Understanding in-cylinder soot reduction in the use of high pressure fuel injection in a small-bore diesel engine Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-25 Lingzhe Rao, Yilong Zhang, Sanghoon Kook, Kenneth S. Kim, Chol-Bum Kweon
This study shows how soot particles inside the cylinder of the engine are reduced due to high pressure fuel injection used in a light-duty single-cylinder optical diesel engine fuelled with methyl decanoate, a selected surrogate fuel for the diagnostics. For various injection pressures, planar laser induced incandescence (PLII) imaging and planar laser-induced fluorescence of hydroxyl (OH-PLIF) imaging were performed to understand the temporal and spatial development of soot and high-temperature flames. In addition, a thermophoresis-based particle sampling technique was used to obtain transmission electron microscope (TEM) images of soot aggregates and primary particles for detailed morphology analysis. The OH-PLIF images suggest that an increase in the injection pressure leads to wider distribution of high-temperature flames likely due to better mixing. The enhanced high-temperature reaction can promote soot formation evidenced by both a faster increase of LII signals and larger soot aggregates on the TEM images. However, the increased OH radicals at higher injection pressure accelerates the soot oxidation as shown in a higher decreasing rate of LII signals as well as dramatic reduction of the sampled soot aggregates at later crank angles. The analysis of nanoscale carbon layer fringe structures also shows a consistent trend that, at higher injection pressure, the soot particles are more oxidized to form more graphitic carbon layer structures. Therefore, it is concluded that the in-cylinder soot reduction at higher injection pressure conditions is due to enhanced soot oxidation despite increased soot formation.
Chemical-looping combustion: Status and research needs Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-23 Juan Adánez, Alberto Abad
Chemical-Looping Combustion (CLC) has emerged in recent years as a very promising combustion technology for power plants and industrial applications with inherent CO2 capture, which circumvent the energy penalty imposed on other competing technologies. The process is based on the use of a metal oxide to transport the oxygen needed for combustion in order to prevent direct contact between the fuel and air. CLC is performed in two interconnected reactors, and the CO2 separation inherent to the process practically eliminates the energy penalty associated with gas separation. The CLC process was initially developed for gaseous fuels, and its application was subsequently extended to solid fuels. The process has been demonstrated in units of different size, from bench scale to MW-scale pilot plants, burning natural gas, syngas, coal and biomass, and using ores and synthetic materials as oxygen-carriers.An overview of the status of the process, starting with the fundamentals and considering the main experimental results and characteristics of process performance, is made both for gaseous and solid fuels. Process modelling of the system for solid and gaseous fuels is also analysed. The main research needs and challenges both for gaseous and solid fuel are highlighted.
Development of a wide range-operable, rich-lean low-NOx combustor for NH3 fuel gas-turbine power generation Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-22 Osamu Kurata, Norihiko Iki, Takahiro Inoue, Takayuki Matsunuma, Taku Tsujimura, Hirohide Furutani, Masato Kawano, Keisuke Arai, Ekenechukwu Chijioke Okafor, Akihiro Hayakawa, Hideaki Kobayashi
Low-NOx NH3-air combustion power generation technology was developed by using a 50-kWe class micro gas-turbine system at the National Institute of Advanced Industrial Science and Technology (AIST), Japan, for the first time. Based on the global demand for carbon-free power generation as well as recent advances involving gas-turbine technologies, such as heat-regenerative cycles, rapid fuel mixing using strong swirling flows, and two-stage combustion with equivalence ratio control, we developed a low-NOx NH3-air non-premixed combustor for the gas-turbine system. Considering a previously performed numerical analysis, which proved that the NO reduction level depends on the equivalence ratio of the primary combustion zone in a NH3-air swirl burner, an experimental study using a combustor test rig was carried out. Results showed that eliminating air flow through primary dilution holes moves the point of the lowest NO emissions to the lesser fuel flow rate. Based on findings derived by using a test rig, a rich-lean low NOx combustor was newly manufactured for actual gas-turbine operations. As a result, the NH3 single fueled low-NOx combustion gas-turbine power generation using the rich-lean combustion concept succeeded over a wide range of power and rotational speeds, i.e., below 10–40 kWe and 75,000–80,000 rpm, respectively. The NO emissions were reduced to 337 ppm (16% O2), which was about one-third of that of the base system. Simultaneously, unburnt NH3 was reduced significantly, especially at the low electrical power output, which was indicative of the wider operating range with high combustion efficiency. In addition, N2O emissions, which have a large Global Warming Potential (GWP) of 298, were reduced significantly, thus demonstrating the potential of NH3 gas-turbine power generation with low environmental impacts.
Dynamics of triple-flames in ignition of turbulent dual fuel mixture: A direct numerical simulation study Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-21 Tai Jin, Kai H. Luo, Xujiang Wang, Kun Luo, Jianren Fan
Pilot-ignited dual fuel combustion involves a complex transition between the pilot fuel autoignition and the premixed-like phase of combustion, which is challenging for experimental measurement and numerical modelling, and not sufficiently explored. To further understand the fundamentals of the dual fuel ignition processes, the transient ignition and subsequent flame development in a turbulent dimethyl ether (DME)/methane-air mixing layer under diesel engine-relevant conditions are studied by direct numerical simulations (DNS). Results indicate that combustion is initiated by a two-stage autoignition that involves both low-temperature and high-temperature chemistry. The first stage autoignition is initiated at the stoichiometric mixture, and then the ignition front propagates against the mixture fraction gradient into rich mixtures and eventually forms a diffusively-supported cool flame. The second stage ignition kernels are spatially distributed around the most reactive mixture fraction with a low scalar dissipation rate. Multiple triple flames are established and propagate along the stoichiometric mixture, which is proven to play an essential role in the flame developing process. The edge flames gradually get close to each other with their branches eventually connected. It is the leading lean premixed branch that initiates the steady propagating methane-air flame. The time required for the initiation of steady flame is substantially shorter than the autoignition delay time of the methane-air mixture under the same thermochemical condition. Temporal evolution of the displacement speed at the flame front is also investigated to clarify the propagation characteristics of the combustion waves. Cool flame and propagation of triple flames are also identified in this study, which are novel features of the pilot-ignited dual fuel combustion.
The oxidation characteristics of furan derivatives and binary TPGME blends under engine relevant conditions Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-20 Shiliang Wu, Dongil Kang, Huiyan Zhang, Rui Xiao, André L. Boehman
Furan and its derivatives have been receiving attention as next generation alternative fuels, related to advanced bio-oil production. However, the ignition quality of furans allows their use only as an additive to diesel fuel in CI engines, which potentially requires the continued use of a fossil-derived base fuel. This study first adopts tri-propylene glycol mono-methyl ether (TPGME) as a substitute for diesel fuel with addition of furan and furan derivatives, including 2-methylfuran, 2,5-dimethylfuran, and furfural, thereby removing fossil-derived fuels from the mixture. With this motivation, gas-phase ignition characteristics of furans were investigated in a modified CFR motored engine, displaying an absence of low temperature heat release (LTHR), while n-heptane as a reference fuel shows a strong two-stage ignition characteristic under the same condition. The structural impact of furans is represented as global oxidation reactivities that are as follows: furan < 2-methylfuran < 2,5-dimethylfuran < furfural < n-heptane. The ranking of individual furans is supported by bond dissociation energies of each fuel's functional group substituent on the furan-ring. Ignition characteristics of TPGME display a strong low-temperature oxidation reactivity; however, its reactivity rapidly diminishes with increasing amounts of furan, shutting down low-temperature oxidation paths. The structural impact of furan and methyl-substituted furans on reactivity is significantly muted when blended with TPGME, as observed in a motored CFR engine and a constant volume spray combustion chamber.
Oxy-combustion of coal in liquid-antimony-anode solid oxide fuel cell system Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-20 Tianyu Cao, Peidong Song, Yixiang Shi, Ahmed F. Ghoniem, Ningsheng Cai
A novel system based on the indirect oxy-combustion of coal in a liquid Sb anode solid oxide fuel cell (SOFC) has been used to produce electricity for over 48 h. Pulverized anthracite was fed to the liquid-antimony-anode of the fuel cell, and a peak power density of 47 mW cm−2 was reached at 1023 K and 35 mW cm−2 at 973 K. The fuel cell was prepared using a porous stainless-steel tube as a support for an LSM cathode, antimony oxide (Sb2O3)/yittria stabilized zirconia (YSZ, Y0.08Z0.92O1.96) composite electrolyte (membrane), while liquid antimony acted as the anode. Liquid antimony/antimony oxide served as the intermediate medium for coal oxidation producing mainly carbon dioxide, which evolved as a separate gas stream. The fuel cell will facilitate carbon capture process, and simultaneously convert the chemical energy of coal directly to electricity. The experiment showed that while the fabricated electrolyte was porous, it became dense during the actual operation, preventing nitrogen leakage into the Sb/C side and producing reasonable open circuit voltage. Analysis of the experimental EIS data illustrates that the Ohmic resistance was the primary loss mechanism in the system. It further suggests approaches to improve the design. Continuous operation of this coal fueled oxy-combustion/fuel cell system achieved an overall efficiency of 28.2% despite of its tiny scale. Simple technologies can be employed to scale up this system at relatively low cost of fabrication and materials.
Flame spread and burning rates through vertical arrays of wooden dowels Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-20 Lin Jiang, Zhao Zhao, Wei Tang, Colin Miller, Jin-Hua Sun, Michael J. Gollner
Fuel loads in real-world fire scenarios often feature discrete elements, discontinuities, or inhomogeneities; however, most models for flame spread only assume a continuous, homogeneous fuel. Because discrete fuels represent a realistic scenario not yet well-modeled, it is of interest to find simple methods to model fire growth first in simple, laboratory-scale configurations. A detailed experimental and theoretical study was therefore performed to investigate the controlling mechanisms of flame spread through arrays of wooden dowels, with dowel spacings of 0.75, 0.875, and 1.5 cm. Flames were found to spread vertically for all spacings; however, for the 1.5 cm spacing, the gap was too large for horizontal flame spread to occur. A radiation-controlled model for horizontal flame spread was developed that predicted the horizontal flame spread rate through various arrays of dowels. Combined with an existing convection-based model for vertical flame spread, both horizontal and vertical flame spread was modeled to predict the number of burning wooden dowels as a function of time. Using models for the burning rate of wooden dowels and boundary-layer theory, a global burning rate model was developed that provided reasonable agreement with experimental results.
Joint probability distribution of Arrhenius parameters in reaction model optimization and uncertainty minimization Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-19 Yujie Tao, Hai Wang
The method of uncertainty minimization by polynomial chaos expansions is extended to Arrhenius prefactor and activation energy co-optimization and uncertainty minimization. A covariance matrix is formulated to describe the joint probability distribution of the reaction rate parameters. The method is tested on a recently proposed foundational fuel chemistry model using 60 H2 and H2/CO flame speeds as the targets. The results show that co-optimizing A and Ea did not produce appreciable improvements in the ability of the reaction model to better predict the flame targets. It does yield reduction in the temperature-dependent uncertainty band of the rate coefficients of several key reactions. The importance of additional experimental and theoretical studies needed for the CO+OH→CO2+H, HO2+H→H2+O2 and HO2+H→2OH reactions is highlighted.
General correlations of high pressure turbulent burning velocities with the consideration of Lewis number effect Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-19 M.T. Nguyen, D.W. Yu, S.S. Shy
This study measures high-pressure turbulent burning velocities (ST) of spherical expanding flames for various liquid and gaseous fuel/air mixtures with different effective Lewis number (Le), i.e. pre-vaporized stoichiometric iso-octane with Le ≈ 1.43 at T = 423 K, hydrogen at the equivalence ratio ϕ= 0.6 with Le ≈ 0.58 at 298 K, and propane at ϕ= 0.7 with Le ≈ 1.62 at 298 K, using the same dual-chamber, fan-stirred cruciform burner capable of generating near-isotropic turbulence. High-speed schlieren imaging is used to obtain the temporal growth of mean flame radii < R(t )> and the observed flame speeds, SF and/or d < R > /dt, where SF is the slope of < R(t )> which equals the average of d < R > /dt within 25 mm ≤ < R(t )> ≤ 45 mm. Using the density correction and Bradley's mean progress variable converting factor for schlieren spherical flames from = 0.1 to 0.5, ST,c=0.5 ≈ (ρb/ρu)SF(< R > c=0.1/ < R > c=0.5)2, where the subscripts b and u indicate the burned and unburned gas. Results show that Le < 1 flames have much higher ST,c=0.5 than that of Le > 1 flames at any given rms turbulent fluctuating velocities (u′) and pressure (p). We find that these very scattering ST,c=0.5 data with Le < 1 and Le>1 together with previous methane data at 300 K/423 K with Le ≈ 1 can be well represented by three modified general correlations originally proposed by Kobayashi et al. (2005), Chaudhuri et al. (2012), and Shy et al. (2012) when their scaling parameters are rescaling and grouping with Le−1, each representing a single curve with small data scattering. This suggests a possible self-similar propagation for turbulent spherical flames, regardless of different fuels, T, p, u′ used. Discussion and comparison with the Bradley's correlation (1992) are offered and future studies identified.
Quantitative NH measurements by using laser-based diagnostics in low-pressure flames Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-18 Nathalie Lamoureux, Laurent Gasnot, Pascale Desgroux
NH is a key short-lived radical involved in the prompt-NO formation. Quantification of NH is thus particularly important for testing the NO kinetic mechanisms. However, quantitative measurements of native NH in hydrocarbon/oxygen/nitrogen flames remain very scarce. Therefore, in this work, the mole fractions of native NH were obtained using a combination of laser-based diagnostics; Laser Induced Fluorescence (LIF) and Cavity Ring-Down Spectroscopy (CRDS). The NH species was probed after exciting the transition R1(6) in the A3Π-X3Σ− (0-0) system at 333.9 nm. The mole fraction profiles of NH were successfully obtained in premixed low-pressure flames of CH4/O2/N2 and C2H2/O2/N2 at two equivalence ratios of 1.00 and 1.25. The estimated detection limit for the NH radical was around 4.5 × 108 molecule cm−3 (i.e. 2 ppb in mole fraction at 1600 K), which is nearly 2 orders of magnitude lower than previous values reported in the literature. These new experimental results were compared with predictions by a recently developed NO model (namely NOMecha2.0). In the case of the CH4 flames, a satisfying agreement between the experiment and model was observed. However, in the case of the C2H2 flames, some discrepancies were observed. Model analysis has highlighted the importance of the HCCO radicals in the NH formation through the HCNO→HNCO→NH2 reactions pathway. Modification of the rate constant values of the reactions C2H2+ O and HCCO + O2, which are key reactions for both the acetylene laminar flame speed and the HCCO predictions, has enabled the model to satisfactorily predict the experimental NH and NO profiles also in the C2H2 flames.
Effects of heat release and fuel type on highly turbulent premixed jet flames Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-18 Laurel Paxton, Jennifer Smolke, Fokion N. Egolfopoulos
An experimental study on the fuel type and attendant heat release effects on piloted turbulent premixed jet flames was carried out. The investigation focused on four fuels, namely methane, ethylene, n-heptane, and toluene, five lean to stoichiometric equivalence ratios, and two jet Reynolds numbers. The average flame height and the global turbulent consumption speed were scaled against the laminar flame speed, the maximum heat release rate, and the laminar flame thickness for all fuels and conditions. Results showed that for the different fuel types and for the lower equivalence ratios, the average flame height does not scale well with any of the aforementioned parameters, while the global consumption speed was determined to scale well with laminar flame speed and maximum heat release rate. The thickness of the shear layer was also characterized through detailed particle image velocimetry measurements and was found to decrease with increasing heat release when the jet diameter is used to scale the axial distance. However, when a density-based momentum diameter is used as the scaling parameter of the axial distance, the effect of heat release is suppressed. Additionally, the growth rates of the shear layer thickness could not be scaled between the different fuels using either the laminar flame speed or the maximum heat release rate. Finally, the turbulent kinetic energy and turbulent shear stress development in the shear layer was determined also to be different between the fuels despite keeping either the laminar flame speed or maximum heat release rate constant, with methane flames most notably showing a sharper decay in intensity with axial distance. The fuel effects on the turbulent kinetic energy and the turbulent shear stress were more pronounced at the higher Reynolds number conditions.
On the relative importance of HONO versus HNO2 in low-temperature combustion Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-18 Mark E. Fuller, C. Franklin Goldsmith
This work investigates whether both HONO and HNO2 are essential in describing the reactivity for NO2-doped ignition experiments or if a strategy could be developed that lumps the two isomers into a single species without adversely affecting the model fidelity. First, the possibility of different product branching fractions is considered; temperature- and pressure-dependent rate constants are computed for H and CH3 addition to the N=O bond in both HONO and HNO2. These results suggest that addition of a radical to HONO and HNO2 do indeed have different products, but that the results are not likely to have a significant effect. Next, two different approaches to simplifying the HONO submechanism are considered. In the first, HNO2 is removed from the mechanism. In the second, HNO2 is replaced with HONO. These two strategies are implemented in different literature mechanisms and then used to compute ignition delay times for H2 and CH4. The results show that removing HNO2 has a modest effect on the ignition delay time, whereas systematically replacing HNO2 with HONO decreases the predicted ignition delay by approximately a factor of two. The recommendation is that for larger fuels, both HONO and HNO2 should be included in the mechanism.
Multi-environment PDF modeling for turbulent piloted premixed jet flames Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-18 Jaehyeon Kim, Namsu Kim, Yongmo Kim
The three-environment PDF approach has been applied to numerically investigate a series of the turbulent piloted premixed flames in the piloted premixed jet burner (PPJB). In this study, computations are made for three flames (PM1-100, PM1-150 and PM1-200). In terms of the unconditional means and conditional statistics, numerical results are reasonably well agreed with experimental data. At the central jet mixture fraction and temperature spaces, the predicted environment-conditional statistics at the near-injector region clearly identify the dual premixed flame modes established by the stoichiometric pilot flame and the lean premixed central jet. The present MEPDF approach together with the IEM micro-mixing model and the skeletal chemistry qualitatively predicts the ignition, local extinction and reignition process in the three-stream highly stretched piloted premixed flame even if there exist certain discrepancies. In terms of [CO][OH]|1400 K and temperature PDFs, the detailed discussions were also made for the effects of micro-mixing models (IEM, EIEM) and chemical mechanism (skeletal, GRI) on the local extinction and reignition characteristics.
Impact of exhaust gas recirculation on ignition delay times of gasoline fuel: An experimental and modeling study Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-18 Liming Cai, Ajoy Ramalingam, Heiko Minwegen, Karl Alexander Heufer, Heinz Pitsch
The ignition delay times of a research grade gasoline, RON95E10, are measured in a rapid compression machine and in a shock tube. The experiments are carried out for fuel/O2/Ar/N2 mixtures with two equivalence ratios of 0.77 and 1.18 at pressures between 20 and 40 bar over the temperature range of 700–1250 K. In particular, data are reported for two exhaust gas recirculation (EGR) loadings of 0 and 25% to demonstrate the impact of EGR on gasoline ignition delay times. The presented data are, to the authors’ knowledge, the first set of ignition delay times of real gasoline with EGR and at engine relevant high pressures covering the entire intermediate temperature range. A published gasoline mechanism (Cai and Pitsch, 2015) is modified as part of this study following the latest kinetic knowledge and is further used to compute the ignition delay times at the conditions experimentally investigated. The simulations employ a four-component surrogate consisting of n-heptane, iso-octane, toluene, and ethanol. It is shown that the proposed model predicts the total ignition delay times of gasoline accurately under various conditions and reflects correctly the influences of pressure, EGR, and equivalence ratio on ignition delay times, while it fails to predict the first stage ignition delay times with very high accuracy due to the neglect of the olefin and naphthene content in the surrogate formulation. Furthermore, the influence of exhaust gas addition on ignition delay times is analyzed and discussed in more detail using results from numerical simulations. The chemical impact of EGR is minor but not negligible and varies at different conditions depending on the EGR composition.
Experimental investigation of soot evolution in a turbulent non-premixed prevaporized toluene flame Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-18 Stephan Kruse, Jingjing Ye, Zhiwei Sun, Antonio Attili, Bassam Dally, Paul Medwell, Heinz Pitsch
The formation, growth, and oxidation of soot in turbulent prevaporized toluene diffusion flames stabilized on a jet-in-hot-coflow (JHC) burner are investigated in this study. Flame structure, local gas temperature as well as local soot volume fraction and primary soot particle diameter, are simultaneously detected by means of OH planar laser-induced fluorescence (PLIF), non-linear excitation regime two-line atomic fluorescence (nTLAF) of indium, and time-resolved (TiRe) laser-induced incandescence (LII), respectively. The collected data sets were used to generate joint statistics of soot properties and flame characteristics and provided new insights into the interaction of the OH layer and soot in turbulent flames. The interaction of OH and soot as a driving mechanism for soot oxidation is of particular interest as it has been proven to be challenging to model. Statistics of soot volume fraction and primary particle size in the OH layer are employed to gain deeper insights into the soot oxidation process. Mean soot volume fraction and primary soot particle size conditioned on temperature and OH signal intensity indicate that, due to differential diffusion of soot with respect to the chemical species, high soot volume fraction and primary soot particle diameter of up to 50 nm are present at low temperatures and low OH concentration. In the soot oxidation region, statistical analysis of the soot parameters disclose that clusters of high soot volume fraction mostly consist of large primary particles. Observations from instantaneous images and the presence of large primary particles inside the OH layer suggest that the oxidation is not sufficiently fast to burn the soot completely.
Predictive kinetics for the thermal decomposition of RDX Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-18 Xi Chen, C. Franklin Goldsmith
The application of Variable Reaction Coordinate Transition State Theory for an energetic material is presented. The homolysis of the N–N bond in RDX is characterized using an embedding methodology in which key atoms in the bond-dissociation process are computed using CASPT2(10e,7o)/jun-cc-pVTZ, while the rest of the molecule is computed using M06-2X/jun-cc-pVTZ. Microcanonical rate theory is used to quantify the temperature and pressure dependent rate constants. The cleavage of the N–N bond is by far the dominant channel, with HONO elimination a distant second. The predicted rate constants are in excellent agreement with the experimental data. The computational approach can be used to provide accurate models for the combustion properties of novel energetic materials.
Effects of low temperature heat release on the aerodynamics of a flat piston rapid compression machine: Impact on velocity and temperature fields Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-17 Moez Ben Houidi, Julien Sotton, Marc Bellenoue, Camille Strozzi
The study of auto-ignition under temperature stratification is of great interest. Indeed, further understanding of the thermo-kinetic interactions and its influence on the combustion propagation regime is needed. In a previous work , experiments in a flat piston Rapid Compression Machine (RCM) demonstrated that the apparent propagation of reaction fronts is highly influenced by the typical temperature stratification observed at inert conditions. Nevertheless, the influence of low temperature heat release (LTHR) on the internal aerodynamics and temperature of the RCM is not well understood. In the present study, we first address the LTHR-flow interaction then address the LTHR-temperature interaction. We performed 2D-PIV experiments at 10 kHz for inert and reactive lean isooctane mixtures. We averaged spatially the acceleration to present the time evolution during the cool flame period. We found that the normalized acceleration has a decreasing trend in both inert and reactive tests. No significant effect of the cool flame was observed on the trend. We performed temperature measurements using thin wire (7.6 µm) type K thermocouples at inert and reactive n-hexane mixtures (same test conditions of Fig. 7 in ). The temperature evolution of the hot (adiabatically compressed) and the colder gases were recorded when cool flame occurs. The corrected gas temperature showed good agreement with the theoretical adiabatic core temperature as well as previous measurements with toluene LIF. In the tested case, we found that the cool flame induces an equal temperature rise of approximately 110 K in both the adiabatically compressed and the colder vortex gases. These results confirm quantitatively that LTHR does not significantly affect the mixing of the temperature stratification of our flat piston RCM. In the studied test conditions, the temperature stratification is conserved globally despite the LTHR.
Determination of aluminum-air burning velocities using PIV and Laser sheet tomography Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-17 R. Lomba, P. Laboureur, C. Dumand, C. Chauveau, F. Halter
In a context of growing concerns over climate change, aluminum has the potential to serve as a dense energy carrier in order to replace fossil fuels and reduce greenhouse gases emissions. Indeed, its combustion in air may provide carbon-free energy for applications in which a high-energy storage capacity is required. However, attempts of designing a metal-fueled combustor will conflict with a relatively large dispersion of the burning velocity values reported in the literature, even when similar powders are used. This uncertainty is partially due to the range of experimental conditions and techniques used on those previous studies. In the present work, an experimental Bunsen-type aluminum-air burner is introduced. It is shown that the setup is capable of generating stable dust suspensions under well-controlled conditions. The stabilized aluminum-air flames are studied using emission spectroscopy, Particle Image Velocimetry, laser sheet tomography, and direct visualization of the AlO(g) emissions. The measured burning velocities are then compared to previous results obtained for similar powders as a function of dust concentration. A reasonable agreement is obtained, and it is shown that metal flame tomography can yield a more precise indicator of the flame front position than AlO(g) emissions, helping to reduce the data scatter regarding dust-air burning velocities.
Three-stage heat release in n-heptane auto-ignition Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-17 S. Mani Sarathy, Efstathios-Al. Tingas, Ehson F. Nasir, Alberta Detogni, Zhandong Wang, Aamir Farooq, Hong Im
Multi-stage heat release is an important feature of hydrocarbon auto-ignition that influences engine operation. This work presents findings of previously unreported three-stage heat release in the auto-ignition of n-heptane/air mixtures at lean equivalence ratios and high pressures. Detailed homogenous gas-phase chemical kinetic simulations were utilized to identify conditions where two-stage and three-stage heat release exist. Temperature and heat release profiles of lean n-heptane/air auto-ignition display three distinct stages of heat release, which is notably different than two-stage heat release typically reported for stoichiometric fuel/air mixtures. Concentration profiles of key radicals (HO2 and OH) and intermediate/product species (CO and CO2) also display unique behavior in the lean auto-ignition case. Rapid compression machine measurements were performed at a lean equivalence ratio to confirm the existence of three-stage heat release in experiments. Laser diagnostic measurements of CO concentrations in the RCM indicate similar concentration-time profiles as those predicted by kinetic modeling. Computational singular perturbation was then used to identify key reactions and species contributing to explosive time scales at various points of the three-stage ignition process. Comparisons with two-stage ignition at stoichiometric conditions indicate that thermal runaway at the second stage of heat release is inhibited under lean conditions. H + O2 chain branching and CO oxidation reactions drive high-temperature heat release under stoichiometric conditions, but these reactions are suppressed by H, OH, and HO2 radical termination reactions at lean conditions, leading to a distinct third stage of heat release.
Magnetic control of flame stability: Application to oxygen-enriched and carbon dioxide-diluted sooting flames Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-17 Agnes Jocher, Jérôme Bonnety, Thomas Gomez, Heinz Pitsch, Guillaume Legros
This present study explores possible stabilization mechanisms in flickering, sooting, ethylene flames burning in varying density coflow and exposed to different levels of an upward gradient of the square of the magnetic flux density (∇(B2)). In normal gravity, flame flickering defines a natural large scale and low frequency flame oscillation that is induced by a so called modified Kelvin–Helmholtz type instability. To assess the potential of the magnetically induced stabilization process, a range of coflow mixtures with varying N2, O2, and CO2 contents in volume is studied. As a result, a domain of controllable flame stability is identified. Its extension depends on the maximum magnitude of ∇(B2), i.e., 18.2 T2/m for the present experimental setup. Spectral emission rate, spectral absorption coefficient, soot volume fraction, and soot temperature fields are measured in the flame by the Modulated Absorption/Emission technique (MAE). In agreement with former studies, the soot content is shown to play a key role in the stabilization process. Due to the magnetic force that is mainly acting on paramagnetic oxygen molecules, opposing gravity, and generated by ∇(B2), the residence time of soot particles in the flame presumably increases with ∇(B2). With growing soot volume fraction, radiative heat losses are enhanced leading to flame cooling. Therefore, flames exposed to the magnetic field exhibit both lower density gradients through the flame sheet and a weaker field of buoyant acceleration in the hot exhaust gas stream. Both mechanisms then reduce the flame vulnerability to the onset of oscillations due to modified Kelvin–Helmholtz type instabilities. The findings may be relevant for designing strategies to control the stability of oxyfuel combustion.
DNS and LES of spark ignition with an automotive coil Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-15 Olivier Colin, Martin Ritter, Corine Lacour, Karine Truffin, Sophie Mouriaux, Sergey Stepanyan, Bertrand Lecordier, Pierre Vervisch
The cycle to cycle combustion variability which is observed in spark-ignition engines is often caused by fluctuations of the early flame development. LES can be exploited for a better understanding and mastering of their origins. For that purpose appropriate models taking into account energy deposition, mixture ignition and transition to propagation are necessary requirements. This paper presents first DNS and LES of spark ignition with a real automotive coil and simplified pin-pin electrodes. The electrical circuit characteristics are provided by ISSIM while the energy deposition is modelled by Lagrangian particles. The ignition model is first evaluated in terms of initial spark radius on a pin-pin ignition experiment in pure air performed at CORIA and EM2C laboratories, showing that it pilots the radius of the torus formed by the initial shock wave. DNS of a quiescent lean propane/air mixture are then performed with this ignition system and a two-step mechanism. The impact of the modelled transferred energy during glow phase as well as the initial arc radius on the minimum ignition energy (MIE) are examined and compared to experimental values. Replacing the two-step chemistry by an analytically reduced mechanism leads to similar MIE but shows a different ignition kernel shape. Finally, LES of turbulent ignition using a Lagrangian arc model show a realistic prediction of the arc shape and its important role on the energy transfer location and thus on the flame kernel shape.
Measurements and modelling of oxy-fuel coal combustion Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-14 Shuiqing Li, Yang Xu, Qi Gao
Oxy-fuel combustion is one of the most promising technologies to isolate efficiently and economically CO2 emissions in coal combustion for the ready carbon sequestration. The high proportions of both H2O and CO2 in the furnace have complex impacts on flame characteristics (ignition, burnout, and heat transfer), pollutant emissions (NOx, SOx, and particulate matter), and operational concerns (ash deposition, fouling/slagging). In contrast to the existing literature, this review focuses on fundamental studies on both diagnostics and modelling aspects of bench- or lab-scale oxy-fuel combustion and, particularly, gives attention to the correlations among combustion characteristics, pollutant formation, and operational ash concerns. First, the influences of temperature and species concentrations (e.g., O2, H2O) on coal ignition, volatile combustion and char burning processes, for air- and oxy-firing, are comparatively evaluated and modelled, on the basis of data from optically-accessible set-ups including flat-flame burner, drop-tube furnace, and down-fired furnace. Then, the correlations of combustion-generated particulate/NOx emissions with changes of combustion characteristics in both air and oxy-fuel firing modes are summarized. Additionally, ash deposition propensity, as well as its relation to the formation of fine particulates (i.e. PM0.2, PM1 and PM10), for both modes are overviewed. Finally, future research topics are discussed. Fundamental oxy-fuel combustion research may provide an ideal alternative for validating CFD simulations toward industrial applications.
Ignition and flame stabilization of a premixed reacting jet in vitiated crossflow Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-14 James W. Dayton, Kyle Linevitch, Baki M. Cetegen
Ignition and flame liftoff height characteristics of a premixed ethylene-air jet injected into a crossflow of hot combustion products are studied experimentally. The windward edge of the flame is found to be lifted from the nozzle exit while the leeward side is almost always located near the nozzle exit. The windward flame liftoff height is characterized as a function of jet-to-crossflow momentum ratio, jet equivalence ratio and jet mixture temperature. It is found that the windward flame liftoff height is best correlated with respect to a Damköhler number defined based on the chemical autoignition time scale for the most reactive mixture fraction. In order to analyze the mixing between the non-reacting part of the jet and the crossflow, planar laser Rayleigh light scattering measurements were conducted to image the temperature field in that region. Mixing between the jet and the crossflow was determined from the Rayleigh data to characterize the mixing characteristics between the jet and the crossflow upstream of the lifted flame location. Scalar dissipation was computed from the Rayleigh data and the time scales associated with scalar dissipation were determined. Experimental results suggest that the flame stabilization is governed by the autoignition characteristics of the jet mixture for the conditions studied in this paper.
Simultaneous high speed PIV and CH PLIF using R-branch excitation in the C2Σ+-X2Π (0,0) band Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-14 Constandinos M. Mitsingas, Stephen D. Hammack, Eric K. Mayhew, Rajavasanth Rajasegar, Brendan McGann, Aaron W. Skiba, Campbell D. Carter, Tonghun Lee
Simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) utilizing R-branch transitions in the C-X (0,0) band were performed at a 10-kHz repetition-rate in a turbulent premixed flame. The CH lines at 310.690 nm (from the R-branch of the C-X band) used here have greater efficiency than A-X and B-X transitions, which allows for high-framerate imaging with low laser pulse energy. Most importantly, the simultaneous imaging of both CH PLIF and PIV is enabled by the use of a custom edge filter, which blocks scattering at the laser wavelength (below ∼311 nm) while efficiently transmitting fluorescence at longer wavelengths. The Hi-Pilot Bunsen burner operated with a turbulent Reynolds number of 7900 was used to demonstrate simultaneous PIV and CH PLIF utilizing this filtered detection scheme. Instances where pockets of products were observed well upstream of the mean flame brush are found to be the result of out-of-plane motion of the flame sheet. Such instances can lead to ambiguous results when interpreting the thickness of reaction layers. However, the temporally resolved nature of the present diagnostics facilitate the identification and proper treatment of such situations. The strategy demonstrated here can yield important information in the study of turbulent flames by providing temporally resolved flame dynamics in terms of flame sheet visualization and velocity fields.
Evaluation of H-atom adsorption on wall surfaces with a plasma molecular beam scattering technique Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-14 Yu Saiki, Ikuya Kinefuchi, Yong Fan, Yuji Suzuki
Toward a precise modeling for wall chemical effects in flame-wall interactions, radical adsorption on different wall surfaces are directly evaluated through a newly-developed molecular beam scattering technique using a non-equilibrium plasma–driven beam source as well as an ultra-high vacuum chamber. Firstly, sensitivities of adsorption rates for each radical species to the wall chemical effect are examined through a series of numerical simulations with detailed gas/surface chemistry for a methane-air premixed flame. Since H-atom has a higher diffusivity and its adsorption significantly inhibits a chain branching reaction of H + O2= O + OH, H is considered to be the most influential radical on the flame characteristics such as heat release rate or CO emission if compared to OH, O and CH3. Based on the sensitivity analysis, H adsorptions are quantified for quartz and SUS321 surfaces with the plasma molecular beam scattering measurements. It is confirmed that H atomic beam can be successfully produced through the plasma dissociation of H2 molecules. Then, the produced H beam is irradiated onto quartz and SUS321 surfaces at different wall temperatures Tw. It is found that H is adsorbed on the quartz and SUS321 surfaces, and reaction probabilities of H PH has its maxima at Tw ∼673 K for both surfaces. This is probably because that the recombination rate increases as Tw increases, while the desorption rate is also promoted and overcomes the recombination rate at Tw > 673 K. The PH for the SUS321 is in agreement with that in our previous combustion experiments and more precise value can be obtained by the present method.
Modeling stratified flames with and without shear using multiple mapping conditioning Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-13 C. Straub, A. Kronenburg, O.T. Stein, R.S. Barlow, D. Geyer
A stochastic sparse particle approach is coupled with an artificial thickening flame (ATF) model for large eddy simulations (LES) to predict a series of turbulent premixed-stratified flames with and without shear and stratification. The thickened reaction progress variable serves as reference variable for the multiple mapping conditioning (MMC) mixing model which emulates turbulent mixing of the stochastic particles. The key feature of MMC is to enforce localness in this reference space when particle pairs are mixed and prevents unphysical mixing of burnt and unburnt fluid across the flame front. We apply MMC-ATF to three flames of a series of turbulent stratified flames and validate the method by comparison with experimental data. The new measurements feature increased accuracy in comparison to previously published data of the same flames due to a better signal-to-noise ratio and a setup which is less prone to beam steering. All flame locations are well predicted by the LES-ATF approach and an analysis of the MMC particle statistics demonstrates that MMC preserves the flamelet-like behaviour in regions where the experiments show low scatter around the flamelet solution. Predicted (local) deviations from the flamelet-solution are comparable to deviations observed in the measurements and variations in the flame structure due to differences in stratification and shear are reasonably well captured by the method.
Self-excited transverse combustion instabilities in a high pressure lean premixed jet flame Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-13 Timo Buschhagen, Rohan Gejji, John Philo, Lucky Tran, J. Enrique Portillo Bilbao, Carson D. Slabaugh
An experimental investigation of self-excited combustion instabilities in a high pressure, lean premixed natural gas jet flame is presented. The combustor is designed with optical access and is instrumented with high frequency pressure transducers at multiple axial and circumferential locations. OH*-chemiluminescence measurements performed at a frequency of 50 kHz were temporally synchronized with the acoustic measurements recorded from the pressure transducer array during the test. Two representative test conditions are analyzed in detail: Flame 1 (F1) that presents longitudinal mode dynamics (p′/pc=3%) and Flame 2 (F2) that presents high amplitude transverse instabilities (p′/pc=15%). Singular Spectrum Analysis (SSA) and Dynamic Mode Decomposition (DMD) analysis indicate a strong correlation of both instabilities to flame-vortex interactions. Longitudinal mode instabilities are correlated with axisymmetric vortex shedding about the combustor axis that result in periodic axial variations in heat release at the 1L frequency. Transverse mode instabilities correspond to asymmetric vortex shedding pattern that drive transverse variations in heat release at the fundamental 1T frequency of the combustion chamber. The phase relationship of the flame emission intensity and the chamber head-end pressure measurement at the 1T frequency indicates presence of a non-stationary transverse mode that rotates about the chamber axis at 55 Hz.
Structure of a stratified CH4 flame with H2 addition Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-12 Silvan Schneider, Dirk Geyer, Gaetano Magnotti, Matthew J. Dunn, Robert S. Barlow, Andreas Dreizler
To explore the effect of H2 addition (20 percent by volume) on stratified-premixed methane combustion in a turbulent flow, an experimental investigation on a new flame configuration of the Darmstadt stratified burner is conducted. Major species concentrations and temperature are measured with high spatial resolution by 1D Raman-Rayleigh scattering. A conditioning on local equivalence ratio (range from ϕ = 0.45 to ϕ = 1.25) and local stratification is applied to the large dataset and allows to analyze the impact of H2 addition on the flame structure. The local stratification level is determined as Δϕ/ΔT at the location of maximum CO mass fraction for each instantaneous flame realization. Due to the H2 addition, preferential diffusion of H2 is different than in pure methane flames. In addition to diffusing out of the reaction zone where it is formed, particularly in rich conditions, H2 also diffuses from the cold reactant mixture into the flame front. For rich conditions (ϕ = 1.05 to ϕ = 1.15) H2 mass fractions are significantly elevated within the intermediate temperature range compared to fully-premixed laminar flame simulations. This elevation is attributed to preferential transport of H2 into the rich flame front from adjacent even richer regions of the flow. Additionally, when the local stratification across the flame front is taken into account, it is revealed that the state-space relation of H2 is not only a function of the local stoichiometry but also the local stratification level. In these flames H2 is the only major species showing sensitivity of the state-space relation to an equivalence ratio gradient across the flame front.
Pareto-efficient combustion modeling for improved CO-emission prediction in LES of a piloted turbulent dimethyl ether jet flame Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-11 Hao Wu, Peter C. Ma, Thomas Jaravel, Matthias Ihme
This study extends the Pareto-efficient combustion (PEC) framework to adaptive LES combustion simulations of turbulent flames. With the focus on improving predictions of CO emissions, PEC is employed to augment a flamelet/progress variable (FPV) model through local sub-model assignment of a finite-rate chemistry (FRC) model. A series of LES-PEC calculations are performed on a piloted partially-premixed dimethyl ether flame (DME-D), using a combination of FPV and FRC models. The drift term is utilized in the PEC framework to estimate the model error for quantities of interest. The PEC approach is demonstrated to be capable of significantly improving the prediction of CO emissions compared to a monolithic FPV simulation. The improved accuracy is achieved by enriching the FPV model with FRC in regions where the low-order model is determined insufficient through the evaluation of the drift term. The computational cost is reduced by a factor of two in comparison to the full finite-rate calculation, while maintaining the same level of accuracy for CO predictions.
Determining fractal properties of soot aggregates and primary particle size distribution in counterflow flames up to 10 atm Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-11 Hafiz M.F. Amin, Anthony Bennett, William L. Roberts
Experimental investigations of soot morphology are performed in counterflow flames of N2-diluted ethylene and air, up to 10 atm. A thermophoretic sampling device is attached to a pressure vessel containing a counterflow burner where flames with an ethylene mole fraction of 0.3 are stabilized at 3, 5, and 6 atm. To allow measurements at higher pressures, the fuel mole fraction is lowered to 0.2 to reduce the soot loading and flames are studied at 5, 7, and 10 atm. Thermophoretic sampling of the soot zone is performed using TEM grids. The sampling process causes minimal flame disturbances. Soot collected on TEM grids is analyzed under transmission electron microscope (TEM). Primary particle size distributions are inferred at each pressure by manually analyzing the primary particles from TEM images. Fractal properties of soot at each pressure are also obtained by analyzing the TEM images at comparatively low magnifications. Mean primary particle diameter increases from 17.5 to 47.1 nm as the pressure is increased from 3 to 10 atm, whereas the fractal dimension and prefactor do not change with pressure up to 10 atm. For the flames studied here, fractal dimension lies between 1.61 and 1.67 whereas fractal prefactor varies between 1.68 and 1.86 without following any apparent trend with pressure.
Direct measurements of channel specific rate constants in OH + C3H8 illuminates prompt dissociations of propyl radicals Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-11 Raghu Sivaramakrishnan, C. Franklin Goldsmith, Sebastian Peukert, Joe V. Michael
OH + molecules are an important class of reactions in combustion and atmospheric chemistry. Consequently, numerous studies have measured rate constants for these processes over an extended temperature range. A large majority of these experimental studies have utilized the decay of [OH] profiles (monitored either by absorption or laser-induced fluorescence) to obtain total rate constants. However, there are limited direct measurements of channel specific rate constants in this important class of reactions, particularly at combustion relevant temperatures. In the present experiments, we have directly measured site-specific rate constants for abstraction of the secondary CH bond in OH + C3H8 at high temperatures. Atomic resonance absorption spectrometry (ARAS) was used to monitor the formation of H-atoms from shock-heated mixtures of tert-butylhydroperoxide and C3H8 at high temperatures. Simulations for the experimental H-atom profiles are sensitive only to abstraction of the secondary CH bond leading to unambiguous measurements of the rate constants for this reaction. Over the T-range, 921 K < T < 1146 K, rate constants from the present experiments for OH + C3H8 → H2O + i-C3H7 can be represented by the Arrhenius expression,k=(3.935±1.387)×10−11exp(−1681±362K/T)cm3molecule−1s−1Simulations of the lower temperature data (T < 1000 K) indicate that the H-atom profiles are also influenced to a minor extent by the thermal dissociation of iso-propyl, i-C3H7 → H + C3H6, at short time-scales. Direct dynamics calculations were performed to examine in greater detail the potential role of prompt dissociations of i-C3H7 and n-C3H7 (formed from the title reaction) in interpreting the lower temperature (< 1000 K) data from the present work. These simulations suggest that prompt dissociation of propyl radicals does not influence the present experimental observations but has a minor influence on higher temperature combustion simulations.
Impact of swirl and bluff-body on the transfer function of premixed flames Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-11 M. Gatti, R. Gaudron, C. Mirat, L. Zimmer, T. Schuller
The frequency response of three lean methane/air flames submitted to flowrate perturbations is analyzed for flames featuring the same equivalence ratio and thermal power, but a different stabilization mechanism. The first flame is stabilized by a central bluff body without swirl, the second one by the same bluff body with the addition of swirl and the last one only by swirl without central insert. In the two last cases, the swirl level is roughly the same. These three flames feature different shapes and heat release distributions, but their Flame Transfer Function (FTF) feature about the same phase lag at low frequencies. The gain of the FTF also shows the same behavior for the flame stabilized by the central insert without swirl and the one fully aerodynamically stabilized by swirl. Shedding of vortical structures from the injector nozzle that grow and rollup the flame tip controls the FTF of these flames. The flame stabilized by the swirler-plus-bluff-body system features a peculiar response with a large drop of the FTF gain around a frequency at which large swirl number oscillations are observed. Velocity measurements in cold flow conditions reveal a strong reduction of the size of the vortical structures shed from the injector lip at this forcing condition. The flame stabilized aerodynamically only by swirl and the one stabilized by the bluff body without swirl do not exhibit any FTF gain drop at low frequencies. In the former case, large swirl number oscillations are still identified, but large vortical structures shed from the nozzle also persist at the same forcing frequency in the cold flow response. These different flame responses are found to be intimately related to the dynamics of the internal recirculation region, which response strongly differs depending upon the injector used to stabilize the flame.
Neural network prediction of cycle-to-cycle power variability in a spark-ignited internal combustion engine Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-11 Andrew Di Mauro, Hao Chen, Volker Sick
Cycle-to-cycle variation (CCV) limits how lean a spark-ignited (SI) internal combustion engine (ICE) can stably operate at, restricts efficiency, and increases emissions through incomplete combustion. Therefore, a way to cleaner, more efficient SI ICEs is to minimize the CCV. Current methods to study CCV include experimental investigations and CFD-based numerical simulations. This study, in contrast, investigates the ability of neural networks to accurately model the indicated mean effective pressure (IMEP) and its coefficient of variation (COV of IMEP). Experimental data from a previous study of spark-ignited propane/air combustion in the TCC-III engine was used to train and evaluate a neural network. An optimized network was generated that utilizes 109 experimental inputs and is operated with 15 neurons in one hidden layer to determine IMEP for 18 engine operating conditions, with 625 individual consecutive engine cycles for each condition. The impact of training set size and the number of input parameters was also investigated. The average deviation for IMEP from the experimental measurements is 0.7–2.2% for the training data set and less than 12% for the entire predicted range of operating conditions. Data sets consisted of tests under rich, lean, and stoichiometric conditions without and with 9% nitrogen dilution. Predicted COV of IMEP strongly correlates with experimental data (R2 = 0.8453). However, a systematic over prediction of COV of IMEP for low COVs was observed while higher COVs were under-predicted by the neural network. The cause for this systematic behavior has not yet been identified but histograms of the predicted IMEP data indicate that this could be related to missing physical parameters that have a significant impact on combustion variability.
Dilution effects on laminar jet diffusion flame lengths Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-11 Zhengyang Wang, Peter B. Sunderland, Richard L. Axelbaum
Many studies have examined the stoichiometric lengths of laminar gas jet diffusion flames. However, these have emphasized normal flames of undiluted fuel burning in air. Many questions remain about the effects of fuel dilution, oxygen-enhanced combustion, and inverse flames. Thus, the stoichiometric lengths of 287 normal and inverse gas jet flames are measured for a broad range of nitrogen dilution. The fuels are methane and propane and the ambient pressure is atmospheric. Nitrogen addition to the fuel and/or oxidizer is found to increase the stoichiometric lengths of both normal and inverse diffusion flames, but this effect is small at high reactant mole fraction. This counters previous assertions that inert addition to the fuel stream has a negligible effect on the lengths of normal diffusion flames. The analytical model of Roper is extended to these conditions by specifying the characteristic diffusivity to be the mean diffusivity of the fuel and oxidizer into stoichiometric products and a characteristic temperature that scales with the adiabatic flame temperature and the ambient temperature. The extended model correlates the measured lengths of normal and inverse flames with coefficients of determination of 0.87 for methane and 0.97 for propane.
Differences between PREMIER combustion in a natural gas spark-ignition engine and knocking with pressure oscillations Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-11 Nobuyuki Kawahara, Yungjin Kim, Hisashi Wadahama, Kazuya Tsuboi, Eiji Tomita
PREMIER (PREmixed Mixture Ignition in the End-gas Region) combustion occurs with auto-ignition in the end-gas region when the main combustion flame propagation is nearly finished. Auto-ignition is triggered by the increases in pressure and temperature induced by the main combustion flame. Similarly to engine knocking, heat is released in two stages when engines undergo this type of combustion. This pattern of heat release does not occur during normal combustion. However, engine knocking induces pressure oscillations that cause fatal damage to engines, whereas PREMIER combustion does not. The purpose of this study was to elucidate PREMIER combustion in natural gas spark-ignition engines, and differentiate the causes of knocking and PREMIER combustion. We applied combustion visualization and in-cylinder pressure analysis using a compression–expansion machine (CEM) to investigate the auto-ignition characteristics in the end-gas region of a natural gas spark-ignition engine. We occasionally observed knocking accompanied by pressure oscillations under the spark timings and initial gas conditions used to generate PREMIER combustion. No pressure oscillations were observed during normal and PREMIER combustion. Auto-ignition in the end-gas region was found to induce a secondary increase in pressure before the combustion flame reached the cylinder wall, during both knocking and PREMIER combustion. The auto-ignited flame area spread faster during knocking than during PREMIER combustion. This caused a sudden pressure difference and imbalance between the flame propagation region and the end-gas region, followed by a pressure oscillation.
Numerical analysis of wedge-induced oblique detonations in two-phase kerosene–air mixtures Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-10 Zhaoxin Ren, Bing Wang, Gaoming Xiang, Longxi Zheng
The oblique detonation wave in two-phase kerosene–air mixtures over a wedge is numerically studied. The gas-droplet reacting flow system is solved by means of a hybrid Eulerian–Lagrangian method. We elucidate the initiation feature of the two-phase oblique detonation. The effects of the spray equivalence ratio on the initiation characteristics of a detonation and the transition from oblique shock to detonation are taken into account. As the mass flow rate of the droplets increases, a shift from a smooth transition to an abrupt one is observed, and the initiation length of the oblique detonation is increased. The initiation length as well as the transition pressure depends on the spray equivalence ratio. The observed distribution is a ∧-shape for an equivalence ratio ranging from 0.4 to 1.4, and has its maximum value around unity. This is mainly owing to the interplay between the evaporative cooling and chemical heat release. The results show that the evaporative cooling effects have more influence in the fuel-lean side, but the heat release effects predominate in the fuel-rich side. In particular, the decrease of the initiation length with the increase of the spray equivalence ratio in the fuel-rich side is also due to the increase of the inflow Mach number and the corresponding increase of the post-shock temperature and pressure.
Spray impingement wall film breakup by wave entrainment Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-10 Xuesong Li, Hujie Pan, Xue Dong, David Hung, Min Xu
Fuel spray impingement on engine wall and piston in the spark-ignition direct-injection (SIDI) setting has been considered a major concern in the aspect of engine emission and combustion efficiency. Excess wall film will result in deterioration of engine friction, incomplete combustion, and substantial cycle-to-cycle variations. These effects are more pronounced during engine cold-start process. Therefore, the formation of wall film on engine wall/piston and the dynamic process of the wall film interacting with impinging spray and spray-induced gas flow are of great significance for reducing wall film mass. However, the dynamic process of wall film was not investigated thoroughly in existing literatures. This work will present a high-speed, simultaneous measurement of a single-hole spray structure, as well as wall film geometry and thickness, via Mie scattering and volumetric laser-induced fluorescence, respectively. Quantitative film thickness measurement was achieved via fluorescence intensity signal calibration with a known, wedge-shape liquid film apparatus. Remarkable wall film droplet entrainment at the leading edge of the liquid film waves was revealed in the measurement, which has not been adequately depicted or analyzed in existing spray impingement studies. A considerable amount of liquid droplets detaches from the liquid film via liquid film fingering, during which process the quantity of liquid mass on the wall is decreased. Quantitative analysis of such phenomenon is performed and we estimated that a liquid mass equivalent to 30–40% of the residual liquid film mass is detached from the liquid film via wave entrainment. Furthermore, through the comparative study of the side view of the spray and the liquid film caused by spray impingement, it is shown that non-uniform spray structure is likely the cause of liquid film wavy motions. These observations suggest that wave entrainment should be considered by numerical models and experimental designs to accurately predict spray impingement phenomenon.
Quantitative measurement of temperature in oxygen enriched CH4/O2/N2 premixed flames using Laser Induced Thermal Grating Spectroscopy (LITGS) up to 1.0 MPa Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-08 Akihiro Hayakawa, Tomohisa Yamagami, Kiyonori Takeuchi, Yasuhiro Higuchi, Taku Kudo, Steven Lowe, Yi Gao, Simone Hochgreb, Hideaki Kobayashi
The application of laser diagnostics to high pressure combustion phenomena is particularly challenging, especially in practical combustors such as rocket motors. In this study, temperature measurements using Laser Induced Thermal Grating Spectroscopy (LITGS) are demonstrated in oxygen enriched CH4/O2/N2 premixed laminar flames at pressures up to 1.0 MPa. We use a previously developed OH absorption LITGS technique to determine product gas temperatures from 0.3 to 1.0 MPa, for both high temperature oxygen-enriched and pure-oxygen flames, for measurements up to 3000 K. Further, we demonstrate how it is necessary to correct the measurements for the local absorption of laser light to obtain accurate temperatures, and offer a technique for producing the correction by using different laser energies. Once the correction is applied, we demonstrate that the measurements at 0.5 MPa are within 1.6% of the adiabatic non-strained flame temperatures, with a standard deviation of about 160 K, thus offering a competitive method for the challenging conditions at high pressures and temperatures. The values obtained at derived temperatures at 1.0 MPa were lower than the adiabatic unstrained flame temperatures, which could possibly be attributed to loss mechanisms.
Joint experimental and numerical study of silica particulate synthesis in a turbulent reacting jet Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-08 Gregor Neuber, Carlos E. Garcia, Andreas Kronenburg, Benjamin A.O. Williams, Frank Beyrau, Oliver T. Stein, Matthew J. Cleary
This paper presents results from a joint experimental and numerical study of silica particulate synthesis for a turbulent reacting jet configuration where a cold jet doped with silane issues into a hot vitiated coflow. The experimental investigation involves simultaneous measurements of elastic light scattering and planar laser-induced fluorescence signals and these are used for validation of a novel computational approach, called PBE-MMC-LES, for the solution of the joint scalar probability density function of the gas phase species and the discretised particulate size distribution. Model validation follows the “paradigm shift” approach which is based on the computation of “predicted signals” which are compared directly with the experimentally-acquired signals. The results demonstrate that PBE-MMC-LES can model particulate inception, surface growth and agglomeration at reasonable computational cost. The agreement between the measured and computed signals is good in the light of the modelling complexities associated with particle flame synthesis, but predictions are rather sensitive to the uncertainties in precursor chemistry leading to nucleation and growth.
Flame dynamics of azimuthal forced spinning and standing modes in an annular combustor Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-08 Håkon T. Nygård, Marek Mazur, James R. Dawson, Nicholas A. Worth
Azimuthal forcing has been applied to flames in a laboratory scale annular combustor in order to accurately control the azimuthal mode of excitation. A new forcing configuration permitted not only the pressure amplitude, but also the spin ratio and mode orientation to be accurately controlled, in order to generate standing modes and for the first time strong spinning modes in both a clockwise (CW) and anti-clockwise (ACW) direction. The phase averaged heat release dynamics of these modes was compared and a number of differences observed depending on the direction of pressure wave propagation, demonstrating characteristic ACW and CW heat release patterns. A new spin compensating averaging method was then introduced to analyse the flame dynamics, and it was shown that through the application of this method the dynamics of standing wave oscillations could be decomposed to recover the characteristic ACW and CW heat release responses. The global heat release response was also assessed during strongly spinning modes, and the magnitude of the response was shown to depend strongly on the direction of propagation, demonstrating the importance of the local swirl direction on the global heat release response, with important implications for the modelling of such flows.
Examination of the electronic structure of oxygen-containing PAH dimers and trimers Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-08 Jennifer A. Giaccai, J. Houston Miller
Interactions of oxygen with polynuclear aromatic hydrocarbons (PAH) can occur both in the flame and during oxidation of soot atmospherically. Past experimental measurements of PAH in soot samples collected either immediately after combustion, from the atmosphere, or in a flame show a variety of oxygen moieties within the PAH structures. This study investigated the electronic structure of oxygen-containing PAH to gain insight into their interaction with light both to better interpret spectroscopic measurements and to recognize the role of oxygen-containing PAH in atmospheric radiative forcing. Our research has shown that oxygen in ethers and hydroxyl moieties on PAH showed little change to the HOMO-LUMO gap (HLG), whereas ketones and aldehydes show a HLG decrease of 0.5 eV. The effect is enhanced when more than one ketone is present on a PAH molecule and further enhanced in subsequent dehydrogenation to a quinone-like structure. The presence of an oxygen-containing PAH with a ketone functional group in a dimer and trimer will substantially lower the HLG of the PAH stack. This may have a significant effect in the interaction of atmospheric soot with solar radiation.
Direct measurement of energy loss due to aging effects in the condensed phase explosive PBX 9404 Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-07 Scott I. Jackson, Eric K. Anderson, Larry G. Hill
The explosive performance of PBX 9404, a condensed phase explosive with HMX and nitrocellulose reactive ingredients, was evaluated using the detonation cylinder expansion test after it had been naturally aged for over 46 years. Nitrocellulose is known to chemically degrade with age, but the corresponding effect on explosive performance is currently unknown. Two new cylinder tests were fielded with the oldest known PBX 9404 explosive (>46 years) and the data was compared with prior PBX 9404 cylinder test data. A method for comparing wall motion data collected by streak camera and laser interferometry diagnostics was also introduced. Analysis of the cylinder motion indicated that the aged explosive exhibits decreased performance, which varied with cylinder expansion radius and product specific volume. This energy decrement was found to be 0.8% of the total initial explosive energy per decade of age at a product volume of 7.0 cc/g. The measured energy decrement in explosives older than 37 years exceeds the chemical energy content of nitrocellulose, indicating that nitrocellulose decomposition radicals are likely degrading the normally stable HMX molecules during the aging process.
In situ monitoring of hydrogen loss during pyrolysis of wood by neutron imaging Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-07 Frederik Ossler, Louis J. Santodonato, Jeffrey M. Warren, Charles E.A. Finney, Jean-Christophe Bilheux, Rebecca A. Mills, Harley D. Skorpenske, Hassina Z. Bilheux
Hydrogen is an element of fundamental importance for energy but hard to quantify in bulk materials. Neutron radiography was used to map in situ loss of elemental hydrogen from beech tree wood samples during pyrolysis. The samples consisted of three wood cylinders (finished dowel or cut branch) of approximately 1 cm in length. The samples were pyrolyzed under vacuum in a furnace vessel that was placed inside a cold neutron imaging beamline using a temperature ramp of 5 °C/min from ambient up to 400 °C. Neutron radiographs with exposures of 30 s were sequentially recorded with a charge-coupled device over the course of the experiment. Relative absorbance/scattering of the neutron beam by each sample was based on intensity (or brightness) values as a function of pixel position. The much larger neutron cross section for hydrogen compared to carbon and oxygen enables almost direct conversion of neutron attenuation into sample hydrogen content for each time step during the pyrolysis experiment. Target and vessel temperatures were recorded concurrently with collection of the radiographs so that changes could be directly correlated to different states of pyrolysis. The most visible change appeared at the initial phase of the 400 °C plateau as evidenced by strong hydrogen loss and primarily diametric shrinking of the samples. The loss of elemental hydrogen between initial and final states of pyrolysis was estimated to be about 70%.
Phase synchronization and collective interaction of multiple flamelets in a laboratory scale spray combustor Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-07 Samadhan A. Pawar, Mahesh V. Panchagnula, R.I. Sujith
In this paper, we investigate the coupled behvior of the acoustic field in the confinement and the unsteady flame dynamics in a laboratory scale spray combustor. We study this interaction during the intermittency route to thermoacoustic instability when the location of the flame is varied inside the combustor. As the flame location is changed, the synchronization properties of the coupled acoustic pressure and heat release rate signals change from desynchronized aperiodicity (combustion noise) to phase synchronized periodicity (thermoacoustic instability) through intermittent phase synchronization (intermittency). We also characterize the collective interaction between the multiple flamelets anchored at the flame holder and the acoustic field in the system, during different dynamical states observed in the combustor operation. When the signals are desynchronized, we notice that the flamelets exhibit a steady combustion without the exhibition of a prominent feedback with the acoustic field. In a state of intermittent phase synchronization, we observe the existence of a short-term coupling between the heat release rate and the acoustic field. We notice that the onset of collective synchronization in the oscillations of multiple flamelets and the acoustic field leads to the simultaneous emergence of periodicity in the global dynamics of the system. This collective periodicity in both the subsystems causes enhancement of oscillations during epochs of amplitude growth in the intermittency signal. On the contrary, the weakening of the coupling induces suppression of periodic oscillations during epochs of amplitude decay in the intermittency signal. During phase synchronization, we notice a sustained synchronized movement of all flamelets with the periodicity of the acoustic cycle in the system.
Cavity flameholding in an optical axisymmetric scramjet in Mach 4.5 flows Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-07 Qili Liu, Damiano Baccarella, Will Landsberg, Ananthanarayanan Veeraragavan, Tonghun Lee
The dynamics of an optical axisymmetric scramjet, preferred for generic combustion and flameholding studies without the corner boundary-layer effects that distort the freestream and flame propagation in rectangular scramjets, is investigated at Mach 4.5. Ethylene fuel is injected into supersonic flow via sixteen sonic fuel nozzles equally spaced in a circumferential direction and inclined at 45° to the freestream. The gaseous fuel is auto-ignited by high-enthalpy flows that are compressed and decelerated by shockwaves and boundary layers while passing through the scramjet inlet and isolator. An axisymmetric cavity located downstream of fuel injectors achieves flame holding by providing slow flow recirculation regions and inducing shockwaves at cavity leading edge and ramp to redirect and recompress flows. Flow and flame behaviors are characterized by high speed flame chemiluminescence imaging and static pressure measurement, while the detailed flame structures are resolved by instantaneous ground-state hydroxyl radical (OH) distributions using planar laser-induced fluorescence (PLIF). Numerical simulation is employed to aid the inlet/isolator design to avoid unexpected unstart introduced by shock-boundary layer interactions in the shock train region. The Mach number profiles in the radial direction are measured using a Pitot probe at the combustor exit. We demonstrate supersonic flameholding with the presence of a cavity at ethylene fueled conditions where stable combustion is achieved without scramjet unstart under both mass and heat loading. The heat addition from the cavity-stabilized flame mainly reduces the flow Mach number in the near-wall flow region. In the core flow region, the flow Mach number is decreased by the jet- and cavity-induced shockwaves and minimally affected by heat addition.
Opposed-flow flame spread in a narrow channel: Prediction of flame spread velocity Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-07 Koshi Funashima, Ayaka Masuyama, Kazunori Kuwana, Genichiro Kushida
This paper presents results of experimental and numerical investigations of flame spread along a thin solid in an opposed oxygen flow in a narrow channel. Experiments are conducted at various oxygen flow speeds and gas-phase heights. For a low gas-phase height or a low oxygen flow speed, a large portion of solid is left unburned, and the burned region forms a finger-like pattern. It is noted that both the flame spread velocity and the fraction burned increase with an increase in the gas-phase height or oxygen flow speed. A simple, two-equation model is then developed to simulate the phenomenon. The original 3-D equations are reduced to 2-D forms, which are solved numerically. To simplify the model, it is assumed that the rate of solid pyrolysis is linearly proportional to that of gas-phase oxidation. A comparison between the numerical predictions and the experimental data, however, indicates that because of this assumption, prediction error tends to increase with increase in the gas-phase height or oxygen flow speed. Nevertheless, model predictions agree reasonably well with the experimental data, thus validating the assumptions of the model, at least qualitatively. A weakly nonlinear stability analysis is finally conducted to derive a relationship between the scaled flame spread velocity and a dimensionless parameter that combines the effects of material properties and experimental parameters such as the gas-phase height and oxygen flow speed. The presented numerical and experimental results support the stability analysis.
A study on the low-to-intermediate temperature ignition delays of long chain branched paraffin: Iso-cetane Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-07 Liang Yu, Yue Qiu, Yebing Mao, Sixu Wang, Can Ruan, Wencao Tao, Yong Qian, Xingcai Lu
Iso-cetane (2, 2, 4, 4, 6, 8, 8-heptamethylnonane) is known as a primary reference fuel for cetane number rating and is regarded as an applicable component for surrogate diesel fuel. In this study, ignition delays for iso-cetane homogeneous mixture were measured at equivalence ratios varying from 0.5 to 2.0, compressed pressures of 10, 15 and 20 bar, and compressed temperatures of 620–880 K in a heated rapid compression machine (RCM). Two-stage ignition characteristic of iso-cetane was observed for all mixtures. Negative temperature coefficient (NTC) behavior of iso-cetane ignition delay appears in the temperature range of 670–730 K, which is significantly lower than that of other large hydrocarbons. Influences of compressed temperature, compressed pressure, and mixture composition on iso-cetane ignition delays were also investigated. It is found that the total ignition delays shorten with the increase of compressed pressure, equivalence ratio and oxygen mole fraction. The first-stage ignition delays exhibit Arrhenius-like dependence on compressed temperature and are relatively insensitive to the change of other parameters. In addition, modeling study was conducted using an updated iso-cetane kinetic model developed from a literature iso-cetane mechanism. Simulation results show that the total ignition delays are underestimated while the temperature range of the NTC behavior is overestimated. Rate of production (ROP) analysis prior to the first-stage ignition was also conducted to identify the controlling reactions generating and consuming OH and HO2 radicals in low-temperature (low-T) reaction pathways.
Propagation of a hemispherical flame over a heat-absorbing surface Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-07 V. Golub, A. Korobov, A. Mikushkin, V. Petukhov, V. Volodin
This paper presents an experimental study of hydrogen–air flame front dynamics and propagation over a heat-absorbing surface. Experimental data on the hemispherical flame propagation in hydrogen–air mixture with hydrogen content of 15% in 4.5 m3 cylindrical volume were obtained at the ignition at the centre of the bottom with energy of 5 J. The flame propagates at atmospheric pressure over a solid aluminium wall or a layer of steel wool. The flame acceleration dynamics were compared at hemispherical and finger flame stages. It was found that in a mixture with a hydrogen content of 15% the flame over the layer of steel wool propagates 2.5 times more slowly than that over the surface of an aluminium wall. Calculation of heat absorption in the steel wool layer shows that the heat losses due to the absorption are the main phenomenon causing the suppression of Darrieus–Landau instability and flame front speed reduction, which was observed in the experiments. Experimental results are compared with analytical model of finger flame propagation from literature.
Butanol–acetone mixture blended with cottonseed biodiesel: Spray characteristics evolution, combustion characteristics, engine performance and emission Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-06 Sattar Jabbar Murad Algayyim, Andrew P. Wandel, Talal Yusaf, Saddam Al-Lwayzy
Increasing energy demands and more stringent legislation relating to pollutants such as nitrogen oxide (NOx) and carbon monoxide (CO) from fossil fuels have accelerated the use of biofuels such as biodiesel. However, current limitations of using biodiesel as an alternative fuel for CI engines include a higher viscosity and higher NOx emissions. This is a major issue that could be improved by blending biodiesel with alcohols. This paper investigates the effect of a butanol–acetone mixture (BA) as an additive blended with biodiesel to improve the latter's properties. Macroscopic spray characteristics (spray penetration, spray cone angle and spray volume) were measured in constant volume vessel (CVV) at two injection pressures. A high-speed camera was used to record spray images. The spray's edge was determined using an automatic threshold calculation algorithm to locate the spray outline (edge) from the binary images. In addition, an engine test was carried out experimentally on a single-cylinder diesel engine. The engine's performance was measured using in-cylinder pressure, brake power (BP) and specific fuel consumption (SFC). Emission characteristics NOx, CO and UHC were also measured. Neat biodiesel and three blends of biodiesel with up to 30% added BA were tested. The experimental data were analyzed via ANOVA to evaluate whether variations in parameters due to the different fuels were significant. The results showed that BA can enhance the spray characteristics of biodiesel by increasing both the spray penetration length and the contact surface area, thereby improving air–fuel mixing. The peak in-cylinder pressure for 30% BA was comparable to neat diesel and higher than that of neat biodiesel. Brake power (BP) was slightly improved for 10% BA at an engine speed of 2000 rpm while SFC was not significantly higher for any of the BA-biodiesel blends because of the smaller heating value of BA. Comparing the effect on emissions of adding BA to biodiesel, increasing the amount of BA reduced NOx and CO (7%) and (40%) respectively compared to neat biodiesel, but increased UHC.
Experimental and numerical investigation of effects of premixing on soot processes in iso-octane co-flow flames Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-06 A. Makwana, A. Jain, M. Linevsky, S. Iyer, R. Santoro, T. Litzinger, Y. Xuan, J. O'Connor
The goal of the current work is to understand the effects of premixing on soot processes in an iso-octane, axisymmetric, co-flow, laminar flame at atmospheric pressure. The flames investigated are non-premixed and partially-premixed (jet equivalence ratios of 24, 15, 12, 9 and 6). The total carbon flow rate is kept constant to facilitate comparison among the six flames. Laser-induced incandescence and laser extinction are applied to obtain two-dimensional soot volume fraction. The experimental results show that the spatial distribution of soot changes with premixing; the peak soot volume fraction location is in the annular region in the non-premixed flame and transitions to the centerline as the jet equivalence ratio is reduced. Numerical simulations are performed using a detailed iso-octane fuel chemistry and bi-variate soot model. The numerical model captures the changes in the spatial distribution of soot due to premixing, as in the experiment. Similar to the change in the soot distribution, the soot production processes, including nucleation, surface growth, and PAH condensation, show the transition behavior with premixing. The simulation shows that the location of peak PAH dimer concentration shifts from the annular region towards the centerline with premixing. As a result, the location where soot nucleation and PAH condensation rates peak show similar transition as observed in the PAH dimer concentration. Furthermore, PAH dimer concentration decreases due to premixing, leading to a decrease in the soot nucleation and soot growth due to PAH condensation. Additionally, soot growth due to surface reactions decrease with premixing due to the reduction in number of active sites on the soot surface.
The role of cellular instability on the critical tube diameter problem for unstable gaseous detonations Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-06 Han Xu, Xiaocheng Mi, Charles B. Kiyanda, Hoi Dick Ng, John H.S. Lee, Chunde Yao
The transmission of detonation waves, propagating in a homogeneous, gaseous, reactive medium, from a tube into an unconfined space is well known to succeed or fail based on the tube diameter. Below a certain diameter, the detonation fails to transition into the unconfined space, while for a large enough geometry, the transition succeeds. This critical diameter is well correlated to the incoming detonation cell size. For common undiluted hydrocarbon mixtures with a strong degree of transverse instability, the ratio of critical tube diameter to cell size has been measured at Dc = 13λ. In this paper, stoichiometric acetylene-oxygen mixture at different initial pressures is detonated in a circular tube that transitions into an effectively unconfined space. The transition is observed with simultaneous schlieren photography and soot foil records to look at the role of transverse cellular instability. Three regimes of transition are observed: supercritical, where the cellular pattern is continuously connected from the donor tube to the larger space; subcritical, where the wave fails and the cellular pattern disappears; and a critical regime, where the wave initially fails, asymptoting to a weakly decoupled shock-reaction front regime, and exhibits a subsequent re-initiation in a critical zone of pre-shocked gas through the onset of an explosion bubble. A substantial amount of transverse instability remains even after the expansion wave reaches the central axis, sustaining the diffracted wave at a critical thermodynamic state for the re-initiation. The location of this critical zone is identified at about 22λ and a small obstacle is used to promote the generation of transverse waves and a re-initiation kernel. The re-initiation is effected by placing an obstacle in the critical region. The role of the resulting instability is also illustrated through a simple numerical simulation using an obstacle in the sub-critical regime to perturb the flow and promote the re-initiation.
The effect of bulk gas diffusivity on apparent pulverized coal char combustion kinetics Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-06 Christopher R. Shaddix, Ethan S. Hecht, Cristina Gonzalo-Tirado, Brian S. Haynes
Apparent char kinetic rates are commonly used to predict pulverized coal char burning rates. These kinetic rates quantify the char burning rate based on the temperature of the particle and the oxygen concentration at the external particle surface, inherently neglecting the impact of variations in the internal diffusion rate and penetration of oxygen. To investigate the impact of bulk gas diffusivity on these phenomena during Zone II burning conditions, experimental measurements were performed of char particle combustion temperature and burnout for a subbituminous coal burning in an optical entrained flow reactor with helium and nitrogen diluents. The combination of much higher thermal conductivity and mass diffusivity in the helium environments resulted in cooler char combustion temperatures than in equivalent N2 environments. Measured char burnout was similar in the two environments for a given bulk oxygen concentration but was approximately 60% higher in helium environments for a given char combustion temperature. To augment the experimental measurements, detailed particle simulations of the experimental conditions were conducted with the SKIPPY code. These simulations also showed a 60% higher burning rate in the helium environments for a given char particle combustion temperature. To differentiate the effect of enhanced diffusion through the external boundary layer from the effect of enhanced diffusion through the particle, additional SKIPPY simulations were conducted under selected conditions in N2 and He environments for which the temperature and concentrations of reactants (oxygen and steam) were identical on the external char surface. Under these conditions, which yield matching apparent char burning rates, the computed char burning rate for He was 50% larger, demonstrating the potential for significant errors with the apparent kinetics approach. However, for specific application to oxy-fuel combustion in CO2 environments, these results suggest the error to be as low as 3% when applying apparent char burning rates from nitrogen environments.
Differential diffusion effects in counter-flow premixed hydrogen-enriched methane and propane flames Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-06 Ehsan Abbasi-Atibeh, Jeffrey M. Bergthorson
The effects of differential diffusion and stretch sensitivity on propagation and stabilization of lean premixed hydrogen-enriched methane-air and propane-air flames are studied in a turbulent counter-flow apparatus. In these experiments, the unstretched laminar flame speed is kept constant through decreasing the mixture equivalence ratio, in order to minimize the effects of chemistry and highlight the effects of differential diffusion during hydrogen-enrichment. Bulk flow properties are also kept constant between laminar and turbulent flames. High-speed particle image velocimetry (PIV) is applied to quantify the flow velocity field using oil droplet seeding, enabling simultaneous flame position and velocity measurements. Data processing tools are developed through this study to quantify instantaneous local measurements of flame position, flame curvature, and apparent turbulent flame velocity within the imaged plane. Probability density functions (PDF) of instantaneous flame position show that, in hydrogen-enriched methane-air flames (effective Lewis number < 1), differential diffusion increases the turbulent burning rates throughout the whole hydrogen-enrichment range. However, in hydrogen-enriched propane-air flames, these effects are only observed at hydrogen content above 60% (by volume), where effective Lewis number falls below unity. PDFs of flame position also illustrated that the effects of differential diffusion become significant when the effective Lewis number < 0.8. In contrast, PDFs of turbulent flame velocities only showed a slight increase in local instantaneous velocities with increasing hydrogen content. Furthermore, it was illustrated that differential diffusion affects the flame front topology by increasing instantaneous flamelet curvature at below unity Lewis numbers, consistent with flame stability theory.
Cellular instability in Le < 1 turbulent expanding flames Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-06 Zirui Liu, Sheng Yang, Chung K. Law, Abhishek Saha
Flow turbulence and intrinsic flamefront cellular instability can each wrinkle a flamefront and thereby increase its surface area and the corresponding global flame speed, with the characteristics of wrinkling depending on the separate and coupled spectra of their respective length scales. Extending our previous study on the interaction between turbulence and the hydrodynamic, Darrieus–Landau cellular instability, and using the same expanding, globally spherical flame as the vehicle of investigation, we report herein experimental results on the interaction between turbulence and diffusional-thermal cellular instability relevant for mixtures with sub-unity Lewis numbers. Results show that the flame acceleration is primarily controlled by cellular instability in the wrinkled flamelet regime and by turbulence in the thickened flamelet regime, respectively, while both mechanisms influence the propagation in the corrugated flamelet regime. It is also noted that while the Lewis number and thus diffusional-thermal instability does not affect the global flame acceleration, it does enhance the total burning rate.
Soot light absorption and refractive index during agglomeration and surface growth Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-06 Georgios A. Kelesidis, Sotiris E. Pratsinis
Optical characterization of soot (e.g., Laser Induced Incandescence, LII) and its impact on climate largely depend on light absorption. As soot grows, its morphology changes affecting its optical or radiative properties. Here, the impact of soot maturity on its light absorption is investigated by coupling Discrete Element Modeling (DEM) with Discrete Dipole Approximation (DDA) during soot surface growth and agglomeration. The Mass Absorption Cross-section, MAC, of nascent and mature soot agglomerates is estimated by DDA and validated against atomistic point dipole interactions and mesoscale DDA calculations. Using a refractive index, RI, for mature soot yields constant average <MAC> and absorption function <E> overestimating the nascent soot light absorption up to 75%. The RI is interpolated between those of nascent and mature soot for wavelengths, λ = 532 and 1064 nm to account for quantum confinement and evolving number of clustered sp2-bonded rings that affect the optical band gap, Eg. This results in excellent agreement of the DEM-derived evolutions of <MAC> , <E> and ratio R = <E(λ=532nm)><E(λ=1064nm)> with the corresponding LII measurements in methane and ethylene premixed flames. The nascent soot Eg decreases during aging and agglomeration, increasing <MAC> and <E> by 65%. The R decreases from 1.34 to 0.95 by aging and coagulation and slowly converges to the asymptotic 0.89 of mature soot measured in diffusion flames. The good agreement between DEM and LII data confirms that soot dynamics by surface growth and agglomeration strongly correlate with soot maturity, composition and RI that are essential for quantifying soot light absorption and scattering.
Validation of PMDI-based polyurethane foam model for fire safety applications Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-06 Sarah N. Scott, Ryan M. Keedy, Victor E. Brunini, Matthew W. Kury, Amanda B. Dodd, James L. Urban, A. Carlos Fernandez-Pello
Under normal operating conditions, polymer foams protect components from mechanical, electrical, and thermal shocks. However, if the protecting polymer foam is exposed to a heat source such as a fire or an over-heating component, the foam will pyrolyze, changing heat paths by creating voids or dripping onto components. Material properties also change as the virgin material becomes char, gas, or liquid. In a sealed system, the gases created by the pyrolysates can also pressurize the system, leading to breach. Understanding the chemistry, heat transfer, and fluid flow of these materials in a fire is vital for safety assessments. To investigate such a scenario, a 2D finite element model with heat transfer, porous media flow, and a pyrolysis chemistry model was created. The gas velocity is solved using Darcy’s approximation, and the heat transfer and pressurization are determined by solving the continuity, species, and enthalpy equations in both the condensed and gas phases. A vapor–liquid equilibrium (VLE) model is used to determine the phase of the pyrolysates. The model was validated using experimental data that showed that the rate of pressurization and the local temperatures are dependent on orientation with respect to gravity. In addition, at the high temperatures and pressures seen in these experiments, it is expected that the organic pyrolysates will exist in both the liquid and gaseous phases. The model reproduces the orientation dependence of the temperature and pressure, as well as the condensation and evaporation of organic pyrolysates. Model uncertainty is analyzed using a Latin Hypercube approach, and sensitivities are ranked using the Pearson correlation. The inverted orientation shows a larger model uncertainty due the buoyant flow. The model was generally sensitive to the density of the steel, the density of the foam, and the pyrolysis reactions.
Assessment of spray combustion models in large-eddy simulations of a polydispersed acetone spray flame Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-05 Qing Wang, Thomas Jaravel, Matthias Ihme
Spray combustion is of practical importance to various applications. To study spray flames, benchmark cases were investigated both experimentally and numerically. Previous numerical studies of these flames have identified sensitivities to gas-phase combustion models and the description of the droplet evaporation. The objective of this work is to examine effects of combustion models on the structure of an acetone spray flame. To this end, three different combustion models are examined, namely a finite-rate chemistry model, a flamelet/progress variable model, and a flame prolongation of intrinsic low dimensional manifold model. In the two flamelet approaches, effects of spray evaporation are considered in the limit of small Stokes number. By examining radial profiles and employing a doubly-conditioned analysis on gaseous mixture fraction and liquid-to-gas mass ratio, it is shown that both flamelet models show differences in the evaporation and subsequent gas phase combustion and temperature field. The use of a finite-rate combustion model in conjunction with a reduced chemical mechanism provide improved predictions of heat release and spray-flame structure.
A simplified CFD model for spectral radiative heat transfer in high-pressure hydrocarbon–air combustion systems Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-05 C. Paul, D.C. Haworth, M.F. Modest
Detailed radiation modeling in piston engines has received relatively little attention to date. Recently, it is being revisited in light of current trends towards higher operating pressures and higher levels of exhaust-gas recirculation (EGR), both of which enhance molecular gas radiation. Advanced high-efficiency engines also are expected to function closer to the limits of stable operation, where even small perturbations to the energy balance can have a large influence on system behavior. Detailed radiation modeling using sophisticated tools like photon Monte Carlo/line-by-line (PMC/LBL) is computationally expensive. Here, guided by results from PMC/LBL, a simplified stepwise-gray spectral model in combination with a first-order spherical harmonics (P1 method) radiative transfer equation (RTE) solver is proposed and tested for engine-relevant conditions. Radiative emission, reabsorption and radiation reaching the walls are computed for a heavy-duty compression-ignition engine at part-load and full-load operating conditions with different levels of EGR and soot. The results are compared with those from PMC/LBL, P1/FSK (P1 with a full-spectrum k-distribution spectral model) and P1/Gray radiation models to assess the proposed model’s accuracy and computational cost. The results show that the proposed P1/StepwiseGray model can calculate reabsorption locally and globally with less than 10% error (with respect to PMC/LBL) at a small fraction of the computational cost of PMC/LBL (a factor of 30) and P1/FSK (a factor of 15). In contrast, error in computed reabsorption by the P1/Gray model is as high as 60%. It is expected that the simplified model should be broadly applicable to high-pressure hydrocarbon–air combustion systems, with or without soot.
An experimental and computational study of hydrogen–air combustion in the LAPCAT II supersonic combustor Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-05 A. Vincent-Randonnier, V. Sabelnikov, A. Ristori, N. Zettervall, C. Fureby
Dual-mode ramjet/scramjet engines are considered a promising propulsion system for the next generation commercial high-speed transport flight vehicles. In this study we combine experimental measurements of high-speed (subsonic and supersonic) combustion at different operating conditions in the LAPCAT-II dual-mode ramjet/scramjet combustor with Large Eddy Simulations (LES) using finite rate chemistry models and skeletal H2–air combustion chemistry. The combustor geometrically consists of four sections, and experiments have been realized for wall injection of H2 in a Mach = 2 vitiated air-flow for total pressures and temperatures of p0 = 0.40 MPa, 1414 K < T0 < 1707 K, and a fixed equivalence ratio of ϕ=0.15. For this p0 the combustor is over-expanded, and the transition from supersonic to subsonic flow occurs at the beginning of the fourth combustor section. The flow and combustion diagnostics include measurements of p0 and T0 upstream of the combustor, wall-pressure profiles and Schlieren as well as OH* chemiluminescence imaging. The computational set-up consists of the full combustor, from the nozzle into the dump-tank. The computational model is composed of a compressible finite rate chemistry LES model, using the mixed subgrid flow model and the Partially Stirred Reactor (PaSR) combustion model, together with a novel skeletal 22 step H2–air reaction mechanism. Qualitative as well as quantitative comparisons between experiments and simulations show reasonably good agreement, but most importantly reveal a high sensitivity of both the LES predictions and the experiments to T0. The LES results are further used to describe the underlying mechanisms of flow, wall-injection, mixing, self-ignition and turbulent combustion, and how these interrelated processes are modified by increasing the total temperature under otherwise identical conditions.
Front shape similarity measure for data-driven simulations of wildland fire spread based on state estimation: Application to the RxCADRE field-scale experiment Proc. Combust. Inst. (IF 5.336) Pub Date : 2018-09-05 C. Zhang, A. Collin, P. Moireau, A. Trouvé, M.C. Rochoux
Data-driven wildfire spread modeling is emerging as a cornerstone for forecasting real-time fire behavior using thermal-infrared imaging data. One key challenge in data assimilation lies in the design of an adequate measure to represent the discrepancies between observed and simulated firelines (or “fronts”). A first approach consists in adopting a Lagrangian description of the flame front and in computing a Euclidean distance between simulated and observed fronts by pairing each observed marker with its closest neighbor along the simulated front. However, this front marker registration approach is difficult to generalize to complex front topology that can occur when fire propagation conditions are highly heterogeneous due to topography, biomass fuel and micrometeorology. To overcome this issue, we investigate in this paper an object-oriented approach derived from the Chan–Vese contour fitting functional used in image processing. The burning area is treated as a moving object that can undergo shape deformations and topological changes. We combine this non-Euclidean measure with a state estimation approach (a Luenberger observer) to perform simulations of the time-evolving fire front location driven by discrete observations of the fireline. We apply this object-oriented data assimilation method to the three-hectare RxCADRE S5 field-scale experiment. We demonstrate that this method provides more accurate forecast of fireline propagation than if either the fire spread model or the observations were taken separately. Results show that when the observation frequency becomes lower than 1/60 s−1, the forecast performance of data assimilation is improved compared to simply extrapolating observation data. This highlights the need of a physics-based forward model to forecast flame front propagation. We also demonstrate that the front shape similarity measure is suitable for both Eulerian and Lagrangian-type front-tracking solvers and thereby can provide a unified framework to track moving structures such as flame front position and topology in combustion problems.
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