reactions identified in our previous study. The new model employed herein simulates the thermal decomposition of ADN under specific heating conditions and successfully predicts heats of reaction and the gases that result from decomposition under those conditions. The thermal behavior predicted from the new model was an excellent match with the experimental behavior observed from thermal analysis using differential scanning calorimetry and Raman spectroscopy. The new kinetic model reveals the mechanism for the decomposition of ADN.
       
  • Visualization of detonation propagation in a round tube equipped with
           repeating orifice plates
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Georgina Rainsford, Deepinder Jot Singh Aulakh, Gaby Ciccarelli Self-luminous, high-speed photography was used to visualize fast-flame and detonation propagation through a transparent round tube equipped with repeating orifice plates, in stoichiometric hydrogen-oxygen mixtures at initial pressures up to 60 kPa. Experiments were conducted in a 1.55 m, 7.6 cm inner-diameter plastic tube filled with equally spaced 5.33 cm and 3.81 cm orifice plates (50% and 75% area blockage ratio, respectively). The unprecedented visualization of quasi-detonation propagation in a round tube was used to identify the propagation mechanisms. For both sets of orifice plates, fast-flames were observed below a critical initial pressure. Fast-flame propagation involved the interaction of an uncoupled shock wave and flame with the orifice plates. Detonation propagation involved repeated detonation failure and initiation along the channel length; the limits measured in the 50% and 75% blockage ratio (BR) orifice plates were 7 kPa and 40 kPa, respectively. The orifice diameter-to-detonation cell size ratio (d/λ) corresponding to these limits are 1.4 and 14, respectively. It is proposed that the significant variance in the d/λ at the two limits is attributable to the difference in the detonation propagation mechanism. For the 50% BR orifice plates, near the limit, detonation initiation occurred on the tube wall between orifice plates following reflection of the lead shock wave. Whereas, for the 75% BR orifice plates, detonation initiation at the tube wall was not possible for initial pressures up to 40 kPa. This is the result of a weaker shock wave at the time of reflection due primarily to the larger distance from the orifice edge to the tube wall. Steady propagation of a curved detonation wave was observed for the 50% BR orifice plates for an initial pressure of 50 kPa (d/λ = 25), or greater; a similar propagation was not observed in the 75% BR orifice plates at initial pressures up to 60 kPa (d/λ = 27). Numerical simulations carried out using a single-step reaction model demonstrated the key processes involved in detonation initiation at the tube wall and the orifice plate but could not predict quantitatively the critical initial pressure required for detonation propagation measured in the experiments.
       
  • Effect of paraffin wax on combustion properties and surface protection of
           Al/CuO-based nanoenergetic composite pellets
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Kyung Joo Kim, Myung Hoon Cho, Ji Hoon Kim, Soo Hyung Kim We systematically investigated the effect of a polymer binder on various combustion properties and surface protection of nanoenergetic composite pellets containing Al and CuO nanoparticles (NPs) as the fuel and oxidizer, respectively. Al/CuO NP-based composite pellets were then fabricated by a pelletization process and the effect of paraffin wax (PW) binder concentration was investigated. The burn rate decreased with increasing PW content as the binder thermochemically interfered with the aluminothermic reaction between Al and CuO. However, the presence of a critical amount of PW (
       
  • Soot formation in shock-wave-induced pyrolysis of acetylene and benzene
           with H2, O2, and CH4 addition
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Alexander Drakon, Alexander Eremin, Ekaterina Mikheyeva, Bo Shu, Mustapha Fikri, Christof Schulz Experiments on the pyrolysis of C2H2/Ar and C6H6/Ar mixtures with addition of H2, O2, and CH4 have been carried out behind reflected shock waves at temperatures ranging from 1400 to 2600 K. Soot formation was measured by laser extinction at 633 nm. Time-resolved temperature measurements were performed via two-color CO absorption on the P(8) and R(21) lines at 2111.54 and 2191.50 cm-1 using quantum-cascade lasers. For this purpose, 0.5–0.8% CO was added to the gas mixtures. The measured temperature dependence of soot formation in experiments with added O2, and CH4 was corrected for the temperature effect caused by the thermochemistry of either endothermic pyrolysis or exothermic oxidation or reactions that cause time-dependent deviation from the initial frozen-shock temperatures. In all mixtures, the addition of H2 resulted in a noticeable decrease of the soot yield. A considerable increase in the soot yield was found with addition of methane to acetylene mixtures. In contrast, in benzene mixtures, the addition of methane caused a decrease of the soot yield. The qualitative analysis of the kinetics of the gas-phase stage of the pyrolysis reactions elucidated the influence of all investigated additives on the change in the key routes of initial stages of PAH and soot formation. We observed that the addition of H2 to acetylene inhibits the initial stages of the pyrolysis reaction, while the addition of CH4 and O2 opens up new ways for the formation of benzene and phenyl and following growth of pyrene. In contrast to that, in benzene all the additives studied lead to the suppression of the kinetics pathways for the formation of pyrene and the subsequent growth of soot.
       
  • ReaxFF simulations of petroleum coke sulfur removal mechanisms during
           pyrolysis and combustion
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Qifan Zhong, Qiuyun Mao, Jin Xiao, Adri C.T. van Duin, Jonathan P. Mathews Green petroleum coke (petcoke) is used as a feedstock for raw carbon material or as a fuel. Petcoke with high sulfur (S) content (>4 wt%) is typically restricted to fuel use unless extensive S removal is successful. Here, the S removal mechanisms during both pyrolysis and combustion were explored using the Reactive Force Field (ReaxFF) MD approach. A structural representation (C1648H772O59N24S47) of a green Qingdao petcoke was generated coupling high-resolution transmission electron microscopy lattice fringe image analysis and analytical data. This structure was consistent with elemental, aromaticity (FT-IR), the pair correlation function (XRD), and functional group (S, O, and N from XPS) data. The ReaxFF pyrolysis simulation produced gas and tar yields of 44.7 and 11.0 wt% at 3000 K after 250 ps of simulation. The combustion simulation on the same initial structure was performed in an O2 environment. During the pyrolysis simulation, the first-step for S-removal was thiophenic sulfur conversion to C1–4S (mostly C2S), COS, or CNS. The heteroatom pyrolysis overlapped, for this structure, at these conditions. However, for the combustion simulation earlier conversion of thiophenic sulfur to COS was observed. No NS containing structures occurred in this O-rich environment, as pyrrolic and pyridinic N quickly oxidized into CON or NO compounds. The S transformation during combustion can be summarized by COS → CO2S → CO3S → CO4S. The H atoms reacted with S-containing gases like COS/C2S/CNS producing HS and H2S rather than with the coke-S.
       
  • Experimental and numerical studies on detonation reflections over
           cylindrical convex surfaces
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Jian Li, Jianguo Ning The detonation reflection over a cylindrical convex surface was investigated experimentally and numerically by focusing on the length-scale effect on the reflection process, such as the triple-point trajectory and the critical wedge angle at which a transition occurs from regular reflection to Mach reflection. The results show that the critical wall angle plots exhibit significant scatter because of the cellular properties of the detonation front. If the transverse spacing is large as compared to the radius of curvature, the scatter range extends. If the transverse spacing is small as compared to the radius of curvature, the scattering is dramatically reduced. The critical wall angle is found to mainly depend on the scaled length i.e., the radius of curvature (R) over the cell size λ (or the reaction zone thickness Δ). Moreover, the critical wall angle increases with the decrease in the detonation thickness or with the increase in the radius. As R/λ increases to approximately ten, the critical wall angle approaches a value calculated using the non-reactive two-shock theory for pseudo-steady flows. The numerical results reveal that the transition to Mach reflection occurs earlier in the case of a ZND detonation than in the case of an inert shock wave because of the higher sound speed due to the release of chemical heat.
       
  • High-pressure 1D fuel/air-ratio measurements with LIBS
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Yue Wu, Mark Gragston, Zhili Zhang, Paul S. Hsu, Naibo Jiang, Anil K. Patnaik, Sukesh Roy, James R. Gord Quantitative, one-dimensional (1D), single-laser-shot, fuel–air ratio (FAR) measurements in both laminar and turbulent methane–air flames were conducted using time-gated nanosecond-laser-induced breakdown spectroscopy (ns-LIBS) line imaging. In the laminar methane–air flames at a pressure of 1–11 bar, hydrogen (Hα) and nitrogen (NII) atomic emission lines at 568 and 656 nm, respectively, were selected to establish a correlation between the line intensities and the local FAR. The spatial calibration profiles of the N/H ratios in the flames at various pressures were obtained in one dimension. The effects of the laser energy and pressure on the stability and precision of the 1D FAR measurements were investigated. It was observed that the N/H correlation is significantly reduced at ∼11 bar, which sets the limits of the 1D LIBS-based FAR measurements. Single-laser-shot 1D FAR measurements were conducted in a turbulent flame at atmospheric pressure, and multiline LIBS was performed to extend the measurement area of interest. Spatially and spectrally resolved line LIBS can provide the local FAR with a spatial resolution of ∼0.1 mm. These results hold promise for the utilization of ns-LIBS for spatially resolved 1D FAR measurements in turbulent flames at elevated pressures.
       
  • Using SIMD and SIMT vectorization to evaluate sparse chemical kinetic
           Jacobian matrices and thermochemical source terms
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Nicholas J. Curtis, Kyle E. Niemeyer, Chih-Jen Sung Accurately predicting key combustion phenomena in reactive-flow simulations, e.g., lean blow-out, extinction/ignition limits and pollutant formation, necessitates the use of detailed chemical kinetics. The large size and high levels of numerical stiffness typically present in chemical kinetic models relevant to transportation/power-generation applications make the efficient evaluation/factorization of the chemical kinetic Jacobian and thermochemical source-terms critical to the performance of reactive-flow codes. Here we investigate the performance of vectorized evaluation of constant-pressure/volume thermochemical source-term and sparse/dense chemical kinetic Jacobians using single-instruction, multiple-data (SIMD) and single-instruction, multiple thread (SIMT) paradigms. These are implemented in pyJac, an open-source, reproducible code generation platform. Selected chemical kinetic models covering the range of sizes typically used in reactive-flow simulations were used for demonstration. A new formulation of the chemical kinetic governing equations was derived and verified, resulting in Jacobian sparsities of 28.6–92.0% for the tested models. Speedups of 3.40–4.08 ×  were found for shallow-vectorized OpenCL source-rate evaluation compared with a parallel OpenMP code on an avx2 central processing unit (CPU), increasing to 6.63–9.44 ×  and 3.03–4.23 ×  for sparse and dense chemical kinetic Jacobian evaluation, respectively. Furthermore, the effect of data-ordering was investigated and a storage pattern specifically formulated for vectorized evaluation was proposed; as well, the effect of the constant pressure/volume assumptions and varying vector widths were studied on source-term evaluation performance. Speedups reached up to 17.60 ×  and 45.13 ×  for dense and sparse evaluation on the GPU, and up to 55.11 ×  and 245.63 ×  on the CPU over a first-order finite-difference Jacobian approach. Further, dense Jacobian evaluation was up to 19.56 ×  and 2.84 ×  times faster than a previous version of pyJac on a CPU and GPU, respectively. Finally, future directions for vectorized chemical kinetic evaluation and sparse linear-algebra techniques were discussed.
       
  • A generalized flamelet tabulation method for partially premixed combustion
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Xu Wen, Xue-Song Bai, Kun Luo, Haiou Wang, Yujuan Luo, Jianren Fan A flamelet tabulation method for partially premixed flames is proposed, in which partially premixed flamelets are incorporated as the archetypal flamelet elements. This method considers triple flame structures with both the partial premixing of fuel in the oxidizer side and the partial premixing of oxidizer in the fuel side, by replacing the pure-air and pure-fuel in the counterflow diffusion flame with a range of fuel-lean and -rich mixtures, respectively. The thermo-chemical quantities in the partially premixed flamelet are stored in a four-dimensional flamelet library as a function of the mixture fraction Z, describing the mixing process, the reaction progress variable YPV, describing the progress of reactions, and the trajectory variables YF and YO, characterizing the partial premixings of fuel and oxidizer, respectively. The performance of the proposed partially premixed flamelet tabulation (PPFT) method is evaluated through both a priori and a posteriori tests on laminar tribrachial flames with different mixture fraction gradients. The PPFT results are compared with those from a premixed flamelet tabulation (PFT) method and a diffusion flamelet tabulation (DFT) method. It is found that the combustion-mode-sensitive species such as CO and H2 can be accurately predicted by the PPFT method for both the low and high mixture fraction gradient flame cases, which cannot be well predicted by the PFT and DFT methods.
       
  • Enhanced ignition of milled boron-polytetrafluoroethylene mixtures
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Trevor D. Hedman, Andrew R. Demko, Joseph Kalman The combustion and physical properties of boron-polytetrafluoroethylene (PTFE) mixtures were modified by ball milling. Examination of the milled material through optical microscopy, scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) reveal that the milled mixtures are more intimately mixed and arranged into larger aggregate particles. Differential scanning calorimetry indicates the appearance of an endothermic reaction, brought on by milling. Fourier transform infrared spectroscopy provides evidence that the milling process enhances the chemical reaction of boron and PTFE. The milled boron-PTFE mixtures are demonstrated to be more reactive than those mixed by hand, despite containing larger particle sizes. Laser-ignition studies of the materials show that milling boron-PTFE mixtures results in the ignition delay times being reduced by a factor of 2. The milled mixtures were able to sustain combustion in air and emitted a strong BO2 signal while simple physical mixtures do not. Enhanced reactivity of the milled materials is attributed to a combination of decreased diffusion lengths and disruption of the boron oxide shell during the milling process.
       
  • An experimental and modeling study of dimethyl ether/methanol blends
           autoignition at low temperature
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Hongfu Wang, Ruozhou Fang, Bryan W. Weber, Chih-Jen Sung New rapid compression machine (RCM) ignition delay data for dimethyl ether (DME), methanol (MeOH), and their blends are acquired at engine-relevant conditions (T = 600 K–890 K, P = 15 bar and 30 bar, and equivalence ratios of ϕ = 0.5, 1.0, and 2.0 in synthetic dry air). The data are then used to validate a detailed DME/MeOH model in conjunction with literature RCM and shock tube data for DME and MeOH. This detailed DME/MeOH model, constructed by systematically merging literature models for the combustion of the individual fuel constituents, is capable of accurately predicting the experimental ignition delay data at a wide range of temperatures and pressures. The experiments and simulations both show a non-linear promoting effect of DME addition on MeOH autoignition. Additional analyses are performed using the merged DME/MeOH model to gain deeper insight into the binary fuel blend autoignition, especially the promoting effect of DME on MeOH. It is found that the unimolecular decomposition of HO2CH2OCHO plays an essential role in low temperature DME/MeOH blend autoignition. The accumulation of HO2CH2OCHO before the first-stage ignition and later quick consumption not only triggers the first-stage ignition, but also causes the non-linear promoting effect by accumulating to higher levels at higher DME blending ratios. These analyses suggest the rate parameters of HO2CH2OCHO unimolecular decomposition are critical to accurately predict the first-stage and overall ignition delay times as well as the first-stage heat release profile for low temperature DME/MeOH oxidation.
       
  • Modelling spark-plug discharge in dry air
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Lucas W.S. Crispim, Patricia H. Hallak, Mikhail S. Benilov, Maikel Y. Ballester This work presents a novel numerical strategy for studying the electric discharge produced by a vehicular spark plug in dry air. For such a task an axial symmetric 2D domain is used. The starting gas mixture is formed by molecular nitrogen and oxygen (8:2 ratio). The mathematical model considers heat and species diffusion and convection jointly with a discrete sub-model for energy transfer in electronic, atomic and molecular collisions. Chemical reactions between species are also included. Solutions of source terms is accomplished in the frame of ZDPlaskin, a zero-dimensional plasma modelling tool. The used plasmo-chemical kinetics model includes 53 species and 430 processes. Experimental properties from an actual spark plug discharge are introduced to the simulation. Spatio-temporal evolution of species concentrations are obtained within this model. Gas temperature evolution and species distribution is discussed and compare with available values in literature.Graphical abstractGraphical abstract for this article
       
  • Effects of jet in crossflow on flame acceleration and deflagration to
           detonation transition in methane–oxygen mixture
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Han Peng, Yue Huang, Ralf Deiterding, Zhenye Luan, Fei Xing, Yancheng You The fluidic jet turbulator has been a novel perturbation generator in the pulse-detonation engines research field for the past few years. In this paper, an experiment is performed to study the deflagration to detonation transition (DDT) process in a detonation chamber with a reactive transverse methane–oxygen mixture jet in crossflow (JICF). The jet injection arrangement is fundamentally investigated, including single jet and various double jets patterns. Corresponding two-dimensional direct numerical simulations with a multistep chemical kinetics mechanism are employed for analyzing details in the flow field, and the interaction between the vortex and flame temporal evolution is characterized. Both the experiments and simulations demonstrate that the JICF can distinctly accelerate flame propagation and shorten the DDT time and distance. The vortex stream induced by the jet distorts and wrinkles the flame front resulting in local flame acceleration. Moreover, the double jet patterns enhance flame acceleration more than the single jet injection because of the intrinsic counter-rotating vortex pairs and enhanced turbulence intensity.
       
  • Study on the ignition mechanism of Ni-coated aluminum particles in air
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Sangmin Kim, Jihwan Lim, Sanghyup Lee, Jaechul Jeong, Woongsup Yoon Since aluminum responds to various oxidizers and has a high energy density, there are high expectations for its usefulness as a fuel. However, it is covered with an aluminum oxide film, which has a high melting point, and thus, its ignition is difficult. One method suggested to solve this problem is nickel coating; however, in contrast to the extensive amount of research conducted on the overall phenomenon of aluminum combustion, research regarding Ni-coated aluminum is still in nascent stages. This study was carried out to further elucidate the ignition mechanism; thus, millimeter-sized (∼2.38 mm) aluminum particles were used to observe the surface where ignition occurs in air. The spatial and temporal resolutions were heightened by prolonging the heating period. The aluminum particles were nickel coated using electro/electroless methods, and surface analysis by SEM, thermal analysis by TGA/DSC, and species analysis by XRD and EDS were carried out. In addition, two-wavelength pyrometry was used to measure the ignition temperature. The results show that regardless of the nickel content in the coating of the aluminum particles, the ignition temperature was approximately 2400 K, similar to the melting point of aluminum oxide. The thermodynamic and thermophysical characteristics of nickel, aluminum, aluminum oxide, and nickel (II) oxide, and the surface/cross-sectional analysis, thermal and species analysis, and high-speed cinematography of the quenched samples provided a detailed explanation of the ignition process. Through this ignition mechanism, the emitted spectrum of AlO (as an intermediate combustion material) was traced to explain the decrease in ignition delay with increase in nickel content.
       
  • Laminar combustion regimes for hybrid mixtures of coal dust with methane
           gas below the gas lower flammability limit
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Chris T. Cloney, Robert C. Ripley, Michael J. Pegg, Paul R. Amyotte Understanding flame propagation in dust clouds and hybrid mixtures requires knowledge of the fundamental combustion processes and their coupling interaction. The objective of this work is to use computational fluid dynamics to classify laminar flame structure in hybrid mixtures where the initial gas concentration is below the lower flammability limit. Particular focus is given to the role of reaction chemistry and overall equivalence ratio on flame structure and burning velocity. Through this study, five flame regimes were determined: fuel-lean flames (Type I), volatile-lean flames (Type II), volatile-rich flames (Type III), transition flames (Type IV), and kinetic-limited flames (Type V). Gas-phase chemistry was found to play a critical role in burning velocity for Type III, IV, and V flames. Burning velocities at hybrid volatile component equivalence ratios less than 0.9, were found to be less sensitive to reaction kinetics. Further research using this model will focus on initial gas concentrations above the lower flammability limit, exploring the flammability limits of hybrid mixtures, and extending the results to turbulent flames in system geometries relevant to industrial safety.
       
  • Impact of engine operating cycle, biodiesel blends and fuel impurities on
           soot production and soot characteristics
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Julie Schobing, Valerie Tschamber, Alain Brillard, Gontrand Leyssens, Eduard Iojoiu, Vincent Lauga The impact of engine operating cycle, Biodiesel blends and fuel impurities on soot production and soot properties are evaluated in the present work. To this end, soot were produced on engine test bench and then collected inside a Diesel Particulate Filter (DPF). Two engine cycles (a Natural Loading and an Accelerated Loading) were tested. A standard Euro VI fuel blended with 7% of Biodiesel (B7) and a pure Biofuel (B100 RME EN 14214) were used. This latter was additivated with potassium and phosphorus at a low (B100+) or at a high (B100++) concentration. Soot characterization through elemental analyses, nitrogen adsorption, Raman spectroscopy, TGA and TPO experiments show that the engine operating cycle impact the soot reactivity through modifications of their texture and structure. Test bench experiments also show that increasing Biodiesel blend from B7 to B100+ divides by five the soot production. Moreover, soot obtained with B100+ are more reactive because of higher oxygen and ash content. When the inorganic content of the fuel is increased, few effects on the soot production are observed but the soot reactivity is significantly increased. In fact, analyses highlight that impurities present in the fuel are retrieved inside the soot composition and then catalyze their oxidation. K has a beneficial effect on both passive and active regenerations. On the contrary, P inhibits the active regeneration but has a significant catalytic impact on the CNO2H2O reaction. Finally, a numerical simulation allows to extract the kinetic constants of real B7- and B100+-soot, whose values confirm the differences of the soot reactivity.
       
  • Fuel particle shape effects in the packed bed combustion of wood
    • Abstract: Publication date: December 2018Source: Combustion and Flame, Volume 198Author(s): Élizabeth Trudel, William L.H. Hallett, Evan Wiens, Jeremiah D. O'Neil, Marina K. Busigin, Dana Berdusco Experiments on the overfeed packed bed combustion and gasification of seven different shapes of parallelepipedal wood particles are presented. Attention is focussed on the part of the bed in which char conversion occurs; results for the pyrolysis zone at the top of the bed are not included. It is shown that fuel particle shape can affect conversion through the sphericity of the particle, through the orientation of the wood grain in the particle, and through the overlap of particles in the bed. These effects were incorporated into an existing numerical model of packed bed combustion and gasification. Particle sphericities for input to the model were determined directly from particle geometry, and particle overlap factors were estimated photographically. Comparison of predicted gas analyses and temperatures in the bed with experimental values then allowed the effect of the orientation of the original wood grain on char conversion to be estimated, with the conclusion that the rate of carbon conversion by the CO2 reduction reaction is faster by a factor of about 5 on surfaces normal to the wood fibres compared to the rate on surfaces parallel to the fibres. The carbon oxidation reaction at the bottom of the bed, on the other hand, is controlled by external gas phase diffusion and is not affected by the fibre orientation.
       
  • A Physics-based approach to modeling real-fuel combustion chemistry
           – III. Reaction kinetic model of JP10
    • Abstract: Publication date: Available online 18 September 2018Source: Combustion and FlameAuthor(s): Yujie Tao, Rui Xu, Kun Wang, Jiankun Shao, Sarah E. Johnson, Ashkan Movaghar, Xu Han, Ji-Woong Park, Tianfeng Lu, Kenneth Brezinsky, Fokion N. Egolfopoulos, David F. Davidson, Ronald K. Hanson, Craig T. Bowman, Hai Wang The Hybrid Chemistry (HyChem) approach has been proposed previously for combustion chemistry modeling of real, liquid fuels of a distillate origin. In this work, the applicability of the HyChem approach is tested for single-component fuels using JP10 as the model fuel. The method remains the same: an experimentally constrained, lumped single-fuel model describing the kinetics of fuel pyrolysis is combined with a detailed foundational fuel chemistry model. Due to the multi-ring molecular structure of JP10, the pyrolysis products were found to be somewhat different from those of conventional jet fuels. The lumped reactions were therefore modified to accommodate the fuel-specific pyrolysis products. The resulting model shows generally good agreement with experimental data, which suggests that the HyChem approach is also applicable for developing combustion reaction kinetic models for single-component fuels.
       
  • A physics-based approach to modeling real-fuel combustion chemistry –
           IV. HyChem modeling of combustion kinetics of a bio-derived jet fuel and
           its blends with a conventional Jet A
    • Abstract: Publication date: Available online 7 August 2018Source: Combustion and FlameAuthor(s): Kun Wang, Rui Xu, Tom Parise, Jiankun Shao, Ashkan Movaghar, Dong Joon Lee, Ji-Woong Park, Yang Gao, Tianfeng Lu, Fokion N. Egolfopoulos, David F. Davidson, Ronald K. Hanson, Craig T. Bowman, Hai Wang A Hybrid Chemistry (HyChem) approach has been recently developed for the modeling of real fuels; it incorporates a basic understanding about the combustion chemistry of multicomponent liquid fuels that overcomes some of the limitations of the conventional surrogate fuel approach. The present work extends this approach to modeling the combustion behaviors of a two-component bio-derived jet fuel (Gevo, designated as C1) and its blending with a conventional, petroleum-derived jet fuel (Jet A, designated as A2). The stringent tests and agreement between the HyChem models and experimental measurements for the combustion chemistry, including ignition delay and laminar flame speed, of C1 highlight the validity as well as potential wider applications of the HyChem concept in studying combustion chemistry of complex liquid hydrocarbon fuels. Another aspect of the present study aims at answering a central question of whether the HyChem models for neat fuels can be simply combined to model the combustion behaviors of fuel blends. The pyrolysis and oxidation of several blends of A2 and C1 were investigated. Flow reactor experiments were carried out at pressure of 1 atm, temperature of 1030 K, with equivalence ratios of 1.0 and 2.0. Shock tube measurements were performed for the blended fuel pyrolysis at 1 atm from 1025 to 1325 K. Ignition delay times were also measured using a shock-tube. Good agreement between measurements and model predictions was found showing that formation of the products as well as combustion properties of the blended fuels were predicted by a simple combination of the HyChem models for the two individual fuels, thus demonstrating that the HyChem models for two jet fuels of very different compositions are “additive.”
       
 
 
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