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  • A comprehensive study of flamelet tabulation methods for pulverized coal combustion in a turbulent mixing layer—Part II: Strong heat losses and multi-mode combustion
    Combust. Flame (IF 4.120) Pub Date : 2020-01-06
    Xu Wen; Martin Rieth; Arne Scholtissek; Oliver T. Stein; Haiou Wang; Kun Luo; Andreas Kronenburg; Jianren Fan; Christian Hasse

    This paper is a continuation of our work done in Part I, in which the a priori and budget analyses were conducted, Wen et al. (2019). In this work, we focus on addressing specific and recurring issues in flamelet modeling for pulverized coal combustion, including strong heat losses, multi-mode combustion and reaction progress variable definition. First, extended flamelet formulations are developed that can take into account strong heat loss effects in pulverized coal combustion systems. Then, to characterize multi-mode combustion in pulverized coal flames, a coupled premixed and non-premixed flamelet model is developed using the combustion mode index. Finally, the effects of reaction progress variable definition on the flamelet predictions are quantified. A state-of-the-art direct numerical simulation database is employed to challenge the newly developed flamelet models. The tabulated thermo-chemical quantities are compared with the reference direct numerical simulation results through a priori analyses. Comparisons show that the newly developed flamelet models which take into account strong heat loss effects can predict the gas temperature and species mass fractions correctly. The adiabatic flamelet models over-predict the corresponding thermo-chemical quantities in regions where the interphase heat transfer is significant. Coupled with a linear extrapolation method, the prediction of the gas temperature with the adiabatic flamelet models can be improved. The performance of the multi-mode flamelet model depends on whether the local combustion mode can be correctly identified. The conventional combustion mode index based on the gradients of fuel and oxidizer species mass fractions cannot correctly identify the combustion mode in the entire combustion field.

    更新日期:2020-01-06
  • Influence of heat-loss on compressibility-driven flames propagating from the closed end of a long narrow duct
    Combust. Flame (IF 4.120) Pub Date : 2019-12-26
    Vadim N. Kurdyumov; Moshe Matalon

    This study focuses on the dynamics of compressibility-driven flames that emerge in narrow tubes, closed at their ignition end, when conductive heat losses through the walls are appreciable. A narrow gap approximation is used to reduce the governing equations to an effectively one-dimensional problem. In long channels this problem admits traveling-wave solutions which we have investigated numerically for finite values of the Zel’dovich number, and asymptotically in the limit of large Zel’dovich numbers. In particular, we describe the flame structure and the dependence of the propagation speed on the physico-chemical parameters, including the heat loss and compressibility parameters, and examine the transition from compressibility-driven to isobaric flames when systematically reducing the representative Mach number.

    更新日期:2019-12-27
  • DMMP pyrolysis and oxidation studies at high temperature inside a shock tube using laser absorption measurements of CO
    Combust. Flame (IF 4.120) Pub Date : 2019-12-26
    Sneha Neupane; Ramees K. Rahman; Jessica Baker; Farhan Arafin; Erik Ninnemann; Kyle Thurmond; Chun-Hung Wang; Artëm E. Masunov; Subith S. Vasu

    Dimethyl methyl phosphonate (DMMP) is an organo-phosphorous compound (OPC) used as a fire suppressant and a simulant for sarin, a chemical warfare agent. There exists a critical need to gather combustion data at high heating rate and high temperatures conditions, similar to what exists during destruction process of chemical weapons. In the present work, DMMP pyrolysis and oxidation were carried out behind reflected shock waves at temperatures of 1300–1700 K and pressures of 1.5–1.8 atm. Lean, stoichiometric, and rich DMMP mixtures (Φ = 0.23, 0.5, 1, 2) were investigated for oxidation experiments. Laser absorption spectroscopy utilizing a quantum cascade laser near 4.9 µm was used to measure intermediate CO concentration formed during the pyrolysis and oxidation processes. To the best of our knowledge, we present the first intermediate concentration data at the reported conditions for DMMP. A tentative kinetic model, based on the AramcoMech2.0 mechanism with Lawrence Livermore National Lab (LLNL)’s OPC incineration chemistry, was utilized in Chemkin-Pro to predict CO yield during the processes. The model provided fair prediction of CO yield during DMMP pyrolysis, however, overpredicted the CO yield for oxidation. Sensitivity and rate of production analyses were carried out to understand important reactions leading to CO formation.

    更新日期:2019-12-27
  • Morphology and internal structure of soot particles under the influence of jet–swirl and jet–jet interactions in a diesel combustion environment
    Combust. Flame (IF 4.120) Pub Date : 2019-12-26
    Lingzhe Rao; Yilong Zhang; Sanghoon Kook; Kenneth S. Kim; Chol-Bum Kweon

    A new multi-location soot sampling method is used to enhance the knowledge about the structural evolution of in-flame particles in a light-duty optical diesel engine. Through thermophoresis-based particle sampling performed at multiple in-bowl locations, the soot structures are shown for both early formation stage and later stage from the same combustion event. Three different jet-spacing angles of 45°, 90° and 180° were studied to analyse how different levels of jet–jet interaction impact the soot particle morphology and internal structure. One selected jet–jet interaction condition was further analysed to show differences in soot structures between the up-swirl side and down-swirl side of the wall jets. From transmission electron microscopes (TEM) images of the sampled soot particles and their statistical size analysis, it was found soot particles initially formed within 45∘ separated jet–jet interaction region have un-solidified premature aggregates due to limited carbonisation in the locally fuel-rich mixtures. When these soot particles travelled on the down-swirl side of the jets, they became solidified and carbonised while the oxidation was evident from the smaller soot primary particle and longer carbon-layer fringe and lower tortuosity. The higher mixing on the up-swirl side of the jets further enhanced the soot oxidation, resulting in even smaller soot primary particle, fragmentation of large soot aggregates, and even longer and less curved carbon-layer fringes. Regarding jet–jet interaction, the 180° jet spacing angle created no jet–jet interaction condition on the soot sampler locations. For smaller jet-spacing angles, the increase in jet–jet interaction promoted the soot formation as evidenced by larger and more complex soot aggregates formed due to more active soot aggregation and agglomeration. The soot oxidation became limited at higher jet–jet interaction conditions, which led to more amorphous soot internal structures.

    更新日期:2019-12-27
  • Effect of premilling Al and CuO in acetonitrile on properties of Al·CuO thermites prepared by arrested reactive milling
    Combust. Flame (IF 4.120) Pub Date : 2019-12-26
    Mehnaz Mursalat; Mirko Schoenitz; Edward L. Dreizin

    Thermite powders with molar composition 8Al·3CuO were prepared in two stages by Arrested Reactive Milling (ARM). In the first stage, the starting materials Al and CuO were milled separately in acetonitrile. Composite powders were then prepared in the second milling stage with hexane as process control agent and in the four possible combinations of one, both, or neither starting material being premilled in acetonitrile. Composites were characterized for morphology, size distribution, surface area, and reactive properties at low heating rates (thermal analysis) and high heating rates (ignition). Whether or not CuO was premilled, dense composites formed without premilling of Al. If Al was premilled in acetonitrile, however, loose agglomerates of refined Al and CuO particles formed in the second milling stage. Premilling changed the low-temperature reactions leading to ignition in the 8Al·3CuO thermites. These changes are attributed to increased porosity of the formed composites if aluminum is premilled with acetonitrile. It is shown that greater refinement and lower ignition temperatures are achievable using two-stage milling.

    更新日期:2019-12-27
  • Experimental observation of diffusive-thermal oscillations of burner stabilized methane-air flames
    Combust. Flame (IF 4.120) Pub Date : 2019-12-18
    S. Nechipurenko; T. Miroshnichenko; N. Pestovskii; S. Tskhai; B. Kichatov; V. Gubernov; V. Bykov; U. Maas

    In this work we experimentally investigate the onset of the diffusive-thermal instabilities for flat burner stabilized methane-air flames at normal pressure. The structure and dynamics of combustion fronts are studied by using visual imaging, including high-speed video recording, spectroscopic and acoustic measurements. Numerical simulations with a one-dimensional model for a burner stabilized flame with detailed reaction mechanisms are also conducted. The critical conditions for the emergence of flame oscillations and the period of pulsations are determined. The experimental data obtained with different techniques are found to be in good agreement and also correlate with the numerical results. The numerically calculated data are shown to be sensitive to the choice of the reaction mechanism. Thus a direct measurement and calculation of the critical conditions and the characteristics of flame oscillations open a new perspective for the verification and validation of detailed reaction mechanisms.

    更新日期:2019-12-19
  • Sensitive and interference-immune formaldehyde diagnostic for high-temperature reacting gases using two-color laser absorption near 5.6 µm
    Combust. Flame (IF 4.120) Pub Date : 2019-12-10
    Yiming Ding, Shengkai Wang, Ronald K. Hanson

    In this work we present a novel two-color differential absorption diagnostic near 5.6 μm using an external-cavity quantum-cascade-laser (EC-QCL) for formaldehyde (CH2O) detection in high-temperature reacting gases. This diagnostic utilizes the narrow mid-infrared absorption features of CH2O in the R branch of the C=O stretching band. Specifically, the online and offline colors were chosen as 1787.05 cm-1 and 1787.85 cm-1, respectively. Compared to the previous Stanford work that targeted the 3.5 μm band of CH2O, this new diagnostic achieved higher sensitivity, better species selectivity and immunity to interference absorption from other combustion gases, especially small hydrocarbons (e.g. methane). Absorption cross sections of CH2O at the two colors were measured in shock-heated 1,3,5-trioxane/argon mixtures over a wide range of temperatures and pressures: 870 K ≤ T ≤ 1800 K, 0.7 atm ≤ P ≤ 4.5 atm. These cross section measurements were further expressed as analytical functions of temperature and pressure with 2σ statistical uncertainties of ± 4.9%. Initial application of the diagnostic was demonstrated in a shock tube study of the thermal decomposition of methanol/argon mixture at 1427 K and 2.77 atm, where the measured CH2O time-history was compared with simulations from multiple combustion mechanisms. We expect this new diagnostic to be useful in future combustion studies.

    更新日期:2019-12-11
  • Spatial distribution of spectrally emitting species in a nitromethane–air diffusion flame and comparison with kinetic models
    Combust. Flame (IF 4.120) Pub Date : 2019-12-09
    Suzanne L. Sheehe, Scott I. Jackson

    Current nitromethane kinetic schemes do not fully capture the intermediate reaction states of deflagrating nitromethane, which is an explosive used in many combustion applications. There is a need for data on these transient species that can be used to validate and improve contemporary kinetic models. This work aims to analyze the evolution of identified transient species present in a nitromethane–air flame stabilized on a wick at ambient pressure. We detect several new emitters in excited electronic states, HNO*(A’), CN* (A2Σ), NO2*, and in ground electronic states, CH, OH, NH, and H2O. We also confirm the presence of previously observed reaction species CH*(A2Δ). Formaldehyde and the characteristic C2*(A3Πg) swan bands are not observed. Results are reported as a function of flame position. Flame simulations for the nitromethane flame using reaction mechanisms by Brequigny et al. (Proc. Combust. Inst. 2014, 35, 703) and Mathieu et al. (Fuel 2016, 182, 597) yield good qualitative agreement to the experimental data for rich stoichiometries. While current mechanisms do not include excited state species, these results also provide insight into potential ground state precursors.

    更新日期:2019-12-09
  • Combustion plasma electrical conductivity model validation for oxy-fuel MHD applications: Spectroscopic and electrostatic probe studies
    Combust. Flame (IF 4.120) Pub Date : 2019-12-06
    Clinton R. Bedick, C. Rigel Woodside, Robert Baylor, Moses Paul-Irudayaraj

    The U.S. Department of Energy has a renewed interest in direct power extraction (DPE) technologies, such as a magnetohydrodynamic (MHD) generator, in conjunction with oxy-fuel combustion. As a topping cycle, this configuration can enable efficient CO2 capture, while offsetting oxygen separation penalties. In order to appropriately evaluate these cycles in the context of modern plant configurations, validated modeling tools are needed. In a prior publication (Bedick et al., 2016), an electrical conductivity model was presented for oxy-fuel MHD applications. However, rigorous validation of the model could not be performed due to a lack of high-quality experimental data at relevant conditions. In this publication, validation experiments were performed and relevant parameters quantified using spectroscopic and electrostatic probe diagnostics. Oxygen-methane flames were generated using a Hencken burner and seeded with K2CO3 to increase ionization and electrical conductivity. The electrical conductivity model from Bedick et al. (2016) was integrated into a 3D CFD simulation of a single burner quadrant and a reaction mechanism including potassium kinetics and ionization was utilized. Lineshape fitting techniques were implemented to determine atomic potassium concentration and gas temperature, while appropriate electrostatic probe theory was applied to derive potassium ion concentrations from experimental current-voltage characteristics. Measured quantities are compared to CFD predictions as a function of seed rate and spatial location within the flame, showing good overall agreement. Indirect validation of electrical conductivity predictions is performed using measured quantities, with results falling well within the bounds of measurement uncertainty.

    更新日期:2019-12-07
  • Particle temperature and flue gas emission of a burning single pellet in air and oxy-fuel combustion
    Combust. Flame (IF 4.120) Pub Date : 2019-12-07
    Chinsung Mock, Hyemin Park, Changkook Ryu, Vasilije Manovic, Seuk-Cheun Choi

    Pelletization of biomass increases the bulk energy density and the uniformity in size and shape with minimized mechanical degradation from transport, storage, and handling. A pellet can be burned in industrial circulating fluidized-bed or fixed-bed furnaces. This experimental study examines the combustion behavior of single pine wood and empty fruit bunch pellets in a laboratory-scale entrained-flow reactor. Individual biomass pellets were oxidized under both N2/O2 (79%/21%) and CO2/O2 (79%/21%) to investigate the burning characteristics during oxy-fuel combustion. The gas temperature was set at 860 K, 970 K, and 1090 K to examine its effect on combustion behavior. The burning pellets were directly observed in a visualization window in the reactor throughout the sequential combustion process. Direct observation, optical pyrometry, and flue gas analysis are synchronized for each pellet to interpret the relation between phenomenological events, particle temperature, and flue gas emission. The experimental results shows an increased homogenous ignition delay and a conical, less sooty flame in the presence of CO2. The particle temperature under CO2/O2 atmosphere is slightly lower than that under N2/O2, especially for the volatile combustion. The CO emission under CO2/O2 is significantly higher and produced from incomplete combustion. These combustion characteristics are attributed to low oxygen diffusion and the high volumetric heat capacity of CO2. The pine wood displayed a fast, homogeneous ignition and a sooty flame with high NO and SO2 emissions compared to the empty fruit bunches.

    更新日期:2019-12-07
  • Detonation initiation by compressible turbulence thermodynamic fluctuations
    Combust. Flame (IF 4.120) Pub Date : 2019-12-07
    Colin A.Z. Towery, Alexei Y. Poludnenko, Peter E. Hamlington

    Theory and computations have established that thermodynamic gradients created by hot spots in reactive gas mixtures can lead to spontaneous detonation initiation. However, the current laminar theory of the temperature-gradient mechanism for detonation initiation is restricted to idealized physical configurations. Thus, it only predicts conditions for the onset of detonations in quiescent gases, where an isolated hot spot is formed on a timescale shorter than the chemical and acoustic timescales of the gas. In this work, we extend the laminar temperature-gradient mechanism into a statistical model for predicting the detonability of an autoignitive gas experiencing compressible isotropic turbulence fluctuations. Compressible turbulence forms non-monotonic temperature fields with tightly-spaced local minima and maxima that evolve over a range of timescales, including those much larger than chemical and acoustic timescales. We examine the utility of the adapted statistical model through direct numerical simulations of compressible isotropic turbulence in premixed hydrogen-air reactants for a range of conditions. We find strong, but not conclusive, evidence that the model can predict the degree of detonability in an autoignitive gas due to turbulence-induced thermodynamic gradients.

    更新日期:2019-12-07
  • Burning properties of redox crystals of ammonium nitrate and saccharides
    Combust. Flame (IF 4.120) Pub Date : 2019-12-06
    Ibukun Oluwoye, Mohammednoor Altarawneh, Jeff Gore, Bogdan Z. Dlugogorski

    Ammonium nitrate (AN, NH4NO3) constitutes the key ingredient of monofuels and civilian-grade explosives, attracting scientific interests aimed at improving their operational and safety performance. This study investigates the combustion properties of redox crystals comprising ammonium nitrate and simple saccharides, with the infrared spectroscopy, X-ray diffraction and molecular modelling. Furthermore, the thermogravimetric measurements afford the isoconversional analysis that yields the overall activation energies of the decomposition process. In addition, the synthesised samples are subjected to elemental and sorption analyses. The results outline (i) the molecular inclusion of the solid fuels within the lattice clusters of AN, (ii) a comparable hygroscopicity behaviour, i.e., a minor increase in affinity towards the absorption of moisture, and (iii) an energetically improved decomposition (and regression) rate, relatively to pristine AN. These features manifest themselves in lower activation energies of redox crystals that enhance the deflagrating properties of these materials for possible application in aviation propellants, and minimise the environmental footprint, especially the emission of nitrogen oxide to the atmosphere, which arises because of inhomogeneities in AN-fuel mixtures commonly used in civilian explosives.

    更新日期:2019-12-06
  • A pressure- or velocity-dependent acceleration rate law for the shock-to-detonation transition process in PBX 9502 high explosive
    Combust. Flame (IF 4.120) Pub Date : 2019-12-05
    Scott I. Jackson

    Shock-to-detonation transition profiles of PBX 9502 explosive are analyzed to develop a rate law for shock acceleration. The shock motion profiles are seen to follow a common trend in the shock acceleration–velocity frame, aside from an early time transient that is dependent on the initiating shock strength. The duration of the early time transient is seen to correlate with the initial shock strength. The common shock acceleration profile is seen to be Arrhenius-like with respect to the local particle velocity or pressure. A dual-rate pressure-dependent Arrhenius-type rate law is developed with the duration of the early rate set by the initial shock strength. The rate law is able to predict the shock motion for all tests well in both particle velocity and pressure space. In addition to directly measuring commonalities in the acceleration profiles of the experimental shock motion, this work provides insight into the functional form of the reaction rate laws for this TATB-based high explosive. The rate law also supports the concept that shock-driven reaction in heterogenous high explosives is driven by localized ignition and growth of hotspots.

    更新日期:2019-12-05
  • Experimental investigation of compartment fires with circular opening: From the aspects of internal temperature and facade flame
    Combust. Flame (IF 4.120) Pub Date : 2019-12-05
    Xiaochun Zhang, Zijian Zhang, Zhenyi Zhang, Wenbin Xu, Quanbing Luo, Haowen Tao, Xing Li

    This paper experimentally investigated the internal temperature and the facade flame for compartment fires involving circular openings. A series of experiments were carried out by using a reduced-scale model which composed of a cube compartment with various circular openings and a vertical fireproofing board (used as building facade). A porous burner with propane fuel was set inside the compartment as fire source. The internal temperature profile was measured by vertical thermocouple arrays at the inner and outer corners, and the facade flame was captured by a CCD camera from a side view. This paper revealed that: (1) With the same area, maximum heat release rate inside the compartment with circular opening is larger than that with square opening. (2) For relatively larger circular openings, facade flames tend to ceiling jet flame extension. With the increasing of heat release rate, facade flame induced by ceiling jet flame extension will convert to spill fire plume. (3) A dimensionless number R, which represents the ratio of total heat release rate to internal maximum heat release rate, is proposed to distinguish spill fire plume and ceiling jet flame extension when facade flames appear at exterior. (4) A non-dimensional correlation is developed to characterize the facade flame heights for both ceiling jet flame extension and spill fire plume. These findings and results can provide fundamental knowledge for understanding compartment fire with a circular opening.

    更新日期:2019-12-05
  • A fast-solving particle model for thermochemical conversion of biomass
    Combust. Flame (IF 4.120) Pub Date : 2019-12-05
    Tian Li, Henrik Thunman, Henrik Ström

    Computational fluid dynamics (CFD) simulations of large-scale furnaces or reactors for thermal conversion of solid fuels remains challenging partially due to the high computational cost related to the particle sub-models. Owing to the thermally thick nature, it is particularly expensive to simulate the conversion of large fuel particles such as biomass particles. To address this issue, a fast-solving particle model was developed in this work with special attention to the computational efficiency. The model spatially discretizes a fuel particle in one homogenized dimension. The conversion process of the fuel particle is treated as a reactive variable-volume one-dimensional transient heat conduction problem. The model also utilizes several features that are typically found in sharp interphase-based models to reduce the computational cost. Validation of the model was carried out by comparing with experimental results under both pyrolysis and combustion conditions. The accuracy and computational efficiency of the model was thoroughly examined by varying the degrees of temporal and spatial discretization. It was found that the model well predicted pyrolysis and combustion of a single biomass particle within a broad range of temporal and spatial discretization. The time used to simulate the conversion of a biomass particle using the developed model can be more than one order of magnitude smaller than the conversion process itself. It was also revealed that a well-predicted conductive heat transfer inside the particle is essential for a precise simulation of the drying and devolatilization process. The char conversion process, however, is less sensitive to the external heat transfer as it is mainly controlled by the mass diffusion process. Further studies showed that a time step of 1×10−3 s and a spatial discretization of 20 cells were sufficient for simulating the conversion of typical fuel particles in grate-fired and fluidized-bed furnaces. We also demonstrated that when the particle model was implemented in a CFD solver, only 2.2% of computational overhead was introduced by the model. As the model can efficiently employ fixed time stepping, optimal load balancing during parallel computing of many simultaneous conversion processes becomes trivial. This performance opens up new possibilities for treating fuel polydispersity in Eulerian CFD simulations of biomass conversion.

    更新日期:2019-12-05
  • CFD simulation of the steam gasification of millimeter-sized char particle using thermally thick treatment
    Combust. Flame (IF 4.120) Pub Date : 2019-12-04
    Tao Chen, Xiaoke Ku, Jianzhong Lin

    A detailed char gasification model is developed using a multiphase Eulerian–Lagrangian algorithm and thermally thick treatment. The model is first validated by both gasification and combustion experiments of a millimeter-sized char particle. Temperature and mass loss histories as well as the particle morphology evolution correspond well with the existing results. Then the steam gasification of a 5 mm char particle is simulated and detailed physical and chemical conversion processes inside the particle are explored. During gasification, three distinct layers, i.e., the outer ash layer, the intermediate layer and the core layer, are identified based on the intraparticle porosity distribution. Simulation results show that the highest H2O and CO2 mass fractions locate in the ash layer, while the intermediate and core layers contain the highest H2 and CO mass fractions, respectively. Moreover, effects of several parameters are also explored. It is found that the Stefan flow caused by the mass transfer plays a key role in determining the diffusion and convection behavior during gasification. The strength of the Stefan flow in the intermediate layer appears to be two orders of magnitude smaller than that of the inflow and has an influence on the shifting from a kinetically-controlled mode to a diffusion-controlled mode. In addition, the char consumption rate in the intermediate layer increases with an increase in steam mass fraction, gasification temperature and inflow velocity while it decreases with increasing particle diameter. Meanwhile, the char consumption rate caused by CO2 is much smaller than that due to steam.

    更新日期:2019-12-04
  • Facilitating Bayesian analysis of combustion kinetic models with artificial neural network
    Combust. Flame (IF 4.120) Pub Date : 2019-12-04
    Jiaxing Wang, Zijun Zhou, Keli Lin, Chung K. Law, Bin Yang

    Bayesian analysis provides a framework for the inverse uncertainty quantification (UQ) of combustion kinetic models. As the workhorse of the Bayesian approach, the Markov chain Monte Carlo (MCMC) methods, however, incur a substantial computational cost. In this work, a surrogate model is employed to improve the traditional MCMC algorithm. Specifically, the test errors of three typical surrogate models are compared, namely Polynomial Chaos Expansion (PCE), High Dimensional Model Representation (HDMR) and Artificial Neural Network (ANN); and ANN is shown to be a relatively more efficient surrogate model for the approximation of combustion reaction systems under extensive conditions. An inverse UQ method, which is the combination of the ANN and traditional MCMC method, and as such termed ANN–MCMC, is adopted. The calculation is performed on the methanol oxidation system and a series of ignition delay data are employed to optimize the rate coefficients of the kinetic model. The estimated posterior distributions of the rate coefficients and the model predictions using the ANN–MCMC are compared with the traditional MCMC methods, with the results showing that the ANN–MCMC can significantly reduce the computational cost needed for the MCMC algorithm, especially on the estimation of the posterior distributions of the input parameters. The rejection rate of the samples in a Markov chain is very high, especially for the calculation of the posterior distribution of less sensitive parameters, thus a large number of samples are needed to reach a desired accuracy for traditional MCMC process. While no samples are rejected when training the ANN surrogate model. Therefore, fewer original samples are needed to get a converged ANN surrogate, which can then generate a large number of low-cost ANN samples for a better accuracy of the MCMC process. The errors for the estimated posterior distributions using ANN–MCMC depend on the accuracy of converged ANN surrogates and more accurate results are obtained with improved settings of ANN. The ANN–MCMC is especially suitable to the computational systems when the computational ability is limited.

    更新日期:2019-12-04
  • Effects of Soret diffusion on turbulent non-premixed H2 jet flames
    Combust. Flame (IF 4.120) Pub Date : 2019-12-03
    Wang Han, Arne Scholtissek, Felix Dietzsch, Christian Hasse

    Recent studies have shown that Soret diffusion (SD), driven by temperature gradients, could play an important role in both laminar and turbulent premixed H2 flames. However, comparatively little effort has been made to investigate SD effects on turbulent non-premixed H2 flames, in spite of the relevance of these flames in safety and their potential application in clean power and propulsion systems. To this end, the impact of SD on turbulent non-premixed H2 combustion is investigated numerically in this work by comparing two three-dimensional direct numerical simulations of temporally evolving turbulent jet flames. In one simulation, a mixture-averaged diffusion (MD) model is used to approximate multicomponent transport, while in the other the MD model is supplemented with a Soret term to consider SD effects. The emphasis is placed on examining and interpreting the impact of SD on flame structure, differential diffusion, and flame-tangential diffusion. It is found that H and OH mass fractions are significantly affected by SD, while SD has a negligible impact on temperature, heat release rate and H2 mass fraction. This is due to the fact that larger SD flux of H radical is strongly coupled with the main chemical reactions. However, for H2 its larger SD flux is located in the fuel-rich zone and decoupled from the main reactions. The difference between a conserved scalar (mixture fraction Z) and a non-conserved scalar (Bilger mixture fraction ZBilger) is employed as a diagnostic parameter to characterize differential diffusion, and the results show that the effects of SD on differential diffusion are reflected in two aspects: (i) increasing the absolute value of Z−ZBilger and (ii) increasing the degree of misalignment between the gradients of Z and ZBilger. Furthermore, the analysis of the contribution of SD to flame-tangential diffusion occurring in mixture fraction isosurfaces indicates that for H radical SD can augment the relative contribution of flame-tangential diffusion, especially in the region of high scalar dissipation rate. On the other hand, for H2, SD has a negligible impact on both flame-normal and flame-tangential diffusion. The present study contributes to providing insights into how SD affects turbulent non-premixed H2 flames and into modeling SD effects with flamelet-based combustion models.

    更新日期:2019-12-03
  • Experimental study of perforated-wall rotating detonation combustors
    Combust. Flame (IF 4.120) Pub Date : 2019-12-03
    Haocheng Wen, Bing Wang

    Perforated walls are potentially applied in rotating detonation combustors (RDCs) to stabilize combustion and perform transpiration cooling. This study involves an experimental investigation on the rotating detonation in perforated-wall combustors for the first time. Five types of walls with different hole sizes and perforated area ratios that range from 0 to 3.5% are examined to analyze acoustics and combustion characteristics, and performance of the RDC. The stable and unstable rotating detonation are both observed in the experiments, and the unstable phenomena mainly correspond to the counter two-wave rotating detonation that co-exists with the acoustic modes of the combustor. The acoustic modes are effectively suppressed by the perforated wall with area ratios over 1.75%, and the stability of rotating detonation significantly improves. The perforated walls significantly weaken the measured detonation pressure peaks and mitigate the impact of rotating detonation on the H2 plenum, while they do not evidently reduce the specific impulse. It is proposed that the acoustic modes are excited by local high-pressure spots generated by the collision of two detonation waves, and they induce the fluctuating pressure peaks and wave velocity by affecting the H2 injection. The perforated holes dissipate high-pressure spots, and thereby suppress the acoustic modes.

    更新日期:2019-12-03
  • Experimental and kinetic modeling study of laminar burning velocities of NH3/syngas/air premixed flames
    Combust. Flame (IF 4.120) Pub Date : 2019-11-29
    Xinlu Han, Zhihua Wang, Yong He, Yanqun Zhu, Kefa Cen

    Ammonia (NH3) can be used as carbon-free alternative fuel for modern energy and transportation systems. Co-firing NH3 with syngas can overcome the high ignition energy and low burning velocities of pure NH3 flames on the one hand, while regarding the characteristics of syngas on the other hand, this strategy may have low-emission potential in real application, and a corresponding research can be helpful for validating or developing NH3 co-firing mechanisms with more complex fuels. The present study experimentally investigated laminar burning velocities of NH3/syngas/air flames at atmospheric pressure and 298 K using the heat flux method. Two types of syngas components were used, i.e., SYN_A: 5 vol% H2 + 95 vol% CO and SYN_B: 50 vol% H2 + 50 vol% CO, and the measured conditions cover wide ranges of mixing ratios and equivalence ratios. Several literature kinetic mechanisms were tested and a new mechanism was proposed. Results calculated by the present mechanism agree well with experimental data of the burning velocities and the ignition delay times of NH3, NH3/H2, NH3/CO, and NH3/syngas flames at various mixing ratios, equivalence ratios, and pressures. The present mechanism also reproduces the trend of NOx emission characteristic in literature. Detailed kinetic analyses using the present mechanism were carried out, showing the NH3 oxidation processes in NH3/syngas/air flames and the most rate-limiting reactions for predicting the laminar burning velocities. Important reactions with different rate parameters from different sources were labeled, which could be helpful for future organization or optimization of NH3 kinetic mechanisms.

    更新日期:2019-11-30
  • Chemical effects of hydrogen addition on soot formation in counterflow diffusion flames: Dependence on fuel type and oxidizer composition
    Combust. Flame (IF 4.120) Pub Date : 2019-11-30
    Lei Xu, Fuwu Yan, Yu Wang, Suk Ho Chung

    We experimentally studied and kinetically modeled the effects of hydrogen addition on soot formation in methane and ethylene counterflow diffusion flames (CDFs). To isolate the chemical effects of hydrogen in such flames, we also ran a set of experiments on flames of the same base fuels but with the addition of helium. Specifically, we measured the soot volume fractions of the flames using the planar laser-induced incandescence technique. We simulated detailed sooting structures by coupling the gas-phase chemistry with the polycyclic aromatic hydrocarbon (PAH)-based soot model, using a sectional method to resolve the soot particle dynamics. Our experimental and numerical results show that hydrogen chemically inhibits soot formation in ethylene CDFs. While in methane flames, it is interesting to observe that the difference in soot production between the hydrogen- and helium-doped cases became much smaller when the oxygen concentration in the oxidizer stream (XO) was reduced. This suggests that for methane CDFs the chemical soot-inhibiting effects of hydrogen was highly dependent on oxidizer composition (i.e., XO). Explanations are provided through detailed kinetic analysis concerning the effects of hydrogen addition on the growth of PAH, soot inception, and soot surface growth processes. Our results suggest that hydrogen's chemical role in soot formation not only depends on fuel type (ethylene or methane), it also may be sensitive to oxidizer composition.

    更新日期:2019-11-30
  • Non-intrusive uncertainty quantification in the simulation of turbulent spray combustion using Polynomial Chaos Expansion: A case study
    Combust. Flame (IF 4.120) Pub Date : 2019-11-30
    Benedict Enderle, Bastian Rauch, Felix Grimm, Georg Eckel, Manfred Aigner

    A major source of input uncertainties in the simulation of turbulent spray combustion is introduced by the need to specify the state of the liquid spray after primary breakup, i.e. a spray boundary condition for the lagrangian transport equations. To further enhance the credibility and predictive capabilities of such simulations, output uncertainties should be reported in addition to the quantities of interest. Therefore, this paper presents results from a comprehensive quantification of uncertainties from the specification of a spray boundary condition and numerical approximation errors. A well characterized lab-scale spray flame is studied by means of an Euler-Lagrange simulation framework using detailed finite rate chemistry. As direct Monte Carlo sampling of the simulation model is prohibitive, non-intrusive Polynomial Chaos expansion (PCE) is used for forward propagation of the uncertainties. Uncertain input parameters are prioritized in a screening study, which allows for a reduction of the parameter space. The computation of probabilistic bounds reveals an extensive uncertainty region around the deterministic reference simulation. In an a posteriori sensitivity analysis, the majority of this variance is traced back to the uncertain spray cone angle of the atomizer. The explicit computation of input uncertainties finally allows for an evaluation of total predictive uncertainty in the case considered.

    更新日期:2019-11-30
  • Ultrafast swelling and shrinking of soot in alkanol–air flames induced by femtosecond laser filamentation
    Combust. Flame (IF 4.120) Pub Date : 2019-11-28
    Hongwei Zang, Yao Fu, Mengyao Hou, Helong Li, Kaoru Yamanouchi, Huailiang Xu

    Soot, which is produced through the rapid growth of hydrocarbon molecules in fuel-rich flames, has been proved to be responsible for global warming and harmful to the respiratory system of human. However, the complex formation process of soot in flames has not been understood yet because of the lack of efficient time-resolved methods for monitoring soot and its precursor species. We present here a viable pump–probe approach to measure ultrafast processes of the soot particles in flames with femtosecond time resolution. The scattering of femtosecond ultraviolet (UV) light pulses is used to monitor soot particles along alkanol–air diffusion flames from the inception phase up to the burnout region after the formation of a femtosecond laser filament in the flames. The time evolution of the scattered UV signal reveals unexpected ultrafast swelling and shrinking of soot particles induced by the femtosecond laser filament in the different flame regions. Our in situ and time-domain measurements of soot particles with the femtosecond time resolution unveil their unique properties in the flames.

    更新日期:2019-11-29
  • Reduction of large-scale chemical mechanisms using global sensitivity analysis on reaction class/sub-mechanism
    Combust. Flame (IF 4.120) Pub Date : 2019-11-28
    Yachao Chang, Ming Jia, Bo Niu, Xue Dong, Pengzhi Wang

    The reliable chemical mechanisms with compact scale play an important role in engine modeling. In this research, a new method was presented to reduce the large-scale reaction kinetic mechanisms through the global sensitivity analysis on the reaction classes/sub-mechanisms in detailed mechanisms. In the present method, the importance of each reaction sub-mechanism in a detailed mechanism was first assessed using the Morris method. Then, the crucial reaction classes in the fuel-specific sub-mechanism were identified based on their significance on the detailed mechanism predictions and their coupling relationship using the Morris method and the path sensitivity analysis. Third, the reactions in the unimportant sub-mechanisms and the isomers in the dominant reaction classes of the fuel-specific sub-mechanism were lumped. The scale of the final mechanism was reduced further by removing the unimportant reactions in the important small-molecule sub-mechanism, and the initial reduced mechanism was obtained. Final, the optimization of the reaction rate constants in the fuel-specific sub-mechanism was performed under their uncertainties to improve the performance of the reduced mechanism over a wide temperature range. With the proposed method, a detailed iso-cetane mechanism including 1107 species and 4469 reactions was reduced. The reduced mechanism contains only 56 species and 131 reactions. Good agreements between the reduced mechanism and the detailed mechanism were achieved on the predicted results of ignition delay times and species concentrations over wide conditions.

    更新日期:2019-11-29
  • An experimental and analytical investigation of canyon fire spread
    Combust. Flame (IF 4.120) Pub Date : 2019-11-28
    Xiaodong Xie, Naian Liu, Jorge R. Raposo, Domingos X. Viegas, Xieshang Yuan, Ran Tu

    This work investigates the fire line evolution in canyon fire spread by analytical and experimental means. The purpose is to understand the mechanism of fire spread acceleration associated with eruptive fire in canyon terrain. A symmetrical model canyon is used for experiments, and in each test the fire is initiated by a point ignition using dead pine needles as the fuels. By experimental observations and data, three distinct types of fire line contour are identified under different central slope angles (the angle between the canyon centerline and the horizontal ground), resulting in different variation modes of rate of spread along the canyon centerline. It is found that the trace of fire head deviates from the line of maximum slope for non-zero central slope angles, and such a deviation increases with increasing central slope angle. The fire spread acceleration associated with eruptive fire occurs when the central slope angle reaches a certain critical value. Experimental data confirm that under higher slope conditions, the significant interaction between the two lateral flame fronts induces marked convective heating ahead of the flame front, which is inferred to be the potential mechanisms of eruptive fire in canyon topography. A model analysis further elucidates how the deflection of the trace of fire head, induced by the fire interaction effect, influences the acceleration of fire spread under higher slope conditions.

    更新日期:2019-11-29
  • A reduced reaction mechanism of biodiesel surrogates with low temperature chemistry for multidimensional engine simulation
    Combust. Flame (IF 4.120) Pub Date : 2019-11-28
    Lei Zhang, Xiaohua Ren, Zhigang Lan

    A reduced biodiesel mechanism composed of 156 species and 589 reactions is reduced from an original complex mechanism (3299 species and 10806 reactions) based on MD, MD9D, and n-heptane as the surrogates. The mechanism reduction is conducted using the path flux analysis method, which considers multiple reaction path generations in the analysis of species interactions, and isomer lumping. Calculations of homogeneous auto-ignition and perfectly stirred reactor (PSR) combustion on a variety of reaction states, including pressures from 1 to 100 atm and equivalence ratios from 0.5 to 2, are the basis of the reduction. The initial temperatures are from 700 to 1800 K for the auto-ignition, and the inlet temperature is 300 K for the PSR. These reaction states cover the high-pressure and low-temperature operating conditions of future engines using advanced combustion technologies characterized by fuel–air premixing and auto-ignition. The fidelity of the resulting reduced mechanism with low-temperature chemistry is examined using a variety of applications. Close agreements between the reduced and original mechanisms are obtained in the predictions of ignition delay, history of mixture temperature, and species mole fraction during homogeneous auto-ignition and the temperature profile in PSR. The reduced mechanism, further integrated with a nitrogen oxides chemistry and a two-step soot model, is implemented into the KIVA/CHEMKIN program for the 3D simulation of biodiesel spray combustion. The predicted liquid and vapor penetrations agree with the experimental data in a non-reactive biodiesel spray simulation, indicating an accurate estimation of biodiesel physical properties. In the simulation of biodiesel spray combustion, predicted spatial distributions of hydroxyl radical and soot also agree with the corresponding experimental data.

    更新日期:2019-11-29
  • An experimental study of n-dodecane and the development of an improved kinetic model
    Combust. Flame (IF 4.120) Pub Date : 2019-11-28
    Yebing Mao, Mohsin Raza, Zhiyong Wu, Jizhen Zhu, Liang Yu, Sixu Wang, Lei Zhu, Xingcai Lu

    This work presents new fundamental combustion experimental datasets and propose an improved kinetic model for n-dodecane, a widely used surrogate component for jet fuels and diesel. In the experiments, ignition delay times for n-dodecane, were measured over low-to-high temperature range by combining a heated rapid compression machine (RCM) and a heated shock tube (ST). The measurements cover a wide range of temperatures (621–1320 K), pressures (8, 15 bar) and equivalence ratios (0.5–1.5). Flow reactor oxidation was also carried out over temperature range of 600–1100 K and at equivalence ratios of 0.5, 1.0 and 1.5 under atmospheric pressure to provide species evolution profiles of O2, CO, CO2 CH4, C2H4 etc. These datasets were used to develop a mechanism for n-dodecane. New rate coefficients were adopted from recent literature to improve the performance of the model, leading to a model composed of 737 species and 3629 reactions. The present experimental data, as well as a wide range of datasets in literature, were used to evaluate the performance of the model. An overall good agreement of the predictions with the experimental results was observed. This model is anticipated to facilitate the development of kinetic models for jet fuel surrogate.

    更新日期:2019-11-29
  • Effects of non-thermal plasma on the lean blowout limits and CO/NOx emissions in swirl-stabilized turbulent lean-premixed flames of methane/air
    Combust. Flame (IF 4.120) Pub Date : 2019-11-28
    Gyeong Taek Kim, Chun Sang Yoo, Suk Ho Chung, Jeong Park

    This study investigates experimentally the effects of non-thermal plasma (NTP) induced by a dielectric barrier discharge (DBD) reactor on the characteristics of swirl-stabilized turbulent lean-premixed methane/air flames in a laboratory scale combustor by systematically varying the applied AC voltage, VAC, and frequency, fAC. Especially, it is elucidated how the NTP influences the lean blowout (LBO) limits and the characteristics of CO/NOx emissions depending on flame configuration. Without applying the NTP as the mixture equivalence ratio, ϕ, decreases from the stoichiometry to an LBO limit, the flame configuration changes from an M-flame (Regime I) to a conical flame (Regime II) and to a columnar flame (Regime III) for the whole range of the mixture nozzle exit velocity, U0, (4–10 m/s). With the NTP, however, it exhibits only Regimes I and II at relatively-low U0 range (4–6 m/s), while all three regimes at relatively-high U0 range (7–10 m/s). For both velocity ranges, the LBO limits are significantly extended by the NTP enhancing the flame stability. Under the relatively-low U0 range, streamers induced by the DBD reactor play a critical role in stabilizing the flames such that the degree of extension of the LBO limit depends linearly on VAC and fAC. Under the relatively-high U0 range, however, ozone generated by the DBD reactor in Regime III is found to be a major reason in extending the LBO limit, which is substantiated by another flame regime diagram with ozone addition only, and hence, the extension of LBO limit minimally depends on fAC. Simultaneously, the NTP considerably reduces CO emission, while slightly increases NOx emission near the LBO limits due to the enhanced combustion by ozone.

    更新日期:2019-11-29
  • Prediction of local extinctions in piloted jet flames with inhomogeneous inlets using unstrained flamelets
    Combust. Flame (IF 4.120) Pub Date : 2019-11-28
    Zhi X. Chen, Ivan Langella, Robert S. Barlow, Nedunchezhian Swaminathan

    Multi-regime turbulent combustion modelling remains challenging and is explored with occurrence of local extinction in this study. A partially premixed model based on unstrained premixed flamelets is used in this work to investigate a piloted jet flame configuration with inhomogeneous inlets. Three different cases are simulated, which differ in the bulk mean velocity that amounts respectively to about 50%, 70% and 90% of the blow off velocity measured experimentally. As the jet velocity approaches the blow off limit, local extinctions start to occur along the flame surface and thus these flames are challenging from a modelling prospective. Two different numerical approaches, involving scaled and unscaled progress variable respectively, are compared to elucidate their abilities and limitations to predict local extinctions and to deal with the three-stream problem at the pilot/coflow interface. The key modelling details for such predictions are indicated and discussed. LES results are systematically compared to two sets of experimental measurements available in the literature for the three flames. The differences observed in the two experimental datasets are also discussed with the help of LES results. Although both approaches show promising agreement for the flame statistics, the scaled progress variable approach better predicts the local extinctions. The unscaled approach shows to naturally handle the three-stream problem without additional treatment for the pilot/coflow interface, which is required for the scaled approach. Furthermore, computed scalar dissipation rate of mixture fraction is compared with the measurements showing good agreement for the conditions investigated. This further suggests that local extinctions can be predicted using unstrained flamelets if the correct scalar mixing and its dissipation are captured.

    更新日期:2019-11-29
  • Effect of ageing on combustion of ammonium perchlorate with copper chromite additive
    Combust. Flame (IF 4.120) Pub Date : 2019-11-28
    Ramanathan Perumal, Kumar Nagendra, P.A. Ramakrishna

    Ageing is an irreversible process during which, the material undergoes a series of slow chemical degradation/oxidation reactions. The aim of this paper is to study the effect of ageing of ammonium perchlorate-copper chromite (AP-CC) pellets on the burn rate. It also focuses on any change in CC properties due to direct contact with an oxidizer (AP). Accelerated ageing with cyclic temperature profile has been used to replicate the natural ageing. The effect of ageing of individual ingredients on the burn rate was studied. The burn rate of AP-CC pellets was found to reduce when AP-CC were aged together. X-ray diffraction (XRD) and Energy Dispersive Spectroscopy (EDS) analysis were used to identify the change in properties of copper chromite during ageing.

    更新日期:2019-11-29
  • Studies of multi-channel spark ignition of lean n-pentane/air mixtures in a spherical chamber
    Combust. Flame (IF 4.120) Pub Date : 2019-11-27
    Hao Zhao, Ningbo Zhao, Tianhan Zhang, Shuqun Wu, Guoming Ma, Chao Yan, Yiguang Ju

    A novel ignition strategy using multi-channel sparks was developed in this paper. Compared with the typical single spark, the three-channel spark ignition technique enables three spatially separated and temporally synchronized discharges to increase the ignition kernel size while maintaining the same total ignition energy. The three-channel discharge characteristics, ignition kernel development, and ignition probability of lean n-pentane/air mixtures were studied and compared with those of single-channel spark in a spherical combustion chamber with different discharge distances, pressures, and CO2 dilution levels. The experimental results show that the three-channel sparks increase the ignition probability and dramatically extend the fuel-lean ignition limits compared to the single-channel discharge under all tested conditions. Moreover, ignition enhancement effects of multi-channel sparks increase with the decrease of pressure, reduction of discharge gap, and increase of CO2 dilution. One-dimensional numerical simulation was performed by using a detailed n-pentane kinetic model (Bugler et al., 2017) and the results revealed that the increase of fuel lean ignition probability and the decrease of the minimum ignition energy by using multi-channel sparks were the outcome of increased ignition kernel size compared to the minimum critical ignition radius. This present study confirms the advantages of using multi-channel sparks on advanced fuel-lean combustion engines.

    更新日期:2019-11-28
  • Experimental and numerical investigations of charge motion and combustion in lean-burn natural gas engines
    Combust. Flame (IF 4.120) Pub Date : 2019-11-25
    Benjamin Korb, Kalyan Kuppa, Hoang Dung Nguyen, Friedrich Dinkelacker, Georg Wachtmeister

    It is important to understand the lean-burn combustion process of large-bore natural gas engines and influences on it in order to improve next generations of gas engines to meet the increasing requirements for high efficiencies and low emissions. The investigations in this study focus on the ignition and early flame propagation phase using optical experiments on a single-cylinder research engine, since both phases highly influence the subsequent main-combustion. Scavenged prechambers with different operating conditions and a tangential and radial nozzle alignment as well as unscavenged prechamber and direct spark plug ignition are compared. Beside the phenomenology of the ignition system itself, the interaction of the main-chamber charge motion and the ignition system is important to understand. Therefore, different valve timings (conventional timings and Miller cycle) as well as a low and high turbulence setup are subject of the study. Numerical simulations of the cold flow are used to understand the charge motion and mixture formation in the prechamber as well as in the main-chamber. The experiments depict an enhancement of the first flame propagation phase using a scavenged prechamber due to hot turbulent jets emerging from the nozzles. Furthermore, an influence of the nozzle geometry and the boundary conditions on the jet development and on the early flame propagation is observed. It is seen by the optical measurements that cycle-to-cycle variations can originate from the hot turbulent jets and its influence on the ignition of the main-chamber charge. Further, the optical measurements show that a low in-cylinder swirl and turbulence due to Miller cycle has an impact on the interaction between the in-cylinder charge motion and the jets. A comparison between high turbulence and low turbulence in-cylinder flows on the combustion is carried out. It is shown that the scavenged prechamber can compensate the lack of turbulence in the main-chamber. Hence, the scavenged prechamber enhances the flame propagation due to induced turbulence in the main-chamber and larger flame front surfaces generated by penetrating flame torches.

    更新日期:2019-11-26
  • Laminar premixed and non-premixed flame investigation on the influence of dimethyl ether addition on n-heptane combustion
    Combust. Flame (IF 4.120) Pub Date : 2019-11-25
    Julia Wullenkord, Isabelle Graf, Martina Baroncelli, Daniel Felsmann, Liming Cai, Heinz Pitsch, Katharina Kohse-Höinghaus

    Several different ether components have been proposed as renewable replacements for diesel fuel in compression ignition engines. From a fundamental point of view, blends of n-heptane and dimethyl ether (DME) have been considered in this study, where n-heptane is a single-component diesel surrogate and DME acts as a representative for oxymethylene ethers. n-Heptane and n-heptane / DME blends have been investigated experimentally under laminar premixed low-pressure and non-premixed atmospheric-pressure counterflow flame conditions. For the premixed case, a series of three fuel-rich flames was studied with oxygen as the oxidizer, at a constant C/O ratio of 0.47, pressure of 4 kPa, with 50% argon dilution, and an equivalence ratio ϕ near 1.5, burning pure n-heptane and mixtures of n-heptane with DME in the ratios 1:1 and 1:4. In the non-premixed counterflow configuration, two fuel/oxygen/argon flames were studied with pure n-heptane and a 1:1 DME–n-heptane mixture as the fuel. Electron ionization molecular-beam mass spectrometry was used for both flame configurations to investigate the flame structure and determine quantitative mole fraction profiles of stable and reactive species formed in the combustion process. Particular attention was given to the changes caused by partial replacement of n-heptane by DME. Notwithstanding the experimental focus of this work, the experimental results were compared with predictions from a model suitable for both n-heptane and DME that was combined for this case from recent mechanisms available in the literature for these fuels. While the overall flame structure was not significantly altered upon DME addition, with some smaller differences mainly due to temperature effects, more prominent changes and interactive effects were found for a number of primary decomposition products, oxygenated species, and higher-molecular hydrocarbon compounds. In most cases, experimentally observed trends, but not always quantitative changes, could be satisfactorily reproduced and explained by the model.

    更新日期:2019-11-26
  • Ignition and combustion characterization of single nitromethane and isopropyl nitrate monopropellant droplets under high-temperature and quasi-steady conditions
    Combust. Flame (IF 4.120) Pub Date : 2019-11-20
    Angela Mbugua, Aman Satija, Robert P. Lucht, Sally Bane

    The dynamics of reacting droplets of the monopropellants nitromethane (NM) and isopropyl nitrate (IPN) are compared to those of methanol. Single droplets of these three fuels were burned in steady flow high-temperature conditions, produced by a McKenna flat-flame burner. Coherent anti-Stokes Raman scattering (CARS) thermometry was employed to characterize the temperature of the flow-field experienced by the droplets. Ignition delay times and droplet burning rates were obtained for droplets with initial diameters ranging between 0.4 and 1.4 mm. Droplet dynamics such as deformation, puffing, stripping and micro-explosions are qualitatively discussed. Lastly, the D2 law and the hybrid combustion model are applied to IPN and NM droplets, and the experimental mass burning rates are compared to theoretical predictions.

    更新日期:2019-11-21
  • NOX reduction in an axially staged gas turbine model combustor through increase in the combustor exit Mach number
    Combust. Flame (IF 4.120) Pub Date : 2019-11-15
    Neil S. Rodrigues, Oluwatobi Busari, William C.B. Senior, Colin T. McDonald, YunTao Chen, Andrew J. North, Walter R. Laster, Scott E. Meyer, Robert P. Lucht

    Advanced combustion technologies to limit NOX production are needed to meet rigorous emissions standards, which are required due to the harmful effect of NOX on the environment. One such advanced concept involves axially staging the fuel to create a staged combustion system. This paper reports on emissions measurements obtained for premixed natural gas and air reacting jets into vitiated crossflow with negligible swirl at conditions corresponding to elevated inlet temperatures of 500–600 K and an elevated combustor pressure of 500 kPa. A significant NOX reduction was achieved when the staged combustor exit Mach number was increased and the axial residence time was reduced. Based on this preliminary investigation, a test matrix was developed to independently vary the exit Mach number for a constant axial residence time by utilizing modular rig hardware to change the length of the axial combustor. Up to 70% reduction in NOX produced by the axial stage was observed when the combustor exit Mach number was increased from about 0.26 to 0.66 at a constant residence time of 1.4 ms. NOX reduction based on variation in the Mach number and at a constant residence time has not been previously reported in the literature to the best of our knowledge. This decrease in NOX is hypothesized to be due to the lower static temperature of a compressible flow and potentially better mixing of the jet with the crossflow due to the interaction occurring at high speeds. The effects of axial residence times on NOX emissions were also investigated at a constant Mach number of 0.44 for axial residence times between 0.8 and 2.3 ms. Varying the axial residence time was observed to have a strong effect on NOX emissions for exit total temperatures greater than 1900 K, which agrees well with existing trends.

    更新日期:2019-11-18
  • A comprehensive study of flamelet tabulation methods for pulverized coal combustion in a turbulent mixing layer — Part I: A priori and budget analyses
    Combust. Flame (IF 4.120) Pub Date : 2019-06-15
    Xu Wen, Martin Rieth, Arne Scholtissek, Oliver T. Stein, Haiou Wang, Kun Luo, Andreas M. Kempf, Andreas Kronenburg, Jianren Fan, Christian Hasse

    In this work, a comprehensive study of flamelet tabulation methods for pulverized coal combustion in a turbulent mixing layer is conducted. At first, a priori analyses are conducted to evaluate the suitability of the premixed and non-premixed flamelet models for the studied pulverized coal flame with multiple combustion modes. Then, to clarify why a certain flamelet model does work or not work in certain regions, a more in-depth investigation of the premixed and non-premixed flamelet models is conducted through a budget analysis. The results show that the first and second derivatives in physical space can be well reproduced by the tabulated manifolds in trajectory variable space for both the premixed and non-premixed flamelet models, between which the non-premixed flamelet model performs slightly better. For the time derivative, large discrepancies can be observed, although the predicted variation trend overall follows the reference results. Through the analysis of the individual budget terms in the trajectory variable space, the individual trajectory variable’s contributions to the convection and diffusion of thermo-chemical variables are quantified. Through the analysis of the individual budget terms for the sensible enthalpy and the CO mass fraction governing equations, the influences of the space transformation on the individual transport process (e.g., convection, diffusion, etc.) are clarified. Overall, the findings obtained from the budget analyses are consistent with those obtained from the a priori analyses.

    更新日期:2019-11-18
  • Titanium enhanced ignition and combustion of Al/I2O5 mesoparticle composites
    Combust. Flame (IF 4.120) Pub Date : 2019-11-14
    Wanjun Zhao, Xizheng Wang, Haiyang Wang, Tao Wu, Dylan J. Kline, Miles Rehwoldt, Hui Ren, Michael R. Zachariah

    Aluminum (Al) is widely used in thermites, and iodine pentoxide (I2O5) is a strong oxidizer that can release gas phase iodine. The result is a potentially very powerful and effective energetic composite with biocidal properties. However, the alumina coating on the Al presents a diffusion barrier that increases the ignition temperature. In this paper, nano titanium powders (nTi) were added at various mass ratios to Al/I2O5 and a composite mesoparticle was assembled by electrospray. Combustion studies showed that nano Ti can enhance the reactivity of Al/I2O5 thermites significantly with increases in pressure, pressurization rate and dramatic decreases in burn time. Part of this behavior can likely be attributed to less sintering enhancing the completeness of reaction in the Ti added cases. Addition of titanium to aluminum was found to have a significant impact on ignition and when Al is replaced with Ti, the ignition temperature was ∼300 °C below that of the corresponding aluminum thermite. These results show that titanium can be used as a fuel to tune energetic behavior and iodine release temperature and in some ways superior to aluminum.

    更新日期:2019-11-14
  • Investigation into pressure dependence of flame speed for fuels with low and high octane sensitivity through blending ethanol
    Combust. Flame (IF 4.120) Pub Date : 2019-11-14
    Qinhao Fan, Yunliang Qi, Yingdi Wang, Zhi Wang

    Spark assistance for homogeneous charge compression ignition (HCCI) can control combustion phasing, improve thermal efficiency, and reduce emissions in gasoline engines. As the characteristics of flame propagation determine the control authority of ignition timing, it is important and necessary to investigate pressure dependence of flame speed in the lean-premixed mixture relative to engine operating conditions. Experimental study in an optical rapid compression machine (RCM) and simulation work were carried out using two fuels comprising n-heptane/iso-octane/ethanol with varied octane sensitivity (S). The effective pressure ranged from 10 to 35 bar, temperature from 715 to 860 K, and equivalence ratios between 0.3 and 0.7 to cover the region of lean flammability limits of low and high S fuels with ethanol blended. Based on pressure profiles, flame speed extracted from images, and sensitivity analysis of flame speed, the dependence of flame speed on the effective pressure in low and high S fuels was discovered and the fundamental mechanism behind this phenomena became to be understood in the negative temperature coefficient (NTC) and non-NTC regions, respectively. In the studied temperature conditions, the flame speed of high S fuel has stronger dependence on the pressure than that of low S fuel does. In the NTC region, this phenomenon is attributed to the dependence of H radical concentration on pressure in the unburned mixture and flame structure. In the non-NTC region, promoting effect of dominant reactions varied with pressure can significantly influence pressure dependence of flame speed. Although quite limited data of laminar burning velocity for studied fuels were obtained in high pressures (>15 bar), the trend of flame speed's dependence on pressure was well predicted by two models with different but well-accepted core mechanisms, showing consistent results with the experimental ones in the RCM.

    更新日期:2019-11-14
  • A physics-based approach to modeling real-fuel combustion chemistry – V. NOx formation from a typical Jet A
    Combust. Flame (IF 4.120) Pub Date : 2019-11-14
    Chiara Saggese, Kevin Wan, Rui Xu, Yujie Tao, Craig T. Bowman, Ji-Woong Park, Tianfeng Lu, Hai Wang

    Real transportation fuels are complex mixtures of a variety of hydrocarbon components. Predicting NOx formation in practical combustors burning real fuels is usually made with the assumption that the NOx submodels developed and tested for small hydrocarbon combustion are applicable to mixtures of large hydrocarbons as found in real fuels. Additionally, NOx data are scarce for flames of real fuels. The aims of the current study are (i) to provide reliable NOx data in flames of a typical jet fuel, and (ii) to test our capability to predict these data by combining a recently proposed HyChem reaction model of jet A combustion (Xu et al., 2018) with the NOx submodel of Glarborg (2018). Specifically, NOx concentrations were measured in stretch-stabilized premixed flames of methane and Jet A (POSF10325) from fuel lean to rich conditions and of ethylene at a fuel-rich equivalence ratio. This range of stoichiometries allows both thermal NO and prompt NO pathways to be tested. The results show reasonably good agreement between the experimental data and model predictions for all flames tested, although the model appears to underpredict NOx concentrations in the Jet A flames under fuel rich conditions. Sensitivity analyses were conducted to illustrate the influence of the reaction pathways and flame boundary conditions on NOx predictions. The analyses also suggest that additional prompt NO reaction pathways may play a role in flames of large hydrocarbons.

    更新日期:2019-11-14
  • Cool-flame dodecane-droplet extinction diameters
    Combust. Flame (IF 4.120) Pub Date : 2019-11-14
    Forman A. Williams, Vedha Nayagam

    Previously published simplified n-alkane cool-flame chemistry is re-evaluated for n-dodecane. Comparison with experimental results produces improved rate-parameter estimates for n-dodecane and indicates deterioration of the simplified chemistry with increasing pressure in predictions of droplet diameters at cool-flame extinction.

    更新日期:2019-11-14
  • Singlet-diradical character in large PAHs triggers spontaneous-ignition of coal
    Combust. Flame (IF 4.120) Pub Date : 2019-11-14
    Mohammednoor Altarawneh, Ibukun Oluwoye, Bogdan Z. Dlugogorski

    Chemical phenomena dictating the spontaneous-ignition of coal remain highly speculative with some of the most intriguing questions appearing unaddressed. In this communication, we deploy a synergistic experimental-theoretical approach to elucidate the definite role of singlet-biradical PAHs in driving the low-temperature oxidation of coal. We establish that, five linearly-fused benzene rings (i.e., pentacene, as a model compound for the PAH content in coal, and an example of short-chain graphene nanoribbons) ignites 200 °C lower than the corresponding temperature for naphthalene.

    更新日期:2019-11-14
  • Effects of organic silicon compounds on syngas auto-ignition behavior
    Combust. Flame (IF 4.120) Pub Date : 2019-11-11
    Rachel A. Schwind, Margaret S. Wooldridge

    Siloxanes are a significant impurity in syngas feedstocks and an important source of silicon in combustion synthesis applications. However, little is known regarding the fundamental combustion chemistry of siloxane compounds. The impact of two organic silicon species with different but related chemical structures, trimethylsilanol (TMSO) and hexamethyldisiloxane (HMDSO), on syngas auto-ignition behavior was investigated in this study using physical and computational experiments. A rapid compression facility (RCF) was used to create temperatures of 1010–1070 K and pressures of 8–10.3 atm for auto-ignition experiments. Experiments with trace concentrations of TMSO (100 ppm, mole basis) or HMDSO (100 ppm) were added to a surrogate syngas blend (CO and H2 with a molar ratio of 2.34:1, air levels of dilution, with molar equivalence ratios of ϕ = 0.1). The measured ignition delay times showed both siloxane species promote ignition behavior with TMSO yielding faster ignition delay times by approximately 37% and HMDSO yielding faster times by approximately 50% compared with the reference syngas mixture which contained no siloxanes. A computational study was conducted to interpret the results of the ignition experiments. Because detailed chemistry does not exist for these organo-silicon compounds, the effects of the addition of CH3, H, and OH radicals to H2 and CO mixtures were explored to simulate the potential rapid decomposition of the siloxanes in the H2 and CO system. Addition of the radicals decreased the predicted ignition delay times when compared with the H2 and CO mixture simulations without the radical species, but the simulations did not fully capture the behavior observed in the experiments, indicating the siloxane chemistry is more complex than providing a rapid source of radicals.

    更新日期:2019-11-13
  • Experimental and kinetic modeling study on 1,3-cyclopentadiene oxidation and pyrolysis
    Combust. Flame (IF 4.120) Pub Date : 2019-11-11
    Hongyan Wang, Zhiqiang Liu, Siyuan Gong, Yujie Liu, Li Wang, Xiangwen Zhang, Guozhu Liu

    The oxidation of 1,3-cyclopentadiene (c-C5H6), a key intermediate formed in the combustion of aromatic and cyclic hydrocarbon fuels and formation of polycyclic aromatic hydrocarbons (PAHs), was studied in a jet-stirred reactor (JSR) under the pressure of 0.1 MPa and temperatures from 623 to 1073 K with three equivalence ratios (φ = 0.5, 1.0 and 1.8). The mole fraction profiles of 25 oxidation products including light hydrocarbons, oxygenated and aromatic species were identified and quantified using online synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) with uncertainties of C, H, and O molar balances about 24%, 20% and 14%, respectively. A detailed kinetic model with 245 species and 1638 reactions was constructed based on the theoretical calculation and reported models in literature and validated with the new JSR oxidation data in this work. The dominant consumption channels of 1,3-cyclopentadiene are through H-atom abstraction reactions by OH, HO2 and CH3 radicals to produce cyclopentadienyl radical (c-C5H5) under all equivalence ratios and the contribution of CH3 attacking increases from φ = 0.5 to 1.8. Subsequently, the c-C5H5 is consumed via the reactions with HO2 and O2 to form small molecule and the recombination reaction with c-C5H6 to yield PAHs. Furthermore, the current model was also validated with a wide range of experimental data in literature of 1,3-cyclopentadiene combustion, including the species concentration profiles in flow reactors pyrolysis and oxidation.

    更新日期:2019-11-11
  • Hypergolic ignition delay studies of solidified ethanol fuel with hydrogen peroxide for hybrid rockets
    Combust. Flame (IF 4.120) Pub Date : 2019-11-11
    Jerin John, Purushothaman Nandagopalan, Seung Wook Baek, Sung June Cho

    The hypergolic interaction and ignition delay times of catalytically promoted solidified ethanol (CPSE) formulated using organic gellant, namely, methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC), has been investigated using rocket grade hydrogen peroxide (90% RGHP; H2O2) as an oxidizer, for the use in hybrid rocket engines. Using iso-conversional Friedman method, the apparent activation energy (Ea) is found to be in the range of 9.06–14.01 kJ/mol for unloaded solidified ethanol samples. The hypergolic ignition delay (ID) of the CPSE fuels with H2O2 are investigated using manganese (III) acetylacetonate (Mnacac) as a catalyst using the drop-test method and found that the ID lies in the range of 49–307 ms. Five-stage events are identified based on the signals from the high-speed camera, photodiode, and microphone obtained from the drop-test. Stage I: prior to of H2O2 drop impact; Stage II: inertial spreading of H2O2 drop over CPSE pellet; Stage III: onset of exothermic decomposition reaction H2O2 droplet by Mnacac particles which leads to the formation of aerosol; Stage IV: local explosion of aerosol and ignition of CPSE pellet occur results in two types of aerosol-CPSE fuel interaction; Stage V: a weak self-sustained flame from CPSE fuel. A key finding of the study reveals that a minimum catalyst concentration ([C]Mn3+,L) is required to ignite CPSE fuel using H2O2 and ID decreases with an increase in the [C]Mn3+. Similarly, there exists an upper catalyst loading ([C]Mn3+,U) above which ID is increased due to the increase in Mnacac particle size by agglomeration. [C]Mn3+,L and [C]Mn3+,U depends on the type and concentration of the gellant and the availability of active hydroxyl (–OH) group in the gellant. Finally, SEM-EDS analysis of the residue indicates the formation of oxides of manganese with a small amount of carbon (~1.66 to 2.53 wt%), hinting complete combustion of the gellant particles.

    更新日期:2019-11-11
  • Chemical pathways for the formation of benzofuran and dibenzofuran in combustion
    Combust. Flame (IF 4.120) Pub Date : 2019-11-11
    Xuetao Shi, Qi Wang, Angela Violi

    Understanding and predicting the formation of polycyclic aromatic compounds (PACs) and their role in the formation of high molecular mass compounds is still an unresolved topic in combustion. PACs characteristics, such as chemical composition, size, and presence of side chains, play an important role not only in terms of environmental and health impact, but also when developing models that describe the formation of nanoparticles and soot. In this paper, we report on a detailed analysis of the reaction pathways describing the chemistry of furan-embedded PACs using ab initio G3-type electronic structure calculations leading to the formation of benzofuran and dibenzofuran from benzene and biphenyl. The 82 elementary reactions, identified in this work, contain unexplored pathways involving triplet oxygen atom and hydroxyl radical addition reactions. A protocol for improving the calculations of reaction energetics from ab initio compound methods is proposed, which consists of the thorough usage of IRCmax scheme to identify the transition state structure and an energy correction ( ~ 0.2 kcal/mol) to the empirical term in G3 formula for systems with open-shell singlet type of electronic configurations. Based on these ab initio calculations, temperature dependent reaction rate constants are calculated according to statistical theories, together with thermodynamics data. Branching ratio analysis based on steady-state approximation is carried out to illustrate the relative importance of the new pathways in an ethylene premixed flame. Results show that the newly discovered benzofuran formation pathways can play a relative important role when in presence of phenol or phenoxyl radicals at various locations in the flame.

    更新日期:2019-11-11
  • Experimental investigation of lean premixed pre-vaporized liquid-fuel combustion in porous media burners at elevated pressures up to 20 bar
    Combust. Flame (IF 4.120) Pub Date : 2019-11-09
    Sadaf Sobhani, Justin Legg, David F. Bartz, Jun J. Kojima, Clarence T. Chang, John D. Sullivan, Jeffrey P. Moder, Matthias Ihme

    Combustion in porous media has been identified as a promising technology for achieving higher burning rates, extending flammability limits, and reducing emissions. To assess the viability of this technology for application to aviation gas-turbine engines, the performance of a Porous Media Burner (PMB) operated with pre-vaporized liquid fuel at high pressures is experimentally examined. The PMB was operated at fuel-lean equivalence ratios between 0.4 and 0.55 at pressures up to 20 bar with fully pre-vaporized and preheated n-heptane as well as gaseous methane at 8 bar for performance comparison. Combustion stability maps are reported along with temperature profiles, pressure drops, and emissions of CO and NOx at stable operating conditions. Results from these experiments show excellent performance of PMBs at high-pressure conditions. Additionally, numerical simulations using the volume-averaged, one-dimensional reacting flow-equations complement the experimental measurements to provide further insight into the effects of the pressure and fuel mixture on the flame structure. Lastly, high-resolution es X-ray Computed Tomography (XCT) is used to examine the structural integrity of the porous matrix during the high-pressure combustion operation, showing evidence of micro-fissures and an increase in the surface roughness due to SiC-oxidation. Large-scale defects were not observed after four days of cyclic high-pressure testing over a wide range of pressures, heating rates, and equivalence ratios.

    更新日期:2019-11-11
  • The kinetic study of excited singlet oxygen atom O(1D) reactions with acetylene
    Combust. Flame (IF 4.120) Pub Date : 2019-11-09
    Chao Yan, Chu C. Teng, Timothy Chen, Hongtao Zhong, Aric Rousso, Hao Zhao, Guoming Ma, Gerard Wysocki, Yiguang Ju

    Understanding the multi-channel dynamics of O(1D) reactions with unsaturated hydrocarbon molecules in low temperature reaction kinetics is critically important in stratospheric chemistry, plasma chemistry, plasma assisted fuel reforming, materials synthesis, and plasma assisted combustion. A photolysis flow reactor coupled with highly selective mid-infrared Faraday Rotation Spectroscopy (FRS) and direct ultraviolet-infrared (UV-IR) absorption spectroscopy (DAS) techniques was developed for the first time to study the multi-channel dynamics of excited singlet oxygen atom O(1D) reactions with C2H2 and the kinetics of subsequent reactions. Time-resolved species concentrations of OH, HO2 and H2O were obtained and used to develop a validated kinetic model of O(1D) reactions with C2H2. The branching ratios of O(1D) reaction with C2H2 and subsequent HO2 kinetics were also quantified. It is found that, contrary to O(1D) reactions with saturated alkanes, OH formation via direct H abstraction by O(1D) is negligible. The results revealed that two chain-branching and propagation reactions via direct O(1D) insertion are the major pathways for radical production. The present study clearly demonstrated the advantage of radical detection and kinetic studies using FRS in the effective suppression of absorption interference from non-paramagnetic hydrocarbons.

    更新日期:2019-11-11
  • Comprehensive kinetic study of combustion technologies for low environmental impact: MILD and OXY-fuel combustion of methane
    Combust. Flame (IF 4.120) Pub Date : 2019-11-09
    Ghobad Bagheri, Eliseo Ranzi, Matteo Pelucchi, Alessandro Parente, Alessio Frassoldati, Tiziano Faravelli

    The development of processes with near-zero emissions such as MILD, flameless, and OXY-fuel combustion are of great interest in various energy scenarios. The assessment and design of new burners to meet energy demands and pollution reduction strongly depends on an accurate description of the chemistry involved in the combustion process. The main outcome of this study is the collection and review of a vast amount of experimental data on MILD and OXY-fuel combustion of methane that have been reported in recent years, together with a thorough kinetic analysis to identify aspects of the mechanism that requires further revision. The CRECK core model presented here is developed upon the Aramco 2.0 mechanism [1], and further extends the validation objectives to MILD and OXY fuel combustion conditions. The aim of this work is not only the mechanism validation but also a better understanding of the combustion characteristics and critical reaction pathways in MILD, flameless, and OXY-fuel combustion.

    更新日期:2019-11-11
  • Reconstructing cellular surface of gaseous detonation based on artificial neural network and proper orthogonal decomposition
    Combust. Flame (IF 4.120) Pub Date : 2019-11-09
    Yining Zhang, Lin Zhou, Hao Meng, Honghui Teng

    Gaseous detonation has complicated cellular surface, whose comprehensive investigation is critical not only to the detonation physics but also the detonation engine development. Because measuring the high-resolution dynamic surface is beyond the present experimental technical skills, we propose a reconstruction method of detonation wave surface based on post-surface flow field. This method combines two technologies, the proper orthogonal decomposition (POD) in fluid research and the artificial neural network (ANN) in machine learning research. POD is employed to extract the main features of flow fields, and the pre-trained ANN builds up the connection between the reduced coefficients of full flow fields and post-surface flow fields. The reconstruction is tested through the numerical results from one-step irreversible heat release model, displaying a good performance in both cellular normal detonations and unstable oblique detonations. The method may provide a universal frame for the detonation research, and has the potential to be employed in other numerical and experimental results.

    更新日期:2019-11-11
  • Development of an optically accessible apparatus to characterize the evolution of spherically expanding flames under constant volume conditions
    Combust. Flame (IF 4.120) Pub Date : 2019-11-09
    Fabien Halter, Z. Chen, G. Dayma, C. Bariki, Y. Wang, P. Dagaut, C. Chauveau

    Flame speed is extremely important as it affects the performances of many industrial systems. Moreover, its significance makes it a major target for the validation of kinetic mechanisms, which explains the necessity to provide ever more accurate data. Flame speed dependence on pressure and temperature conditions is interestingly assessed using, among others, spherically expanding flame in constant volume chambers. In these conditions, the flame speed derivation, based solely on the pressure evolution in the chamber, requires empirical models. The current study describes a perfectly spherical chamber with full optical access allowing simultaneous recording of the pressure inside the chamber and, fully innovative, of the flame radius evolution until the flame vanishes at wall. A careful description of the new set-up and of the accuracy of the measurements, in particular of the flame radius, is presented here. In parallel with experiments, one-dimensional transient simulations were carried out to identify the limits of the proposed new method. Then, the simultaneous use of pressure and flame radius information is compared to the traditional constant volume method based on empirical models. A first advantage relies in the direct detection of the development of instabilities during the flame propagation. In addition, although the flame speed is extremely sensitive to the flame radius determination, the actual experimental accuracy allows significant improvements in terms of accuracy, notably as initial pressure and temperature are elevated. This new set-up will allow major advances in the measurement of laminar flame velocity under extreme thermodynamic conditions.

    更新日期:2019-11-11
  • Characteristics of laser ignition and spark discharge ignition in a cavity-based supersonic combustor
    Combust. Flame (IF 4.120) Pub Date : 2019-11-09
    Bin An, Leichao Yang, Zhenguo Wang, Xipeng Li, Mingbo Sun, Jiajian Zhu, Wei Yan

    An experimental study was conducted to compare laser ignition (LI) and spark discharge ignition (SDI) in an ethylene fueled supersonic combustor operating at a global equivalence ratio of 0.23. The Mach number, total pressure, and stagnation temperature of the inflow were 2.92, 2.6 MPa, and 1650 K, respectively. The flame kernel generated by the laser pulses experiences a rapid growth in size when it propagates towards the cavity leading edge and resides there. The cavity recirculation flow plays an important role at this stage because the velocity of it far outstrips the turbulent flame speed. In comparison, the ignition process of SDI is significantly slower because the long pulse duration of the spark discharge decreases the energy density of the plasma and makes the flame kernel more vulnerable to flame quenching. The extra heat loss resulting from the electrodes also delay the ignition process. After reaching the cavity leading edge, the flame kernel forms a small self-sustained flame there. Since the plasmas have transformed into flames, LI and SDI are similar in the aspect of flame propagation. The competition between the chemical reaction and the losses of heat and radicals determines the evolution of the flame. Prior to the reaction zone, the hot products generated by the flame are transported downstream via the cavity shear layer, which reduces the ignition delay time of the fuel/air mixture and promotes the accumulation of radicals in the flame base. The enhanced flame base further accelerates the propagation of the cavity shear layer flame. At the rapid propagation stage, the cavity shear layer flame spreads downstream prior to the flame base because the cavity shear layer dominates the evolution of the flame.

    更新日期:2019-11-11
  • Confined spherically expanding flame method for measuring laminar flame speeds: Revisiting the assumptions and application to C1C4 hydrocarbon flames
    Combust. Flame (IF 4.120) Pub Date : 2019-11-01
    Ashkan Movaghar, Robert Lawson, Fokion N. Egolfopoulos

    The spherically expanding flame under constant volume method was introduced in 1934 by Lewis and von Elbe as a means to study laminar flame propagation at engine-relevant conditions. Despite its potential, this method has not been utilized extensively due to concerns regarding the underlying assumptions and data uncertainty. In the present study, the intricacies of the experimental approach as well as the models and assumptions involved during data interpretation were reassessed with the aid of direct numerical simulations. Results confirmed that stretch effects are negligible during the compression stage of the experiment for a wide range of Lewis numbers. Additionally, it was shown that the laminar flame speed is sensitive to the flame radius stressing thus the requirement that the modeling of flame radius needs to be done with the highest possible accuracy by accounting properly for product dissociation and thermal radiation from the burned gases. It was also shown, that the equilibrium assumption is valid for modeling the flame radius as a function of pressure and, as expected, the kinetic and transport effects are negligible. Subsequently, laminar flame speeds were measured for methane, ethane, ethylene, propane, propylene, n-butane, 1-butene, and isobutene flames for 8–30 atm pressures and 400–520 K unburned mixture temperatures. A hybrid thermodynamic/radiation model was utilized to interpret the experimental observables and derive the laminar flame speed by accounting for spectrally dependent emission from and absorption by the burned gases as well as product dissociation during compression. The data were found to be consistent with measurements obtained in spherically expanding flame experiments under constant pressure conditions and predictions using a number of current kinetic models.

    更新日期:2019-11-04
  • Transient ellipsoidal combustion model for a porous burner in microgravity
    Combust. Flame (IF 4.120) Pub Date : 2019-11-02
    Akshit Markan, Howard R. Baum, Peter B. Sunderland, James G. Quintiere, John L. de Ris

    The current study develops a transient combustion model formulated in oblate ellipsoidal coordinates to analyze the behavior of non-buoyant burner-generated diffusion flames. The combustion model is axially symmetric and considers a porous gas-fueled burner called the Burning Rate Emulator (BRE), which is idealized as an ellipsoidal disk. An approximate analytical transient solution for the flame shape and heat transfer to the surface of the burner is generated as a product of the exact steady-state result and the asymptotic transient result that becomes exact far from the burner. Microgravity BRE experiments conducted at NASA Glenn's 5.18-s Zero Gravity Research Facility indicated the evolution of an approximately ellipsoidal flame moving away from the burner with steady state not achieved during the 5-second test period. The microgravity experimental results are shown to be in good agreement with the mathematical model, which can help predict the flame behavior beyond the duration of the test.

    更新日期:2019-11-04
  • An experimental and modeling study on the reactivity of extremely fuel-rich methane/dimethyl ether mixtures
    Combust. Flame (IF 4.120) Pub Date : 2019-11-02
    S. Porras, D. Kaczmarek, J. Herzler, S. Drost, M. Werler, T. Kasper, M. Fikri, R. Schießl, B. Atakan, C. Schulz, U. Maas

    Chemical reactions in stoichiometric to fuel-rich methane/dimethyl ether/air mixtures (fuel air equivalence ratio ϕ = 1–20) were investigated by experiment and simulation with the focus on the conversion of methane to chemically more valuable species through partial oxidation. Experimental data from different facilities were measured and collected to provide a large database for developing and validating a reaction mechanism for extended equivalence ratio ranges. Rapid Compression Machine ignition delay times and species profiles were collected in the temperature range between 660 and 1052 K at 10 bar and equivalence ratios of ϕ = 1–15. Ignition delay times and product compositions were measured in a shock tube at temperatures of 630–1500 K, pressures of 20–30 bar and equivalence ratios of ϕ = 2 and 10. Additionally, species concentration profiles were measured in a flow reactor at temperatures between 473 and 973 K, a pressure of 6 bar and equivalence ratios of ϕ = 2, 10, and 20. The extended equivalence ratio range towards extremely fuel-rich mixtures as well as the reaction-enhancing effect of dimethyl ether were studied because of their usefulness for the conversion of methane into chemically valuable species through partial oxidation at these conditions. Since existing reaction models focus only on equivalence ratios in the range of ϕ = 0.3–2.5, an extended chemical kinetics mechanism was developed that also covers extremely fuel-rich conditions of methane/dimethyl ether mixtures. The measured ignition delay times and species concentration profiles were compared with the predictions of the new mechanism, which is shown to predict well the ignition delay time and species concentration evolution measurements presented in this work. Sensitivity and reaction pathway analyses were used to identify the key reactions governing the ignition and oxidation kinetics at extremely fuel-rich conditions.

    更新日期:2019-11-04
  • A comprehensive mechanism for liquid-phase decomposition of 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX): Thermolysis experiments and detailed kinetic modeling
    Combust. Flame (IF 4.120) Pub Date : 2019-11-01
    Lalit Patidar, Mayank Khichar, Stefan T. Thynell

    The nitramines 1,3,5,7-tetranitro-1,3,5,7-tetrazoctane (HMX), and 1,3,5-trinitro-1,3,5-triazinane (RDX) are energetic materials commonly used in solid propellants and explosives. In order to predict ignition and deflagration of propellants containing these ingredients, their thermal decomposition behaviors must be thoroughly understood. In this study, the thermal decomposition of HMX was investigated using synergetic application of experimental and computational methods. Mole fraction profiles of the gaseous decomposition products evolving from the liquid-phase HMX were obtained using Fourier transform infrared (FTIR) spectroscopy for two types of thermolysis experiments – thermogravimetric analysis (TGA) and confined rapid thermolysis (CRT). Four heating rates (5, 10, 15, and 20 K/min) in TGA experiments and four set temperatures (290, 300, 310 and 320°C) in CRT experiments were considered. In the TGA and differential scanning calorimetry (DSC) results, steep mass loss and rapid decomposition were observed after the melting of the HMX at 280°C. CH2O and N2O were identified as the major decomposition products. Smaller quantities of H2O, HCN, NO and NO2, CO and CO2 were also formed. In the complementary computational study, liquid-phase elementary reactions were investigated using quantum mechanics calculations at B3LYP/6-311++G(d,p) level of theory with the conductor-like polarizable continuum model (CPCM). A zero-dimensional model was developed to simulate the TGA and CRT experiments based on conservation of mass and species in the condensed-phase and the gas-phase control volumes. The predicted mass loss and gas-phase mole fraction profiles of the decomposition products are in good agreements with the corresponding experimental results, indicating that the comprehensive mechanism proposed here captures the important reactions occurring during liquid-phase decomposition of HMX.

    更新日期:2019-11-01
  • Characteristics of counterflow premixed flames with low frequency composition fluctuations
    Combust. Flame (IF 4.120) Pub Date : 2019-10-30
    Takuya Tomidokoro, Takeshi Yokomori, Hong G. Im, Toshihisa Ueda

    The response of laminar methane/air counterflow premixed flames under sinusoidal equivalence ratio oscillation was investigated numerically. The timescales of the oscillation were chosen to be sufficiently longer than the flame timescale so that the flame responds quasi-steadily. The response of periodically stratified flame (SF) with a detailed reaction mechanism exhibited the “back-support” effect, in that the consumption speed Sc response deviated increasingly from Sc of steady homogeneous flames (HFs) at higher oscillation frequencies. It was shown that even when the imposed oscillation timescale is much longer than the flame timescale, the flame response can still be delayed under a sufficiently large equivalence ratio gradient. Subsequently, the above results were compared with those obtained with a global four-step mechanism that omits back-diffusion radicals into the reaction zone. As a result, SFs with the global mechanism displayed a much smaller back-support effect in both lean and rich mixtures. Further analysis with modified diffusion coefficients revealed the dominant roles of H2 and radical species diffusion in inducing the back-support effect. Contrary to the previous findings, variations in burned gas temperature were found to play a negligible role in modifying Sc. Additionally, the hysteresis of the back-support effect under periodical stratification was found to be more prominent on the richer side because of the presence of a larger H2 pool.

    更新日期:2019-11-01
  • Analysis of the spray flame structure in a lab-scale burner using Large Eddy Simulation and Discrete Particle Simulation
    Combust. Flame (IF 4.120) Pub Date : 2019-10-30
    Damien Paulhiac, Bénédicte Cuenot, Eleonore Riber, Lucas Esclapez, Stéphane Richard

    The numerical study of an academic lab-scale spray burner using Large Eddy Simulation coupled with a Discrete Particle Simulation is presented. The objectives are first, to validate current turbulent combustion modeling approach for two-phase flames, and second, to bring new insight on two-phase flame structure in a complex flow, representative of real configurations. The comparison with the experiment shows a good quantitative prediction of the velocity field of the gas and the liquid phases, in both non-reacting and reacting cases. Experimental and numerical results of the spray flame are also in good agreement. The detailed study of the interaction between the flame front and the droplets shows that both single droplet and group combustion regimes occur in the present configuration. These regimes are investigated from the numerical and physical points of view, highlighting the necessity to further investigate their possible importance for the modeling of two-phase combustion.

    更新日期:2019-11-01
  • Modeling pre-spark heat release and low temperature chemistry of iso-octane in a boosted spark-ignition engine
    Combust. Flame (IF 4.120) Pub Date : 2019-10-31
    Dan A. DelVescovo, Derek A. Splitter, James P. Szybist, Gurneesh S. Jatana

    Recent trends among automotive manufacturers towards downsized, boosted engines make it imperative to understand specific fuel chemistry interactions encountered in this new operating regime. At these elevated pressure conditions a phenomenon called pre-spark heat release has recently been discovered, and is characterized by kinetically controlled heat release before spark, with resultant changes in end-gas thermodynamic state and composition. These reactions typically occur in the end-gas during normal operation, but are obscured by the deflagration heat release and therefore cannot be easily studied. A 2-zone spark-ignition engine model was utilized to determine whether chemical kinetic mechanisms are able to predict this phenomenon, and whether they accurately capture end-gas thermodynamic history. Experimental engine data at a range of boosted operating conditions demonstrating pre-spark heat release were compared with simulations using mechanisms representing the latest developments in gasoline kinetic modeling. The results demonstrated significant discrepancies between mechanisms, and between experimental and simulated results in terms of low-temperature heat release magnitude, end-gas thermodynamic state, and autoignition propensity. The results highlight shortcomings in low-temperature reaction pathways, and indicate the necessity of simultaneously matching first-stage ignition delay and heat release magnitude, in addition to second-stage ignition delay, in order to accurately predict end-gas thermodynamics and autoignition.

    更新日期:2019-11-01
  • Near-lean blowoff dynamics in a liquid fueled combustor
    Combust. Flame (IF 4.120) Pub Date : 2019-10-31
    Nicholas Rock, Ben Emerson, Jerry Seitzman, Tim Lieuwen

    This paper describes an analysis of the near-lean blow off (LBO) dynamics of spray flames, including the influence of fuel composition upon these dynamics. It is motivated by the fact that, while reasonable correlations exist for predicting blowoff conditions, the fundamental reasons for why flames supported by flow recirculation actually blow off are not well understood. Prior work on gaseous systems has shown that the blowoff event is a culmination of several intermediate processes, initiating with local extinction of reactions (“stage 1”), followed by large scale changes in flame and flow dynamics (“stage 2”), finally leading to blowoff. In this study, near-LBO dynamics were characterized for ten liquid fuels with widely varying kinetic and physical properties. Results were compared at two air inlet temperatures, 450 and 300 K, as this influences the relative importance of physical and kinetic properties in controlling LBO. Extinction, re-ignition, and recovery of the flame are evident from these data, and grow in frequency as blowoff is approached. Results show that after a near-blowoff event, the flame can move upstream at velocities much faster than the flow velocity, corresponding to re-ignition. Nonetheless, the majority of the flame recovery events appear to be associated with convection of hot products back upstream, not re-ignition. In contrast, downstream motion of the flame faster than the flow, which would correspond to bulk flame extinction, was never observed. This indicates that “extinction events” actually correspond to convection of the flame downstream by the flow when it loses its stabilization point. The dependence of the equivalence ratio when these events appear, their frequency, and event duration were quantified as a function of fuel composition and air inlet temperature. For example, the data shows a higher percentage of recovery from near-blowoff events through re-ignition for high DCN fuels at the 450 K air temperature condition. These extinction/re-ignition results suggest that high DCN fuels are harder to blow off than low DCN fuels through two mechanisms: (1) by delaying the onset of LBO precursor events, and (2) because they are able to recover from these precursor events through re-ignition more often.

    更新日期:2019-11-01
  • Potential Explosion Hazard of Carbonaceous Nanoparticles: Screening of Allotropes.
    Combust. Flame (IF 4.120) Pub Date : 2016-07-29
    Leonid A Turkevich,Joseph Fernback,Ashok G Dastidar,Paul Osterberg

    There is a concern that engineered carbon nanoparticles, when manufactured on an industrial scale, will pose an explosion hazard. Explosion testing has been performed on 20 codes of carbonaceous powders. These include several different codes of SWCNTs (single-walled carbon nanotubes), MWCNTs (multi-walled carbon nanotubes) and CNFs (carbon nanofibers), graphene, diamond, fullerene, as well as several different control carbon blacks and graphites. Explosion screening was performed in a 20 L explosion chamber (ASTM E1226 protocol), at a concentration of 500 g/m3, using a 5 kJ ignition source. Time traces of overpressure were recorded. Samples typically exhibited overpressures of 5-7 bar, and deflagration index KSt = V1/3 (dP/dt)max ~ 10 - 80 bar-m/s, which places these materials in European Dust Explosion Class St-1. There is minimal variation between these different materials. The explosive characteristics of these carbonaceous powders are uncorrelated with primary particle size (BET specific surface area).

    更新日期:2019-11-01
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