No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Soot, PAH and OH measurements in vaporized liquid fuel flames
... Many researchers have made great efforts to explore the formation process of soot particles in flames through experiments and numerical simulation [3][4][5][6][7][8]. Besides, polycyclic aromatic hydrocarbons (PAHs) are considered as the precursors of soot [9][10][11][12] and play an important role in soot formation. ...
... As important non-intrusive techniques, laser-induced incandescence (LII) and laser-induced fluorescence (LIF) were widely utilized in combustion diagnostics due to their high temporal and spatial resolution. LII can be used for the measurement of sooting tendency [3,8,[16][17][18][19], while LIF is usually employed to measure PAHs in sooting flames [5,6,11,[20][21][22][23][24]. Vander wal et al. [14] measured PAHs and soot in ethylene laminar flame by using LIF and LII respectively and found that LIF signal decreases and LII signal begins to increase when reaching the height above burner (HAB) of 33-34 mm. ...
... Since it tends to excite PAHs with large molecular weight by the 532 nm laser, the signals of PAHs-LIF and Soot-LII are easily interfered with each other [3]. Lasers with the wavelengths of 266 nm and 355 nm have been applied to measure PAHs-LIF signals [3,5,6,8,23,24,26,27]. Vander wal et al. [12] pointed out that both the UV and visible light can excite the soot signal in the flame as long as the laser energy is high enough. ...
As Polycyclic Aromatic Hydrocarbons (PAHs) are the main precursors of soot formation, the investigation of PAHs formation and PAHs transforming to soot is essential for understanding the soot formation and soot reduction in combustion. This study investigated PAHs and soot formation in laminar co-flow diffusion flames fueled by n-heptane at elevated pressures. By using the combination of Laser-induced Incandescence (LII) and Laser-induced Fluorescence (LIF) techniques, the distributions of PAHs and soot concentration in flames were obtained with the energy of 6 and 48 mJ at various detection times. The flame stability at different pressures was analyzed, and the relationship between the integrated signal intensities of Soot-LII and PAHs-LIF in the region near the flame centerline was discussed. The results revealed that, with the increase of pressure, the flame stability becomes worse and both the soot and PAHs concentrations are increased. The integrated intensities of Soot-LII and PAHs-LIF in the region near the centerline of the flame scale with p n. In addition, the integrated signal intensity of Soot-LII has a good linear correlation with that of the PAHs-LIF, and the slopes of correlations are 0.33 and 1.81 at the laser energy of 6 and 48 mJ respectively.
... The signal intensity can also be used to qualitatively evaluate the concentrations of these different classes of PAHs. This method is particularly useful in evaluating fuel effects on PAH formation by comparing the relative concentrations of PAHs as represented by LIF signal intensity among flames with different fuels or fuel additives [441][442][443][444][445][446][447]. Nevertheless, it is important to note that PAH LIF signal can be highly dependent on temperature [448]. ...
... More recently, research interest has gradually shifted towards pre-vaporized liquid fuels such as ethanol [785], C6 hydrocarbon and oxygenated species [21], n-heptane [486,786,787] and n-decane [445]. The effects of pressure on soot properties other than SVF were also tackled, using light scattering [69] and/or thermophoretic sampling [509]. ...
Many practical soot-emitting combustion systems such as diesel and jet engines rely on diffusion flames for efficient and reliable operation. Efforts to mitigate soot emissions from these systems are dependent on fundamental understanding of the physicochemical pathways leading from fuel to soot in laminar diffusion flames. Existing diffusion flame−based soot studies focused primarily on over-ventilated coflow flame where the fuel gas (or vapor) issues from a cylindrical tube into a co-flowing oxidizer, and counterflow flame, where a reacting zone is established between two opposing streams of fuel and oxidizer. As a canonical diffusion flame configuration, laminar counterflow diffusion flames have been widely used as a highly controllable environment for soot research, enabling significant progress in the understanding of soot formation for several decades. In view of the possibility of fuel/oxidizer premixing in practical systems, counterflow partially premixed flames have also been studied. In the present work we intend to provide a comprehensive review of the researches on various aspects of soot formation utilizing counterflow flames. Major processes of soot formation (formation of gas phase soot precursors, soot inception and surface reactions, as well as particle-particle interactions) are examined first, with focus on the most recent (post-2010) research progress. Experimental techniques and associated challenges for the measurement of soot-related properties (some knowledge of which is helpful for full appreciation of the experimental data to be reviewed) are then introduced. This is followed by a detailed description of soot evolution in counterflow flames, which is complemented by a discussion on the similarity and differences of the sooting structures between counterflow and coflow diffusion flames. Parametric studies of the effects of fuel molecular structure, fuel additive, partial-premixing, pressure, temperature, stoichiometric mixture fraction, and residence time on soot formation in counterflow flames will then be addressed in detail. This review concludes with a summary of the knowledge and challenges gathered and demonstrated through decades of research, and an outlook on opportunities for future counterflow flame−based soot research towards a more complete understanding of soot formation and the development of novel techniques for soot mitigation in practical combustion devices.
... Unfortunately, the experimental set-up is not designed to allow phase-correlated measurements of the soot distributions, because the maximum repetition rates of the CCD camera (Hamamatsu C4880) recording the LII signals from the streak unit (0.5 frames per second) as well as the Nd:YAG laser (10 Hz) are limited. By applying Planck's law (assuming grey properties of the soot particles), the temporal evolution of the particleensemble temperature can be calculated from the measured ratio of the time-resolved LII signals obtained at two different wavelengths [7][8][9][10][11][12][13][14]: ...
... 450 to 650 nm [7][8][9][10][11][12][13][14][15][16][17][18][19]. This assumption seems to be reasonable in a diffusion flame, even though some restrictions are found in recent investigations in sooting premixed laminar ethylene/air flames at lower heights above the burner [20,21]. ...
The response of non-premixed swirling flames to acoustic perturbations at various frequencies (0-350 Hz) and the impact of imposed air inlet velocity oscillations on the formation and oxidation of soot are investigated. The results obtained from these flames are of special interest for “rich-quenched-lean” (RQL) combustion concepts applied in modern gas turbines. In RQL combustion, the fuel is initially oxidized by air under fuel-rich conditions in a first stage followed by a fuel-lean combustion step in a second stage. To mimic soot formation and oxidation in RQL combustion, soot particle measurements in highly turbulent, non-premixed swirling natural gas/ethylene-confined flames at imposed air inlet velocity oscillations are performed using simultaneous 2-Colour-Time-Resolved-Laser-Induced Incandescence (simultaneous 2-Colour-TIRE-LII). The latter technique is combined with line-of-sight averaged OH*-chemiluminescence imaging, measurements of the velocity field by high-speed particle imaging velocimetry under reactive combustion conditions and measurements of the mean temperature field obtained by a thermocouple. A natural gas/ethylene mixture (Φ = 1.56, 42 % C2H4, 58 % natural gas, P th = 17.6 kW at atmospheric pressure) is used as a fuel, which is oxidized by air under fuel-rich conditions in the first combustion chamber.
... Several studies report the visualization of PAH LIF and soot LII in laminar diffusion flames [169][170][171][172][173]. Figure 21 shows (a) images and (b) axial profiles of OH LIF, soot LII, and PAH LIF, excited with a laser light-sheet at 248.5 nm, in the center plane of a laminar propane diffusion flame (see caption for excitation and detection details). PAH are generated close to the nozzle exit and found completely inside the flame within in the fuel-rich region. ...
Late-evaporating liquid fuel wall-films are considered a major source of soot in spark-ignition direct-injection (SIDI) engines. In this study, a direct-injection model experiment was developed, and optical diagnostics were used to image fuel-film evaporation and soot formation. Fuel is injected by a multi-hole injector into the optically accessible test section of a constant-flow facility. Some of the liquid fuel impinges on a quartz-glass window and forms fuel films. After spark ignition, a turbulent flame front propagates through the chamber, and subsequently sooting combustion arises near the evaporating fuel films.
Laser-induced fluorescence (LIF) of toluene (fluorescent tracer) added to iso-octane (surrogate fuel) in small concentration, excited by laser pulses at 266 nm, is used to image the fuel-film thickness and the air/fuel equivalence ratio in the gas phase. The LIF images of fuel films show that the films remain on the wall surface long after the flame front has passed and that the fuel accumulates to thick fuel blobs in the films throughout the evaporation. Generally, and consistent with results from a Computational Fluid Dynamics (CFD) simulation, the evaporation rate is highest early after start of injection (aSOI) and then decreases and remains on a constant level, with the magnitude strongly depending on the wall temperature. Combustion and thus convective heat transfer show a minor effect on the fuel-film evaporation rate compared to the wall temperature in either CFD or LIF. A low-dimensional model (LDM), developed in this work, shows that the quartz wall only slightly cools down in the impingement region. Consistent with results from the CFD, the fuel-film temperature rapidly approaches the wall temperature, independent of the initial film temperature. Gas-phase LIF finds fuel-rich vapor plumes emerging from the fuel films until late aSOI without combustion. Interestingly, when combustion is initiated fuel vapor plumes are not detected, indicating that the fuel vapor pyrolytically decomposes spatially very close to the fuel films.
The second part of this work deals with the visualization of soot formation from evaporating fuel films by multiple laser-based and high-speed imaging diagnostics. Overlapping laser light sheets at 532 and 1064 nm excited LIF of polycyclic aromatic hydrocarbons (PAH) -potential soot precursors- and laser-induced incandescence (LII) of soot, respectively. For preliminary measurements, the constant-flow facility was replaced with a Santoro or Yale burner, providing a steady, sooting, laminar co-flow diffusion flame. In complementary line-of-sight integrated imaging, the fuel spray, chemiluminescence, and soot incandescence were captured with a high-speed color camera. PAH LIF is found in close vicinity of the evaporating fuel films. Soot is found spatially separated from, but adjacent to the PAH, both with high spatial intermittency. Average images indicate that soot is formed with a much higher spatial intermittency than PAH. Images from the color camera show soot incandescence earlier and in a similar region compared to soot LII. Chemiluminescence downstream of the soot-forming region is thought to indicate the subsequent oxidation of fuel, soot, and PAH. High-speed imaging and the CFD simulation predict the inception of soot pockets to similar times, close to the evaporating fuel films and in consistent spatial extent. Excitation of PAH LIF with 266 and 532 nm and a variation of the detection bandpass-filter show PAH of different size classes systematically in a thin layer between the fuel films and soot.
... Several works have been carried out on the sooting behaviour of liquid diesel-like fuels, fossil fuel surrogates and biodiesel surrogates under various conditions. The set-ups employed include shock tubes [55,56], diffusion flames [57][58][59] and premixed flames [60,61]. The studies on soot formation in neat diesel flames are restricted to mainly spray flames. ...
Soot emitted by combustion engines is a dangerous environmental pollutant and it must be kept as low as possible also to meet emission limit regulations. PM reduction strategies include reformulation / modification of existing fuels by the use of fuel additives. The principle goal of the present work is to isolate and comprehend the effect of fuel’s additives on particulate emission. During this study a self-designed burner has been implemented and optimized to obtain a laminar diesel flame. Subsequently, the combustion of pure diesel fuels and diesel fuels additised with different ether compounds (oxymethylene dimethyl ethers, OMEs, and tripropylene glycol methyl ether, TPGME), inserted in increasing concentration, has been investigated by optical methods, namely laser-induced incandescence (for evaluation of soot volume fraction), elastic light scattering (for the characterization of soot aggregate size) and two-color pyrometry (for temperature estimation). The optical diagnostics have been combined with a non-optical one, the scanning mobility particle sizer that was employed for particles diameter evaluation. The role of the additives during the soot formation process was investigated on the burner set-up and on spray flames generated in a constant volume combustion chamber (the latest was employed only for soot concentration analysis). The results found show that the production of soot (i.e. soot volume fraction) is inhibited by the addition of oxygenated additives, especially OME with longer molecular chain. The reduction of soot concentration observed, compared to the pure diesel, goes up to a maximum of 36% for the long chain OME-diesel blend and 27% for the TPGME-diesel blend, in the laminar flame conditions. This trend has been confirmed by the measurements on spray flames where the conditions are more similar to a realistic diesel engine. In this case the amount of soot produced by the diesel additised with the oxygenated species decreases up to 88% for OME-diesel blend and 84% for the TPGME-diesel blend, compared to the neat diesel (in both cases the maximum soot drop was observed at higher temperature/pressure conditions). Soot aggregate size is also found to be affected by additives addition. In particular, the soot aggregate size obtained for OME-diesel blend is ~20% smaller than the one obtained for the pure diesel, while TPGME decreases aggregate size of ~13% compared to pure diesel. The same trend has been obtained from scanning mobility particle sizer measurements. On the contrary, soot temperature does not seem to vary significantly when additives are inserted. Overall, the burner implemented and the laminar flame obtained allowed a successful characterization of soot properties by using optical and non-optical diagnostics. Generally, the additive’s effect on soot concentration and morphology results to be in most cases proportional to the additive’s concentration and the number of C-O bonds in the additive’s molecule. Furthermore, the results of this study show that, although the effect of additives is observed during the whole combustion, the maximum reduction of soot concentration and size is achieved at the end of the combustion process. The additives seem to enhance the oxidation process due to their molecular structure that can release oxygen.
... 20 Qualitative and quantitative measurements of the soot volume concentration, OH, and PAHs were performed in vaporized liquid combustion experiments by using LIF and laser-induced laser techniques in a codirectional laminar flow burner. 21 Franzelli et al. observed the spatiotemporal evolution of turbulent diffusion flame soot using image diagnostic methods, providing spaceresolved information on turbulent flame surfaces and the interaction region of soot precursors. 22 An and Bobba et al. studied the formation process and spatiotemporal evolution of soot precursor PAHs in an engine. ...
The bright spot phenomenon during the gas explosion was because of the soot particles of high heat radiation characteristics generated during the explosion process. The formation mechanism of soot and precursor polycyclic aromatic hydrocarbons (PAHs) of the methane explosion was numerically simulated using CHEMKIN-PRO. The methane explosion soot of the CH4–air premixed gas explosion experiments with volume concentrations of 8% was collected, and the pore size distribution and surface structure of the soot were analyzed by low-pressure nitrogen gas adsorption (LP-N2GA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that C2 and C3 play an important role in the formation of PAHs in the early stage of the explosion reaction. The LP-N2GA isotherms demonstrate that the pore type of the soot particles is mostly wedge-shaped, which was verified with SEM observations. The SEM analysis showed that the methane explosion soot is composed of a large number of spherical soot aggregates with diameters between 4 and 50 μm and the pores at the particle surface are well developed, some of the particles exhibit a melt sintering feature. Soot aggregates collide with each other with a chain-branched structure, and the diameters of the majority of the particles are of 100 nm according to TEM images. In addition, graphite-like lattice stripes can be clearly seen inside the particles when magnified to 8 nm. This work will provide the basis for further analysis of soot formation in the gas explosion process.
... Detailed experiments on the soot formation processes in partially premixed and premixed n-heptane flames were performed by de Andrade Oliveir et al. [19] and by Inal and Senkan [20] , respectively. D'Anna et al. [21,22] investigated the effect of fuel/air ratio and aromatic addition on the soot formation in premixed n-heptane flames. ...
Soot formation is experimentally and numerically investigated in laminar counterflow diffusion flames burning ethylene and three typical gasoline surrogate components; n-heptane, iso-octane, and toluene. Laser-induced incandescence and a light extinction technique are employed to determine the soot volume fraction within the well-controlled region of the burner. The experiments are performed across a wide range of strain rates and stoichiometric mixture fractions. From the experimental data, sensitivities of soot formation on strain rate and stoichiometric mixture fraction are derived for each fuel. The fuels show significantly different sensitivities. For iso-octane and n-heptane, a higher sensitivity of soot production on the strain rate is observed as compared to ethylene and toluene. Moreover, the sensitivities of soot formation on the strain rate increase with increasing stoichiometric mixture fraction. One-dimensional simulations of the flames investigated experimentally were performed using two different detailed chemical kinetic mechanisms, detailed chemical soot models, and the hybrid method of moments as well as a discrete sectional method to describe soot dynamics. The models are capable of predicting the soot volume fraction of the ethylene flames with remarkable accuracy, whereas for the gasoline surrogate components, the overall soot volume fractions are overpredicted for all tested models. In iso-octane flames, soot nucleation and PAH condensation rates are particularly enhanced. A reaction pathway analysis shows that in ethylene flames, the formation of benzene mostly originates from acetylene, while for iso-octane, large amounts of iso-butenyl form propyne, propargyl, and then benzene.
... The sooting tendency of liquid fuels under various conditions has been the topic of a number of studies. Fundamental investigations on fossil fuel surrogates as well as on biodiesel surrogates have been performed both in shock tubes [11,12], premixed [13,14], and diffusion flames [15][16][17]. The studies of soot formation in diesel flames are limited; recent investigations, including the work of Solero [18] and Lemaire et al. [2,19], employed an atomizer to produce a fuel spray for direct vaporization of the fuel in the flame itself. ...
In the present work, a novel burner capable of complete pre-vaporization and stationary combustion of diesel fuel in a laminar diffusion flame has been developed to investigate the effect of the chemical composition of diesel fuel on soot formation. For the characterization of soot formation during diesel combustion we performed a comprehensive morphological characterization of the soot and determined its concentration by coupling elastic light scattering (ELS) and laser-induced incandescence (LII) measurements. With ELS, radii of gyration of aggregates were measured within a point-wise measurement volume, LII was employed in an imaging approach for a 2D-analysis of the soot volume fraction. We carried out LII and ELS measurements at different positions in the flame for two different fuel types, revealing the effects of small modifications of the fuel composition on soot emission during diesel combustion.
... The LII technique setup is shown in Fig. 1b. For simultaneous polycyclic aromatic hydrocarbons (PAH) fluorescence and soot incandescence, the excitation is performed with the 4 th harmonic (266 nm) of a Brilliant b (Quantel) Nd:YAG laser, with 80 mJ nominal energy, and operated at 10 Hz (Vander Wal, 1996;Vander Wal et al., 1997;de Andrade Oliveira et al., 2013). A dichroic mirror is used to filter the second harmonic (532 nm) residual component. ...
... In particular, the laser-induced incandescence (LII) technique is widely used for the in-situ and non-intrusive measurement of f v [5]. It has been applied in a range of laminar and turbulent, premixed, and nonpremixed flames [6][7][8][9], as well as specifically in counterflow flames [10][11][12]. The LII technique has also been extended to provide a measure of soot primary particle size, both at a single point [13,14] and two-dimensional imaging [15,16]. ...
Beam steering is often encountered in laser diagnostic measurements, especially in flame environments, due to changes in refractive index caused by thermal and species gradients. It can negatively affect the accuracy of the results. In this work, the effects of beam steering on laser-induced incandescence (LII) measurements of pre-vaporized-liquid counterflow flames are assessed. The focus on counterflow flames is to facilitate future detailed experimental campaigns on one-dimensional nonpremixed sooty flames. It is found that the temperature and species gradients in the counterflow configuration have a much more significant impact on the beam profile than in laminar flat flames, especially for heavier fuels. As a result of the changes in the beam profile, for the same applied laser energy, the local fluence shifts markedly with fuel type, therefore, having a direct impact on the LII measurements. A procedure is developed for ensuring accurate measurements and it is shown that, for a specific fuel, it is possible to tailor the laser energy, such that the collected LII signal in the counterflow flames is nearly independent of beam-steering effects.
... A number of experiments of liquid fuels were also conducted, while most of them were at atmospheric pressure. Oliveira et al. [16,17] combined LII and LIF methods to conduct the measurement of liquid fuels, n-heptane and n-decane at atmospheric pressure, and the results indicated that the maximum PAH-LIF signal was a good predictor of maximum volume fraction of soot obtained from the LII signal. It was found that there was a linear correlation between the volume fraction of soot and the PAH-LIF and delayed LII signals (50 ns). ...
... 28 They have been known as atmospheric pollutants since the 1980s, 29 and are often generated in most types of hydrocarbon combustion. [30][31][32][33] Anions and PAH anions, in particular, are also known to be created in similar reactions. 34 Since the atmospheres of carbon-rich stars have been observed to contain many of the similar types of hydrocarbons as are present in combustion, 35 it has been hypothesized that PAHs are significant players in the chemistry of space. ...
Some anions are known to exhibit excited states independent of external forces such as dipole moments and induced polarizabilities. Such states exist simply as a result of the stabilization of valence accepting orbitals whereby the binding energy of the extra electron is greater than the valence excitation energy. Closed-shell anions are interesting candidates for such transitions since their ground-state, spin-paired nature makes the anions more stable from the beginning. Consequently, this work shows the point beyond which deprotonated, closed-shell polycyclic aromatic hydrocarbons (PAHs) and those PAHs containing nitrogen heteroatoms (PANHs) will exhibit valence excited states. This behavior has already been demonstrated in some PANHs and for anistropically-extended PAHs. This work establishes a general trend for PAHs/PANHs of arbitrary size and directional extension, whether in one dimension or two. Once seven six-membered rings make up a PAH/PANH, valence excited states are present. For most classes of PAHs/PANHs, this number is closer to four. Even though most of these excited states are weak absorbers, the sheer number of PAHs present in various astronomical environments should make them significant contributors to astronomical spectra.
... This result indicates that the graphitic crystallite size and the amorphous content decrease due to the soot oxidation by OH. The OH must be the primary oxidant of soot in the test flame because the soot surface oxidation within the laminar co-flow sooting flames is dominated by OH [36,[46][47][48]. It is well known that a carbon atom at the edge site is much more reactive than that of the basal plane carbon atom in graphitic layers; therefore, oxidation of the graphitic crystallite proceeds at the edge site and thereby reduces I D /I G in the OH-LIF region. ...
The growth and oxidation behaviors of graphitic crystallites in soot particles within a laminar co-flow propane diffusion flame were investigated experimentally. Soot was sampled along the flame axis and collected on a quartz-fiber filter. The soot nanostructure, i.e., the graphitic crystallite size and the amorphous carbon content in the soot particles, was characterized by Raman spectroscopy. The spatial distributions of the fuel, PAHs, soot and OH measured by laser diagnostic techniques are presented to examine the effect of the flame structure on the soot nanostructure. The Raman spectra of the soot show that both the graphitic crystallite size and the amorphous carbon content in the soot particles increase during the soot growth process. In addition, the crystallite size and the amorphous carbon content simultaneously decrease during the soot oxidation process by OH. HRTEM images of soot particles support that these findings obtained from the Raman spectroscopy are reasonable. The influences of the soot formation and oxidation history within the flame on the soot nanostructure are validated.
Late-evaporating liquid fuel wall-films are considered a major source of soot in spark-ignition direct-
injection (SIDI) engines. In this study, a direct-injection model experiment was developed to visualize soot
formation in the vicinity of evaporating fuel films. Isooctane is injected by a multi-hole injector into the
optically accessible part of a constant-flow facility at atmospheric pressure. Some of the liquid fuel impinges
on the quartz-glass windows and forms fuel films. After spark ignition, a turbulent flame front propagates
through the chamber, and subsequently sooting combustion is detected near the fuel films. Overlapping laser
light sheets at 532 and 1064 nm excite laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons
(PAH) -potential soot precursors- and laser-induced incandescence (LII) of soot, respectively. The 532 nm
light sheet has low fluence to avoid the excitation of LII. The LII and LIF signals are detected simultaneously
and spectrally separated on two cameras. In complementary line-of-sight imaging, the fuel spray, chemiluminescence,
and soot incandescence are captured with a high-speed color camera. In separate experiments,
toluene is added to the isooctane as a fluorescent tracer and excited by pulsed 266 nm flood illumination. From
images of the LIF signal, the fuel-films’ thickness and mass evolutions are evaluated. The films survive the entire
combustion event. PAH LIF is found in close vicinity of the evaporating fuel films. Soot is found spatially
separated from, but adjacent to the PAH, both with high spatial intermittency. Average images additionally
indicate that soot is formed with a much higher spatial intermittency than PAH. Images from the color
camera show soot incandescence earlier and in a similar region compared to soot LII. Chemiluminescence
downstream of the soot-forming region is thought to indicate the subsequent oxidation of fuel, soot, and
PAH.
div class="section abstract"> Although accumulated in-cylinder soot can be measured by various optical techniques, discerning soot formation rates from oxidation rates is more difficult. Various optical measurements have pointed toward ways to affect in-cylinder soot oxidation, but evidence of effects of operational variables on soot formation is less plentiful. The formation of soot and its precursors, including polycyclic aromatic hydrocarbons (PAHs), are strongly dependent on temperature, so factors affecting soot formation may be more evident at low-temperature combustion conditions. Here, in-cylinder PAHs are imaged by planar laser-induced fluorescence (PAH-PLIF) using three different excitation wavelengths of 355, 532, and 633 nm, to probe three different size-classes of PAH from 2-3 to 10+ rings. Simultaneous planar laser-induced incandescence of soot (soot-PLII) using 1064-nm excitation provides complementary imaging of soot formation near inception. To achieve low combustion temperatures at the threshold of PAH and soot formation, the engine operating conditions are highly diluted, with intake-O<sub>2</sub> mole-fractions as low as 7.5%. The optical diagnostics show that increasing dilution delays the inception of PAH by over 2.5 ms as the intake-O<sub>2</sub> mole-fraction decreases from 15.0% to 9.0%. At 7.5% intake-O<sub>2</sub>, no large PAH or soot are formed, while the 9.0% intake-O<sub>2</sub> condition forms PAH but virtually no detectable soot. Conditions with 10.0% or more intake-O<sub>2</sub> form both PAH and soot. For the threshold-sooting condition with 10.0% intake-O<sub>2</sub>, large PAH typically forms broadly throughout the cross-section of the downstream jets and along the bowl-wall. Soot appears after PAH, and in narrower ribbons in the jet-jet interaction region. These soot ribbons are on the periphery of the PAH, near the diffusion flame, where the highest temperatures are expected. With increasing intake-O<sub>2</sub>, the delay time between soot and PAH shortens, and soot tends to shift upstream to the jet region prior to wall impingement, though still on the periphery of the PAH. The spatial distributions of PAH and soot overlap slightly under these threshold-sooting conditions, with soot typically surrounding the PAH. This may indicate that temperatures are only high enough for soot formation on the jet periphery, near the diffusion flame. The minimal overlap also suggests that PAHs are rapidly consumed and/or adsorbed when soot is formed. Additionally, increasing the fuel-injection pressure from 533 to 800 and then to 1200 bar increases soot and large PAH formation, which is opposite to the trend for conventional diesel combustion.
</div
Exhaust gas recirculation (EGR) has been widely used in engine to meet current emission regulations. Investigating the chemical mechanism of EGR on PAHs (polycyclic aromatic hydrocarbons, the precursor of soot) formation in premixed flames contributes to understanding the EGR-dependence on soot formation in engine. In this study, the influences of flame temperature, equivalence ratio and CO2 addition on the formation of PAHs was systematically investigated in premixed C2H4/O2/Ar/CO2 flames using laser induced fluorescence (LIF) technology. The temperature dependence of PAHs formation was studied at fixed equivalence ratio and dilution ratio. It was found that the LIF signal of PAH reaches the maximum value around 1730 K, and decreases at lower or higher temperature in this study. The LIF signal of PAHs almost increases linearly with equivalence ratio, as the maximum flame temperature and dilution ratio are kept constant. The experimental results show that the CO2 addition in the inlet gas suppresses PAHs formation due to the chemical inhibition effect. The thermal effect of CO2 addition on PAHs formation is highly sensitive to flame temperature. The PAHs reaction mechanisms proposed by Appel et al. and Wang et al. are used to clarify the experimental results. The first-order temperature sensitivity analysis showed that the hydrogen-abstraction-carbon-addition pathway with high reaction reversibility should account for temperature effects on PAHs formation. The pathway sensitivity analysis showed that CO2 inhibition chemical effect is realized thought the route CO2 (+H) → OH → C3H3 (C2H2) → A1 → PAHs with the assistance of the entrance reaction CO2+H=CO+OH.
The goal of this work is to understand the effect of fuel molecular structure on soot precursors and soot in an axisymmetric, co-flow, laminar flame configuration at atmospheric pressure with partially-premixed fuel jets. Five fuels with varying molecular structure are investigated: methylcyclohexane/. n-dodecane mixture, n-heptane/. n-dodecane mixture, iso-octane/. n-dodecane mixture, m-xylene/. n-dodecane mixture and pure n-dodecane. The flames investigated have jet equivalence ratios of 24 and 6. The total carbon flow rate and carbon fraction of the two components are kept constant to facilitate comparison among fuels. A laser-induced fluorescence technique is used to obtain spatially-resolved polycyclic aromatic hydrocarbons, soot precursors, in the jet flames. The polycyclic aromatic hydrocarbons are identified into two classes: single/two ring aromatics (small) and aromatics having three to five rings (large). A laser-induced incandescence and laser extinction technique are applied to obtain two-dimensional soot volume fraction for all the flames. The experimental results indicate that the level of soot is highest for the m-xylene/. n-dodecane fuel at approximately three times the peak soot levels in the paraffinic fuels. The data show the effects of premixing on the spatial distribution of aromatic species and soot, including the shift from a soot field that peaks near the flame front to one that has maximum soot volume fractions near the centerline in m-xylene/. n-dodecane flame. The iso-octane/. n-dodecane and methylcyclohexane/. n-dodecane fuels show a similar transition of soot field that has an annular peak in the non-premixed flame to a more uniform soot field under premixed conditions. Normalizing the maximum soot volume fraction by the maximum for the n-dodecane base fuel, the data shows that, within measurement uncertainty, the effect of fuel structure on maximum soot volume fraction is independent of the equivalence ratio of the fuel jet.
A gliding arc discharge can be generated between two diverging electrodes and extended by a turbulent gas flow to form a plume of stable non-thermal plasmas sustained at atmospheric pressure. Gliding arc discharge is rather complicated since it involves plasma chemistry, flow dynamics and discharge-turbulence interaction. Optical techniques, especially laser-based methods, are able to perform in-situ and non-intrusive diagnostics with high temporal and spatial resolutions, as well as species-specific visualization, for gaining a deeper inside of discharge characteristics. Spatiotemporally resolved characteristics of the gliding arc discharge were investigated by recording the voltage and current waveforms simultaneously synchronized with instantaneous images of the plasma columns captured by high-speed photography. The true instantaneously optical and electrical parameters of the highly dynamic plasma columns can be revealed. Three-dimensional (3D) particle tracking velocimetry (PTV) has been used to determine the 3D flow velocity in the gliding arc discharge. Instantaneous 3D velocities of the tracer particles and the 3D structure of the plasma columns are reconstructed so as to determine the 3D plasma length and the 3D slip velocity. OH is an important radical that can be generated by gliding arc discharges in humid air. Planar laser-induced fluorescence measurements demonstrate that ground-state OH is distributed around the plasma column with a hollow structure and that the thickness of the OH is much greater than that of the plasma column. The gas temperature of the gliding arc discharge was obtained using planar laser-induced Rayleigh scattering. Turbulent effects were found to play an important role in determining the OH distribution and the dynamics of the gliding arc discharge.
Polycyclic Aromatic Hydrocarbons (PAHs) play a significant role in the chemistry of the interstellar medium (ISM) as well as in hydrocarbon combustion. These molecules can have high levels of diversity with the inclusion of heteroatoms and the addition or removal of hydrogens to form charged or radical species. There is an abundance of data on the cationic forms of these molecules, but there have been many fewer studies on the anionic species. The present study focuses on the anionic forms of deprotonated PAHs. It has been shown in previous work that PAHs containing nitrogen heteroatoms (PANHs) have the ability to form valence excited states giving anions electronic absorption features. This work analyzes how the isoelectronic pure PAHs behave under similar structural constructions. Singly-deprotonated forms of benzene, naphthalene, anthracene, and teteracene classes are examined. None of the neutral-radicals possess dipole moments large enough to support dipole-bound excited states in their corresponding closed-shell anions. Even though the PANH anion derivatives support valence excited states for three-ringed structures, it is not until four-ringed structures of the pure PAH anion derivatives that valence excited states are exhibited. However, anisotropically-extended PAHs larger than tetracene will likely exhibit valence excited states. The relative energies for the anion isomers are very small for all of the systems in this study.
The electronic emission characteristics of 13 gas-phase PAHs, ranging from phenlylacetylene to rubicene, were investigated to diagnose laser induced fluorescence (LIF) spectra of PAHs in flame by DFT, TD-DFT and premixed flame modeling methods. It was found that the maximum emission wavelengths of the PAHs with 5-membered ring are located in visible region and insensitive to the number of C atom. However, the fluorescence wavelengths of the PAHs without 5-membered ring increase with the number of C atom due to the reduced HOMO-LUMO gap. In addition, the fluorescence wavelength of the PAHs without 5-membered ring with linear arrangement is longer than that of PAHs with non-linear arrangement. According to the Franck-Condon principle, the vibrationally-resolved electronic fluorescence spectra were obtained. The results show that fluorescence bandwidth of the PAHs with 5-membered ring is much broader than that of the PAHs without 5-menbered ring. The concentration of PAHs was calculated using the premixed flat-flame model with KM2 mechanism. Based on the fluorescence bandwidth and the concentration of the PAHs, the potentially fluorescence distribution of PAHs in flame was mapped. One can distinguish the specific PAHs according to the mapped fluorescence distribution of PAHs in this study. It was found that naphthalene should be responsible for the fluorescence located in 312-340 nm region in flame. 1-ethynylnaphthalene is the most possible candidate to emit the fluorescence located in 360-380 nm region. The fluorescence signals with the wavelength longer than 500 nm are likely emitted by the PAHs with 5-membered ring. This study contributes to enhance the selectivity of PAHs in LIF technology, especially the visible region.
A skeletal reaction mechanism of n-heptane combustion is developed and validated. The axisymmetric laminar co-flow diffusion flames of n-heptane/oxygen are simulated using the skeletal mechanism at elevated pressure, and the chemical reaction paths in three reaction zones of flame are presented. The n-heptane/oxygen flame height decreases from 8.8 to 5.6 mm with the increase of pressure from 0.1 to 2 MPa, and the Roper's formulation is not expected for the prediction of n-heptane/oxygen flame height. The maximum volume fraction of soot increases with pressure as fv,max ∞ P2. The ratio of flame height to soot oxidation length of the n-heptane/oxygen flames is close to unity (1.037-1.121).
Computational fluid dynamic (CFD) simulations that include realistic combustion/emissions chemistry hold the promise of significantly shortening the development time for advanced high-efficiency, low-emission engines. However, significant challenges must be overcome to realize this potential. This paper discusses these challenges in the context of diesel combustion and outlines a technical program based on the use of surrogate fuels that sufficiently emulate the chemical complexity inherent in conventional diesel fuel. The essential components of such a program are discussed and include: (a) surrogate component selection; (b) the acquisition or estimation of requisite elementary chemical kinetic, thermochemical, and physical property data; (c) the development of accurate predictive chemical kinetic models, together with the measurement of the necessary fundamental laboratory data to validate these mechanisms; and (d) mechanism reduction tools to render the coupled chemistry/flow calculations feasible. In parallel to these efforts, the need exists to develop similarly robust models for fuel injection and spray processes involving multicomponent mixtures of wide distillation character, as well as methodologies to include all of these high fidelity submodels in computationally efficient CFD tools. Near- and longer-term research plans are proposed based on an application target of premixed diesel combustion. In the near term, the recommended surrogate components include n-decane, iso-octane, methylcyclohexane, and toluene. For the longer term, n-hexadecane, heptamethylnonane, n-decylbenzene, and 1-methylnaphthalene are proposed.
A high pressure vessel and diffusion burner (HPVB) were designed, constructed and integrated with an evaporation system, to enable laminar flames studies of gaseous and vaporized liquid fuels. This experimental setup is designed to offer ample optical accessibility for laser diagnostics techniques; its capabilities and specifications are described. Soot volume fraction measurements are performed with line-of-sight attenuation (LOSA) and validated using data of ethylene flames from literature. The first measurements of a vaporized liquid fuel (n-heptane) are reported. A stable laminar n-heptane flame is achieved with a maximum standard deviation in soot volume fraction of 0.04 ppm.
A sooting, ethylene coflow diffusion flame has been studied both experimentally and computationally. The fuel is diluted with nitrogen and the flame is slightly fifted to minimize the effects of the burner. Both probe (thermocouple and gas-sampling techniques) and optical diagnostic methods (Rayleigh scattering and laser-induced incandescence) are used to measure the temperature, gas species, and soot volume fractions. A detailed soot growth model in which the equations for particle production are coupled to the flow and gaseous species conservation equations has been used to investigate soot formation in the flame. The two-dimensional system couples detailed transport and finite-rate chemistry in the gas phase with the aerosol equations in the sectional representation. The formulation includes detailed treatment of the transport, inception, surface growth, oxidation, and coalescence of soot particulates. Effects of thermal radiation and particle scrubbing of gas-phase growth and oxidation species are also included. Predictions and measurements of temperature, soot volume fractions, and selected species are compared over a range of heights and as a function of radius. The formation of benzene is primarily controlled by the recombination of propargyl radicals, and benzene production rates are found to limit the rate of inception, as well as the net rate of soot growth. The model predicted soot volume fractions well along the wings of the flame but underpredicted soot volume fractions by a factor of four along the centerline. Oxidation of particulates is dominated by reactions with hydroxyl radicals that attain levels approximately ten times higher than calculated equilibrium levels. Gas cooling effects due to radiative loss are shown to have a very significant effect on predicted temperatures.
Laminar, sooting, ethylene-fuelled, co-flow diffusion flames at atmospheric pressure have been studied experimentally and theoretically as a function of fuel dilution by inert nitrogen. The flames have been investigated experimentally using a combination of laser diagnostics and thermocouple-gas sampling probe measurements. Numerical simulations have been based on a fully coupled solution of the flow conservation equations, gas-phase species conservation equations with complex chemistry and the dynamical equations for soot spheroid growth. Predicted flame heights, temperatures and the important soot growth species, acetylene, are in good agreement with experiment. Benzene simulations are less satisfactory and are significantly under-predicted at low dilution levels of ethylene. As ethylene dilution is decreased and soot levels increase, the experimental maximum in soot moves from the flame centreline toward the wings of the flame. Simulations of the soot field show similar trends with decreasing dilution of the fuel and predicted peak soot levels are in reasonable agreement with the data. Computations are also presented for modifications to the model that include: (i) use of a more comprehensive chemical kinetics model; (ii) a revised inception model; (iii) a maximum size limit to the primary particle size; and (iv) estimates of radiative optical thickness corrections to computed flame temperatures.
This paper provides an overview of a workshop focused on fundamental experimental and theoretical aspects of soot measurements by laser-induced incandescence (LII). This workshop was held in Duisburg, Germany in September 2005. The goal of the workshop was to review the current understanding of the technique and identify gaps in this understanding associated with experimental implementation, model descriptions, and signal interpretation. The results of this workshop suggest that uncertainties in the understanding of this technique are sufficient to lead to large variability among model predictions from different LII models, among measurements using different experimental approaches, and between modeled and measured signals, even under well-defined conditions. This article summarizes the content and conclusions of the workshop, discusses controversial topics and areas of disagreement identified during the workshop, and highlights recent important references related to these topics. It clearly demonstrates that despite the widespread application of LII for soot-concentration and particle-size measurements there is still a significant lack in fundamental understanding for many of the underlying physical processes.
“The laser-induced incandescence (LII) signal is proportional to soot volume fraction” is an often used statement in scientific
papers, and it has – within experimental uncertainties – been validated in comparisons with other diagnostic techniques in
several investigations. In 1984 it was shown theoretically in a paper by Melton that there is a deviation from this statement
in that the presence of larger particles leads to some overestimation of soot volume fractions. In the present paper we present
a detailed theoretical investigation of how the soot particle size influences the relationship between LII signal and soot
volume fraction for different experimental conditions. Several parameters have been varied; detection wavelength, time and
delay of detection gate, ambient gas temperature and pressure, laser fluence, level of aggregation and spatial profile. Based
on these results we are able, firstly, to understand how experimental conditions should be chosen in order to minimize the
errors introduced when assuming a linear dependence between the signal and volume fraction and secondly, to obtain knowledge
on how to use this information to obtain more accurate soot volume fraction data if the particle size is known.
The accuracy of laser-induced incandescence (LII) measurements is significantly influenced by the calibration process and
the laser profile degradation due to beam steering. Additionally, the wavelength used for extinction measurements, needed
for LII calibration, is critical and should be kept as high as possible in order to avoid light absorption by molecular species
in the flame. The influence of beam steering on the LII measurement was studied in turbulent sooting C2H4/air flames at different pressures. While inhomogeneities in the laser profile become smoothed out in time-averaged measurements,
especially at higher pressure, the corresponding single-shot beam profiles reveal an increasing effect of beam steering. In
the current configuration it was observed that the resulting local laser fluence remains within certain limits (30% to 200%)
of the original value. Asufficiently high incident laser fluence can thus prevent the local fluence from dropping below the
LII threshold value of approximately 0.3J/cm2 at the cost of increased soot surface vaporization. Aspatial resolution in the dimension of the sheet thickness of below
1mm cannot be guaranteed at increased pressure of 9 bars due to beam steering. Afeasibility study in a combustor at technical
conditions demonstrates the influence of both effects beam steering and choice of calibration wavelength and led to the conclusion
that, however, ashot-to-shot calibration of LII with simultaneously measured extinction can be realized.
A non-sooting lifted methane/air coflowing non-premixed flame has been studied experimentally and computationally. The flame structure was computed using a model that solves the fully elliptic governing equations, utilizes detailed transport coefficients and a chemical kinetic mechanism with C1 to C6 hydrocarbons, and includes an optically thin radiation submodel. Gas temperature, major species mole fractions, and non-fuel hydrocarbon concentrations were experimentally mapped in two dimensions with both probe techniques (thermocouples and on-line mass spectrometry) and optical diagnostics (Rayleigh and Raman scattering). A differential polarization strategy was used to remove C2 and polycyclic hydrocarbon fluorescence interferences from the Raman scattering signals, which dramatically improved the quality of the laser diagnostic images over what had previously been possible. Good agreement was observed between the probe and laser images: this validates the Rayleigh-Raman data processing procedure, and it shows that the probes produce negligible perturbations to the flame structure. The spatial precision of the data and range of measured quantities provides a sensitive test of the computations. Nonetheless, the model reproduces most of the experimental observations, including the overall flame height and liftoff height, the peak concentrations and spatial distributions of major species, and the peak concentrations of oxygenated hydrocarbon intermediates such as ketene and soot precursors such as benzene and acetylene.
This paper proposes a design space abstraction, in order to decouple the exploration algorithm from the design space, which allows the application of the design space exploration (DSE) tool in different design scenarios and is appropriate for representing simultaneous and interdependent design alternative. From this new abstraction, the Model-Driven Engineering (MDE) approach is exploited to extract design information and compose the design space to be explored. Our approach uses model-to-model transformation rules as DSE constraints, which prune the available design space. These constraints are automatically generated from the UML model by translating design decisions pre-specified in UML diagrams into model transformation rules. In addition, non-functional requirements specified in UML as stereotypes are used to generate constraints in order to remove invalid solutions proposed during the DSE process. Finally, our approach offers an easy way for the designer to extend the set of constraints by using a well-accepted MDE toolset. A real application running on top of an MPSoC is used as case study to illustrate the proposed method.
A non-sooting lifted methane/air coflowing non-premixed flame has been studied experimentally and computationally. The flame structure was computed using a model that solves the fully elliptic governing equations, utilizes detailed transport coefficients and a chemical kinetic mechanism with C-1 to C-6 hydrocarbons, and includes an optically thin radiation submodel. Gas temperature, major species mole fractions, and non-fuel hydrocarbon concentrations were experimentally mapped in two dimensions with both probe techniques (thermocouples and on-line mass spectrometry) and optical diagnostics (Rayleigh and Raman scattering). A differential polarization strategy was used to remove C-2 and polycyclic hydrocarbon fluorescence interferences from the Raman scattering signals, which dramatically improved the quality of the laser diagnostic images over what had previously been possible. Good agreement was observed between the probe and laser images: this validates the Rayleigh-Raman data processing procedure, and it shows that the probes produce negligible perturbations to the flame structure. The spatial precision of the data and range of measured quantities provides a sensitive test of the computations. Nonetheless, the model reproduces most of the experimental observations, including the overall flame height and liftoff height, the peak concentrations and spatial distributions of major species and the peak concentrations of oxygenated hydrocarbon intermediates such as ketene and soot precursors such as benzene and acetylene.
We investigate experimentally and computationally the OH and CH radical fields in a twodimensional, axisymmetric, laminar, methane-air diffusion flame in which a cylindrical fuel stream is surrounded by a coflowing oxidizer jet. Experimentally, laser induced fluorescence is used to generate profiles of the minor species. Computationally, we apply a detailed chemistry, complex transprot combustion model to predict the temperature, veocity and species concentrations as a function of the two coordinate directions. Unlike some models in which diffusion in the axial direction is neglected, we treat the fully elliptic problem. Results of the study include a two-dimensional comparison of the location and magnitude of the OH and CH radicals and a mole fraction versus mixture fraction correlation between radial slices of the coflow flame and a set of corresponding counterflow flames.
Fluorescence spectra were obtained in laminar, gaseous, rich premixed and diffusion flames employing different harmonics of a pulsed Nd: YAG laser. Spectra obtained with the fourth harmonic at lambda0 = 266 nm exhibit a peak in the u.v. at lambda = 320 nm and a broader one in the visible with a maximum beyond 400 nm. The relative role of these two features has been followed in rich premixed flames with different C:O ratios and heights above the burner and along the radius of a cylindrical, coflowing diffusion flame. The u.v. fluorescence detected in our flames should be attributed to structures containing no more than two aromatic rings, while the fluorescence in the visible is attributed to larger aromatic structures. Fluorescence and absorption spectra obtained from the different fractions of the material sampled into the flames showed qualitative agreement with the ''in situ'' spectra. Fluorescence spectra in the visible, originating from structures heavier than 300 a.m.u., has been also detected in the sampled material but are not considered the main cause of the visible fluorescence because their low quantum efficiency for u.v. excitation.
We have measured sooting tendencies of 72 nonvolatile aromatic hydrocarbons, only five of which have been previously reported in the literature. The tested compounds include long-chain alkylbenzenes up to tridecylbenzene, methyl-substituted benzenes, naphthalenes, biaryls, and polycyclic aromatic hydrocarbons (PAH) with up to four rings. Sooting tendency was defined as the maximum soot concentration fv,max in a methane/air coflow nonpremixed flame with 5–80ppm of the aromatic added to the fuel. The fv,max were converted into Yield Sooting Indices (YSI’s) by the equation YSI=C∗fv,max+D, where C and D are constants chosen so that YSI-2-heptanone=17 and YSI-phenanthrene=191. The aromatics were dissolved in 2-heptanone and added to the fuel mixture with a syringe pump. Soot concentrations were measured with laser-induced incandescence (LII). The burner and fuel lines were heated; time-resolved soot measurements verified that all of the test compounds were quantitatively transmitted to the flame without losses to the walls. The uncertainties in the results range from ±3 to ±10%.
Comparison between laser-induced fluorescence (LIF) and laser-induced incandescence (LII) images and axial intensity profiles derived from these images of a laminar ethylene—air diffusion flame distinguish the polycyclic aromatic hydrocarbon (PAH) and soot-containing regions along the axial flow streamline. Examination of the temporal evolution of the combined fluorescence plus incandescence signal along the axial streamline reveals the transformation of soot precursor material into solid carbonaceous soot. Bright field transmission electron microscopy (TEM) of thermophoretically sampled material shows soot precursor material and more generally nascent soot particles and their evolution towards solid carbonaceous soot. Corresponding dark field TEM images track the increasing crystallinity of the soot precursor material as carbonization proceeds with increasing integrated temperature-time history within the flame. Both bright and dark field TEM confirm interpretation of the optical signals as reflecting the overall material transformation from soot precursor material toward solid carbonaceous soot.
Centerline profiles of gas temperature, C1 to C12 nonfuel hydrocarbon concentrations, polycyclic aromatic hydrocarbon (PAH) laser-induced fluorescence (LIF), and soot volume fraction are reported for coflowing ethylene nonpremixed and partially premixed flames with primary equivalence ratios ranging from 24 to 3. Concentrations of acetylene and C4 hydrocarbons were lower in nearly all of the partially premixed flames than in the nonpremixed flame, whereas concentrations of methane and C3H4 were larger in all of the partially premixed flames than in the nonpremixed flame. These results indicate that the primary effect of partial premixing is not to uniformly increase the concentrations of pyrolysis products, but to shift the pyrolysis mechanism towards odd-carbon species. The concentration of benzene was larger in several of the richer partially premixed flames than in the nonpremixed flame, probably because the shift in pyrolysis mechanism enhances self-reaction of C3H3 radicals. Increases in soot volume fraction and other aromatics were observed that matched the increases in benzene. Profiles of PAH fluorescence agreed closely with those for specific gas-phase PAH such as naphthalene, and the maximum PAH signals were a good predictor of the eventual maximum soot volume fractions. Concentrations of oxygenated hydrocarbons such as formaldehyde and ketene were dramatically increased in the partially premixed flames; for formaldehyde this trend was confirmed with in situ LIF measurements.
Soot concentration and temperature distributions within the flame envelope of laminar diffusion flames of methane and ethane at elevated pressures were measured in a high-pressure combustion chamber. Methane measurements were made with two different fuel flow rates: 0.43mg/s (0.32mg/s carbon flow rate) for the pressure range of 15–60atm, and 0.83mg/s for the pressure range of 5–20atm (0.62mg/s carbon flow rate). For the ethane flames, the flow rate was 0.78mg/s (0.62mg/s carbon flow rate) and the pressure range was 2–15atm. From the soot concentration distribution, soot yields were calculated as a function of flame height and pressure. Maximum soot yields from the current study and the previous measurements in similar flames with methane, ethane, and propane flames were shown to display a unified behaviour. Maximum soot yields, when scaled properly, were represented by an empirical exponential function in terms of the reduced pressure, actual pressure divided by the critical pressure of the fuel. The maximum soot yield seems to reach a plateau asymptotically as the pressure exceeds the critical pressure of the fuel.
Using synchrotron radiation as a continuum light source, we have measured the absolute photoabsorption cross sections of methane (CH4) and ethane (C2H6) from their respective absorption thresholds to , with a spectral bandwidth (FWHM) of and at three different temperatures, i.e., 370, 295, and . Only moderate temperature effects are observed in the changes of cross-section values of these two molecules and are attributed to their high vibrational frequencies of the ground electronic states and their repulsive potential surfaces of the excited electronic states. When the gas temperature decreases from 360 to , the percentage changes of cross sections amount to a maximum of ±30% in CH4 at and ±20% in C2H6 at 142.3 and . The well-known vibrational progressions of C2H6 exhibit pronounced temperature effects in their band profiles which become narrower and sharper as the gas temperature decreases. The data presented are an extension of our effort to provide the required data to the planetary atmospheres community and will have an important impact on our understanding of the atmospheres of the giant planets.
Simultaneous images of laser-induced fluorescence (LIF) due to polycyclic aromatic hydrocarbons (PAHs) and laser-induced incandescence (LH) visualization of soot concentrations are presented for laminar gas-jet diffusion flames. Spatially integrated measurements reveal similar spectral emission for LIF and LII, but vastly different time scales associated with radiative decay. Comparison of spatially resolved images using either 266-nm or 1064-nm excitation light reveals distinct regions of molecular fluorescence and soot incandescence. Consideration of photophysical properties of PAHs suggests that the fluorescence wavelength distribution is dependent on the size of the PAH. Using different detection spectral bands, spatially resolved regions attributed to different-sized PAHs are presented. The spatial distribution of PAH size is consistent with the putative growth mechanisms of PAHs. In the region between the LIF due to PAHs and LII due to soot, a dark zone is observed that is attributed to the presence of soot precursor particles. Current understanding of soot formation indicates that through both physical and/or chemical condensation, large PAHs react first to form soot precursor particles prior to the formation of soot particles. Transmission electron microscopy analysis of thermophoretically collected material from within this dark region confirmed the presence of soot precursor particles 2 to 5 nm in diameter.
Two critical steps towards soot production in combustors are the decomposition of the fuel and the subsequent formation of aromatic hydrocarbons with one to three benzenoid rings. Traditionally, flame studies of these processes have used small hydrocarbons such as methane, ethylene, and acetylene as the fuel. However, recent research, which is reviewed in this article, has begun to close the ‘gap’ between these small hydrocarbons and the larger, more complex hydrocarbons that constitute all liquid combustion fuels.
Laminar nonpremixed methane–air flames were studied over the pressure range of 0.5 to 4 MPa using a new high-pressure combustion chamber. Flame characterization showed very good flame stability over the range of pressures, with a flame tip rms flicker of less than 1% in flame height. At all pressures, soot was completely oxidized within the visible flame. Spectral soot emission (SSE) and line-of-sight attenuation (LOSA) measurements provided radially resolved measurements of soot volume fraction and soot temperature at pressures from 0.5 to 4.0 MPa. Such measurements provide an improved understanding of the influence of pressure on soot formation and have not been reported previously in laminar nonpremixed flames for pressures above 0.4 MPa. SSE and LOSA soot concentration values typically agree to within 30% and both methods exhibit similar trends in the spatial distribution of soot concentration. Maximum soot concentration depended on pressure according to a power law, where the exponent on pressure is about 2 for the range of pressures between 0.5 and 2.0 MPa, and about 1.2 for 2.0 to 4.0 MPa. Peak carbon conversion to soot also followed a power-law dependence on pressure, where the pressure exponent is unity for pressures between 0.5 and 2.0 MPa and 0.1 for 2.0 to 4.0 MPa. The pressure dependence of sooting propensity diminished at pressures above 2.0 MPa. Soot concentrations measured in this work, when transformed to line-integrated values, are consistent with the measurements of Flower and Bowman for pressures up to 1.0 MPa [Proc. Combust Inst. 21 (1986) 1115–1124] and Lee and Na for pressures up to 0.4 MPa [JSME Int. J. Ser. B 43 (2000) 550–555]. Soot temperature measurements indicate that the overall temperatures decrease with increasing pressure; however, the differences diminish with increasing height in the flame. Low down in the flame, temperatures are about 150 K lower at pressures of 4.0 MPa than those at 0.5 MPa. In the upper half of the flame the differences reduce to 50 K.
The effects of pressure on soot formation and the structure of the temperature field were studied in co-flow methane–air laminar diffusion flames over a wide pressure range, from 10 to 60 atm in a high-pressure combustion chamber. The selected fuel mass flow rate provided diffusion flames in which the soot was completely oxidized within the visible flame envelope and the flame was stable at all pressures considered. The spatially resolved soot volume fraction and soot temperature were measured by spectral soot emission as a function of pressure. The visible (luminous) flame height remained almost unchanged from 10 to 100 atm. Peak soot concentrations showed a strong dependence on pressure at relatively lower pressures; but this dependence got weaker as the pressure is increased. The maximum conversion of the fuel’s carbon to soot, 12.6%, was observed at 60 atm at approximately the mid-height of the flame. Radial temperature gradients within the flame increased with pressure and decreased with flame height above the burner rim. Higher radial temperature gradients near the burner exit at higher pressures mean that the thermal diffusion from the hot regions of the flame towards the flame centerline is enhanced. This leads to higher fuel pyrolysis rates causing accelerated soot nucleation and growth as the pressure increases.
A detailed soot growth model in which the equations for particle production have been coupled to the flow and gaseous species conservation equations has been developed for an axisymmetric, laminar, coflow diffusion flame. Results from the model have been compared to experimental data for a confined methane–air flame. The two-dimensional system couples detailed transport and finite rate chemistry in the gas phase with the aerosol equations in the sectional representation. The formulation includes detailed treatment of the transport, inception, surface growth, oxidation, and coalescence of soot particulates. Effects of thermal radiation and particle scrubbing of gas-phase growth and oxidation species are also included. Predictions and measurements of temperature, soot volume fractions, and selected species are compared over a range of heights and as a function of radius. Flame heights are somewhat overpredicted and local temperatures and volume fractions are underpredicted. We believe the inability to reproduce accurately bulk flame parameters directly inhibits the ability to predict soot volume fractions and these differences are likely a result of uncertainties in the experimental inlet conditions. Predictions of the distributions of particle sizes indicate the existence of (relatively) low-molecular-weight species along the centerline of the burner and trace amounts of the particles that escape from the flame, unoxidized. Oxidation of particulates is dominated by reactions with hydroxyl radicals which attain levels approximately 10 times higher than calculated equilibrium levels. Gas cooling effects due to radiative loss are shown to have a very significant effect on predicted soot concentrations.
Simultaneous combined laser-induced fluorescence and laser-induced incandescence (LIF-LII) images are presented for both a normal and inverse diffusion flame. The excitation wavelength dependence distinguishes the LIF and LII signals in images from the normal diffusion flame while the temporal, decay distinguishes the signals in images of the inverse diffusion flame. Each flame presents a minimum in the combined LIF-LII intensity in a region separating the fuel pyrolysis and soot containing regions. Opacity, geometric definition, and extent of crystallinity measured through both bright and dark field transmission electron microscopy (TEM) characterizes the thermophoretically sampled material from within this minimal LIF-LII intensity region. TEM analysis reveals rather different soot processes occurring within the normal and inverse diffusion flame. In the normal diffusion flame, rapid chemical and physical coalescence of PAHs results in initial formation of soot precursor particles that are highly crystalline and evolve toward fully formed soot. In the inverse diffusion flame, rapid coalescence of pyrolysis products occurs, producing tarlike, globular structures equivalent in size to fully formed soot aggregates but with markedly less crystallinity than normal-appearing soot. These different material properties are interpreted as reflecting different relative rates of chemical and physical coalescence of fuel pyrolysis products versus carbonization. Significantly, these TEM images support qualitative photophysical arguments suggesting that, in general, this “dark” region observed in the LIF-LII images demarcates a transitional region in which a fundamental change in the material chemical/physical properties occurs between solid carbonaceous soot and condensed or gaseous molecular growth material.
Laminar, sooting, coflow diffusion flames at atmospheric pressure have been studied experimentally and theoretically as a function of fuel dilution by inert nitrogen. The flames have been investigated with laser diagnostics. Laser extinction has been used to calibrate the experimental soot volume fractions and an improved gating method has been implemented in the laser-induced incandescence (LII) measurements resulting in differences to the soot distributions reported previously. Numerical simulations have been based on a fully coupled solution of the flow conservation equations, gas-phase species conservation equations with complex chemistry, and the dynamical equations for soot spheroid growth. The model also includes the effects of radiation reabsorption through an iterative procedure. An investigation of the computed rates of particle inception, surface growth, and oxidation, along with a residence time analysis, helps to explain the shift in the peak soot volume fraction from the centerline to the wings of the flame as the fuel fraction increases. The shift arises from changes in the relative importance of inception and surface growth combined with a significant increase in the residence time within the annular soot formation field leading to higher soot volume fractions, as the fuel fraction increases.
In this study, we extended previous numerical and experimental investigations of an axisymmetric laminar diffusion flame to assess the role of buoyancy and dilution in flame properties such as temperature, fuel and oxygen concentration, and soot volume fraction. Measurements were made both in normal gravity and on the NASA KC-135 reduced-gravity aircraft. Computations of temperature and major species were performed with a two-dimensional, axisymmetric flame model using a 26-species C2 hydrocarbon mechanism. This set of temperature and major species measurements affords the most extensive set of comparisons with flame computations to date. Results indicate that the predicted temperature profiles are in excellent agreement with measurement in both normal gravity and microgravity flames at low dilution levels. In these well-predicted flames, however, subtle differences existed in fuel/air mixing between measurement and computation, in which the contrast was most visible in normal gravity. As the fuel stream was diluted, the computations began to lose their predictive ability, again most markedly in normal gravity.Additionally, relative soot volume fraction was measured with laser-induced incandescence (LII) in both normal gravity and in the time-varying gravitational field provided by the KC-135. Results indicate that the soot concentration and distribution are extremely sensitive to the “g-jitter” present, and that the peak soot volume fraction can increase by as much as a factor of 15 in the absence of gravity.
The absorption spectra of gaseous n-alkanes have been investigated for photon energies from 8 to 35 eV where excitations of the σ electrons are expected to occur. For methane and ethane the spectra are compared to reflectance measurements on solid films. A new monochromator designed for work with synchrotron radiation and a windowless absorption cell made it possible to resolve considerable new structures.
Fluorescence spectra excited with the lines of a high-pressure Hg/Xe lamp and of an Argon ion laser have been taken from the pyrolysis region of an n-heptane diffusion flame. An experimental characterization of the pyrolysis region at a given flame's front in terms of broadband fluorescence spectra is represented. An initial attempt of analyzing the fluorescence in terms of the spectral responses of the polycyclic aromatic hydrocarbons (PAH) formed in the combustion process has been performed, and the concentrations of such molecular species have been estimated at different heights over the burner by employing this method of analysis. The fluorescence cross sections and other spectroscopic data for the most fluorescing PAH in diffusion flames are reported. The fluorescence measurements as well as the estimated concentration levels agree with previous determinations in diffusion flames.
Sooting tendencies have been determined for aromatic hydrocarbons using a new definition: the maximum soot volume fraction fv,max measured in a coflow methane/air nonpremixed flame whose fuel is doped with 400 ppm of the test hydrocarbon. These fv,max were converted into apparatus-independent yield sooting indices (YSIs) by the equation YSI=C×fv,max+D, where C and D are apparatus-specific parameters chosen so that YSI-benzene = 30 and YSI-1,2-dihydronaphthalene = 100. The dopants were added to the fuel mixture with a syringe pump and fv,max was measured with laser-induced incandescence. YSI was determined for 6 cycloaliphatics and for 62 aromatics, which included 28 alkylbenzenes, 10 alkenylbenzenes, 10 alkynylbenzenes, 25 multiply substituted benzenes, 6 two-ring aromatics, and 6 substituted benzenes with heteroatoms in the side chains. The YSIs correlate well with literature values of threshold sooting index (TSI), which is a more traditional sooting tendency based on the height of pure-fuelled flames at the smoke point. This agreement indicates that fv,max and smoke height are equivalent measures of sooting tendency and that YSI is largely apparatus-independent. However, the YSIs have a total uncertainty of ±3%, which is substantially better than the TSIs, and the number of aromatic YSIs reported here is more than double the number of aromatic TSIs in the literature. The YSIs depend strongly on molecular structure; thus they provide information about the chemical kinetic reaction mechanisms responsible for fuel decomposition and hydrocarbon growth from a broad cross section of one-ring aromatics. Important naphthalene formation pathways appear to include acetylene addition to ethynylphenyl, propargyl addition to benzyl, and methyl addition to indenyl. This last pathway is particularly significant because it converts indene quantitatively to naphthalene and because the side chains in many alkenylbenzenes and alkynylbenzenes cyclize to form five-membered rings.
As the drive towards a better understanding of airline fuel combustion, and its associated emission characteristics continues, there is a need for fundamental numerical and experimental jet fuel studies. In the present work, a numerical and experimental study is conducted for a complex blended liquid fuel, Jet-A1, in an atmospheric pressure, laminar sooting coflow diffusion flame. The numerical model uses a surrogate mixture, comprising 69% n-decane, 20% n-propylbenzene, and 11% n-propylcyclohexane (by mole). The combustion chemistry and soot formation are solved using a detailed chemical kinetic mechanism with 304 species and 2265 reactions, detailed transport, and a sectional soot model including soot nucleation, heterogeneous surface growth and oxidation, soot aggregate coagulation and fragmentation, and PAH surface condensation. The problem is intractable by serial processing; therefore, distributed-memory parallelization is used, employing 192 CPUs. Experimentally, soot volume fraction and gaseous species concentration profiles are determined by a Laser Extinction Measurement method and Gas Chromatography, respectively, in a coflow diffusion flame of vaporized Jet-A1. These data are used to validate the model. Centerline species concentrations are satisfactorily reproduced by the model. The order of magnitude of the peak soot volume fraction is well predicted without calibrating any of the model constants to the experimental data, but discrepancies remain between numerical and experimental results on the radial locations of the peaks and the centerline soot concentration levels.
The time-delayed detection of soot incandescence is demonstrated to discriminate against other laser-induced signals that have shorter decay times. This technique exhibits high sensitivity and no need for any verification of the spectral content of the signal; it is promising for two-dimensional imaging applications in hostile environments, such as in practical flame and combustion chambers, in which it permits an easy visualization of sooty regions.
Soot measurements in high-pressure diffusion flames of gaseous and liquid fuels
- G Intasopa
Intasopa G. Soot measurements in high-pressure diffusion flames of gaseous
and liquid fuels. Master's thesis, Graduate Department of Aerospace Science
and Engineering, University of Toronto; 2011.
Nonpremixed CH 4 /air laminar flames at pressures up to 4 MPa
- K Thomson
- O Gülder
- E Weckman
- R Fraser
- G Smallwood
- D Snelling
Thomson K, Gülder O, Weckman E, Fraser R, Smallwood G, Snelling D, et al.
Nonpremixed CH 4 /air laminar flames at pressures up to 4 MPa. Combust Flame
2005;140:222-32.
ethane and propane laminar diffusion flames at high pressures
- O Gülder
- G Intasopa
- H Joo
- P Mandatori
- D Bento
- M Vaillancourt
Gülder O, Intasopa G, Joo H, Mandatori P, Bento D, Vaillancourt M, et al. ethane
and propane laminar diffusion flames at high pressures. Combust Flame
2011;158:2037-44.
Motor gasolines technical review
- Chevron
Chevron. Motor gasolines technical review. Tech. rep., Chevron (San Ramon,
CA, USA); 2007.
Modeling non-premixed laminar co-flow flames using flamelet-generated manifolds
- F Beretta
- D 'alessio
- D Orsi
- A Minutolo
Beretta F, D'Alessio A, D'Orsi A, Minutolo P. Modeling non-premixed laminar
co-flow flames using flamelet-generated manifolds. Combust Sci Technol
1992;85:455-70.
- H. De Andrade Oliveira
H. de Andrade Oliveira et al. / Fuel 112 (2013) 145–152
Computational and experimental study of soot formation in a coflow, laminar ethylene diffusion flame
- C Mcenally
- A Schaffer
- M Long
- L Pfefferle
- M Smooke
- M Colket
McEnally C, Schaffer A, Long M, Pfefferle L, Smooke M, Colket M.
Computational and experimental study of soot formation in a coflow,
laminar ethylene diffusion flame. In: The combustion institute 27th
symposium on combustion, vol. 9, 1998. p. 1497-505.
Diesel fuels technical review
- Chevron
Chevron. Diesel fuels technical review. Tech. rep., Chevron (San Ramon, CA,
USA); 2007.
Soot measurements in laminar flames of gaseous and (prevaporized) liquid fuels
- M De Andrade Oliveira
- C Luijten
- L De Goey
de Andrade Oliveira M, Luijten C, de Goey L. Soot measurements in laminar
flames of gaseous and (prevaporized) liquid fuels. In: Szentannai P, Winter F,
editors. Proceedings of the 4th ECM (European combustion meeting), 14-17
april 2009, Vienna, Austria, pp.1-6.