A reactive molecular dynamics simulation study of methane oxidation assisted by platinum/graphene-based catalysts Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 Muye Feng, Xi Zhuo Jiang, Kai H. Luo
Platinum-decorated functionalized graphene sheet (Pt@FGS) is a promising nanoparticle additive for catalytic fuel combustion. In this study, four cases involving pure methane oxidation and methane oxidation in the presence of various Pt/graphene-based nanoparticle catalysts are investigated using the reactive force field molecular dynamics (ReaxFF MD) simulations to reveal catalytic mechanisms and kinetics of methane oxidation. The results demonstrate that Pt@FGS is the most effective catalyst among all the nanoparticle candidates involved in this research. Compared with pure methane oxidation, the combination of Pt and FGS in the Pt@FGS reaction improves the catalytic activity by dramatically lowering the activation energy by approximately 73%. Additionally, the catalytic methane oxidation is initiated by the cleavage of C<img border="0" alt="single bond" src="https://cdn.els-cdn.com/sd/entities/sbnd">H bond and the production of hydroxyl. The observed H transfer process suggests that enhanced dehydrogenation of Pt@FGS and interatomic exchanges activate the catalytic cycle and dominate the catalytic process. Moreover, FGS can be further oxidized mostly at the edge of the sheet to increase the functionality. In summary, this research sheds light on the catalytic mechanisms for enhanced fuel combustion in the presence of Pt@FGS.
Ignition delay measurements of a low-octane gasoline blend, designed for gasoline compression ignition (GCI) engines Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 Mohammed AlAbbad, Jihad Badra, Khalil Djebbi, Aamir Farooq
A blend of low-octane (light and heavy naphtha) and high-octane (reformate) distillate fuels has been proposed for powering gasoline compression ignition (GCI) engines. The formulated ‘GCI blend’ has a research octane number (RON) of 77 and a motor octane number (MON) of 73.9. In addition to ∼64 mole% paraffinic components, the blend contains ∼20 mole% aromatics and ∼15 mole% naphthenes. Experimental and modeling studies have been conducted in this work to assess autoignition characteristics of the GCI blend. Ignition delay times were measured in a shock tube and a rapid comparison machine over wide ranges of experimental conditions (20 and 40 bar, 640–1175 K, ϕ = 0.5, 1 and 2). Reactivity of the GCI blend was compared with experimental measurements of two surrogates: a multi-component surrogate (MCS) and a two-component primary reference fuel (PRF 77). Both surrogates capture the reactivity of the fuel quite well at high and intermediate temperatures. The MCS does a better job of emulating the fuel reactivity at low temperatures, where PRF 77 is more reactive than the GCI blend. Ignition delay times of the two surrogates are also simulated using detailed chemical kinetic models, and the simulations agree well with the experimental findings. The results of rate-of-production analyses show important role of cycloalkane chemistry in the overall autoignition behavior of the fuel at low temperatures.
Effect of pressure on the combustion of an aqueous urea and ammonium nitrate monofuel Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 Bar Mosevitzky, Michael Epstein, Gennady E. Shter, Gideon S. Grader
Temperature profiles and effluent concentrations were measured during combustion of aqueous urea and ammonium nitrate at pressures of 1–15 MPa. Pollutant levels decreased, while the combustion temperature showed a non-monotonic change with increasing pressure. Experimental temperature profiles were applied in kinetic gas-phase simulations, and resulting species concentrations were in good agreement with experimental values. Sensitivity analyses indicated the kinetic parameters of isocyanic acid hydrolysis are the main source of uncertainty, possibly leading to the lower agreement observed for experimental and simulation carbon species. Rate of production analyses indicated that isocyanic acid is mainly consumed by hydrolysis to carbon dioxide, while nitric acid reacts with nitrous acid to produce water. Ammonia exhibited two channels of decomposition, reacting with either hydroxyl or nitrogen dioxide to form amidogen and either water or nitrous acid, respectively. Nitrogen was mainly formed by the three-body reaction of diazenyl. As pressure increased, the aforementioned pathways became increasingly dominant.
Experimental and kinetic study of 2,4,4-trimethyl-1-pentene and iso-octane in laminar flames Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 Erjiang Hu, Geyuan Yin, Jinfeng Ku, Zhenhua Gao, Zuohua Huang
Laminar flame speeds of 2,4,4-trimethyl-1-pentene are investigated at equivalence ratios of 0.7–1.6, initial temperatures of 298–453 K and initial pressures of 0.1–0.5 MPa. The comparison between 2,4,4-trimethyl-1-pentene and iso-octane is also performed. Results show that 2,4,4-trimethyl-1-pentene has faster laminar flame speed than iso-octane. Chemical kinetic models (Metcalfe model, Modified model I) were tested against the present experimental data. The laminar flame speeds are apparently over-estimated by the Metcalfe model and under-predicted by the Modified model I. Therefore, high-level quantum mechanical calculations were used to revise the Modified model I to obtain Modified model II and it can give fairly good prediction at various conditions on laminar flame speeds. In addition, the chemical kinetic analysis was conducted. The analysis indicates both thermal and kinetic effects result in the discrepancy of laminar flame speeds between 2,4,4-trimethyl-1-pentene and iso-octane. Furthermore, IC4H8 plays a dominant role in laminar flame speeds of 2,4,4-trimethyl-1-pentene and iso-octane.
MnO2-coated graphene foam micro-structures for the flame speed enhancement of a solid-propellant Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 Shourya Jain, Li Qiao
In this work, the functionalized form of a highly-conductive, porous, three-dimensional graphene foam (F-GF) was used to enhance the flame speed of a solid propellant, nitrocellulose (NC). The graphene foam (GF) micro-structures, synthesized by the chemical vapor deposition (CVD) technique, were coated with a transition metal oxide (TMO), manganese dioxide (MnO2), using a hydrothermal approach. Average flame speeds as a function of both the NC-GF and MnO2<img border="0" alt="single bond" src="https://cdn.els-cdn.com/sd/entities/sbnd">NC loadings were studied. Overall, flame speed enhancement up to 9 times that of the bulk NC flame speed was observed. An optimum MnO2<img border="0" alt="single bond" src="https://cdn.els-cdn.com/sd/entities/sbnd">NC loading corresponding to the maximum flame speed was obtained for each NC-GF loading, which was found to shift to the right as the NC-GF loading was decreased. In addition, thermogravimetric (TG) and differential scanning calorimetry (DSC) analysis were also conducted to determine the effect of NC-GF and MnO2<img border="0" alt="single bond" src="https://cdn.els-cdn.com/sd/entities/sbnd">NC loadings on the activation energy (E) and peak thermal decomposition (PTD) temperatures of the propellant NC. Similar to the flame speed results, for each NC-GF loading tested, an initial decrease in both E and PTD temperatures was obtained as a function of the MnO2<img border="0" alt="single bond" src="https://cdn.els-cdn.com/sd/entities/sbnd">NC loading but above a certain MnO2 concentration, a slight rise and a plateaued behavior was observed, respectively.
H2 production from partial oxidation of CH4 by Fe2O3-supported Ni-based catalysts in a plasma-assisted packed bed reactor Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 Yaoyao Zheng, Rob Grant, Wenting Hu, Ewa Marek, Stuart A. Scott
H2-rich gas production from CH4 at mild temperature (673 K), was achieved in a single step without introducing a separate oxygen stream. This was conducted in a plasma-assisted packed bed reactor in the presence of Ni-based catalysts doped on an active support, i.e. Fe2O3. Among the tested materials, NiO/Fe2O3 was found to be very promising and its excellent catalytic properties seemed to be induced by the presence of Fe2O3, which suppressed the formation of deposited carbon, and thus maintained the catalytic effect of metallic Ni (formed during NiO reduction by the CH4/Ar plasma). This work demonstrates the potential of plasma-assisted chemical looping partial oxidation for H2 production.
Ignition characteristics of hypergolic fuels with various N-substituents Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 S.V. Khomik, S.V. Usachev, S.P. Medvedev, A.A. Cherepanov, S.V. Stovbun, V.N. Mikhalkin, G.L. Agafonov
The effect of an N-alkyl substituent R on the characteristics of ignition of hypergolic ionic liquids (ILs) upon contact with white fuming nitric acid (WFNA) is examined by a modified drop test method that simultaneously combines high-speed shadow imaging with high-speed two-color pyrometry. The number of carbon atoms in the synthesized N-alkylated propargyl-imidazolium dicyanamide with alkyl substituents varied from 1 to 7. The ILs under study can be grouped in two categories depending on the initial location of the flame zone. When R has 1 to 3 carbon atoms, ignition is visualized as a glow inside a bubble (formed after droplet fall) and above the liquid surface. When the number n of carbon atoms in R is 4 to 6, no glow is recorded inside the bubble. In the case of a heptyl substituent, ignition does not occur since no flame is observed during approximately one minute. It is shown that the value of ignition delay can be interpreted as an Arrhenius-type function where the exponent depends linearly on n. An analysis of high-speed video records shows that the evaporation starting time of WFNA measured from the first IL–WFNA contact is also an Arrhenius-type function of n. Furthermore, it is found that the time for a bubble to reach its maximum diameter is independent of n. A possible mechanism of reaction between ILs of this class and WFNA is suggested and discussed.
Multi-parameter diagnostics for high-resolution in-situ measurements of single coal particle combustion Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 Jan Köser, Tao Li, Nikita Vorobiev, Andreas Dreizler, Martin Schiemann, Benjamin Böhm
To study volatile combustion processes of single coal particles non-intrusive simultaneous multi-parameter measurements were performed. The experiment was carried out in a fully premixed flat flame burner with well-defined boundary conditions. For flame visualization high-speed luminescence imaging was combined with high-resolution high-speed OH-PLIF. To address particle size and shape a stereoscopic high-resolution backlight-illumination system was set up. Due to simultaneous recording of individual particle events the volatile combustion duration related to particle size, shape and velocity was measured. A comparison of luminescence imaging and OH-PLIF for flame visualization was investigated to define their application areas in coal combustion. The stereoscopic backlight-illumination setup was benchmarked to a well characterized bituminous coal. With a pixel resolution of ∼2.5 µm fine particle contours were resolved. The particle diameter and eccentricity were evaluated by an ellipse approximation. The experimental setup can be used to investigate different coal ranks and biomass in N2/O2 and CO2/O2 atmospheres in future.
An analytical study of the effect of flame response to simultaneous axial and transverse perturbations on azimuthal thermoacoustic modes in annular combustors Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 Chunyan Li, Dong Yang, Suhui Li, Min Zhu
Transverse mean flows and transverse acoustic perturbations are factors that may influence the flame response, and thus change the frequency, growth rate and mode nature (standing, spinning or mixed) of the azimuthal thermoacoustic modes in annular combustors. Previous analytical and low-order network models for annular combustors usually consider only axial flame response. This work identifies the linear response of an asymmetric planar Bunsen flame under simultaneous axial and transverse perturbations. A linearized analytical method for studying the response of an inclined flame under perturbations using G-equation is applied. The relation between the flame responses under a 2-D perturbation and separate axial/transverse perturbations is studied to identify a linear 2-D flame response superposition, with the response under simultaneous perturbations being equal to the sum of two responses under each single perturbation. The effect of this linear 2-D flame response on the azimuthal thermoacoustic modes is then investigated by incorporating it into a linear low-order network methodology . An annular combustor both with and without mixing burners and transverse mean flows is studied. The results are verified by comparing with FEM (Helmholtz solver) simulations. It is found that even though the transverse flame response is usually much smaller than the axial flame response, it may strongly affect the spin ratio of the azimuthal thermoacoustic modes. The asymmetries (e.g. due to burner difference) in both the transverse and axial flame response, and the effect of transverse mean flow on the acoustic propagation may all significantly change the spin ratio of the azimuthal modes, and thus need to be considered simultaneously.
Effects of fuel stratification on ignition kernel development and minimum ignition energy of n-decane/air mixtures Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 Yuan Wang, Wang Han, Zheng Chen
Fuel-stratified combustion has broad application due to its promising advantages in extension of lean flammability limit, improvement of flame stabilization, enhancement of lean combustion, etc. In the literature, there are many studies on flame propagation in fuel-stratified mixtures. However, there is little attention on ignition in fuel-stratified mixtures. In this study, one-dimensional numerical simulation is conducted to investigate the ignition and spherical flame kernel propagation in fuel-stratified n-decane/air mixtures. The emphasis is placed on assessing the effects of fuel stratification on the ignition kernel propagation and critical ignition condition. First, ignition and flame kernel propagation in homogeneous n-decane/air mixture are studied and different flame regimes are identified. The minimum ignition energy (MIE) of the homogeneous n-decane/air mixture is obtained and it is found to be very sensitive to the equivalence ratio under fuel-lean conditions. Then, ignition and flame kernel propagation in fuel-stratified n-decane/air mixture are investigated. The inner equivalence ratio and stratification radius are found to have great impact on ignition kernel propagation. The MIEs at different fuel-stratification conditions are calculated. The results indicate that for fuel-lean n-decane/air mixture, fuel stratification can greatly promote ignition and reduce the MIE. Six distinct flame regimes are observed for successful ignition in fuel-stratified mixture. It is shown that the ignition kernel propagation can be induced by not only the ignition energy deposition but also the fuel-stratification. Moreover, it is found that to achieve effective ignition enhancement though fuel stratification, one needs properly choose the values of stratification radius and inner equivalence ratio.
Mechanism on the contribution of coal/char fragmentation to fly ash formation during pulverized coal combustion Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 Qi Gao, Shuiqing Li, Yingqi Zhao, Qiang Yao
In this paper, the correlations between coal/char fragmentation and fly ash formation during pulverized coal combustion are investigated. We observed an explosion-like fragmentation of Zhundong coal in the early devolatilization stage by means of high-speed photography in the Hencken flat-flame burner. While high ash-fusion (HAF) bituminous and coal-derived char samples only undergo gentle perimeter fragmentation in the char burning stage. Simultaneously, combustion experiments of two kinds of coals were conducted in a 25 kW down-fired combustor. The particle size distributions (PSDs) of both fine particulates (PM1-10) and bulk fly ash (PM10+) were measured by Electrical Low Pressure Impactor (ELPI) and Malvern Mastersizer 2000, respectively. The results show that the mass PSD of residual fly ash (PM1+) from Zhundong coal exhibits a bi-modal shape with two peaks located at 14 µm and 102 µm, whereas that from HAF coal only possesses a single peak at 74 µm. A hybrid model accounting for multiple-route ash formation processes is developed to predict the PSD of fly ash during coal combustion. By incorporating coal/char fragmentation sub-models, the simulation can quantitatively reproduce the measured PM1+ PSDs for different kinds of coals. The sensitivity analysis further reveals that the bi-modal mass distribution of PM1+ intrinsically results from the coal fragmentation during devolatilization.
Experimental and numerical study of variable oxygen index effects on soot yield and distribution in laminar co-flow diffusion flames Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 Abhishek Jain, Dhrubajyoti D. Das, Charles S. McEnally, Lisa D. Pfefferle, Yuan Xuan
We study experimentally and numerically a series of methane-fueled laminar co-flow diffusion flames to investigate the effects of variable Oxygen Index (OI) on soot yield and distribution. OI is defined as the mole fraction of oxygen in the oxidizer. Sixteen flames are studied with OI ranging from 21% (air) to 76.3%, so that OI varies in small increments and its effects are precisely resolved. The soot volume fraction distribution is measured experimentally for all flames using color-ratio pyrometry. Simulations are carried out using an extensively validated chemical kinetic mechanism and an aggregate-based soot model that accounts for all major processes of soot inception, growth, and oxidation. The experimental measurements show that the visible flame height decreases with increasing OI, which is consistent with theoretical estimates and the numerical simulations. The measurements also indicate that increasing OI (from 21% to 36.8%) first results in an increase in the maximum soot concentration, but a further increase in OI leads to a decrease in the soot yield. Additionally, the maximum soot concentration occurs on the flame centerline for low OI flames (below 36.8%), but for higher OI, the peak soot yield occurs in the flame wings. All of these experimental observations are well reproduced by the simulations, with the maximum soot volume fraction magnitudes lying within the error bounds of the experimental measurements. The computational results are used to reveal the underlying physical mechanisms, by examining soot evolution along characteristic Lagrangian trajectories through flame regions. We find that increasing OI leads to higher flame temperature, which results in a stronger soot production rate, but also reduced soot residence time in flame regions, which allows less time for soot production. These competing effects cause the initial increase and subsequent decrease in the maximum soot yield and the shift in the maximum soot yield location with increasing OI.
High-speed video analysis of flame oscillations along a PMMA rod after stagnation region blowoff Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 Sandra L. Olson, Paul V. Ferkul, Jeremy W. Marcum
During blowoff extinction of clear cast PMMA rods in concurrent axial flow for microgravity BASS-II experiments, a dynamic flame oscillation was observed after the flame was blown off of the stagnation point but briefly stabilized on the periphery of the rod. Complementary normal gravity experiments were conducted and flame oscillations were tracked using a high-speed color camera at 240 frames per second. The side-stabilized flame oscillated up and down the rod with increasing amplitude until the entire flame extinguished. In none of the BASS-II or normal gravity tests could the side-stabilized flames persist (Hopf subcritical bifurcation). Since the oscillations occurred even in microgravity, the mechanism does not depend on gravity. For the larger fuel radius tests, the flame developed asymmetric oscillation (pitchfork bifurcation). The oscillation time and the number of oscillations scale with the inverse square of the rod radius (∼ Fourier no.) for the preheated microgravity rods. The average flame oscillation frequency is found to be linearly dependent on the mixed convective stretch rate (inverse of the flow time). The flame intensity varied in concert with its direction, either increasing or decreasing as the flame moved upstream or downstream, respectively. The oscillation frequency decreased as the amplitude increased and the flame slipped slightly farther down the rod with each oscillation. The flame speed increased with each subsequent oscillation, both flashing forward upstream and retreating downstream. The oscillations were found to closely follow a power law log-periodic dependence similar to those that describe systems approaching a critical point, such as diffusion-limited aggregation clusters, earthquakes, ruptures, and even stock market crashes. The net flame speeds varied linearly with ambient oxygen concentration, and linearly with the mixed convective stretch rate. Based on these observations, a mechanistic theory of the oscillations is described, and is consistent with the thermodiffusive instability.
CO2 reduction and methane partial oxidation on surface catalyzed La0.9Ca0.1FeO3-δ oxygen transport membranes Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-21 Xiao-Yu Wu, Ahmed F. Ghoniem
In this paper, we demonstrate CO2 thermochemical reduction to CO in a La0.9Ca0.1FeO3-δ oxygen ion transport membrane reactor. For process intensification, we also show that methane can be used on the sweep side, producing two streams: a CO stream from CO2 reduction on the feed side, and a syngas stream on the other. We show that surface reactions are the rate-limiting steps for fuel-assisted CO2 reduction on a flat LCF-91 membrane. To improve productivity, we study how that adding catalytic porous layers can accelerate these steps and hence, increase the CO2-to-fuel conversion rates. Adding LCF-91 porous layers onto the membrane surface raised the oxygen flux by 1.4X. Secondly, different catalysts (Ce0.5Zr0.5O2 on the feed side and (La0.6Sr0.4)0.95Co0.2Fe0.8O3 on the sweep side) were added onto the porous layers to further accelerate the surface reaction rates. As a result, the oxygen flux was further increased especially at lower temperatures, e.g., at 850°C, oxygen flux was raised by one order of magnitude as compared to the unmodified membrane. Process intensification was tested on the latter membrane configuration, and the syngas produced on the sweep side had a H2:CO ratio very close to 2, ideal for production of fuels. Carbon species balance showed that higher methane concentration on the sweep side could lead to coke formation. Results also show that the selectivity to CO2 near the membrane surface is higher than that at the reactor outlet due to the availability of lattice oxygen and the favorable water-gas shift reactions.
Toward accurate high temperature anharmonic partition functions Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 David H. Bross, Ahren W. Jasper, Branko Ruscic, Albert F. Wagner
We use four different methods to calculate an anharmonic correction factor fvib to the conventional RRHO partition functions for H2O, HO2, 3CH2, H2O2, and CH4 over a temperature range up to 3000 K. The exact quantum mechanical method benchmarks the other three approximate methods that are based on classical Monte Carlo phase space integrals, on vibrational perturbation theory, and on conventional harmonic partition functions evaluated with fundamental, rather than harmonic, frequencies. The last two of these methods converge on the exact partition function below temperatures that vary from 1500 K for the least anharmonic system (H2O) to 250 K for the most anharmonic system (H2O2). For 3CH2 and H2O2, both these methods are qualitatively incorrect because they are insensitive to a low energy barrier for internal motion. The classical method qualitatively overestimates quantum mechanical results at low temperatures because of the exclusion of zero point energy. However, here anharmonic corrections are small. At high temperatures, our anharmonic corrections can be large (up to 40% for CH4 at 3000 K) and at high enough temperatures the classical and exact quantum results will converge. Comparing perturbation theory and the classical method, the classical method becomes the approximate method of choice above ∼750 K for H2O2 and CH4, ∼2100 K for 3CH2, ∼2700 K for HO2, and > 3000 K for H2O.
The thermal decomposition of furfural: molecular chemistry unraveled Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Florence H. Vermeire, Hans-Heinrich Carstensen, Olivier Herbinet, Frédérique Battin-Leclerc, Guy B. Marin, Kevin M. Van Geem
The thermal decomposition of furfural is investigated experimentally and through theoretical calculations at the CBS-QB3 level of theory. Furfural is a major product observed during biomass pyrolysis, but despite its importance there are many speculations about the thermal decomposition channels of this compound. To address these open questions new experiments are performed in a jet-stirred reactor at atmospheric pressure and temperatures ranging from 900 to 1100 K with a furfural inlet mole fraction of 0.005 and He as diluent. The residence time is set to 2 s. The main products observed by GC analysis are CO, CO2, α-pyrone, furan, 3-furaldehyde, propyne, propadiene, acetylene, methane and benzene. Small amounts of other aromatics, e.g. toluene, styrene, benzaldehyde and phenol are observed as well.Theoretical calculations at the CBS-QB3 level are used to extend the furfural potential energy surface and to identify possible reaction pathways to the observed products. The unimolecular non-radical decomposition channel through α-pyrone as proposed in literature is confirmed as the main channel, but carbene pathways are found to make small contributions as well. Furthermore, pericyclic reactions are suggested to contribute to the molecular elimination of CO in open-chain molecules and Diels Alder reactions are found to be important for the formation of CO2 and aromatics. Finally, even radical chemistry initiated by homolytic scission of the approximately 380 kJ/mol strong C<img border="0" alt="single bond" src="https://cdn.els-cdn.com/sd/entities/sbnd">H bond in the furfural carbonyl group has a non-negligible influence.
Effect of feedstock water leaching on ignition and PM1.0 emission during biomass combustion in a flat-flame burner reactor ☆ ✰✰ Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Xuebin Wang, Adewale Adeosun, Zhongfa Hu, Zhenghang Xiao, Dishant Khatri, Tianxiang Li, Houzhang Tan, Richard L. Axelbaum
In this work, the effects of feedstock water leaching on ignition and PM1.0 emission during biomass combustion were studied, for the first time, in a Hencken flat-flame burner reactor (HFFBR). A high-speed video camera and high-resolution electrical low-pressure impactor were respectively employed to diagnose ignition and PM1.0 along the height of the burner. The mineral composition of PM10+ was measured as a function of height to demonstrate the potassium release during the early stage of biomass combustion. The results show that water leaching does not change the functional group of the biomass (straw), but increases the BET surface area and pore volume. Water leaching removes 90% of the potassium and all the chlorine, reducing the same amount of PM1.0 emission. The effect of water leaching on ignition delay observed in the flat-flame burner reactor agrees with the delay of biomass-devolatilization in TGA. Profiles of mineral composition in the PM10+ with height shows that a large amount of the potassium is released before biomass ignition. This indicates that, at realistic heating rates, the catalytic promotion of water-soluble minerals on biomass ignition is primarily through promoting devolatilization. The ignition delay of biomass particles caused by water leaching is more significant at lower temperature, e.g., ignition is delayed from 20 to 24 ms at 1000 °C, and from 9.2 to 10.2 ms at 1300 °C.
An experimental and numerical study of thermal and chemical structure of downward flame spread over PMMA surface in still air Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 O.P. Korobeinichev, A.I. Karpov, A.A. Bolkisev, A.A. Shaklein, M.B. Gonchikzhapov, A.A. Paletsky, A.G. Tereshchenko, A.G. Shmakov, I.E. Gerasimov, A. Kumar
This study presents the results of a comprehensive experimental investigation and numerical simulation of the downward flame spread over PMMA slabs. For the first time, in the case of downward flame spread over PMMA slab 9.6 mm thick, temperature and species concentration fields in the gas-phase flame, temperature profiles in the condensed phase and dependence of the heat flux to the burning surface on the distance from the flame front were obtained. A coupled model of heat and mass transfer involving two-dimensional elliptic conservation equations both for gas phase and solid fuel has been used with the fuel surface approximation of the samples burnout. This allowed us to state, for the indefinite intermediate mode (in terms of the sample thickness, which are not neither thermally thin nor thermally thick), a mathematical model ensuring good agreement between the experimental and calculated macro parameters of combustion. The results of comparing the experimental and calculated data allowed us to determine a number of facts, which, despite the satisfactory agreement between the simulation and the experimental data in the main macro parameters, indicate the necessity of further improvement of the model derived. Such facts are: the increasing disagreement between the calculation and the experiment in the position of the maxima of the temperature in the gas phase as the distance from the flame front grows; essential difference in the width of the MMA and O2 consumption zone between the calculation and the experiment; identification in the experiment of CO as an intermediate product. Further improvement of the model should be aimed at more detailed development of the combustion reaction mechanism, which should consider at least two steps.
Predicting kinetic parameters for coal devolatilization by means of Artificial Neural Networks Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Jiangkuan Xing, Kun Luo, Heinz Pitsch, Haiou Wang, Yun Bai, Chunguang Zhao, Jianren Fan
The chemical percolation devolatilization (CPD) model has been shown to represent the devolatilization process of different coals and heating conditions with good accuracy. However, its use in computational fluid dynamics is limited because of its relatively high computational cost. Here, an Artificial Neural Network (ANN) based model for predicting coal devolatilization kinetics is developed based on a database constructed with the CPD model for a wide range of coals and heating rates. The heating rates and the information of ultimate and proximate analysis are chosen as inputs of the ANN model to consider the effects of coal types and heating conditions on coal devolatilization; the outputs are the kinetic parameters for the two-step kinetic model. The learning, validation, and application results show that the proposed ANN model has a competitive prediction capability on both the total volatile release and release rates when compared with the CPD model, but has obvious computational efficiency advantages. Furthermore, the relative impact of the coal type and heating rate on each kinetic parameter for coal devolatilization is quantitatively evaluated through the Garson equation. It is found that the heating rate has the strongest effect on the pre-exponential factor, while the coal types show significant influence on the activation energy and final yield of the two reactions in the two-step model.
Continuous catalytic pyrolysis of oily sludge using U-shape reactor for producing saturates-enriched light oil Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Bingcheng Lin, Qunxing Huang, Mujahid Ali, Fei Wang, Yong Chi, Jianhua Yan
Continuous catalytic pyrolysis of oily sludge was carried out in a special U-shape reactor for producing saturates-enriched light oil. The sludge underwent thermal pyrolysis first and then catalytic pyrolysis. During the thermal pyrolysis, chain hydrocarbons were first cracked and further polymerized into aromatics. The effect of temperatures (400–800 °C) on the products was investigated and the maximum liquid yield (67.7%) was obtained at 500 °C. High temperature promoted polymerization, thus the distribution of aromatics in the liquid product was increased and was more concentrated in polyaromatics at 800 °C. In the catalytic upgrading stage, dolomite was used as catalyst and aromatics were adsorbed on it, either aggregated or decomposed. As a result, a light oil product with 57.0% saturates was obtained at the residence time of 8.9 s due to the conversion of aromatics and heavy hydrocarbons into light aliphatic hydrocarbons such as straight chain hydrocarbons. Compared with the oil phase in the raw sludge sample, the content of saturates was increased by 45.0% and that of the asphaltenes was reduced by 88.5%. Meanwhile, the inherent moisture in the oily sludge could participate in the steam reforming reaction, promoting the decomposition of aromatics and leading to an increase in the H2 generation. Moreover, the release of H2S was reduced from 0.132 to 0.005 mol per kg sludge and the sulfur content of the oil product was also decreased in the presence of dolomite. The deactivation of dolomite can be attributed to the carbonization of CaO and deposition of polyaromatic coke on the catalyst surface.
Stationary combustion regimes and extinction limits of one-dimensional stretched premixed flames in a gap between two heat conducting plates Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Roman Fursenko, Sergey Mokrin, Sergey Minaev
Stationary combustion regimes, their linear stability and extinction limits of stretched premixed flames in a narrow gap between two heat conducting plates are studied by means of numerical simulations in the framework of one-dimensional thermal-diffusion model with overall one-step reaction. Various stationary combustion modes including normal flame (NF), near-stagnation plane flame (NSF), weak flame (WF) and distant flame (DF) are detected and found to be analogous to the same-named regimes of conventional counterflow flames. For the flames stabilized in the vicinity of stagnation plane at moderate and large stretch rates (which are NF, NSF and WF) the effect of channel walls is basically reduced to additional heat loss. For distant flame characterized by large flame separation distance and small stretch rates intensive interphase heat transfer and heat recirculation are typical. It is shown that in mixture content / stretch rate plane the extinction limit curve has ε-shape, while for conventional counterflow flames it is known to be C-shaped. This result is quite in line with recent experimental findings and is explained by extension of extinction limits at small stretch rates at the expense of heat recirculation. Analysis of the numerical results makes possible to reveal prime mechanisms of flame quenching on different branches of ε-shaped extinction limit curve. Namely, two upper limits are caused by stretch and heat loss. These limits are direct analogs of the upper and lower limits on conventional C-shaped curve. Two other limits are related with weakening of heat recirculation and heat dissipation to the burner. Thus, the present study provides a satisfactory explanation for the recent experimental observations of stretched flames in narrow channel.
Investigation on spherically expanding flame temperature of n-butane/air mixtures with tunable diode laser absorption spectroscopy Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Guoqing Wang, Bowen Mei, Xunchen Liu, Guoyong Zhang, Yuyang Li, Fei Qi
A joint schlieren imaging, pressure recording and tunable diode laser absorption spectroscopy (TDLAS) thermometry technique was developed to simultaneously determine the flame radius, pressure and line-of-sight averaged temperature of spherically expanding flames of n-butane/air mixtures at initial temperature of 298 K, initial pressure of 1 atm and equivalence ratios of 0.9–1.5. To probe the flame temperature, a mid-infrared interband cascade laser at 4.2 µm was used to measure the time-resolved direct absorption spectra of CO2 which are strongly related to flame temperature, CO2 mole fraction, flame radius and pressure. Quantitative line-of-sight averaged temperatures of burnt gas were obtained by fitting the normalized absorbance spectra. Three typical stages, including the spark affected initial stage, quasi-steady stage and the pressure induced growing stage are determined from the evolution of measured temperature as a function of time and flame radius. The relation between flame temperature, stretch rate and burning velocity of burnt gas are analyzed. Stretch rate is found to have minor effect on the measured temperature in the quasi-steady stage. The relative variation of temperature is much smaller than that of velocity. The flame with lower normalized temperature tends to propagate slower.
Redox reaction process between hydrocarbon and adsorbed NOx over lean NOx trap catalyst Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Bo Li, Chonglin Song, Gang Lv, Chenyang Fan, Xuyang Zhang, Hong Wen, Jingyao Liu
In this study, we investigated the redox reaction between a hydrocarbon (HC) and adsorbed NOx over Cu<img border="0" alt="single bond" src="https://cdn.els-cdn.com/sd/entities/sbnd">Pt<img border="0" alt="single bond" src="https://cdn.els-cdn.com/sd/entities/sbnd">Ba/Al2O3 lean NOx trap (LNT) catalyst. Experimental characterizations included temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), in situ diffuse reflectance Fourier transform spectroscopy (in situ DRIFTs) and Raman spectroscopy. Density functional theory (DFT) calculations were also performed. The model C3H6 first adsorbed to the catalyst surface, and the resulting C3H6-adsorbed species reacted with adsorbed NOx species to form various intermediates such as: carboxylates, enolic compounds, carbonato complexes, Pt-carbonyls, acid anhydrides, basic hydroxyls, nitrogen-containing organic species, acyl chlorides, and ammonium salts. These intermediates participated in subsequent reactions and finally produced the effluent gases CO2, N2, NH3, and H2O. The decomposition ability of adsorbed NOx species indicated that the NOx species participated in the C3H6 oxidation in the order: adsorbed N2O4 > monodentate nitrates > free ionic nitrates > bulk free ionic nitrates. Carbonaceous materials were generated during the C3H6 oxidation process and were consumed as intermediates by gaseous NOx released upon decomposition of the adsorbed NOx species. In summary, the redox reaction between C3H6 and adsorbed NOx followed the Langmuir–Hinshelwood mechanism, and three reaction routes were proposed for the redox process over the studied catalyst. The activation energy barriers determined by DFT + U calculations for the three routes indicated that the initial dissociation of NO3− species in R(2) occurred more easily than the oxidation of C3H6 species in R(1), delivering the active oxygen species that participated in R(1) and R(3). As the reactions proceeded, the higher energy barrier for the complete dissociation of NO2* indicated that the temperature determined the further decomposition of adsorbed NOx species, and R(1) and R(3) were constrained owing to the limited surface active oxygen species from R(2).
Downward flame spreading over electric wire under various oxygen concentrations Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Yusuke Konno, Nozomu Hashimoto, Osamu Fujita
The downward flame spread over laboratory electric wire under various oxygen concentrations has been investigated experimentally to improve our knowledge of electric-wire combustion. Two kinds of electrical wire (low-density-polyethylene (LDPE)-insulated copper (Cu) and nickel-chrome (NiCr)) are used in this study. The oxygen concentration of the mixture stream (O2 and N2) in the test section is varied between 15 and 41 vol%. Opposed-flow velocity in the test section is fixed at 15 cm/s. For NiCr wire, the flame spread rate (Vf) and flame length (Lf) monotonically increase with oxygen concentration. For Cu wire, both Vf and Lf show non-monotonic behavior against oxygen concentration. Most interestingly, Vf decreases with oxygen concentration increase in the 25–31% range. Theoretical analysis shows two regimes of variation of Vf with oxygen concentration: the “temperature-dependent regime (TDR)” and the “negative-oxygen-dependent regime (NOR)”. The non-monotonic behavior of Vf against oxygen concentration for Cu can be explained by the controlling mechanism behind TDR and NOR. However, experimental results show one more regime above 31% oxygen concentration that cannot be explained by the theory proposed in this work, namely the “soot-generation-dependent regime (SGR)”; here, radiation from the flame and soot deposit plays a dominant role in flame spread.
Ignition and volatile combustion behaviors of a single lignite particle in a fluidized bed under O2/H2O condition Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Lin Li, Lunbo Duan, Dong Zeng, Dennis Y Lu, Changsheng Bu, Changsui Zhao
O2/H2O combustion, as a new evolution of oxy-fuel combustion, has gradually gained more attention recently for carbon capture in a coal-fired power plant. The physical and chemical properties of steam e.g. reactivity, thermal capacity, diffusivity, can affect the coal combustion process. In this work, the ignition and volatile combustion characteristics of a single lignite particle were first investigated in a fluidized bed combustor under O2/H2O atmosphere. The flame and particle temperatures were measured by a calibrated two-color pyrometry and pre-buried thermocouple, respectively. Results indicated that the volatile flame became smaller and brighter as the oxygen concentration increased. The ignition delay time of particle in dense phase was shorter than that in dilute phase due to its higher heat transfer coefficient. Also, the volatile flame was completely separated from particles (defined as off-flame) in dense phase while the flame lay on the particle surface (defined as on-flame) in dilute phase. The self-heating of fuel particles by on-flame in dilute phase was more obvious than that in dense phase, leading to earlier char combustion. At low oxygen concentration, the flame in the H2O atmosphere was darker than that in the N2 atmosphere because the heat capacity of H2O is higher than that of N2. With the increase of oxygen concentration, the flame temperature in the O2/H2O atmosphere was dramatically enhanced rather than that in the O2/N2 atmosphere, where the diffusion rate of oxygen in O2/N2 atmosphere became the dominant factor.
Investigation of sampling-probe distorted temperature fields with X-ray fluorescence spectroscopy Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 N. Hansen, R.S. Tranter, J.B. Randazzo, J.P.A. Lockhart, A.L. Kastengren
Flame-sampling experiments, especially in conjunction with laminar low-pressure premixed flames, are routinely used in combustion chemistry studies to unravel the identities and quantities of key intermediates and their pathways. In many instances, however, an unambiguous interpretation of the experimental and modeling results is hampered by the uncertainties about the probe-induced, perturbed temperature profile. To overcome this limitation, two-dimensional perturbations of the temperature field caused by sampling probes with different geometries have been investigated using synchrotron-based X-ray fluorescence spectroscopy. In these experiments, which were performed at the 7-BM beamline of the Advanced Photon Source (APS) at the Argonne National Laboratory, a continuous beam of hard X-rays at 15 keV was used to excite krypton atoms that were added in a concentration of 5 vol.-% to the unburnt gas mixture and the resulting krypton fluorescence at 12.65 keV was subsequently collected. The highly spatially resolved signal was converted into the local flame temperature to obtain temperature fields at various burner-probe separations as functions of the distance to the burner surface and the radial distance from the centerline. Multiple measurements were performed with different probe geometries and because of the observed impact on the temperature profiles the results clearly revealed the need to specify the sampling probe design to enable quantitative and meaningful comparisons of modeling results with flame-sampled mole fraction data.
The role of molecular properties on the dimerization of aromatic compounds Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Paolo Elvati, Kirk Turrentine, Angela Violi
Recent results have shown the presence and importance of oxygen chemistry during the growth of aromatic compounds, leading to the formation of oxygenated structures that have been identified in various environments. Since the formation of polycyclic aromatic compounds (PAC) bridge the formation of gas-phase species with particle inception, in this work we report a detailed analysis of the effects of molecular characteristics on physical growth of PAC via dimerization. We have included oxygen content, mass, type of bonds (rigid versus rotatable), and shape as main properties of the molecules and studied their effect on the propensity of these structures to form homo-molecular and hetero-molecular dimers. Using enhanced sampling molecular dynamics techniques, we have computed the free energy of dimerization in the temperature range 500–1680 K. Initial structures used in this study were obtained from experimental data. The results show that the effects of shape, presence of oxygen, mass, and internal bonds are tightly intertwined, and that their relative importance changes with temperature. In general, mass and the presence of rotatable bonds are the most influential factor to predict dimerization propensity among the one considered. The results provide knowledge on the inception step and the role that particle characteristics play during inception. In addition, our study highlights the fact that current models that use stabilomers as monomers for physical aggregation are overestimating the importance of their dimerization during particle nucleation.
Sooting tendencies of co-optima test gasolines and their surrogates Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Charles S. McEnally, Yuan Xuan, Peter C. St. John, Dhrubajyoti D. Das, Abhishek Jain, Seonah Kim, Thomas A. Kwan, Lance K. Tan, Junqing Zhu, Lisa D. Pfefferle
This study characterized the sooting tendencies of a set of gasolines and their surrogates using both experimental and computational methods. Sooting tendency was defined in terms of the soot yield when 1000 ppm of the test fuel is doped into the fuel of a methane/air flame, and it provides a measure of the intrinsic chemical tendency of the fuels to form soot in a generic combustion environment. The test fuels were real gasolines containing enhanced concentrations of alkanes, aromatics, cycloalkanes, olefins, and ethanol. These compositional differences caused the experimentally measured sooting tendencies of the fuels to vary by 240%. The surrogates were 3 mixtures defined by Szybist et al. (2017) and 3 alternative formulations modified for greater experimental convenience. The sooting tendencies measured for the surrogate mixtures agreed with the real fuels to within 15%, and varied with composition in the same order. The sooting tendencies of the surrogates could be predicted to within experimental error with an empirical quantitative structure-property relationship and a linear mixing model. The experimental flames were computationally simulated with a 743-species mechanism, and sooting tendencies derived from the results agreed with the measured values to within 11%. Overall, these results show that the sooting behavior of gasoline can vary considerably within the range of acceptable compositions, and that these variations can be accurately predicted with empirical models and computational simulations.
Concurrent flame spread over externally heated Nomex under mixed convection flow Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Maria Thomsen, Xinyan Huang, Carlos Fernandez-Pello, David L. Urban, Gary A. Ruff
Fire-resistant materials are used in multiple applications where protection from fire is needed. Their fire-resistant capacity is often tested under specific conditions that might not represent the situation in an actual fire. Particularly relevant for this work is the application for astronaut spacesuit, since a spacecraft environment may be different than earth atmospheres. There, a material is exposed to low velocity flows, microgravity, reduced pressure, and enriched oxygen concentration. Under these conditions, material flammability can be altered. In addition, flammability tests are based primarily on the exposure of the material to an external radiant flux to simulate an adjacent fire, but not a real flame. In this work, an experimental study was performed to investigate the effect of ambient pressure and oxygen concentration on the concurrent/upward flame spread over a fire-resistant fabric (Nomex HT90-40) exposed to two different external heat sources. One is the radiation from infrared lamps, and the other is the flame from a burning polymethyl methacrylate (PMMA) sheet placed below the fabric. The experimental results show that an external heat flux extends the limiting oxygen concentration (24% LOC) of Nomex. This effect is more pronounced when the PMMA flame provides the heat flux (17% LOC). For oxygen concentrations larger than the Nomex LOC, the flame spread rate decreases as the ambient pressure is decreased, indicating that reducing buoyancy reduces the flame spread rate. A simple analysis of concurrent flame spread that incorporates mixed flow heat transfer correlates well with the experimental data. This suggest that flame spread in microgravity can be predicted in terms of a mixed flow velocity that includes the Reynolds and Grashof numbers. The results of this work provide further information about the effect of the type of external heating on material flammability. They may also guide future fire safety design in space exploration.
On-the-fly ab initio calculations toward accurate rate coefficients Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Ruben Van de Vijver, Kevin M. Van Geem, Guy B. Marin
During automatic kinetic model generation a large number of reactions need to have kinetic data assigned to them. One of the main challenges is on how to account for the large scarcity of accurate data. The ever-growing computational power and availability of high performance computing solutions allows to envisage implementing on-the-fly ab initio calculations for a large number of reaction rate coefficients. Today extensive user knowledge and involvement is required to develop fast and effective ab initio based kinetic models. The present work introduces a series of automation procedures to minimize user involvement. It allows automated quantum chemical calculations for a large number of reactions for gas-phase processes with a wide variety in chemical species. 3D coordinates are calculated for all the reactants, products and transition states and automatically submitted to quantum chemistry packages. The latter yield accurate high-pressure limit rate coefficients. All steps are automated without the need for manual interventions, even for recognizing rotational frequencies and for verifying whether the saddle point searches lead to the appropriate transition state. A good agreement to literature data is found, i.e., most of the rate coefficients are within a factor of 2 of literature data.
Shock tube study of the rate constants for H + O2 + M → HO2 + M (M = Ar, H2O, CO2, N2) at elevated pressures Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Jiankun Shao, Rishav Choudhary, Adam Susa, David F. Davidson, Ronald K. Hanson
The rate constants for the reaction H + O2 + M→HO2 + M were investigated at elevated pressures from 12 to 33 atm using ignition delay time (IDT) measurements behind reflected shock waves in H2/O2/M mixtures with different collision partners M = Ar, H2O, N2 and CO2. The temperature and pressure ranges where the rate constants of the reactions H + O2 + M→HO2 + M and H + O2- > OH + O dominate the IDT sensitivity were selected as optimum test conditions using the detailed H2/O2 mechanism of Hong et al. (2011). The current study thus provides a quantitative and relatively direct method for determining the rate constants for H + O2 + M→HO2 + M using simple IDT measurements. The rate constants found are consistent with earlier studies, but with reduced uncertainties.
A thermogravimetric method for the measurement of CO/CO2 ratio at the surface of carbon during combustion Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Wenting Hu, Ewa Marek, Felix Donat, John S. Dennis, Stuart A. Scott
This work presents a new method of measuring the CO/CO2 ratio at the surface of carbon particles during combustion. This thermogravimetric method deduces the ratio of CO to CO2 by comparing the rate of consumption of carbon with the rate of oxidation of an external reference material with fast oxidation kinetics, in this case Cu. The method is useful when combustion is controlled by external mass transfer, commonly encountered in large-scale processes. The viability of this method has been demonstrated experimentally with graphite and a lignite char. It was found that in an atmosphere of ∼ 1% O2, the graphite produced CO2 between 700 and 900 °C whilst the lignite char produced a mixture of CO and CO2 between 700 and 800 °C with the proportion of CO increasing with temperature, and above 850 °C, only CO was produced. It was also found that for this particular lignite char, the ratio of CO/CO2 increased with decreasing pO2 in the environment.
The influence of hydrogen and methane on the growth of carbon particles during acetylene pyrolysis in a burnt-gas flow reactor Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 S. Peukert, A. Sallom, A. Emelianov, T. Endres, M. Fikri, H. Böhm, H. Jander, A. Eremin, C. Schulz
The growth of carbon particles was studied in heated flows of a burnt-gas flow reactor containing mixtures of N2/C2H2, and N2/C2H2 with addition of H2 or CH4 surrounded by a rich C2H4/air flame. Soot particle sizes and volume fractions were measured by laser-induced incandescence (LII) between 50 and 130 mm above the nozzle exit. The measurements indicate a soot-inhibiting effect of adding H2 to the C2H2/N2 flow on both, particle sizes and soot volume fractions. The effect of CH4 addition to the C2H2/N2 flows was ambivalent, depending on the methane-to-acetylene ratio. At gas mixtures with N2:CH4:C2H2 = 0.42:0.35:0.23 and 0.39:0.32:0.29 by volume at fixed total flow rates, the measured soot volume fractions were substantially increased in presence of CH4, while the mean diameters of the particles were slightly decreased. Gas temperatures were measured by a generalized line-reversal method with Abel transformation. Temperatures of the surrounding C2H4/air flame were around 1600 K, and temperatures of the inner flows, where soot formation was measured, were between 1550 and 1630 K. Plug-flow reactor calculations provided a qualitative understanding of the influence of CH4 on the soot particle growth.
An experimental laminar flame investigation of dual-fuel mixtures of C4 methyl esters with C2–C4 hydrocarbon base fuels Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Maurin Salamanca, Julia Wullenkord, Isabelle Graf, Steffen Schmitt, Lena Ruwe, Katharina Kohse-Höinghaus
Blending petroleum-based fuels with biofuel components is deemed attractive to reduce soot and CO2 emissions, but fundamental studies of the combustion behavior of such fuel blends suited for model development and validation remain rather scarce. To contribute to the understanding of the combustion chemistry effects of such blending strategies, we have investigated laminar premixed low-pressure flames of three hydrocarbon base fuels, namely 1-butene (1-C4H8), isobutene (i-C4H8), and ethene (C2H4), blended each with two different ester fuels, namely methyl crotonate (C5H8O2, MC) and methyl butanoate (C5H10O2, MB). A series of 13 flames with different argon dilution was investigated to study effects of the specific fuel structure on the combustion chemistry. Full speciation analyses were performed for fuel-rich (ϕ = 1.6) conditions by electron ionization molecular-beam mass spectrometry (EI-MBMS). More than 35 species in the range of C0–C7 were identified and quantified in these flames, resulting in ∼450 mol fraction profiles. The experimental data were compared to simulations by the kinetic model reported by Yang et al. [Proc. Combust. Inst. 2011, Phys. Chem. Chem. Phys. 2013] that was chosen because it includes basic mechanisms of all studied fuels. Overall, the agreement of experiment and this model seems satisfactory but calls for further improvements regarding ester as well as hydrocarbon sub-mechanisms. It was noted that the unsaturation degree in the methyl esters affects the formation of hydrocarbons, that depend mainly on the structure of the respective base fuel, and of oxygenated intermediates. The methyl esters have different decomposition pathways leading to some specific oxygenated species. Both methyl esters promote the formation of formaldehyde and methanol, while acetic acid is significantly increased by the presence of MB. The effect of the ester addition is also influenced by the species pool of the respective hydrocarbon base fuel.
High-temperature gas-phase kinetics of the thermal decomposition of tetramethoxysilane Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 P. Sela, S. Peukert, J. Herzler, Y. Sakai, M. Fikri, C. Schulz
The decomposition of tetramethoxysilane (Si(OCH3)4, TMOS) was studied in shock-tube experiments in the 1131–1610 K temperature range at pressures ranging from 1.9 to 2.3 bar behind reflected shock waves combining gas chromatography/mass spectrometry (GC/MS) and high-repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS). The initial reaction is a four-center elimination to form methanol. At elevated temperatures, TMOS also decomposes via a O–C bond scission forming a methyl (CH3) and the corresponding OSi(OCH3)3 radical. The main observed products were methane (CH4), methanol (CH3OH), ethylene (C2H4), and ethane (C2H6). The yields of these products increase with temperature. A kinetics mechanism from literature (Chu et al. 1995), which quantitatively accounts for the observed products in the decomposition of TMOS, was adopted and updated. The mechanism contains 13 silicon species and 24 reactions with silicon-containing species. It was combined with the methanol mechanism of Burke et al. (2006). The measured global rate constant for TMOS decomposition was found to be koverall[TMOS→products] = 2.9 × 1011exp(− 225 kJ mol−1/RT)s−1.
Kinetic modeling of ignition in miniature shock tube ☆ Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Mingyuan Tao, Patrick T. Lynch, Peng Zhao
The ignition delay times of toluene, n-heptane/iso-octane primary reference fuel blends, and toluene/n-heptane/iso-octane toluene primary reference fuel blends were studied in the miniature high repetition rate shock tube (HRRST). In the HRRST, the temperature and pressure change during the test time, complicating interpretation of ignition delay times. Two methods of chemical kinetic modeling were employed in order to account for the changing thermodynamic states. One was based on direct simulation using the pressure profile and the kinetic mechanism, and the other was based on a Livengood-Wu type approach using modeled constant state ignition delay times. Two different kinetic mechanisms were used in simulating the data. Both methods showed agreement with experimental data over a range of temperatures, pressures, and blends. While overall agreement was fair, agreement was poorer for one of the mechanisms for the primary reference fuel blends. An evaluation of the ignition delay iso-contours from these mechanisms for representative HRRST experiment thermodynamic trajectories revealed that the HRRST measured IDTs are controlled by the high temperature chemistry and are more sensitive to temperature. The different performance of mechanisms is therefore attributed to their different high temperature reactivity.
A high-temperature study of 2-pentanone oxidation: experiment and kinetic modeling Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-20 Julia Pieper, Christian Hemken, Rene Büttgen, Isabelle Graf, Nils Hansen, K. Alexander Heufer, Katharina Kohse-Höinghaus
Small methyl ketones are known to have high octane numbers, impressive knock resistance, and show low emissions of soot, NOx, and unburnt hydrocarbons. However, previous studies have focused on the analysis of smaller ketones and 3-pentanone, while the asymmetric 2-pentanone (methyl propyl ketone) has not gained much attention before. Considering ketones as possible fuels or additives, it is of particular importance to fully understand the combustion kinetics and the effect of the functional carbonyl group. Due to the higher energy density in a C5-ketone compared to the potential biofuel 2-butanone, the flame structure and the mole fraction profiles of species formed in 2-pentanone combustion are of high interest, especially to evaluate harmful species formations. In this study, a laminar premixed low-pressure (p = 40 mbar) fuel-rich (ϕ = 1.6) flat flame of 2-pentanone has been analyzed by vacuum-ultraviolet photoionization molecular-beam mass-spectrometry (VUV-PI-MBMS) enabling isomer separation. Quantitative mole fraction profiles of 47 species were obtained and compared to a model consisting of an existing base mechanism and a newly developed high-temperature sub-mechanism for 2-pentanone. High-temperature reactions for 2-pentanone were adapted in analogy to 2-butanone and n-pentane, and the thermochemistry for 2-pentanone and the respective fuel radicals was derived by ab initio calculations. Good agreement was found between experiment and simulation for the first decomposition products, supporting the initial branching reactions of the 2-pentanone sub-mechanism. Also, species indicating low-temperature chemistry in the preheating zone of the flame have been observed. The present measurements of a 2-pentanone flame provide useful validation targets for further kinetic model development.
Polar curved polycyclic aromatic hydrocarbons in soot formation Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 Jacob W. Martin, Kimberly Bowal, Angiras Menon, Radomir I. Slavchov, Jethro Akroyd, Sebastian Mosbach, Markus Kraft
In this paper, we consider the impact of polar curved polycyclic aromatic hydrocarbons (cPAH) on the process of soot formation by employing electronic structure calculations to determine the earliest onset of curvature integration and the binding energy of curved homodimers. The earliest (smallest size) onset of curvature integration was found to be a six ring PAH with at least one pentagonal ring. The σ bonding in the presence of pentagons led to curvature, however, the π bonding strongly favored a planar geometry delaying the onset of curvature and therefore the induction of a flexoelectric dipole moment. The binding energies of cPAH dimers were found to be of similar magnitude to flat PAH containing one or two pentagons, with an alignment of the dipole moments vectors. For the more curved structures, steric effects reduced the dispersion interactions to significantly reduce the interaction energy compared with flat PAH. Homogeneous nucleation of cPAH at flame temperatures then appears unlikely, however, significant interactions are expected between chemi-ions and polar cPAH molecules suggesting heterogeneous nucleation should be explored.
An experimental study of the stability and performance characteristics of a Hybrid Solar Receiver Combustor operated in the MILD combustion regime Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 A. Chinnici, G.J. Nathan, B.B. Dally
This study describes the performance and stability characteristics of a Hybrid Solar Receiver Combustor operated in the Moderate or Intense Low oxygen Dilution (MILD) combustion regime, in which the functions of a solar receiver and a combustor are integrated into a single device. The device was built and tested at a nominal capacity of 20 kWth for both the combustion-only (MILD and conventional combustion) and mixed-mode (a combination of both solar and combustion). Here, a 5 kWel xenon-arc solar simulator and natural gas were used as the energy sources, while the combustion mode was operated in the MILD combustion regime. The thermal efficiency, wall cavity temperature, heat flux distribution within the cavity and pollutant emissions are reported for the two modes of operation for a range of energy input, equivalence ratio, heat extraction, air preheat and solar-to-fuel energy input ratio. The stability limits for stable operations are also identified for each mode of operation. It was found that MILD combustion can be successfully stabilised within the HSRC in a wide range of operating conditions with and without air preheating, and in the mixed-mode of operation, providing ultra-low NOx and CO emissions. Also, the stability limits were found to increase by adding concentrated solar radiation to the combustion process. The thermal performance was found to be similar in both combustion-only (conventional combustion and MILD) and mixed-mode (up to ≈ 88% assuming reasonable heat recovery from the exhaust gas), confirming that an overall benefit can be derived from the device.
Pyrolysis of dimethoxymethane and the reaction of dimethoxymethane with H atoms: A shock-tube/ARAS/TOF-MS and modeling study Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 Leonie Golka, Isabelle Weber, Matthias Olzmann
Dimethoxymethane (CH3OCH2OCH3, DMM) is the smallest oxymethylene ether and currently discussed as a promising alternative diesel fuel. For an adequate modeling of the DMM combustion chemistry, reliable kinetic parameters are needed. In the present work, shock-tube studies are presented on the kinetics of the DMM + H reaction (R1) and the kinetics of the unimolecular decomposition of DMM (R2). In the case of reaction (R1), ethyl iodide pyrolysis was used as H-atom source. For both reactions, rate coefficients were determined from H-atom concentration-time profiles monitored with atom resonance absorption spectroscopy (ARAS). The following overall rate coefficients were obtained: k1(T) = (1.5 ± 0.7) × 1013 exp(− 1040 K/T) cm3 mol−1 s−1 (T = 850 − 1100 K, p ∼ 1.1 bar, bath gas: Ar) and k2(T) = (6.2 ± 1.9) × 1013 exp(− 31830 K/T) s−1 (T = 1100 − 1550 K, p ∼ 1.1 bar, bath gas: Ar). Additionally, the pyrolysis of DMM was investigated in shock-tube experiments with high-repetition time-of-flight mass-spectrometric (TOF-MS) detection (T = 1100 − 1700 K, p = 0.9 − 1.3 bar, bath gas: Ne). Concentration-time profiles of DMM, CO, CH2O, C2H6, C2H4, and CH4 were recorded simultaneously. Both the ARAS and the TOF-MS profiles can be reproduced with reasonable accuracy by simulation with a recent DMM oxidation mechanism from the literature. Mechanistic aspects are elucidated and possible modifications are proposed and discussed.
Experimental and modelling study of the impacts of n-butanol blending on the auto-ignition behaviour of gasoline and its surrogate at low temperatures Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 Inna Gorbatenko, Alison S. Tomlin, Malcolm Lawes, Roger F. Cracknell
The study investigates the impacts of n-butanol addition to a reference gasoline (RON 95, MON 86.6) and a gasoline surrogate on ignition delay times at various blending ratios (10%, 20%, 40% and 85% vol n-butanol) in a rapid compression machine, through experimental measurements and numerical modelling (T = 678–916 K, P = 2 MPa, stoichiometric conditions). The surrogate measurements are used to evaluate a recent chemical mechanism describing the combustion of the blends. The toluene reference fuel (TRF) surrogate showed adequate performance in replicating the ignition response of gasoline for all conditions tested, with closest agreement for the 85% blends. Some discrepancies existed within the negative temperature coefficient (NTC) region, suggesting that better matching of both MON and RON or additional surrogate components may be required. At low temperatures, increasing n-butanol concentration led to increases in ignition delay times. Here, n-butanol acted as an octane enhancer even at low concentrations, with marginal additional effects for blends above 40%. A brute force sensitivity analysis of the surrogate model suggested that the main reaction inhibiting ignition at low temperatures is H abstraction from the α-site of n-butanol, even for the 10% blend. At higher temperatures, the chain branching routes from H abstraction by OH from the γ-site of n-butanol, and from the α-site by HO2, become more dominant, promoting ignition. For the lower blends, the largest discrepancies between simulations and experiment were seen in the NTC region where a larger number of reactions contributed to the uncertainty in predicting τign. For the higher blends, the largest discrepancies were noted at low temperatures, indicating that uncertainties within the low temperature n-butanol chemistry need to be resolved. Accurate, temperature and pressure dependent reaction rates for site specific H abstraction by OH and HO2 for each of the fuel blend components are necessary to improve agreement between simulations and experimental data.
Development of a semi-global reaction mechanism for thermal decomposition of a polymer containing reactive flame retardant Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 Yan Ding, Kyungok Kwon, Stanislav I. Stoliarov, Roland H. Kraemer
To enable intelligent design of fire resistant polymeric materials, it is highly important to have a quantitative understanding of the relation between the composition of these materials and chemical and physical properties that control the process of fire growth. This paper presents a methodology that provides a capability to determine the core subset of these properties including the kinetics and thermodynamics of the thermal decomposition of the condensed-phase constituents and enthalpy of combustion of gaseous pyrolyzates. This methodology is based on three experimental techniques – Thermogravimetric Analysis, Differential Scanning Calorimetry and Microscale Combustion Calorimetry – and inverse numerical modeling of the results of these tests. The material system analyzed in this study is polyamide 66 reinforced with chopped glass fiber and flame retarded with red phosphorous. This system shows a complex decomposition behavior that is highly dependent on the phosphorus concentration. First, a semi-global thermal decomposition mechanism consisting of a set of first- and second-order (two components) reactions was developed using three material specimens containing different phosphorus concentrations. Subsequently, the extrapolative power of the developed reaction model was demonstrated by accurately predicting the experimental measurements obtained for several compositions not used in the model development process.
Concerning shock-tube ignition delay times: An experimental investigation of impurities in the H2/O2 system and beyond Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 Clayton R. Mulvihill, Eric L. Petersen
Shock-tube ignition delay time data have recently been called into question on the basis of possible hydrocarbon impurities that may serve to artificially accelerate ignition delay times. To assess potential sources of impurities, systematic tests were performed in highly dilute H2/O2/Ar and CH4/O2/Ar mixtures using H2O laser absorption at 1.388 µm and OH* emission at 307 nm near 1 atm and between 1257 and 2108 K. Factors investigated included common sources of impurity, namely shock-tube cleanliness, Ar purity, diaphragm fragments, leftover cleaning substances, turbopumping duration, and mixing tank cleanliness, but only mixing tank cleanliness was found to have any quantifiable effect on measured ignition delay times. Rate measurements of H + O2⇆OH + O using H2O time-histories were found to be unperturbed by impurity effects and were in excellent agreement with the Hong et al. measurement. Mixing tank impurities were found to perturb ignition delay times of a stoichiometric H2/O2 mixture in 98% Ar but had no effect on a stoichiometric CH4/O2 mixture in 99% Ar. Even with clean mixing tank conditions, H2/O2 ignition delay times in 98% and 99% Ar displayed a 25–30% discrepancy with predicted values but showed remarkable agreement with two sets of historical data. This discrepancy was modeled by including trace hydrocarbons (within certified purity levels) in simulations. For less-dilute (≤ 94% Ar) H2/O2 mixtures, literature data seem to agree rather well and were insensitive to trace hydrocarbons according to simulations. Furthermore, experimental and modeling evidence strongly suggests that almost every hydrocarbon C1 and higher is insensitive to impurities with respect to ignition delay times. It is thus concluded that the only data for which an impurity effect seems to exist is for highly dilute (98–99% Ar) H2/O2 data, while H2/O2 data below 94% dilution and almost every hydrocarbon C1 and greater are insensitive to common impurities.
Influences of stoichiometry on steadily propagating triple flames in counterflows Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 Prabakaran Rajamanickam, Wilfried Coenen, Antonio L. Sánchez, Forman A. Williams
Most studies of triple flames in counterflowing streams of fuel and oxidizer have been focused on the symmetric problem in which the stoichiometric mixture fraction is 1/2. There then exist lean and rich premixed flames of roughly equal strengths, with a diffusion flame trailing behind from the stoichiometric point at which they meet. In the majority of realistic situations, however, the stoichiometric mixture fraction departs appreciably from unity, typically being quite small. With the objective of clarifying the influences of stoichiometry, attention is focused on one of the simplest possible models, addressed here mainly by numerical integration. When the stoichiometric mixture fraction departs appreciably from 1/2, one of the premixed wings is found to be dominant to such an extent that the diffusion flame and the other premixed flame are very weak by comparison. These curved, partially premixed flames are expected to be relevant in realistic configurations. In addition, a simple kinematic balance is shown to predict the shape of the front and the propagation velocity reasonably well in the limit of low stretch and low curvature.
Laminar flame speeds of DEMP, DMMP, and TEP added to H2- and CH4-air mixtures Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 Travis Sikes, Olivier Mathieu, Waruna D. Kulatilaka, M. Sam Mannan, Eric L. Petersen
Organophosphorus compounds (OPCs) have long been known to have significant fire suppression capabilities but were outclassed by Halon 1301 due to toxicity concerns. Recent interest in finding replacements for Halon 1301 has provided an impetus to reconsider OPCs. To better understand the mechanism by which OPCs suppress flames, more information about how they interact with fuel/air mixtures via chemical kinetics is needed. In this study, dimethyl methylphosphonate (DMMP), diethyl methylphosphonate (DEMP), and trimethyl phosphate (TEP) were added to hydrogen/air and methane/air mixtures to assess their suppression capabilities at 0.1% and 0.3% (DMMP only) of the total mixture volume. Laminar flame speed experiments were performed in an optically tracked, spherically expanding flame setup at 1 atm and 120 °C. The resulting laminar flame speed data are the first to be recorded using these compounds. Results show a 30% decrease in laminar flame speed for all OPCs at 0.1% on the methane/air parent mixture, and the laminar flame speed curves, as a function of equivalence ratio, tend to be broader than for un-doped mixtures. For the hydrogen/air mixtures, the OPCs differentiate themselves by having an increasing suppression effect corresponding with higher carbon moiety, i.e., TEP (15% overall reduction) > DEMP (13%) > DMMP (9%). The OPCs also have an increasing effect with increasing equivalence ratio on hydrogen/air, but with methane/air, they have a non-monotonic effect. The reduction of laminar flame speeds is comparable to twice the concentration of Halon 1301 and 10 times as much for previously investigated Halon 1301 replacements. These results are ideal for improving existing OPC chemical kinetics mechanisms, and possible applications include both fire suppression technologies and destruction of dangerous OPC compounds.
Darrieus–Landau instability and Markstein numbers of premixed flames in a Hele-Shaw cell Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 Elias Al Sarraf, Christophe Almarcha, Joël Quinard, Basile Radisson, Bruno Denet, Pedro Garcia-Ybarra
Hydrodynamic flame instabilities are studied in a Hele-Shaw burner. By studying the development of perturbations, starting from a 2D Bunsen flame at the top of the burner, growth rates are measured for propane and methane–air mixtures, and compared to theoretical predictions. It is found that the dispersion relation in a Hele-Shaw cell has the same dependence with wavenumber σ=k−k2σ=k−k2 as the one predicted in tubes. Markstein numbers relative to fresh gases are obtained for propane and methane flames and compared to the literature.
H2S conversion in a tubular flow reactor: Experiments and kinetic modeling Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 J.M. Colom-Díaz, M. Abián, M.Y. Ballester, Á. Millera, R. Bilbao, M.U. Alzueta
Oxidation of H2S at atmospheric pressure has been studied under different reaction atmospheres, varying the air excess ratio (λ) from reducing (λ = 0.32) to oxidizing conditions (λ = 19.46). The experiments have been carried out in a tubular flow reactor, in the 700-1400 K temperature range. The concentrations of H2S, SO2 and H2 have been determined and the experimental results have been simulated with a detailed chemical mechanism compiled in the present work. The experimental results obtained indicate that H2S consumption is shifted to lower temperatures as the stoichiometry increases, starting at 925 K for reducing conditions and at 700 K for the most oxidizing ones. The model reproduces well, in general, the experimental data from the present work, and those from the literature at high pressures. Supported by theoretical calculations, the isomerization of HSOO to HSO2 has been determined as an alternative and possible pathway to the final product SO2, from the key SH + O2 reaction.
A three-equation model for the prediction of soot emissions in LES of gas turbines Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 B. Franzelli, A. Vié, N. Darabiha
The design of new low-emission systems requires the development of models providing an accurate prediction of soot production for a small computational cost. In this work, a three-equation model is developed based on mono-disperse closure of the source terms from a sectional method. In addition, a post-processing technique to estimate the particles size distribution (PSD) from global quantities is proposed by combining Pareto and log-normal distributions. After validation, the developed strategy is used to perform a large eddy simulation of soot production in a model combustor representative of gas turbine combustion chambers. It is shown that the three-equation model is able to provide a good estimation of soot volume fraction and information on PSD in complex geometries for a low computational time.
Direct numerical simulations of turbulent catalytic and gas-phase combustion of H2/air over Pt at practically-relevant Reynolds numbers Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 Behrooz Ostadmohammadi Arani, Christos Emmanouil Frouzakis, John Mantzaras, Konstantinos Boulouchos
Three-dimensional direct numerical simulations of turbulent catalytic and gas-phase H2/air combustion at a fuel-lean equivalence ratio φ=0.18 φ = 0.18 were performed in platinum-coated planar channels at two industrially-relevant flow conditions (inlet friction Reynolds numbers Reτ= 182 and 385) using detailed hetero-/homogeneous chemical reaction mechanisms. The preferential diffusion of hydrogen and oxygen, which was responsible for creating significantly higher surface equivalence ratios φw compared to the bulk gas-phase φ, was appreciably suppressed by turbulence at Reτ= 385. The higher turbulence intensity at this Reτ resulted in larger near-wall hydrogen excess that in turn yielded shorter homogeneous ignition distances compared to the lower Reτ case. Gas-phase ignition proceeded from isolated ignition kernels that subsequently formed axially elongated flames confined close to the catalytic walls. The coupling of catalytic and gas-phase chemistry inhibited homogeneous ignition, since at the vicinity of the ignition kernels the OH, H and O radical fluxes to the underlying catalytic wall were net-adsorptive and furthermore hydrogen was depleted by the catalytic reactions. The flame topology included alternating vigorously-burning and extinguished elongated streamwise stripes at Reτ=182 R e τ = 182 or islands at Reτ=385 R e τ = 385 . The extinguished gas-phase reaction zones at Reτ=385 R e τ = 385 were characterized by underlying intense catalytic reaction rates. The flame topology and spatiotemporal correlation of the isolated burning and extinguished gaseous zones indicated that significant surface temperature non-uniformities could be obtained in practical catalytic reactors.
Can a spreading flame over electric wire insulation in concurrent flow achieve steady propagation in microgravity? Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 Masashi Nagachi, Fumiya Mitsui, Jean-Marie Citerne, Hugo Dutilleul, Augustin Guibaud, Grunde Jomaas, Guillaume Legros, Nozomu Hashimoto, Osamu Fujita
Concurrent flame spread over electric wire insulation was studied experimentally in microgravity conditions during parabolic flights. Polyethylene insulated Nickel-Chrome wires and Copper wires were examined for external flow velocities ranging from 50 mm/s to 200 mm/s. The experimental results showed that steady state flame spread over wire insulation in microgravity could be achieved, even for concurrent flow. A theoretical analysis on the balance of heat supply from the flame to the unburned region, radiation heat loss from the surface to the ambient and required energy to sustain the flame propagation was carried out to explain the presence of steady spread over insulated wire under concurrent flow. Based on the theory, the change in heat input (defined by the balance between heat supply from flame and radiation heat loss) was drawn as a function of the flame spread rate. The curve intersected the linear line of the required energy to sustain the flame. This balance point evidences the existence of steady propagation in concurrent flow. Moreover, the estimated steady spread rate (1.2 mm/s) was consistent with the experimental result by considering the ratio of the actual flame length to the theoretical to be 0.5. Further experimental results showed that the concurrent flame spread rate increased with the external flow velocity. In addition, the steady spread rate was found to be faster for Copper wires than for Nickel-Chrome wires. The experimental results for upward spreading (concurrent spreading) in normal gravity were compared with the microgravity results. In normal gravity, the flame did not reach a steady state within the investigated parameter range. This is due to the fact that the fairly large flame spread rate prevented the aforementioned heat balance to be reached, which meant that such a spread rate could not be attained within the length of the tested sample.
Modeling of large-scale under-expanded hydrogen jet fires Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-19 Jean-Louis Consalvi, Fatiha Nmira
Large-scale hydrogen under-expanded jet flames and lab-scale H2 and CH4 jet flames are simulated by using the standard k-ε model coupled to a hybrid flamelet/transported PDF method and a narrow band correlated-k gas radiation model. The set of flames considered covers a wide range of residence times and optical thicknesses. The notional nozzle concept is adopted to determine injection conditions for the large-scale chocked flames. Model predictions in terms of flame geometry, flame structure, radiant fraction and radiative flux are consistent with the experimental data whatever the scales. Model results show that these flames do not verify the optically-thin approximation since the part of emitted radiant power re-absorbed within the flame ranges from 11% for the smallest H2 flame to about 70% for the largest H2 flame. Neglecting the turbulence-radiation interaction underestimates significantly the radiant fraction whatever the scale and these discrepancies are enhanced with increasing residence time and optical thickness. A simple analysis based on the assumption of homogenous flame is used to correlate the experimental radiant fraction as a function of τ G E m ( 1 − Q ˙ a b s / Q ˙ e m ) where τG, Em and Q ˙ a b s / Q ˙ e m represent the residence time, an equivalent emission term and the part of emitted radiant power re-absorbed within the flame. Model results are used to provide a proper estimation of the equivalent emission term and self-absorption.
Mechanistic investigation into particulate matter formation during air and oxyfuel combustion of formulated water-soluble fractions of bio-oil Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-18 Chao Feng, Hongwei Wu
This paper reports a systematic study on the formation of particulate matter with diameter of <10 µm (i.e., PM10) during the combustion of two formulated water-soluble fractions (FWSFs) of bio-oil in a drop-tube-furnace (DTF) at 1400 °C under air or oxyfuel (30%O2/70%CO2) conditions. FWSF-1 was an organic-free calcium chloride solution with a calcium concentration similar to that in the bio-oil. FWSF-2 was formulated from the compositions of major organics in bio-oil WSF, doped with calcium chloride at the same concentration. The results suggest that similar to bio-oil combustion, the FWSF combustion produces mainly particulate matter with diameter of between 0.1 and 10 µm (i.e., PM0.1–10). Since there are no combustibles in the organic-free FWSF-1, the PM is produced via droplet evaporation followed by crystallization, fusion and further reactions to form CaO (in air or argon) or partially CaCO3 (under oxyfuel condition). With the addition of organics, FWSF-2 combustion produces PM10 shifting to smaller sizes due to extensive break up of droplets via microexplosion. Sprays with larger droplet size produce PM10 with increased sizes. The results show that upon cooling CaO produced during combustion in air can react with HCl gas to form CaCl2 in PM0.1. The predicted PSDs of PM10 based on the assumption that one droplet produces one PM particle is considerably larger than experimentally-measured PSDs of PM10 during the combustion of FWSFs, confirming that breakup of spray droplets takes place and such breakup is extensive for FWSF-2 when organics are present in the fuel.
A molecular beam mass spectrometric investigation of plasma assisted oxidation and pyrolysis of methane Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-18 Ruzheng Zhang, Handong Liao, Jiuzhong Yang, Xuefeng Fan, Bin Yang
A new experimental setup coupled a dielectric barrier discharge flow reactor with a molecular beam mass spectrometer (MBMS) was developed for detailed species diagnostics on plasma assisted oxidation and pyrolysis system. The mixture of methane and oxygen diluted by argon was selected as the feed gas. The MBMS technique with photoionization or electron ionization provided access to not only stable molecules but also highly reactive species involved in the methane plasma, including ions, excited species and radicals. The qualitative identification of various components is achievable based on the information of exact mass and ionization energy. The corresponding quantitative information can be obtained by integrated ion signal and ionization cross-sections. The experimental results showed that oxidation and pyrolysis of methane were triggered by plasma at room temperature. For the pyrolysis case, hydrocarbons ranging from C2 up to C5 were detected. While for the oxidation case, the formation of acetaldehyde and ketene together with formaldehyde and methanol was observed. The parameters of applied voltage and oxygen concentration in the feed gas were found to have effective control on the reactivity and selectivity of the plasma respectively. The degree of methane consumption is the largest when the oxygen concentration in the feed gas is less than half of that at stoichiometric conditions and a higher voltage is applied.
High-speed imaging of the ignition of ethanol at engine relevant conditions in a rapid compression machine Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-15 Rene Daniel Büttgen, Thomas Raffius, Gerd Grünefeld, Hans-Jürgen Koß, Alexander Heufer
The rapid compression machine (RCM) is a great tool for investigating fuel properties under engine relevant conditions (high-pressures, low temperatures). The most common diagnostics is measuring the pressure over time and determining the ignition delay time (IDT). In this study, for the first time, the OH* luminescence of ethanol/air mixture is measured within an RCM experiment at 15 and 20 bar for Φ = 0.5. Combining the common pressure measurements with the simultaneously recorded high-speed images (up to 74.5 kHz framerate) gives a first insight into understanding the ignition modes and the corresponding pressure traces. At 74.5 kHz, in contrast to findings in literature, the ethanol ignition did not show to be purely homogeneous. Four different propagating fronts of OH* luminescence have been recorded. Besides a flame kernel and a detonation-like ignition front two further fronts prior to main ignition have been observed. The propagating speeds of the fronts have been determined and depend on the overall IDT.
The growth of PAHs and soot in the post-flame region Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-15 Peng Liu, Zepeng Li, William L. Roberts
The PAHs-C2H2 pathway (PAHs + C2H2 → intermediate → product + H2) has been shown, in theory, to be the important contributor to the growth of polycyclic aromatic hydrocarbons (PAHs) and soot in the post-flame region where H atoms are rare. Calculations of the potential energy surface (PES) using the DFT B3LYP 6-311 + G(d,p) method, and the reaction rate coefficients using the RRKM theory, reveal that armchair and bridge surface sites share similar kinetic characteristics, and are more likely to be the targets of C2H2 molecules in flames compared to zig-zag and 5-membered ring surface sites. Results show that the energy barrier of a 2-H elimination reaction (14–23.8 kcal/mol) is much lower than that of a 1-H elimination (typically 30–40 kcal/mol) for some molecules. The formation of pyrene from phenanthrene via HACA (PAHs + H → PAHs radical (+ C2H2) → intermediate → product + H) and PAHs-C2H2 pathways is investigated using a closed homogeneous zero-dimensional reactor with combustion parameters abstracted from the premixed stagnation C2H4/O2/Ar sooting flame. Results show that the HACA pathway is the dominant pathway for the formation of PAHs and soot surface growth in the main-flame region where H atoms are abundant, but that the PAHs-C2H2 pathway is the preferred pathway in the post-flame region. Our study also suggests that the soot nucleation involving a chemical coalescence of moderate-sized PAHs into a crosslinked three-dimensional structure via the addition reactions of PAHs and PAH radicals in the main-flame region should be considered for inclusion in any soot modeling.
Simultaneous measurement of multiple thermal hazards associated with a failure of prismatic lithium ion battery Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-15 Ahmed O. Said, Christopher Lee, Xuan Liu, Zhibo Wu, Stanislav I. Stoliarov
Lithium ion batteries (LIBs) are efficient, high-density electrical energy storage devices utilized in a perpetually increasing range of applications. One of the weaknesses of LIBs is that a small deviation from the normal operating conditions may cause an irreversible failure accompanied by a rapid self-heating and ejection of combustible gases and aerosols. The information on how much energy is released upon failure is critical for design of energy storage systems. In the current study, a novel technique, Copper Slug Battery Calorimetry (CSBC), was combined with oxygen consumption calorimetry to measure the rate of heat generated inside an LIB cell (PIHG) and the rate of heat generated as a result of combustion of ejected battery materials (PFlaming). A short duct equipped with a perforated plate was added to the original design of the CSBC apparatus to collect gases and aerosols ejected from the cell, reduce their flow speed and deliver them to a hot-wire igniter, which was used to initiate a diffusion flame. The exhaust from this flame was collected to measure the oxygen consumed in the combustion process and compute PFlaming. This approach to handling the ejected materials increased their combustion efficiency and eliminated thermal feedback from the flame to the cell, which enabled simultaneous measurement of PIHG and PFlaming. The new setup was employed to investigate thermally induced failure of an 1880 mA h prismatic LIB at various states of charge (SOC). It was determined that, at 100% SOC, this LIB released 33 ± 1.0 kJ of energy into the body of the cell and 113 ± 19 kJ was produced as a result of combustion of the ejected battery materials. The latter value is significantly greater than those previously reported for similarly sized cells, which can be explained by a more complete combustion achieved in this new apparatus.
Volatile–char interactions: Roles of in situ volatiles with distinctly-different chemistry in determining char structure and reactivity Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-15 Xujun Chen, Hongwei Wu
This study reports the roles of volatiles with distinctly-different chemistry in determining char reactivity and char structure during in situ volatile–char interactions under non-catalytic conditions. Volatiles were generated in situ from polyethylene (PE), double-acid washed biosolid (DAWB), polyethylene glycol (PEG) or cellulose and interacted with char prepared from DAWB that is free of catalytically-active inorganic species in a two-stage reactor at 1000 °C. The experimental results show that both H- and O-containing reactive species play different roles during in situ volatile–char interactions. It has been found that char reactivity decreases substantially after in situ volatile–char interactions. Results from Raman analysis of the char after in situ interactions with the PE volatiles show H-containing reactive species substantially enhance the condensation of the aromatic ring systems within the char, thus slightly decreasing the H content in char and also making char carbon structure considerably less reactive. It has also been found that the reactivity of char after in situ volatile–char interactions increases with increasing O/H molar ratio of volatiles. The results indicate that O-containing reactive species in volatiles can react with char to form C O complex oxides that mitigate the carbon structure from condensing into large aromatic ring systems, thus increasing O and H contents in char and enhancing char reactivity.
A comparative laser absorption and gas chromatography study of low-temperature n-heptane oxidation intermediates Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-15 Alison M. Ferris, Jesse W. Streicher, Adam J. Susa, David F. Davidson, Ronald K. Hanson
A novel, multi-species, combined laser absorption/gas chromatography (GC) speciation diagnostic was used to quantify intermediate species present in the low-temperature oxidation of 1.0% n-heptane in 20.8% O2/Ar and 20.8% O2/0.21% CO2/Ar (equivalence ratio 0.53) at 760 K, 4.9 atm. Laser absorption techniques were used to measure initial fuel and time-resolved temperature, CO2, H2O, and C2H4. Sampled-gas GC analysis was used in conjunction with a variable-test-time shock tube facility to obtain quasi-time-resolved measurements of n-heptane, C2H4, CO, H2, C3H6, and CO2 in the same experiments. Measurements obtained using both techniques are compared to each other, and to initial results predicted by a detailed kinetic model using the experimentally measured pressure trace to constrain the model. Discrepancies between measured and predicted ignition delay times indicate the overestimation of three primary RO2 isomerization reaction rates. The three reaction rates were modified to improve agreement of modeled ignition delay times with the measurements. Final results produced using the modified mechanism are compared to the experimental results; the comparison shows close agreement between the two experimental measurement techniques, and measured species yields confirm the low-temperature reaction pathways that govern n-heptane decomposition and C2H4, CO, H2, and C3H6 production.
Flame spray pyrolysis synthesized CuO-TiO2 nanoparticles for catalytic combustion of lean CO Proc. Combust. Inst. (IF 3.214) Pub Date : 2018-06-12 Xin Chen, Zuwei Xu, Fan Yang, Haibo Zhao
In this work, CuO-TiO2 nanoparticles with different CuO mass contents of 2%, 8%, 12%, and 20% are synthesized by flame spray pyrolysis (FSP) method and applied to catalytic combustion of lean CO. The nano-catalyst is characterized by N2-physisorption isotherms, X-ray diffraction (XRD), transmission electron microscopy (TEM), H2-TPR (temperature-programmed reduction) and X-ray photoelectron spectroscopy (XPS). All the catalysts possess a high specific surface area, of which the CuO-TiO2 nanoparticles with 2 wt.% Cu (2CT) is as high as 98 m2/g, and exhibits a spherical structure with a diameter of 15–20 nm. Compared with other methods, the FSP method can significantly improve the loading of CuO without producing large crystalline CuO particles on the catalyst surface. Interestingly, the addition of CuO will essentially change the lattice structure of TiO2 for all catalysts, including its crystal spacing and XRD diffraction angle. Copper cations are embedded in TiO2 lattice to promote the transformation from anatase to rutile by producing oxygen defect at high flame temperature. The interaction between CuO and TiO2 has significant influence on its physicochemical properties. A lower onset reduction temperature on the sample with higher CuO loading is obtained due to the hydrogen spillover effect in H2-TPR test. Moreover, the loaded CuO increases the content of more stable rutile phase in the materials, so that it reduces the strong metal-support interaction (SMSI) effect of CuO and anatase phase to improve the properties of CO catalytic combustion. The synthesized CuO-TiO2 nanoparticles can achieve complete combustion conversion of lean CO at lower temperature of 120°C.
Inorganic PM10 emission from the combustion of individual mallee components and whole-tree biomass Proc. Combust. Inst. (IF 3.214) Pub Date : 2016-10-06 Xiangpeng Gao, Muhammad Usman Rahim, Xixia Chen, Hongwei Wu
This contribution reports the emission of inorganic particulate matter (PM) with an aerodynamic diameter <10 µm (PM10) from the combustion of both individual mallee components and whole-tree biomass. Three major components of a mallee tree, namely bark, leaf, and wood, were size-reduced to 75 – 150 µm and mixed at a dry mass ratio of 15% bark:35% leaf:50% wood, which is close to the real mallee's composition, to prepare a whole-tree biomass. The three individual mallee components and the whole-tree biomass were combusted in a laboratory-scale drop-tube furnace at 1400 °C in air to produce inorganic PM10 for further quantification and characterization. The results demonstrate that, whereas the particle size distributions of the PM10 from the combustion of the bark, leaf and wood components generally follow a bimodal distribution, the yields of PM0.1, PM0.1–1, PM1, PM1–10, PM2.5, and PM10 from the three mallee components are quite different. On the bases of dry biomass and useful energy input, the yields of the PM of various size fractions studied follow a sequence of the bark > the leaf > the wood, consistent with that of the ash contents in the three components. Oppositely, the ash-based yields of PM0.1, PM0.1–1, PM1, PM1–10, PM2.5, and PM10 from the wood are substantially higher than those from the bark and the leaf. No obvious synergetic effect among different mallee components in PM10 emission is observed during the whole-tree biomass combustion, enabling the prediction of the PM10 yield from the whole-tree biomass combustion based on that from the individual mallee components.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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- Curr. Opin. Colloid Interface Sci.
- Curr. Opin. Environ. Sustain
- Curr. Opin. Solid State Mater. Sci.
- Ecotox. Environ. Safe.
- Electrochem. Commun.
- Electrochim. Acta
- Energy Environ. Sci.
- Energy Fuels
- Energy Storage Mater.
- Environ. Impact Assess. Rev.
- Environ. Int.
- Environ. Model. Softw.
- Environ. Pollut.
- Environ. Res.
- Environ. Sci. Policy
- Environ. Sci. Technol.
- Environ. Sci. Technol. Lett.
- Environ. Sci.: Nano
- Environ. Sci.: Processes Impacts
- Environ. Sci.: Water Res. Technol.
- Eur. J. Inorg. Chem.
- Eur. J. Med. Chem.
- Eur. J. Org. Chem.
- Eur. Polym. J.
- J. Acad. Nutr. Diet.
- J. Agric. Food Chem.
- J. Alloys Compd.
- J. Am. Ceram. Soc.
- J. Am. Chem. Soc.
- J. Am. Soc. Mass Spectrom.
- J. Anal. Appl. Pyrol.
- J. Anal. At. Spectrom.
- J. Antibiot.
- J. Catal.
- J. Chem. Educ.
- J. Chem. Eng. Data
- J. Chem. Inf. Model.
- J. Chem. Phys.
- J. Chem. Theory Comput.
- J. Chromatogr. A
- J. Chromatogr. B
- J. Clean. Prod.
- J. CO2 UTIL.
- J. Colloid Interface Sci.
- J. Comput. Chem.
- J. Cryst. Growth
- J. Dairy Sci.
- J. Electroanal. Chem.
- J. Electrochem. Soc.
- J. Environ. Manage.
- J. Eur. Ceram. Soc.
- J. Fluorine Chem.
- J. Food Drug Anal.
- J. Food Eng.
- J. Food Sci.
- J. Funct. Foods
- J. Hazard. Mater.
- J. Heterocycl. Chem.
- J. Hydrol.
- J. Ind. Eng. Chem.
- J. Inorg. Biochem.
- J. Magn. Magn. Mater.
- J. Mater. Chem. A
- J. Mater. Chem. B
- J. Mater. Chem. C
- J. Mater. Process. Tech.
- J. Mech. Behav. Biomed. Mater.
- J. Med. Chem.
- J. Membr. Sci.
- J. Mol. Catal. A Chem.
- J. Mol. Liq.
- J. Nat. Gas Sci. Eng.
- J. Nat. Prod.
- J. Nucl. Mater.
- J. Org. Chem.
- J. Photochem. Photobiol. C Photochem. Rev.
- J. Phys. Chem. A
- J. Phys. Chem. B
- J. Phys. Chem. C
- J. Phys. Chem. Lett.
- J. Polym. Sci. A Polym. Chem.
- J. Porphyr. Phthalocyanines
- J. Power Sources
- J. Solid State Chem.
- J. Taiwan Inst. Chem. E.
- Macromol. Rapid Commun.
- Mass Spectrom. Rev.
- Mater. Chem. Front.
- Mater. Des.
- Mater. Horiz.
- Mater. Lett.
- Mater. Sci. Eng. A
- Mater. Sci. Eng. R Rep.
- Mater. Today
- Meat Sci.
- Med. Chem. Commun.
- Microchem. J.
- Microchim. Acta
- Micropor. Mesopor. Mater.
- Mol. Biosyst.
- Mol. Cancer Ther.
- Mol. Catal.
- Mol. Nutr. Food Res.
- Mol. Pharmaceutics
- Mol. Syst. Des. Eng.
- Nano Energy
- Nano Lett.
- Nano Res.
- Nano Today
- Nano-Micro Lett.
- Nanomed. Nanotech. Biol. Med.
- Nanoscale Horiz.
- Nat. Catal.
- Nat. Chem.
- Nat. Chem. Biol.
- Nat. Commun.
- Nat. Energy
- Nat. Mater.
- Nat. Med.
- Nat. Methods
- Nat. Nanotech.
- Nat. Photon.
- Nat. Prod. Rep.
- Nat. Protoc.
- Nat. Rev. Chem.
- Nat. Rev. Drug. Disc.
- Nat. Rev. Mater.
- Natl. Sci. Rev.
- Neurochem. Int.
- New J. Chem.
- NPG Asia Mater.
- npj 2D Mater. Appl.
- npj Comput. Mater.
- npj Flex. Electron.
- npj Mater. Degrad.
- npj Sci. Food
- Pharmacol. Rev.
- Pharmacol. Therapeut.
- Photochem. Photobiol. Sci.
- Phys. Chem. Chem. Phys.
- Phys. Life Rev.
- PLOS ONE
- Polym. Chem.
- Polym. Degrad. Stabil.
- Polym. J.
- Polym. Rev.
- Powder Technol.
- Proc. Combust. Inst.
- Prog. Cryst. Growth Ch. Mater.
- Prog. Energy Combust. Sci.
- Prog. Mater. Sci.
- Prog. Photovoltaics
- Prog. Polym. Sci.
- Prog. Solid State Chem.