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Mixtures of few representative hydrocarbon species are often used in computational studies as surrogates of market petroleum fuels, which contain hundreds of components. While this simplification is imperative for computational costs of reaction kinetics, it introduces unavoidably uncertainties in simulations. Differences between the real and the surrogate fuels regarding mixture formation, oxidation chemistry, and fuel composition contribute to such uncertainties. An evaluation of the underlying concept of a surrogate fuel model in terms of engine performance is thus of high interest. This paper presents an experimental study with a spark-ignition (SI) single-cylinder engine (SCE) to compare the combustion and emission characteristics between a market gasoline fuel and its corresponding four-component surrogate (iso-octane, n-heptane, toluene, ethanol). The measurements cover a wide range of operating points in terms of engine load, speed, air-to-fuel ratio, and operating conditions. Together with standard performance and emission measurements, a non-standard hydrocarbons (HC) analysis has been performed with a fast flame ionization detector and an ion-molecule-reaction mass spectrometer. The comparison reveals very good agreement between the market gasoline and the surrogate fuel regarding combustion and global gaseous emission behaviors, with an average deviation for almost all of the analyzed quantities below 2%. The comparison of CO emissions in stoichiometric operation presents a higher scatter, due to the high sensitivity of the CO emissions on mixture formation and fuel volatility. The different compositions of the two fuels also lead to deviations of speciated-HC emissions, which is confirmed by mass spectrometry. Additionally, the sooting tendency of the surrogate fuel is found to be more than 10 times lower compared to the market gasoline fuel.

This paper presents a study of the oxidation of iso-octane, ethanol, toluene and their mixtures in a pressurised turbulent flow reactor operating at 900–930 K, 10 bar, and an equivalence ratio of 0.058. A large set of fuels is investigated, including neat iso-octane, ethanol, toluene, their binary mixtures, gasoline reference fuels (PRF91 and TRF91), and their mixtures with ethanol. The resulting species are measured along the length of the reactor and simulated using existing kinetic models from the literature. The existing models are found to reproduce measurements of the major oxidation products of iso-octane, ethanol, their binary mixtures, as well as that of PRF91 and PRF91/ethanol mixtures. However, significant differences are observed between the measurement and simulation of neat toluene. Adjustment is then made to the rate constants of key reactions in the toluene model. The adjusted model, whilst more accurately reproducing neat toluene oxidation, does not significantly improve the modelling of toluene containing mixtures. This suggests that further investigations should focus on the oxidation of neat toluene, as well as the chemical interactions of toluene containing mixtures.

A turbulent flame in an ethanol droplet-laden uniform mixture is investigated at overall equivalence ratios ($\textbf{ϕ}$$_{ov}$) of 0.62, 0.72 and 0.82, using a piloted Bunsen burner. Imaging of OH* chemiluminescence and simultaneous imaging of OH PLIF and Mie scattering, both at 5 kHz, and imaging of CH$^{2}$O-fuel PLIF at 5 Hz, were used to obtain instantaneous and time-averaged images, temporal sequences and 2-D estimates of flame surface density and curvature. 1-D PDA and LDA measurements were used to obtain droplet size and velocity statistics. At $\textbf{ϕ}$$_{ov}$ = 0.62, the flame takes a cylindrical shape, and changes to a cone shape with increasing fuel loading to obtain higher $\textbf{ϕ}$$_{ov}$. Larger droplets are generally observed to have lower average and RMS axial velocities than smaller droplets. Profiles of droplet size distributions indicate a decreasing droplet number density downstream together with a shift to larger droplet diameters. The flame structure is observed to be relatively smooth at locations near the burner exit, and becomes more contorted with distance downstream. In general, droplets are observed to coincide with low-to-intermediate regions of OH. Occasionally, droplets appear to penetrate the flame front, and are detected in regions of intermediate-to-high OH. This occurs particularly at the downstream locations where the flame closes across the jet, with no significant averaged droplet penetration observed past 2 mm in the direction normal to the flame front. Measurements show a gradual reduction in flame surface density and higher flame front curvature with both distance downstream and increasing fuel loading. Estimates of the average droplet evaporation rate increase with both distance downstream and $\textbf{ϕ}$$_{ov}$, as droplets appear in higher mean progress variable regions. The measurements reported here are useful for model validation of flame propagation in dilute sprays.

The octane sensitivity (S), defined as the difference between the Research Octane Number (RON) and the Motor Octane Number (MON), is of increasing interest in spark ignition (SI) engines because of its relevance to knock resistance at boosted high-load conditions. In this study, a set of three fuels was designed to hold RON nearly constant (RON = 99.2–100) and to vary S (S = 0, 6.5, and 12). These fuels were operated at the knock-limited spark advance (KLSA) at nominal engine loads of 10, 15, and 20 bar indicated mean effective pressure (IMEP) in a single cylinder SI engine with side-mounted direct injection fueling, at λ = 1 stoichiometry. At each load condition, the intake manifold temperature was swept from 35 °C to 95 °C to alter the temperature and pressure history of the charge. At the 10 bar IMEP condition, knock resistance was inversely proportional to S, with the S = 0 fuel being the most knock resistant. As load increased the trend reversed and knock resistance became proportional to fuel S, with the S = 12 fuel being the most knock resistant. The fuel S knock resistance reversal with load is attributed to changing fuel ignition delay. At increased load, intermediate temperature heat release (ITHR) for the S = 0 fuel was observed several crank angles prior to spark command, and ITHR magnitude was proportional to intake temperature. As intake temperature continued to increase, the S = 0 fuel transitioned from ITHR to low-temperature heat release (LTHR) prior to spark command. At the highest load and intake temperature, 20 bar IMEP and 95 °C, the S = 0 fuel exhibited distinct LTHR and negative temperature coefficient (NTC), and the intermediate S value fuel (S = 6.5) exhibited distinct ITHR behavior several crank angles prior to spark command. However, for all tested conditions, the S = 12 fuel exhibited neither ITHR nor LTHR. To understand the measured trends, chemical kinetic modeling was used to elucidate the fuel-specific dependencies on in-cylinder pressure–temperature history. An island of low temperature reactivity was identified at temperatures between 700 K and 825 K and at pressures higher than 17 bar. The size and magnitude of this island was found to be fuel-specific, decreasing with increasing S. The combined findings illustrate the commonality and utility of fuel S, ITHR, LTHR, and NTC across a wide range of conditions and the associated implications of fuel S in boosted modern gasoline direct injection SI engines relative to the RON and MON tests.

The combustion of shale, a porous sedimentary rock, has been reported at times in outcrop deposits and mining piles. However, the initiating event of most of these fires is unknown. It could be that, under the right conditions, shale rock undergoes spontaneous exothermic reactions in the presence of oxygen. This work studies experimentally and for the first time the self-heating behavior of shale rock. Because shale has high inert content, novel diagnostics such as mass loss measurements and visual observation of charring are introduced to detect self-heating ignition in respect to other self-heating materials with lower inter content. Using field samples collected from the outcrop at Kimmeridge Bay (UK) and the Frank-Kamenetskii theory of ignition, we determine the effective kinetic parameters for two particle-size distributions of shale. These parameters are then used to upscale the results to geological deposits and mining piles of different thicknesses. We show that for fine particles, with diameter below 2 mm, spontaneous ignition is possible for deposits of thickness between 10.7 m and 607 m at ambient temperatures between −20 ᵒC and 44 ᵒC. For the same ambient temperature range, the critical thickness is in excess of 30 km for deposits made of coarse particles with diameter below 17 mm. Our results indicate that shale rock is reactive, with reactivity highly dependent on particle diameter, and that self-ignition is possible for small particles in outcrops, piles or geological deposits accidentally exposed to oxygen.

The integrated processes of flame acceleration, deflagration-to-detonation transition (DDT), and the resulting detonation propagation in micro- and macro-scale channels are simulated. It is found that the modes of flame acceleration and DDT in these two channels are different, being primarily controlled by viscosity and turbulent flame development, respectively. Furthermore, while boundary layer ignition is crucial for DDT in both channels, viscous wall friction is the key to self-sustained propagation in micro-channels, leading to momentum loss and consequently deficit of the detonation velocity. In macro-channels, the strong overdriven detonation decays and gradually evolves into the Chapman–Jouguet detonation.

Fossil fuel combustion should be decreased in future years in order to lower the CO2 emissions of energy production. The reduction can be achieved by increasing the amount of CO2-neutral fuels in energy production. Here 6-13% of coal was substituted with industrial or roasted pellets in a pulverized fuel-fired power plant without making any changes to fuel grinding or low-NOx burners. The effect of pellet addition for the flue gas particles was studied with direct sampling from the boiler super heater area. Based on primary dilution ratio tests, transmission electron microscope images, and the natural electric charge of the particles, it was observed that particles in the flue gas are spherical and have been formed in the boiler at high temperatures. The pellet addition lowered the total particle number concentrations with all of the studied pellet-coal mixtures in comparison to the coal combustion. The 10.5% industrial pellet addition caused a second mode in the particle number size distribution. In addition, based on the chemical analysis of the collected size-fractioned particle samples, results indicated that the pellet addition did not increase the corrosion risk of the boiler. However, the changes in the particle number size distribution and total particle number concentration can affect the operation of electrostatic precipitators and flue gas cleaning.

This study reports the autoignition delay times of methyl butanoate in argon–air (i.e. Ar/O2 = 3.76 by mole) mixtures under thermodynamic conditions relevant to compression ignition engines, using a rapid compression machine (RCM). The ignition delay times were obtained for the compressed temperature range of 833–1112 K and compressed pressures corresponding to 15, 30, 45, and 75 bar. In addition, the effect of fuel mole fraction on ignition delay times were experimentally studied by covering equivalence ratios corresponding to 0.25, 0.5, and 1.0 under realistic fuel loadings without additional dilution. For the range of conditions investigated, the ignition delay times exhibit an Arrhenius dependence on temperature and an inverse relationship with the compressed pressure. No evidence of two-stage ignition was observed in the current experiments, nor was a negative temperature coefficient trend seen in the ignition delay times. The experimental results were compared to zero-dimensional simulations taking into account the full compression stroke and the heat loss characteristics of the RCM, using chemical kinetic models reported in the literature. The literature models were found to predict significantly higher reactivity when compared to the current experiments. Chemical kinetic analysis was then conducted to identify the reactions responsible for the mismatch between the experiments and the simulated results. Key reactions that could help obtain a better match between the experimental and simulated results were identified using both brute-force and global sensitivity analyses. In view of the large uncertainties associated with the low-temperature chemistry of methyl butanoate, further studies are needed to update the kinetic parameters of the key reactions in order to improve the model comprehensiveness.

Using the planar laser-induced fluorescence (PLIF) technique in a counterflow non-premixed flame configuration, the formation of the polycyclic aromatic hydrocarbons (PAHs) for the butane isomers and the butanol isomers was investigated. For these C4 fuels, the PAHs of various aromatic ring size groups (2, 3, 4, and larger aromatic rings) have been characterized and compared in non-premixed combustion configuration. In particular, the formation and growth of the PAHs of different aromatic ring sizes in these counterflow flames was examined by tracking the PAH-PLIF signals at various detection wavelengths. The fuel structure effects on the PAH formation and growth processes were also analyzed by comparing the PAH growth pathways for these C4 fuels. Furthermore, PAH-PLIF experiments were conducted, by blending each of the branched-chain isomers with the baseline straight-chain isomer, in order to study the synergistic effects, as well as identify the relative importance of the H-abstraction-C2H2-addition (HACA) mechanism and the propargyl (C3H3) recombination/addition pathways in the PAH formation and growth processes. A chemical kinetic model available in the literature that includes both the fuel oxidation and the PAH chemistry was also used to simulate and compare the PAH species up to A4 rings. At the incipient stage of the PAH formation, the simulated results exhibited similar behavior to the experimental observations. The PAH formation pathways considered in the chemical kinetic model were further analyzed. In addition to propargyl, the present results demonstrated the important role of acetylene in the PAH formation and growth processes.

This work explores the mechanisms by which a post injection can reduce unburned hydrocarbon (UHC) emissions in heavy-duty diesel engines operating at low-temperature combustion conditions. Post injections, small, close-coupled injections of fuel after the main injection, have been shown to reduce UHC in the authors’ previous work. In this work, we analyze optical data from laser-induced fluorescence of both CH2O and OH and use chemical reactor modeling to better understand the mechanism by which post injections reduce UHC emissions. The results indicate that post-injection efficacy, or the extent to which a post injection reduces UHC emissions, is a strong function of the cylinder pressure variation during the post injection. However, the data and analysis indicate that the pressure and temperature rise from the post injection combustion cannot solely explain the UHC reduction measured by both engine-out and optical diagnostics. The fluid-mechanic, thermal, and chemical interaction of the post injection with the main-injection mixture is a key part of UHC reduction; the starting action of the post jet and the subsequent entrainment of surrounding gases are likely both important processes in reducing UHC with a post injection.

A three-dimensional direct numerical simulation (DNS) of a turbulent lifted dimethyl ether (DME) slot jet flame was performed at elevated pressure to study interactions between chemical reactions with low-temperature heat release (LTHR), negative temperature coefficient (NTC) reactions and shear generated turbulence in a jet in a heated coflow. By conditioning on mixture fraction, local reaction zones and local heat release rate, the turbulent flame is revealed to exhibit a “pentabrachial” structure that was observed for a laminar DME lifted flame [Krisman et al., (2015)]. The propagation characteristics of the stabilization and triple points are also investigated. Potential stabilization points, spatial locations characterized by preferred temperature and mixture fraction conditions, exhibit autoignition characteristics with large reaction rate and negligible molecular diffusion. The actual stabilization point which coincides with the most upstream samples from the pool of potential stabilization points fovr each spanwise location shows passive flame structure with large diffusion. The propagation speed along the stoichiometric surface near the triple point is compared with the asymptotic value obtained from theory [Ruetsch et al., (1995)]. At stoichiometric conditions, the asymptotic and averaged DNS values of flame displacement speed deviate by a factor of 1.7. However, accounting for the effect of low-temperature species on the local flame speed increase, these two values become comparable. This suggests that the two-stage ignition influences the triple point propagation speed through enhancement of the laminar flame speed in a configuration where abundant low-temperature products from the first stage, low-temperature ignition are transported to the lifted flame by the high-velocity jet.

This paper presents a statistical analysis of edge flames in a turbulent lifted flame using direct numerical simulation (DNS). To investigate the dynamics of edge flames, a theoretical framework describing the edge-flame propagation velocity as a function of propagation velocities of mixture-fraction and product-mass fraction iso-surfaces at the flame base is used. The correlations between these propagation velocities and several other variables are then studied, including iso-surface curvatures, iso-surface orientations, strain rates, scalar dissipation rate and gradients of product mass fraction. The contribution of these parameters to the overall behaviour of the edge flame is also investigated using conditional averaging on two-dimensional spatial locations at the flame base. The analysis reveals that the tangential and normal strain rates in addition to the curvatures and scalar dissipation rates have significant contributions to the overall behaviour of the edge flame. The elliptical motion of the flame base described in our earlier study [1] is extended to provide a clearer picture of how these various parameters affect the large fluctuations of edge-flame velocity observed at the flame base.

The behavior of transient, compressible and combusting pre-mixed methane–air jets was experimentally studied with high speed imaging in a rapid compression machine. The jets were generated with a turbulent jet ignition system, which is a prechamber initiated combustion system. The absence of physical analyses of the characteristics of premixed turbulent jets was the motivation for the present study. Experiments were completed for turbulent jet ignition system orifice diameters of 2.0, 2.5 and 3.0 mm each at lean-to-stoichiometric equivalence ratios of ϕ = 0.67, 0.8 and 1.0. The hot jet velocity at the orifice exit was calculated using mathematical correlations. The Mach number and Reynolds number were also computed. The high speed imaging shows the influence of orifice diameter on the flame propagation and the shape and structure of vortices resulting from the turbulent jet. Results revealed a direct relationship between orifice exit area reduction and a decrease in hot jet penetration speed. There was a reduction in hot jet penetration speed with an increase in the equivalence ratio. For the orifice diameters and equivalence ratios tested here, results showed that the jet evolved downstream of the orifice exit in partial agreement with existing correlations. Moreover, the jet was turbulent with calculated Reynolds numbers of around 20,000 or greater.

Implementation of spray-guided stratified-charge direct-injection spark-ignited (DISI) engines is inhibited by the occurrence of misfire and partial burns. Engine-performance tests demonstrate that increasing engine speed induces combustion instability, but this deterioration can be prevented by generating swirling flow during the intake stroke. In-cylinder pressure-based heat-release analysis reveals that the appearance of poor-burn cycles is not solely dependent on the variability of early flame-kernel growth. Cycles can experience burning-rate regression during later combustion stages and may or may not recover before the end of the cycle. Thermodynamic analysis and optical diagnostics are used here to clarify why swirl improves the combustion repeatability from cycle to cycle. The fluid dynamics of swirl/spray interaction was previously demonstrated using high-speed PIV measurements of in-cylinder motored flow. It was found that the sprays of the multi-hole injector redistribute the intake-generated swirl flow momentum, thereby creating a better-centered higher angular-momentum vortex with reduced variability. The engine operation with high swirl was found to have significant improvement in cycle-to-cycle variations of both flow pattern and flow momentum. This paper is an extension of the previous work. Here, PIV measurements and flame imaging are applied to fired operation for studying how the swirl flow affects variability of ignition and subsequent combustion phases. PIV results for fired operation are consistent with the measurements made of motored flow. They demonstrate that the spark-plasma motion is highly correlated with the direction of the gas flow in the vicinity of the spark-plug gap. Without swirl, the plasma is randomly stretched towards either side of the spark plug, causing variability in the ignition of the two spray plumes that are straddling the spark plug. In contrast, swirl flow always convects the spark plasma towards one spray plume, causing a more repeatable ignition. The swirl decreases local RMS velocity, consistent with an observed reduction of early-burn variability. Broadband flame imaging demonstrates that with swirl, the flame consistently propagates in multiple directions to consume fuel–air mixtures within the piston bowl. In contrast, operation without swirl displays higher variability of flame-spread patterns, occasionally causing the appearance of partial-burn cycles.

With a view to understanding ignition and combustion behaviours in diesel engines, this study investigates several aspects of ignition and combustion of an n-dodecane spray in a high pressure, high temperature chamber, known as Spray A, using data resulting from modelling using the transported probability density function (TPDF) method. The model has been validated comprehensively with good to excellent agreement in our previous work against all available experimental data including for mixture-fraction and velocity fields in non-reacting cases, and flame lift-off length and ignition delay in reacting cases. This good agreement encourages further investigation of the numerical model results to help understand the structure of this flame, which serves to complement the experimental information that is available, which is very limited due to the difficult experimental conditions in which this flame exists. For example, quantitative experimental measurements of local mixture-fraction, temperature, velocity gradients, etc. are not yet possible in reacting cases. Analysis of the model results shows that two-stage ignition is found to occur across the ambient temperature conditions considered: the first stage is rapidly initiated on the lean side where temperatures are high and sequentially moves to richer, cooler conditions. The first stage is extremely resilient to turbulence, occurring in a region of very low Damkohler number. The second stage of ignition occurs first in rich mixtures in a region behind the head of the fuel jet where mixture gradients are low, and appears to be influenced strongly by turbulence. Relative to a homogeneous reactor, it is delayed on the lean side but advanced on the rich side, suggesting entrainment and mixing from the early igniting lean regions into richer mixtures is an important moderator of the ignition process. The second-stage ignition front propagates at very high velocities initially, suggesting it is a sequential ignition moving according to gradients of ignition delay and/or residence time. The flame stabilises however on the lean side in a region of much lower velocity, where turbulent velocity fluctuations are sufficiently high such that turbulent transport influences the propagation. It stabilises in a region of low Damkohler number which implies that a competition of chemistry versus micro-mixing might also be involved in stabilisation. The stabilisation mechanism is investigated by an analysis of the transport budgets, showing the flame is stabilised by autoignition but moderated by turbulent diffusion. Further analysis of the flame index supports this stabilisation mechanism, and demonstrates the simultaneous existence of non-premixed and premixed combustion modes in the same flame. Analysis of the flow fields also reveals that local entrainment and dilatation are important flow features near the flame base.

Boundary layer flashback of swirling turbulent lean-premixed methane–hydrogen–air flames is investigated in a model combustor featuring a mixing tube with center body. The focus of our work is on improving the understanding of the flow–flame interaction during flashback. We combine high-speed chemiluminescence imaging, stereoscopic and tomographic particle image velocimetry, and a three-dimensional flame front reconstruction technique to reveal the time-resolved, volumetric velocity field in the vicinity of the flame front during flashback. We find two different ways in which a flame front propagates upstream along the center body wall. The first mode concerns small-scale bulges counter-propagating into the approach flow, similar to channel-flow flashback, but is found not to be a dominant propagation mechanism. Instead, flashback occurs primarily in the form of large-scale flame tongues swirling in the bulk flow direction as they propagate upstream. The approach flow is modified significantly in both cases, but the scale and nature of the resulting velocity fields differ fundamentally. A key characteristic of the approach flow found previously, both in channel and swirl flame flashback, is regions of negative axial velocity upstream of the flame front. We reveal, however, that in the case of swirl flames the region of negative axial velocity is the result of a primarily swirling motion ahead of the leading flame tongue in contrast to the reverse flow pockets ahead of small-scale bulges. The boundary layer neither separates nor does fluid recirculate in the negative axial velocity region upstream of the flame tongues. Instead, flame tongues impose a local blockage effect causing significant deflection of the approach flow, which results in a constant region of negative axial velocity for the leading side of the flame tongue to propagate into.

Two families of PCA based combustion model were recently developed: score-PCA and MG-PCA. This paper presents the first comparative study of the two aftermentioned methods. Both methods are first benchmarked on 1-D laminar flames of hydrogen-air and syngas-air, to verify the consistency of the approaches. The sensitivity to differential diffusion is carefully addressed and a rotation technique is proposed, to allow for using score-PCA with differential diffusion. Then, 2-D flame-turbulence interactions are considered, spanning a wide range of velocity and length scale ratios, to demonstrate the ability of PC-based combustion models to efficiently capture the behavior of different combustion regimes, from flamelet-like flames to distributed reaction regimes., SCOPUS: ar.j, info:eu-repo/semantics/published

Gas-producing energetic materials that can be readily ignited with a photoflash are typically opaque sensitive primary explosives. In this study, we explore the photoactivity of select high-nitrogen (HiN) compounds that are much less sensitive than primary explosives. These HiN materials produce large amounts of gas upon decomposition and this makes them suitable for use in actuators, igniters, or micro-thrusters. This paper presents ignition experimental results using similar shaped pulses at two different wavelengths, specifically using a xenon photoflash and a single wavelength CO 2 laser. Several select HiN materials were tested for flash ignitability, and those that were found to be flash ignitable were further ignited with CO 2 laser heating. By comparing ignition behavior at various laser and flash intensities, some ignition mechanisms are suggested. Thermal heating, regardless of source, appears to be the dominant mechanism responsible for ignition and photochemical effects appear to be negligible in the ignition of the materials considered in this study. Higher laser and photoflash irradiance is shown to require less energy, and is therefore more efficient. The opacity of the material is an important consideration in ignitability, but not a sufficient criterion. We find that opaque materials that successfully propagate well in small capillary tubes are also more likely to successfully flash ignite. We suggest this is due to the higher burning rate of these materials and also in part due to the exothermic reaction occurring at or near the burning surface, allowing the reactions to proceed without quenching.

Isobutene is an important intermediate in the pyrolysis and oxidation of higher-order branched alkanes, and it is also a component of commercial gasolines. To better understand its combustion characteristics, a series of ignition delay time (IDT) and laminar flame speed (LFS) measurements have been performed. In addition, flow reactor speciation data recorded for the pyrolysis and oxidation of isobutene is also reported. Predictions of an updated kinetic model described herein are compared with each of these data sets, as well as with existing jet-stirred reactor (JSR) species measurements.IDTs of isobutene oxidation were measured in four different shock tubes and in two rapid compression machines (RCMs) under conditions of relevance to practical combustors. The combination of shock tube and RCM data greatly expands the range of available validation data for isobutene oxidation models to pressures of 50 atm and temperatures in the range 666-1715 K. Isobutene flame speeds were measured experimentally at 1 atm and at unburned gas temperatures of 298-398 K over a wide range of equivalence ratios. For the flame speed results, there was good agreement between different facilities and the current model in the fuel-rich region. Ab initio chemical kinetics calculations were carried out to calculate rate constants for important reactions such as H-atom abstraction by hydroxyl and hydroperoxyl radicals and the decomposition of 2-methylallyl radicals.A comprehensive chemical kinetic mechanism has been developed to describe the combustion of isobutene and is validated by comparison to the presently considered experimental measurements. Important reactions, highlighted via flux and sensitivity analyses, include: (a) hydrogen atom abstraction from isobutene by hydroxyl and hydroperoxyl radicals, and molecular oxygen; (b) radical-radical recombination reactions, including 2-methylallyl radical self-recombination, the recombination of 2-methylallyl radicals with hydroperoxyl radicals; and the recombination of 2-methylallyl radicals with methyl radicals; (c) addition reactions, including hydrogen atom and hydroxyl radical addition to isobutene; and (d) 2-methylallyl radical decomposition reactions. The current mechanism accurately predicts the IDT and LFS measurements presented in this study, as well as the JSR and flow reactor speciation data already available in the literature.The differences in low-temperature chemistry between alkanes and alkenes are also highlighted. in this work. In normal alkanes, the fuel radical (R) over dot adds to molecular oxygen forming alkylperoxyl (R(O) over dot(2)) radicals followed by isomerization and chain branching reactions which promote low-temperature fuel reactivity. However, in alkenes, because of the relatively shallow well (similar to 20 kcal mol(-1)) for R(O) over dot(2) formation compared to similar to 35 kcal mol(-1) in alkanes, the (R) over dot+O-2 (sic) R(O) over dot(2) equilibrium lies more to the left favoring (R) over dot+O-2 rather than R(O) over dot(2) radical stabilization. Based on this work, and related studies of allylic systems, it is apparent that reactivity for alkene components at very low temperatures (1300 K), the reactivity is mainly governed by the competition between hydrogen abstractions by molecular oxygen and OH radicals. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved. The work at NUI Galway was supported by Saudi Aramco under the FUELCOM program. Collaboration between NUI Galway and LRGP enters in the frame the COST Action CM1404. peer-reviewed 2017-03-17

Direct numerical simulations of turbulent premixed flames at high Karlovitz numbers are performed using detailed chemistry. Different fuels, chemical mechanisms, and equivalence ratios are considered and their effects on turbulent flame speed, geometry of the reaction zone, and fuel burning rate are analyzed. Differential diffusion effects are systematically isolated by performing simulations with both non-unity and unity Lewis numbers. Heavy fuels with above unity Lewis numbers are considered. In the unity Lewis number limit, the n-heptane, iso-octane, toluene, and methane flames at a given reaction zone Karlovitz number present similar normalized turbulent flame speeds and fuel burning rates close to their respective laminar values. When differential diffusion effects are included, the turbulent flame speeds are lower than their unity Lewis number counterparts due to a reduction in the fuel burning rate. The turbulent reaction zone surface areas increase with the turbulence intensity but are not strongly affected by fuel, equivalence ratio, chemical mechanism, or differential diffusion. The geometry of the reaction zone is studied through the probability density functions of strain rate and curvature which are very similar when normalized by Kolmogorov scales at the reaction zone. The dependence of the chemical source terms on the scalar dissipation rate in the unity Lewis number case is shown and the distributions of scalar dissipation rate on the reaction surface are similar to those of passive scalars in homogeneous isotropic turbulence. The reduced burning rates in the presence of differential diffusion are discussed. The present results indicate that mean turbulent flame properties such as burning velocity and fuel consumption can be predicted with the knowledge of only a few global laminar flame properties. Once normalized by the corresponding laminar flame quantities, fuel and chemistry effects in high Karlovitz number premixed turbulent flames are mostly limited to differential diffusion.

Laser Induced Breakdown Spectroscopy (LIBS) was applied to homogenous methane–air mixtures of a wide range of compositions ranging from only air to only fuel in order to establish a new calibration scheme suitable for LIBS measurement of mixture fraction in turbulent non-premixed flames. Both a portable low resolution spectrometer equipped with a CCD detector with wide temporal detection window and a monochromator with a fast gated ICCD camera were employed to monitor the plasma emission. The results obtained using the two detection systems were fully consistent, suggesting that LIBS can be used successfully with the CCD detector that is more suitable for industrial applications. From the spectroscopic analysis, it was shown that the ratio of the intensities of Hα (656.3 nm) over O (777.3 nm) (Hα/Ο) and of C2 (Δυ=0, d3Πg –#x03B1;3Πu) over CN (Δυ=0, Β2Σ+–Χ2Σ+) (C2/CN) depend monotonically on the mole fraction of methane in the ranges of 0.0–0.3 and 0.3–1.0 respectively, therefore providing a new scheme for measurement of mixture fraction in non-premixed systems spanning the complete range of equivalence ratios. The technique was also applied to a swirling recirculating premixed flame and it was found that the equivalence ratio is successfully measured in both reactants and products. Based on the above, LIBS experiments were then carried out in turbulent axisymmetric non-reacting and reacting jets, and the mean mixture fraction determined by the aforementioned calibration curves was in good agreement with empirical correlations, while the rms measurement showed the expected trends, demonstrating hence the usefulness of the technique.

When thermite mixtures combust considerable amounts of gas can be released which impedes their application in the field of joining. Diluting the thermite mixture with excess metal can lower the reaction temperature and thereby minimize mass ejection. However, increasing the degree of dilution reduces reaction efficiency. This paper demonstrates that the reduction of reaction efficiency is significantly less for the Al/NiO thermite system if the excess metal (Ni) is added using a fuel/core-shell mixture in which Al fuel particles are mixed with Ni/NiO core-shell particles. The use of Ni/NiO core-shell particles reduces the average distance for aluminum and oxygen intermixing by placing the fuel (Al) in direct contact with the oxide (NiO), separate from the diluent (Ni). Mixtures comprised of Al and Ni/NiO core-shell powders were fabricated, characterized, and compared to compacts fabricated using conventional Al, NiO, and Ni powders for dilutions up to 40 wt% Ni. The fuel/core-shell particle geometry increased combustion front velocities for all Ni dilutions. Mass ejection decreased uniformly for both particle geometries as the degree of dilution increased.

The transient injection and subsequent auto-ignition of a methane jet issuing into a laminar coflow of hot exhaust gas from a lean premixed hydrogen air flame was studied using high-speed planar Rayleigh scattering, yielding two-dimensional measurements of mixture fraction, temperature and scalar dissipation rate with high spatio-temporal resolution. The temporal development of the mixing field between the transient fuel jet and the surrounding coflow prior to the occurrence of auto-ignition was examined at a sampling rate of 10 kHz. The impact of the transient jet development on numerical modeling of this test case is discussed. It was found that auto-ignition occurred after the jet transitioned from a transient state into the steady state, thus eliminating the need to model the complete transient fuel injection when the primary focus is on the onset of auto-ignition. Simultaneous high-speed OH * chemiluminescence from two viewing angles was applied to gain 3D-information of the ignition kernel location. This information allowed the selection and analysis of ignition events where the initial kernel formed inside the laser light sheet. Detailed analysis of the dynamics of a single ignition event, as well as statistical analysis of multiple ignition events based on a joint probability density approach, indicated that the ignition kernels occurred at very lean mixture fractions and at locations with low scalar dissipation rates.

This paper elucidates the behavior and dynamics of non-premixed flames responding to bulk fluctuations in flow velocity. It expands previous work on this problem by consistently incorporating finite Peclet number ( ) effects, and differentiating inflow boundary and dynamical effects on the flame dynamics. For analytical tractability, prior treatments of this problem generally prescribe the inflow boundary conditions into the domain. This paper shows, however, that prescribed inflow conditions, such as a step or constant local diffusive flux boundary condition, neglect axial diffusion effects in the region where their effects are most important; i.e., in the near-burner exit region where high transverse gradients and mass burning rates control the heat release dynamics. As the burning rate of non-premixed flames are controlled by mixture fraction gradients, the influence of axial diffusion substantively influences several burning rate characteristics of the flame. In addition, these effects cause the leading edge position of the flame front to oscillate, even for infinitely fast chemistry. Even in Pe ≫ 1 flames, axial diffusion introduces several fundamentally new features to the problem, resulting in exponentially decaying, dispersive flame wrinkle propagation with downstream distance. Also investigated here are asymptotic results for the heat release dynamics. It is shown that axial diffusion introduces a triple-zone asymptotic structure into the unsteady heat release characteristics, resulting in O(1) flame transfer function trends for St≪ 1 (to which an n–τ model is developed), O(1/St1/2) for intermediate Strouhal numbers, and O(1/St) for high Strouhal numbers. Finally, it is shown that the phase of the heat release response is approximately half that of a premixed flame with the same length, due to the concentration of unsteady heat release near the burner outlet, where transverse gradients are largest.

A droplet stream flame was used to measure the burning rate of ethanol droplets with the addition of graphite nanoparticles. Two particle sizes, 50 nm and 100 nm, were used for this study. Results indicate that as particle concentration is increased, the burning rate of the resulting nanofluid droplet also increases. The maximum enhancement of 62% was observed with the addition of 3 wt.% 50 nm graphite nanoparticles. To understand the burning-rate-enhancement phenomenon, a model was developed to estimate the radiation absorptivity by the hybrid droplet from the stream flame. The computational models determine the ratio of radiation retention by the entire depth of the fluid (volumetric absorptivity) using optical properties of both the particles and the fluid along with the penetration of radiation within the nanofluid using the well-known Monte Carlo algorithm that incorporates the aforementioned calculated optical properties of the nanofluid. Results indicate that radiation absorption by the hybrid droplet does play a role in the enhancement of burning rate. More importantly, the absorption is not uniform within the hybrid droplet. It is localized in the region near the droplet surface, promoting boiling at the droplet surface. This mechanism is believed to be responsible for the observed increased burning rate.

A comparative study, including experiments and molecular dynamic simulation, of the combustion waves in the Ni/Al multilayer reactive nanofoils reveals unknown mechanisms of the process. High speed macro-video recording, brightness pyrometry and thermal imaging proved that the combustion wave consists of two stages; the first stage can propagate independently with the same velocity as the complete wave. Products of the first stage of combustion are nano-grains of NiAl separated by liquid gaps of the Al-Ni melt. SEM and TEM study of the intermediate and final products of combustion allow us to suggest a new mechanism of “mosaic” dissolution-precipitation describing nano-heterogeneous reaction at the first stage of the process, which explains dynamics of the combustion wave and resultant microstructure. It was shown also that most of the heat released in the second stage of combustion is generated by grain coarsening. Thus, a conclusion is made that the combustion wave in the Ni/Al reactive multilayer nanofoil is proposed to be a sequential, two stage process involving chemical (first stage) and physical (second stage) exothermic transformations.

Autoignition study of two well-characterized reference diesel fuels with similar cetane ratings but different compositional characteristics has been conducted using a heated rapid compression machine (RCM). The two diesel blends used in the current study were a commercial grade ultra-low-sulfur diesel (ULSD) #2 reference fuel, ULSD#2, and Fuels for Advanced Combustion Engines (FACE) research diesel, FD9A. Ignition delay time measurements were conducted at compressed pressures of 10 bar, 15 bar, and 20 bar covering low-to-intermediate temperatures between 678 K and 938 K. In addition, experiments were carried out with diesel/O2/N2 mixtures at varying equivalence ratios of ϕ = 0.5, 0.69, and 1.02. Equivalence ratio was varied by changing the oxygen mole percentage while keeping the fuel mole percentage fixed at 0.514%. Therefore, the present ignition delay time measurements illustrate the effect of oxygen concentration on diesel autoignition. The experimental results showed that diesel blends with similar cetane ratings and different compositional makeups exhibited varying ignition propensities in different temperature regimes, thereby demonstrating the effect of molecular composition on autoignition characteristics. In particular, the difference in ignition propensities was observed at temperatures at which the low temperature branching reactions are active. Furthermore, the ignition delay time measurements from the current RCM study were found to complement well with the existing shock tube data in the literature, and hence this investigation provides additional experimental database of diesel blends needed for development and validation of diesel surrogate models.

This work reports the measurements of three-dimensional (3D) flame topography and curvature of a premixed turbulent Bunsen flame at a rate of 5 kHz, using a technique combining chemiluminescence and tomography. Line-of-sight images of chemiluminescence (termed projections) of the target flame were recorded by six cameras from different orientations simultaneously at 5 kHz. Based on these projections, a tomography algorithm reconstructed the 3D flame structure, based on which 3D curvature was then calculated. Due to the 3D nature of the data, statistics of flame properties can then be extracted both in temporal and spatial domains. Probability density function (PDF) of flame topography was extracted from a series of 3D measurements, and the PDFs of the flame at different spatial locations were examined. Furthermore, the instantaneously measured 3D flame topography also enabled the calculation of 3D flame curvature (or 2D curvature along an arbitrary orientation). The PDFs of curvature in 2D and 3D were then extracted and compared. These results provide quantification of the flame surface shape in 3D (cylindrical, elliptic, or hyperbolic), illustrating the utility of 3D diagnostics to fully resolve the dynamics of turbulent flames.

In a recent paper (Kamal, et al., 2015), we reanalyzed single shot species and temperature measurements from non-swirling flames stabilized on the Cambridge/Sandia stratified burner by conditioning measurements on the local equivalence ratio, and found that the state space structure of the flames was closely approximated by that of a laminar flame at the given equivalence ratio. In the present communication, we show that the same state space relationships remain robust for species CH 4 , O 2 , H 2 O, and CO 2 in premixed and stratified flames under high swirl. Conditioned mass fractions of CO and H 2 in the stratified swirl flame show a greater effect of stratification than was observed in the non-swirling cases, and this is attributed to larger gradients in equivalence ratio that occur with the addition of swirl. Aside from this modest effect of stratification, major species mass fractions in the swirling flame are also closely approximated by laminar flame results at the local equivalence ratio.

Surrogate fuels are often used in place of real fuels in computational combustion studies. However, many different choices of hydrocarbons to make up surrogate mixtures have been reported in the literature, particularly for jet fuels. To identify the best choice of surrogate components, the capabilities of different surrogate mixtures in emulating the combustion kinetic behavior of the real fuel must be examined. To allow extensive assessment of the combustion behavior of these surrogate mixtures against detailed experimental measurements for real fuels, accurate and compact kinetic models are most essential. To realize this goal, a flexible and evolutive component library framework is proposed here, which allows mixing and matching between surrogate components to obtain short chemical mechanisms with only the necessary kinetics for the desired surrogate mixtures. The idea is demonstrated using an extensively validated multi-component reaction mechanism developed in stages (Blanquart et al., 2009; Narayanaswamy et al., 2010, 2014, 2015), thanks to its compact size and modular assembly. To display the applicability of the component library framework, (i) a jet fuel surrogate consisting of n-dodecane, methylcyclohexane, and m-xylene, whose kinetics are described in the multi-component chemical mechanism is defined, (ii) a chemical model for this surrogate mixture is derived from the multi-component chemical mechanism using the component library framework, and (iii) the predictive capabilities of this jet fuel surrogate and the associated chemical model are assessed extensively from low to high temperatures in well studied experimental configurations, such as shock tubes, premixed flames, and flow reactors.

In the present study, a plasma flow reactor (PFR) facility designed to perform both ex situ and in situ experiments of stable (H2 and O2) and intermediate (OH radicals) species detection was used to examine the plasma-assisted characteristics of hydrogen oxidation at 1 atm pressure for temperatures ranging from 420 K to 1100 K. Experiments were performed at nearly isothermal conditions, by heavily diluting reactive mixtures in argon, in an attempt to mitigate temperature changes from exothermic chemical reactions. This technique allows experimental results to be interpreted from a perspective that plasma and thermal (neutral) heat release effects are decoupled, essentially isolating the effects of the plasma-chemistry and its influence on the neutral-chemistry. Results showed no thermal reaction until 860 K, at which point hydrogen was rapidly consumed within the flow residence time associated with the reactor. With the plasma discharge, the onset of oxidation was extended to lower temperatures (T < 860 K), while exhibiting a steady increase in the rate of oxidation starting from 470 K, and eventually consuming all the initial hydrogen by 800 K. Absolute measurements of OH mole fraction reveal that at conditions well below the dominance of the thermal chain-branched chemistry (at T = 668 K), the plasma induced a chain-propagating effect on OH formation, which was entirely confined to the boundaries of the plasma discharge section of the reactor. Furthermore, temporal measurements were also performed, showing that the extent of OH formation and subsequently its global effect on the fuel consuming chemistry can be manipulated based on the plasma perturbation timescale (i.e., the time scale at which the high-voltage pulses are administered to the reactive flow to generate the plasma discharge). Experimental results are compared to modeling calculations and show relatively good agreement, with the model predicting similar kinetic trends as a function of temperature. These results demonstrate new insight into the kinetics governing plasma-assisted combustion, and provide new experimental data to facilitate the development and validation of PAC-specific kinetic mechanisms.

The present work demonstrates the feasibility of heat release rate imaging using the laser-induced fluorescence (LIF) of atomic hydrogen (H-atom) and formaldehyde (CH2O) in laminar premixed flames. The product of H-atom LIF and CH2O LIF signals is evaluated on a pixel-by-pixel basis and is compared with that of the OH × CH2O technique. These results for equivalence ratio ranging from 0.8 to 1.1 are compared with computations of one-dimensional freely-propagating flames. The performance of these markers is studied based on the following two aspects: the spatial accuracy of the local heat release rate and the trend in the total heat release rate with equivalence ratio. The measured trend in the spatial distribution of radicals and the deduced heat release rate agree well with the computational values. The variation in the spatially integrated heat release rate as a function of equivalence ratio is also investigated. The results suggest that the trend in the variation of the integrated heat release rate and the spatial location of heat release rate can be evaluated by either of these markers. The OH-based marker showed certain sensitivity to the chemical mechanism as compared to the H-atom based marker. Both the OH-based and H-atom based techniques provide close estimates of heat release rate. The OH based technique has practical advantage when compared to the H-atom based method, primarily due to the fact that the H-atom LIF is a two-photon process.

Simultaneous laser ignition and spectroscopy is a scheme that enables rapid determination of the local equivalence ratio and condensed fuel concentration during a reaction in a two phase spray flame. In parallel with laser ignition, the equivalence ratio and droplet characteristics such as the concentration, size, and distribution of spray combustion are simultaneously obtained for a feedback control system. The plasma characteristics of fuel droplets are evaluated initially by shadowgraph, and the high-speed imaging of air and spray breakdown provides visualization of the transition from the plasma to a flame kernel. The spectrum in the spray is evaluated according to droplet characteristics such as size and number density. The probability density function is used to analyze the interaction between the fuel droplets and the laser plasma with laser-induced breakdown spectroscopy (LIBS) measurements. In this paper, we have conducted quantitative analysis of the LIBS signals according to the equivalence ratio, droplet size, droplet number density and droplet concentration for development of a control strategy for flame ignition and stabilization with simultaneous in situ combustion flow diagnostics.

A key component of the thermoacoustic instability feedback mechanism is the excitation of hydrodynamic flow disturbances by narrowband acoustic fluctuations. In earlier work, we considered the response of time-averaged axisymmetric flames to helical disturbances. That study showed that although all helical modes cause local flame wrinkling, only the axisymmetric hydrodynamic mode, m = 0, contributes to the global heat release rate fluctuations. This paper extends this work to consider time-averaged flames that are non-axisymmetric. We show that distortions of the unforced flame shape changes its receptivity to helical disturbances, as it does not allow for the perfect destructive interference seen in axisymmetric flames. This results in a non-zero global flame response contribution from helical modes. Given that helical modes often have the largest amplitudes in jet flows, particularly those with swirl, these results show that the degree of non-axisymmetry of the flame has an important influence in determining which hydrodynamic modes, axisymmetric or helical, control the global heat release response of the flame. These points have been illustrated with example calculations that show how different control parameters and the degree of time-averaged non-axisymmetry influence the global flame response. An important implication of these results relates to scaling results from simplified geometries where a round jet is placed inside a round enclosure, to more realistic ones (such as where multiple jets are placed next to each other) where the flame shape will be distorted from being perfectly circular.

The modeling of mixing by molecular diffusion is a central aspect for transported probability density function (tPDF) methods. In this paper, the newly-proposed shadow position mixing model (SPMM) is examined, using a DNS database for a temporally evolving di-methyl ether slot jet flame. Two methods that invoke different levels of approximation are proposed to extract the shadow displacement (equivalent to shadow position) from the DNS database. An approach for a priori analysis of the mixing-model performance is developed. The shadow displacement is highly correlated with both mixture fraction and velocity, and the peak correlation coefficient of the shadow displacement and mixture fraction is higher than that of the shadow displacement and velocity. This suggests that the composition-space localness is reasonably well enforced by the model, with appropriate choices of model constants. The conditional diffusion of mixture fraction and major species from DNS and from SPMM are then compared, using mixing rates that are derived by matching the mixture fraction scalar dissipation rates. Good qualitative agreement is found, for the prediction of the locations of zero and maximum/minimum conditional diffusion locations for mixture fraction and individual species. Similar comparisons are performed for DNS and the IECM (interaction by exchange with the conditional mean) model. The agreement between SPMM and DNS is better than that between IECM and DNS, in terms of conditional diffusion iso-contour similarities and global normalized residual levels. It is found that a suitable value for the model constant c that controls the mixing frequency can be derived using the local normalized scalar variance, and that the model constant a controls the localness of the model. A higher-Reynolds-number test case is anticipated to be more appropriate to evaluate the mixing models, and stand-alone transported PDF simulations are required to more fully enforce localness and to assess model performance.

In this paper, we investigate the effects of nanosecond, repetitively-pulsed, non-equilibrium plasma discharges on laminar, low-pressure, premixed, burner-stabilized hydrogen/O 2 /N 2 and hydrocarbon/O 2 /N 2 flames using CH * chemiluminescence and quantitative OH laser-induced fluorescence (LIF) diagnostics. Two different plasma sources, both of which generate uniform, low-temperature, volumetric, non-equilibrium plasma discharges, are used to study changes in chemiluminescence, temperature, and OH concentration when non-equilibrium plasmas are directly coupled to conventional hydrogen/hydrocarbon oxidation and combustion chemistry. Qualitative imaging of CH * chemiluminescence indicates that during plasma discharge, the luminous flame zone is shifted upstream towards the burner surface with little change in the CH * zone thickness. For the same plasma discharge and flame conditions, quantitative results using spatially-resolved OH LIF and multi-line, OH-LIF thermometry show significant increases in ground-state OH concentrations in the preheating zones of the flame. More specifically, for a particular axial position downstream of the burner surface, the OH concentration increases, which can be viewed as an effective “shift” of the OH profiles towards the burner surface. The increase in OH concentration is conceivably due to an enhancement of the lower-temperature kinetics including O atom, H atom and OH formation kinetics and temperature rise due to the presence of the low-temperature, non-equilibrium plasma.

We present a direct comparison between the predicted and measured probability density functions (PDF) of the reaction progress variable and conditioned values of the scalar dissipation rates (SDR) in premixed turbulent flames. The predictions are based on simulations of premixed flames using the linear-eddy model (LEM), parameterised by a wide range of integral length scales and turbulent Reynolds numbers. The experimental results are highly spatially resolved temperature and species data from the Cambridge–Sandia swirl burner. The LEM simulations display remarkable accuracy in capturing the features observed experimentally. Further, the results reveal that the LEM calculated PDF and SDR for premixed flames remain relatively steady under a variety of turbulent conditions, including variations in the integral length and turbulent Reynolds number. In general, it appears to be practical to use representative pseudo-turbulent PDF and SDR models for a range of turbulence intensities and length scales.

UV Raman scattering is applied to measure fuel/air mixing and combustion of a Mach-2 air stream flowing over a step-ramp cavity fueled with 70% methane/30% hydrogen. Average and RMS fluctuations of temperature and major species profiles as well as scatter plots of simultaneous temperature and chemical scalars are determined from single-shot Raman scattering measurements along a 6-mm line transverse to the main supersonic flow. In the fuel-rich regions of the pilot cavity, the 248-nm KrF laser induces broadband fluorescence interference that reduces the number of analyzable Raman spectra; nonetheless, on the whole, a significant fraction of the spectra were reducible. In the cavity, hydrogen fuel reacts quickly resulting in a uniform water concentration in the recirculation zone. Methane reacts slowly to carbon monoxide/carbon dioxide in the cavity, leading to non-uniform concentrations of these species. Mean and instantaneous mixture fraction data inside the shear layer were indicative of oxygen transport across the shear layer. Temperature, water, and oxygen fluctuations are fairly constant throughout the combustor due to recirculation/turbulent transport across the shear layer and the slow reaction of methane.

This work reports the study of the ignition processes in a Mach-2 cavity combustor based on three-dimensional (3D) measurements at 20 kHz. The 3D measurements were obtained by a combination of tomographic chemiluminescence and fiber-based endoscopes. Measurements of 3D flame and flow properties were reported under two fueling conditions of the combustor. The properties included 3D volume, surface area, shape factor, and 3D3C (three-dimensional and three-component) velocity of the ignition kernel. These results clearly distinguished the ignition stage from the stable combustion stage of the combustor and enabled the determination of a transition time to quantify both stages. The analysis of the change of the ignition kernel's shape, when combined with the 3D3C velocity measurements, also illustrated flame-flow interactions in the cavity combustor. These results demonstrated the utility of the 3D diagnostics to overcome some of the limitations of established planar diagnostics and to resolve the dynamics of high-speed combustion devices both spatially and temporally.

Using n-butane, n-heptane and iso-octane as representative fuels exhibiting NTC (negative temperature coefficient) behavior, comprehensive computational studies with detailed mechanisms and theoretical analysis were performed to investigate the upper stationary point, denoted as turnover states, on the NTC curve near the higher temperature regime, where the ignition delay τ exhibits a local maximum. It is found that the global behavior of the turnover states exhibits distinctive thermodynamic and kinetic characteristics under different pressures, in that the ignition delay at the turnover states shows an Arrhenius dependence on the temperature T and an approximate inverse quadratic power law dependence on the pressure P. These global behaviors imply that the temperature and pressure of the turnover states are not independent and can be correlated by Arrhenius dependence, as ln P ∝ 1/T. Further theoretical analyses demonstrate that such turnover states result from the competition between the low-temperature chain branching reactions and the decomposition of the intermediate species, and therefore correspond to a critical value, α, of the ratio of OH production from low-temperature chemistry. In addition, the ignition delay at the turnover state can be well correlated by the analytical expression derived by Peters et al., with the further demonstration that the pressure dependence of the turnover ignition delay mainly result from the H2O2 decomposition reaction. Comparison of the present results with the literature experimental data of n-heptane ignition delay time shows very good agreement.

The combustion of methane in air results in large amounts of CO 2 and NO X emissions. In order to reduce the NO X emissions, one possible solution is the oxy-methane combustion with large CO 2 dilution so that the combustion products can be reduced mainly to CO 2 and H 2 O. However, there are very few studies on the chemical kinetics of oxy-methane combustion in a CO 2 diluted environment. In this study, methane time-histories, CH * emission profiles, and pressure time-histories measurements were conducted behind reflected shock waves to gain insight into the effects of CO 2 dilution of the gas mixtures on the ignition of methane. The measurements were carried out for mixtures of CH 4 , CO 2 and O 2 in argon bath gas at temperatures of 1577–2144 K, pressures of 0.53–4.4 atm, equivalence ratios (Φ) of 0.5, 1, and 2, and CO 2 mole fractions (X CO2 ) of 0, 30, and 60%. The laser absorption measurements were conducted using a continuous wave distributed feedback interband cascade laser (DFB ICL) centered at 3403.4 nm. The results showed the decrease of activation energy and the increase of ignition delay time as the amount of CO 2 dilution was increased. However, the changes were minor and within the experimental uncertainties of the measurements. Also, the results were compared to the predictions of two different natural gas mechanisms: GRI 3.0 and AramcoMech 1.3 mechanisms. In general the predictions were reasonable when compared to the experimental data; however, there were discrepancies at some conditions. Three different influences of CO 2 addition to the argon bath gas in regards to chemistry, collision efficiencies, and heat capacities were examined. In addition, the present study included experimentally obtained correlations for absorption cross sections of methane for its P(8) line in the v 3 band in argon bath gas with and without carbon-dioxide dilutions at temperatures between 1200

The effects of an externally applied electric field on the burning characteristics of a spherically symmetric fuel drop, including the flame structure, flame standoff distance, mass burning rate and flame extinction characteristics of the diffusion flame are studied. A reduced three-step chemical kinetic mechanism that reflects the chemi-ionization process for general hydrocarbon fuels has been proposed to capture the production and destruction of ions inside the flame zone. Due to the imposed symmetry, the effect of the ionic wind is simply to modify the pressure field. Our study thus focuses exclusively on the effects of Ohmic heating and kinetic effects on the burning process. Two distinguished limits of weak and strong field are identified, highlighting the relative strength of the internal charge barrier compared to the externally applied field. For both limits, significantly different charged species distributions are observed. An increase in the mass burning rate is noticed with increasing the strength of the electric field in both limits, with a small change in flame temperature. Increasing external voltages pushes the flame away from the droplet and causes a strengthening of the flame with a reduction in the extinction Damkholer number.

Individual particles of pulverized coals exhibit strikingly different combustion phenomena depending on their rank (anthracite, semi-anthracite, bituminous, sub-bituminous and lignite). Herein a concise review is presented on pertinent findings in the literature to contrast ignition and combustion behavior of such fuels at the particle-level. Emphasis is given to recent investigations performed in the laboratory of the authors, where combustion of a variety of coal particles of the same size-cut took place in the same apparatus under identical operating conditions. Such behaviors were then compared to those reported in the literature to verify their replication under often different, but yet relevant, conditions. The objective has been to relate the effect of coal rank to a number of key qualitative and quantitative parameters, such as modes of ignition and combustion, ignition temperatures, ignition delay times, combustion temperatures and burnout times (both those encountered in the volatile and the char combustion phases), volatile flame sizes as well as extents of particle fragmentation. Besides reviewing combustion behaviors in air, analogous behaviors under simulated dry oxy-combustion conditions were also highlighted. Then the coal rank dependence of the required oxygen mole fraction in dry O2/CO2 blends to match the intensity of air-fired combustion was examined.

In order to develop soot models that include effects due to particle aggregation, data must be obtained from flames containing all of the relevant chemical and physical processes that affect the evolution of a soot particle. In this study, the effective radius of gyration of aggregated soot is experimentally determined in nitrogen-diluted ethylene coflow diffusion flames using 2-D multi-angle light scattering (2D-MALS). High spatial resolution is achieved by minimizing signal blur that results from imaging light scattering from the probe volume at oblique angles. The results are validated at six locations in the flame through comparison to an effective radius of gyration derived from transmission electron microscope (TEM) analysis of thermophoretically-sampled soot. Simulations are used to model the path taken by a soot particle to reach the TEM grid to ensure the sampling measurements have the spatial resolution necessary to differentiate between aggregate properties at different radial locations in the flame. A radial distance δ is determined from which the front face of the probe is offset from the desired sample location. The ability of the probe to separately capture wing and centerline soot morphology is confirmed by comparison to a previous time-resolved laser-induced incandescence (TiRe-LII) measurement of primary particle diameter. The observed increase in polydispersity and maximum diameter of primary particles along the flame wing compared to the centerline is thought to be a result of surface growth. Excellent agreement is found between the effective radius of gyration derived from 2D-MALS and TEM analysis. The TEM results not only confirm the optical measurement but also elucidate the effective radius of gyration, which depends on aggregate size and polydispersity.

The present work focuses on the early stages of flame-kernel development in laser-induced spark ignited mixtures issuing out of a Bunsen burner. The time-scale of 3 µs to 1 ms associated with the flame-kernel evolution stage of an ignition event is targeted in this work. A CH4/air mixture (equivalence ratio ϕ = 0.6) is studied as a base case, and compared with CH4/CO2/air (mole fractions = 0.059/0.029/0.912, respectively) and CH4/H2/air (mole fractions = 0.053/0.016/0.931, respectively) mixtures for nearly the same adiabatic flame temperature of 1649 K. The spatio-temporal flame-kernel evolution is imaged using planar laser induced fluorescence of the OH radical (OH-PLIF), simultaneously with H-alpha emission from the plasma. The H-alpha emission suggests that the plasma time-scale is well below 1 µs. The PLIF images indicate all the stages of kernel development from the elongated kernel to the toroidal formations and the subsequent appearance of a front-lobe. The different time-scales associated with these stages are identified from the rate of change of the kernel perimeter. The plasma is followed by a supersonic kernel-perimeter growth. Larger flame-kernel spread is found in the case of CH4/H2 mixtures. A distinct shift in the trends of evolution of LIF intensity and kernel perimeter is observed as the fuel concentration is varied near the lean flammability limit in CH4/air (ϕ = 0.35–0.65) and H2/air (ϕ = 0.05–0.31) mixtures. The flow velocity (Reynolds number, Re) effect in both laminar and turbulent flow regimes (Re = ∼600–6000) indicates that the shape of the flame-kernel changes at higher velocities, but the size of the kernel does not change significantly for a given time from the moment of ignition. This could be due to a balance between two competing effects, namely, increase in the strain rate that causes local extinction and thus decreases the flame-kernel growth, and increase in the turbulence levels that facilitates increased flame-kernel surface area through wrinkling, which in turn increases the flame-kernel growth.

© 2015 The Combustion Institute. Chain-branching reactions represent a general motif in chemistry, encountered in atmospheric chemistry, combustion, polymerization, and photochemistry; the nature and amount of radicals generated by chain-branching are decisive for the reaction progress, its energy signature, and the time towards its completion. In this study, experimental evidence for two new types of chain-branching reactions is presented, based upon detection of highly oxidized multifunctional molecules (HOM) formed during the gas-phase low-temperature oxidation of a branched alkane under conditions relevant to combustion. The oxidation of 2,5-dimethylhexane (DMH) in a jet-stirred reactor (JSR) was studied using synchrotron vacuum ultra-violet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). Specifically, species with four and five oxygen atoms were probed, having molecular formulas of C8H14O4(e.g., diketo-hydroperoxide/keto-hydroperoxy cyclic ether) and C8H16O5(e.g., keto-dihydroperoxide/dihydroperoxy cyclic ether), respectively. The formation of C8H16O5species involves alternative isomerization of OOQOOH radicals via intramolecular H-atom migration, followed by third O2addition, intramolecular isomerization, and OH release; C8H14O4species are proposed to result from subsequent reactions of C8H16O5species. The mechanistic pathways involving these species are related to those proposed as a source of low-volatility highly oxygenated species in Earth's troposphere. At the higher temperatures relevant to auto-ignition, they can result in a net increase of hydroxyl radical production, so these are additional radical chain-branching pathways for ignition. The results presented herein extend the conceptual basis of reaction mechanisms used to predict the reaction behavior of ignition, and have implications on atmospheric gas-phase chemistry and the oxidative stability of organic substances.

Soot formation processes for butane and butanol isomers have been experimentally investigated and compared in a counterflow non-premixed flame configuration at atmospheric pressure. Non-intrusive laser-based diagnostic techniques, including Laser Induced Incandescence and Light Extinction, have been adopted for soot volume fraction measurements. Experimental results of these C4 fuels have been compared to illustrate the effects of hydroxyl (–OH) group and the isomeric structures on soot formation process. Under the investigated conditions, butane isomers were observed to form more soot than butanol isomers, thereby showing the effect of the hydroxyl group. The effects of isomeric structural differences on sooting propensity were also observed within the butane and butanol isomers. In addition, while soot volume fraction was seen to increase with increasing fuel mole fraction, the ranking of sooting propensity for these C4 fuels remained unchanged. Effects of varying strain rate and oxygen mole fraction on soot formation were studied herein as well. For each isomeric class, two chemical kinetic models available in the literature were used to simulate and compare the spatially-resolved profiles of the soot precursors. The amount of soot precursors predicted by the models was different and the initial fuel breaking pathways were also found to differ significantly. Furthermore, no direct correlation between the computed mole fractions of soot precursors and the measured amount of soot formed in the present experimental conditions was observed.

A numerical scheme describing the oxidation process induced by single and multiple dielectric barrier discharges in a H2/O2/Ar mixture is developed. Path flux and sensitivity analyses are performed in order to identify important reaction paths. The analyses are conducted for an initial mixture of 2000 ppm H2 and 3000 ppm O2 balanced in Ar at a constant pressure of 1 atm and temperatures ranging from 474 K to 1008 K, covering both below and above the second explosion limit. Results with only one discharge as well as with repetitive discharges at rates between 1 and 5 kHz are discussed. Three sets of reaction schemes leading to H2O formation are identified: two short-time-scale schemes involving negative ions and O(1D), respectively, and a long-time-scale scheme involving ground state radicals, which is responsible for the majority of the H2O formation and driven by plasma dissociation of H2 and O2. At temperatures below the second explosion limit, the last scheme drives the straight chain propagation mechanism without causing exponential growth of radicals, where the reactions of HO2 play important roles on the overall fuel oxidation. At temperatures above the second explosion limit, H and O atoms produced from the dissociation of reactants by deactivation of excited argon subsequently trigger ignition significantly reducing the ignition delay. The consumption of H2 and O2 which follows is governed by conventional high temperature chain branching kinetics and occurs at a rate equivalent to that of the purely thermal reaction. Increasing the pulse repetition rate of the discharge at low temperatures results in a shift of the rate-limiting steps from reactions of H atoms to those of the OH radical. Furthermore, the kinetics of metastable Ar*m, HO2, O2(a1Δg), and O(1D) are analyzed and their implications to other systems are discussed.

Staged combustion has been explored for power generation gas turbine engines to increase engine efficiency with minimal contribution to pollutant formation. Secondary fuel injection into the vitiated flow from the primary combustion process is one approach. In this work, advanced diagnostic measurements were performed on an experimental representation of such a system, with a transverse jet injection into a swirling vitiated crossflow. High-repetition-rate simultaneous particle image velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) were performed at three measurement planes perpendicular to the jet axis. The measurements provided qualitative as well as quantitative information on the evolution of complex flow structures and transient events such as re-ignition, local extinction and vortex-flame interactions in the turbulent reacting flow. Transverse jets composed of H 2 mixed with N 2 and premixed natural gas and air were injected through a tube protruding into the crossflow. The vitiated crossflow is produced by a low swirl burner (LSB). The crossflow exhibits considerable swirl at the location of the transverse jet injection. Two momentum flux ratios, J = 3 and J = 8 were employed to study the effect of momentum flux ratio on the stabilization of reaction fronts. The time-averaged flow field show a pair of steady wake vortices, which were found to be much larger for J = 8 than for the J = 3 case. The region with the maximum time-averaged OH-PLIF intensity was found to be localized between the wake vortex pairs. The wake Strouhal number was found to be invariant with respect to the momentum flux ratio. Based on the experimental data, it is hypothesized that the shear layer and wake field vortices play a significant role in stabilizing a steady reaction front within the near-wake region of the jet.