Now showing items 1-20 of 944

• #### Three-stage auto-ignition of n-heptane and methyl-cyclohexane mixtures at lean conditions in a flat piston rapid compression machine

(Proceedings of the Combustion Institute, Elsevier BV, 2020-07-28) [Article]
One approach to enhancing the thermal efficiency of combustion systems is to burn fuels at ultra-lean conditions (equivalence ratio below 0.5). It has been recently reported that the auto-ignition of some hydrocarbon fuels, under specific temperature, pressure, and mixture conditions, releases heat in three distinctive stages. The three auto-ignition stages can be divided as a first low-temperature auto-ignition stage with conventional low temperature, and a high-temperature stage separated into two sub-stages. This study presents ignition delay time measurements of n-heptane and methyl-cyclohexane (MCH) mixtures in a flat piston rapid compression machine (RCM) under ultra-lean conditions. It provides experimental evidence of three-stage auto-ignition. This phenomenon of delayed high-temperature heat release is seldom reported in the literature and this is the first time to be reported for these types of fuels. The experiments cover two binary n-heptane/MCH mixtures of 15/85 and 70/30 by volume, pressures of 11 bar and 16 bar, temperature range of 700 to 900 K, and equivalence ratio of 0.4. The RCM optical access was utilized for high-speed chemiluminescence imaging. Detailed chemical kinetic simulations in a homogenous batch reactor with variable volume were conducted to further interrogate the three-stage auto-ignition phenomenon. Chemiluminescence shows that three-stage auto-ignition occurs in the adiabatically compressed end-gas, which indicates that this phenomenon is chemically-driven and is not induced by a thermal stratification in the RCM experiments. The model predicts the features of three-stage auto-ignition, which were experimentally observed at temperatures approximately below 750 K. As expected, significant discrepancies are observed in the ignition delays of experiment and simulation in the negative temperature coefficient (NTC) region. The simulation of the n-heptane/MCH 70/30 mixture shows better agreement with experiments in the Positive Temperature Coefficient (PTC) region compared to the 15/85 mixture.
• #### Chemical structure of atmospheric pressure premixed laminar formic acid/hydrogen flames

(Proceedings of the Combustion Institute, Elsevier BV, 2020-07-28) [Article]
The work presents an experimental and kinetic modeling study of laminar premixed formic acid [HC(O)OH]/H2/O2/Ar flames at different equivalence ratios (φ=0.85, 1.1 and 1.3) stabilized on a flat burner at atmospheric pressure, as well as laminar flame speed of HC(O)OH/O2/Ar flames (φ=0.5–1.5) at 1 atm. Flame structure as well as laminar flame speed were simulated using three different detailed chemical kinetic mechanisms proposed for formic acid oxidation. The components in the fuel blends show different consumption profiles, namely, hydrogen is consumed slower than formic acid. According to kinetic analysis, the reason of the observed phenomenon is that the studied flames have hydrogen as a fuel but also as an intermediate product formed from HC(O)OH decomposition. Comparison of the measured and simulated flame structure shows that all the mechanisms satisfactorily predict the mole fraction profiles of the reactants, main products, and intermediates. It is noteworthy that the mechanisms proposed by Glarborg et al., Konnov et al. and the updated AramcoMech2.0 adequately predict the spatial variations in the mole fractions of free radicals, such as H, OH O and HO2. However, some drawbacks of the mechanisms used were identified; in particular, they predict different concentrations of CH2O. As for laminar flame speed simulations, the Konnov et al. mechanism predicts around two times higher values than in experiment, while the Glarborg et al. and updated AramcoMech2.0 show good agreement with the experimental data.
• #### A statistical analysis of developing knock intensity in a mixture with temperature inhomogeneities

(Proceedings of the Combustion Institute, Elsevier BV, 2020-07-27) [Article]
Knock formation and its intensity for a stoichiometric ethanol/air mixture under a representative endgas auto-ignition condition in IC engines with temperature inhomogeneities are investigated using multidimensional direct numerical simulations (DNS) with a 40-species skeletal mechanism of ethanol. Two- and three-dimensional simulations are performed by systematically varying temperature fluctuations and its most energetic length scale, lT. The volumetric fraction of the mixture regions that have the propensity to detonation development, FD, is proposed as a metric to predict the amplitude of knock intensity.It isfound that with increasing lT, FD shows a good agreement with the heat release fraction of the mixture regions with pressure greater than equilibrium pressure, FH. The detonation peninsula is well captured by FD and FH when plotting them as a function of the volume-averaged ξ , ξ, (ξ = a/Ssp is the ratio of the acoustic speed, a to the ignition front speed, Ssp). Decreasing lT is found to significantly reduce the super-knock intensity. The results suggest that decreasing lT, as in engines with tumble desig
• #### Understanding multi-stage HCCI combustion caused by thermal stratification and chemical three-stage auto-ignition

(Proceedings of the Combustion Institute, Elsevier BV, 2020-07-23) [Article]
The Homogeneous Charge Compression Ignition (HCCI) concept shows great potential for improving engine efficiency and reducing pollutant emissions. However, the operation with this concept in Internal Combustion (IC) engines is still limited to low speed and load conditions, as excessive Pressure Rise Rates (PRR) are generated with its fast auto-ignition. To overcome this limitation, the use of moderate thermal and charge stratification has been promoted. This leads to multi-stage ignition, and thus a potentially acceptable PRR. Recently Sarathy et al. (2019), three-stage auto-ignition has been emphasized as a chemical phenomenon where the thermal runaway is inhibited during the main ignition event. The current paper demonstrates experimental evidence on this phenomenon observed during n-heptane and n-hexane auto-ignition at lean diluted conditions in a flat piston Rapid Compression Machine (RCM). Multi-stage ignition events caused by either chemical kinetics or by the well-known thermal stratification of this type of RCM are clearly identified and differentiated. The combination of these two factors seems to be a suitable solution to overcome PRR limitations.
• #### Height of turbulent non-premixed jet flames at elevated pressure

(Combustion and Flame, Elsevier BV, 2020-07-22) [Article]
Non-premixed turbulent jet flames are encountered in a variety of practical scenarios including furnaces, industrial flares, and pressurized fuel tanks in case of a leak. For design purposes, predicting the flame height/length is desirable. Therefore, predictive models have been developed (e.g., [1], [2], [3], [4], [5], [6], [7], [8]). Their validity depends whether the flames are buoyancy-controlled, momentum-controlled, or in the buoyancy-momentum transition and is a function of the nozzle geometry [3,6,9,10].
• #### Determination of absolute photoionization cross-sections of some aromatic hydrocarbons.

(Rapid communications in mass spectrometry : RCM, Wiley, 2020-07-18) [Article]
RATIONALE:Aromatic hydrocarbons play an important role in the formation and growth of polycyclic aromatic hydrocarbon (PAH) and soot particles. Measurements of their absolute photoionization cross-sections (PICSs), that benefits the quantitative investigation of mass spectrometry, are still lacking, however. METHODS:PICSs of some aromatic hydrocarbons were measured with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Nitric oxide and benzene were chosen as standard references for PICS calibration, since their photoionization cross-sections are well-known in literature. Binary liquid mixtures of the investigated molecules and their specific solvents were used in the measurements. RESULTS:The investigated aromatics include naphthalene, phenanthrene, 1-methylnaphthalene, indene, 2-/3-/4-methylphenylacetylene, 2-methylindene, diphenylacetylene, 1-/2-ethynylnaphthalene and acenaphthylene. Photo-induced fragments from the molecules were also observed with increasing photon energy. CONCLUSIONS:Based on our measurements and literature data, PICSs of most aromatic molecules have very similar values beyond their ionization energies. However, molecules that contain phenylacetylene structure have PICSs higher than other aromatics.
• #### Dual-camera high-speed imaging of the ignition modes of ethanol, methanol and n-hexane in a shock tube

(Combustion and Flame, Elsevier BV, 2020-07-17) [Article]
Shock tubes are used as homogeneous batch reactors to measure ignition delay times, reaction rate coefficients and species time-histories of a variety of chemical systems. Any non-ideality or inhomogeneity in the shock tube experiment would affect the quality and usefulness of measured data. Experimental and computational efforts have previously been carried out to characterize the regimes of ideal operation of shock tubes. High-speed imaging has proven to be a highly useful tool to assess ignition homogeneity. In this work, a dual-camera setup has been used with an optical end-section in a circular shock tube to obtain simultaneous high-speed images from the shock tube endwall and sidewall, thus providing visualization of the ignition phenomenon in three dimensions. Two case studies are presented herein to demonstrate the quality of data and insights that are obtained using this diagnostic technique to study the ignition modes of different fuels. The first is a comparison of the ignition of two alternative fuels, methanol and ethanol, and the second is a study of the ignition dependence on the fuel concentration of a representative paraffinic fuel, n-hexane. The unique dual-camera imaging diagnostic enabled deeper insights into the ignition homogeneity, with all fuels exhibiting localized ignition at low temperatures. Methanol showed a higher propensity than ethanol to ignite far from the endwall, and the high concentration of n-hexane led to inhomogeneous ignition.
• #### Probing hydrogen–nitrogen chemistry: A theoretical study of important reactions in NxHy, HCN and HNCO oxidation

(International Journal of Hydrogen Energy, Elsevier BV, 2020-07-16) [Article]
As an indirect storage medium of hydrogen, ammonia (NH3) has drawn significant attention from academia and industry. Understanding nitrogen combustion chemistry is a major challenge in applying ammonia for converting chemical energy to thermal energy. Diazene (N2H2), diazenyl radical (NNH), amidogen radical (NH2), hydrogen cyanide (HCN) and isocyanic acid (HNCO) are the crucial intermediate species in the combustion of NH3 or its mixtures with other hydrocarbons. In light of that, this study provides advanced theoretical treatment of 14 important reactions in the oxidation of these intermediates, including isomerization, dissociation and abstraction reactions. The rate constants of all these reactions, and the temperature-dependent thermochemistry of the species involved in the reactions, were calculated utilizing high level quantum chemical methods. Ro-vibrational properties of the reactants, products and stationary points were determined at the M06–2X/6–311++G (d,p) level of theory. Coupled cluster (CCSD(T)) methods were employed, with two large basis sets (cc-pVTZ and cc-pVQZ), and complete basis set of extrapolation techniques to compute the energies of the resulting geometries. All calculated results were compared with experimental and theoretical results in the literature. Finally, the implications of this work for combustion modeling were investigated, and the simulated species’ profiles of HCN and HNCO demonstrated the influence of the updated rate coefficients on kinetic model predictions.
• #### Bending modes metrology in the 14-15 $μ$m region

(arXiv, 2020-07-16) [Preprint]
Frequency combs have triggered an impressive evolution of optical metrology across diverse regions of the electromagnetic spectrum, from the ultraviolet to the terahertz frequencies. An unexplored territory, however, remains in the region of vibrational bending modes, mostly due to the lack of single-mode lasers in the long-wavelength (LW) part of the mid-infrared (MIR) spectrum. We fill this gap through a purely MIR-based nonlinear laser source with tunability from 12.1 to 14.8 $\mu$m, optical power up to 110 $\mu$W, MHz-level linewidth and comb calibration. This enables the first example of bending modes metrology in this region, with the assessment of several CO$_2$-based frequency benchmarks with uncertainties down to 30 kHz, and the accurate study of the $\nu_{11}$ band of benzene, which is a significant testbed for the resolution of the spectrometer. These achievements pave the way for LW-MIR metrology, rotationally-resolved studies and astronomic observations of large molecules, such as aromatic hydrocarbons.
• #### Collision Efficiency Parameter Influence on Pressure-Dependent Rate Constant Calculations Using the SS-QRRK Theory

(The Journal of Physical Chemistry A, American Chemical Society (ACS), 2020-07-14) [Article]
The system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory [J. Am. Chem. Soc. 2016, 138, 2690] is suitable to determine rate constants below the high-pressure limit. Its current implementation allows incorporating variational effects, multi-dimensional tunneling, and multi-structural torsional anharmonicity in rate constant calculations. Master equation solvers offer more rigorous approach to compute pressure-dependent rate constant, but several implementations available in the literature do not incorporate the aforementioned effects. However, SS-QRRK theory coupled with a formulation of the modified strong collision model underestimates the value of unimolecular pressure-dependent rate constants in the high temperature regime for reactions involving large molecules. This underestimation is a consequence of the definition for collision efficiency, which is part of the energy transfer model. The selection of the energy transfer model and its parameters constitute a common issue in pressure-dependent calculations. To overcome this underestimation problem, we evaluated and implemented in a bespoke Python code two alternative definitions for the collision efficiency using the SS-QRRK theory, and tested their performance by comparing the pressure-dependent rate constants with Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) results. The modeled systems were the tautomerization of propen-2-ol and the decomposition of 1-propyl, 1-butyl, and 1-pentyl radicals. One of the tested definitions, which Dean et al. explicitly derived [Z. Phys. Chem. 2000, 214, 1533], corrected the underestimation of the pressure-dependent rate constants and, in addition, qualitatively reproduced the trend of RRKM/ME data. Therefore, the used SS-QRRK theory with accurate definitions for the collision efficiency can yield results that are in agreement with those from more sophisticated methodologies such as RRKM/ME.
• #### Soot formation in laminar flames of ethylene/ammonia

(Combustion and Flame, Elsevier BV, 2020-07-12) [Article]
Co-firing NH3 with other fuels is receiving growing interest as a feasible solution to improve its combustion and emission properties. Previous studies mainly focused on NOx emission, and paid less attention to soot formation which is investigated here using a laminar counterflow flame configuration with ethylene fuel (75% by mole) mixed with different proportions of ammonia (between 0 and 25% by mole while the remainder is nitrogen). Soot volume fraction (SVF) was measured using planar laser induced incandescence (PLII). It was found that the addition of ammonia significantly reduced the measured SVF by 4–6% per 1% ammonia addition as compared to the reference flame (25% nitrogen). To rule out temperature effects, the experiments were simulated using Chemkin Pro and it was found that there were negligible differences in temperature between each condition implying that temperature was not responsible for the reduction in SVF. To investigate the chemical effects of ammonia addition, polycyclic aromatic hydrocarbons (PAH) were measured using planar laser induced fluorescence (PLIF) at 4 wavelengths (350 nm, 400 nm, 450 nm, and 500 nm). PLIF intensities at 350 nm is deemed to correlate with PAHs of 2–3 rings and measured profiles at this wavelength were nearly overlapping for all cases. These findings were supported by GC–MS measurements of acetylene and benzene with the latter showing little change in the peak for the cases studied here. At longer wavelengths, PLIF intensities began to show the same trends found for SVF measurements. Additionally, a specific nitrogen detector was used during GC–MS measurements and several nitrogen containing hydrocarbon species were detected with the 25% addition of ammonia. The combined results indicate that the nitrogen containing hydrocarbon species are likely to account for soot reduction, with the precise mechanism yet to be elucidated.
• #### A Data Science Approach to Estimate Enthalpy of Formation of Cyclic Hydrocarbons

(The Journal of Physical Chemistry A, American Chemical Society (ACS), 2020-07-10) [Article]
In spite of increasing importance of cyclic hydrocarbons in various chemical systems, fundamental properties of these compounds, such as enthalpy of formation, are still scarce. One of the reasons for this is the fact that the estimation of thermodynamic properties of cyclic hydrocarbon species via cost-effective computational approaches, such as group additivity (GA), has several limitations and challenges. In this study, a machine learning (ML) approach is proposed using support vector regression (SVR) algorithm to predict standard enthalpy of formation of cyclic hydrocarbon species. The model is developed based on a thoroughly selected dataset of accurate experimental values of 192 species collected from the literature. The molecular descriptors used as input to the SVR are calculated via alvaDesc software, which computes in total 5255 features classified into 30 categories. The developed SVR model has an average error of approximately 10 kJ/mol. In comparison, the SVR model outperforms the GA approach for complex molecules, and can be therefore proposed as a novel data-driven approach to estimate enthalpy values for complex cyclic species. A sensitivity analysis is also conducted to examine the relevant features that play a role in affecting the standard enthalpy of formation of cyclic species. Our species dataset is expected to be updated and expanded as new data is available in order to develop a more accurate SVR model with broader applicability.
• #### Techno-Economic Analysis of Pressurized Oxy-Fuel Combustion of Petroleum Coke

(Energies, MDPI AG, 2020-07-06) [Article]
Petroleum coke (petcoke) is a by-product of heavy petroleum refining, with heating values comparable to that of coal. It is readily available in oil-producing countries such as the United States of America (USA) and the Kingdom of Saudi Arabia (KSA) at minimum costs and can be used as an inexpensive fossil fuel for power generation. Oxy-petcoke combustion is an attractive CO2 capture option as it avoids the use of additional absorption units and chemicals, and results in a CO2 + H2O flue gas stream that is compressed and dehydrated in a CO2 capture and purification unit (CO2CPU). The additional cost of the CO2CPU can be reduced through high pressure combustion. Hence, this paper reports a techno-economic analysis of an oxy-petcoke plant with CO2 capture simulated at pressures between 1 and 15 bars in Aspen PlusTM based on USA and KSA scenarios. Operating at high pressures leads to reduced equipment sizes and numbers of units, specifically compressors in CO2CPU, resulting in increased efficiencies and decreased costs. An optimum pressure of ~10 bars was found to maximize the plant efficiency (~29.7%) and minimize the levelized cost of electricity (LCOE), cost of CO2 avoided and cost of CO2 captured for both the USA and KSA scenarios. The LCOE was found to be moderately sensitive to changes in the capital cost (~0.7% per %) and increases in cost of petcoke (~0.5% per USD/tonne) and insensitive to the costs of labour, utilities and waste treatment.
• #### Autoignition of diethyl ether and a diethyl ether/ethanol blend

(Fuel, Elsevier BV, 2020-07-04) [Article]
Binary blends of fast-reacting diethyl ether (DEE) and slow-reacting ethanol (EtOH) are quite promising as renewable replacements for conventional fuels in modern compression ignition engines. In this work, pure diethyl ether and a 50/50 M binary blend of diethyl ether and ethanol (DEE/EtOH) were investigated in a shock tube and a rapid compression machine. Ignition delay times were measured over the temperature range of 550–1000 K, pressures of 20–40 bar, and equivalence ratios of 0.5–1. Literature reaction mechanisms of diethyl ether and ethanol were combined to simulate the reactivity trends of the blends. Species rate-of-production and sensitivity analyses were performed to analyze the interplay between radicals originating from the two fuels. Multistage ignition behavior was observed in both experiments and simulations, with peculiar 3-stage ignition visible at fuel-lean conditions. Kinetic analyses were used to identify the reactions controlling various stages of ignition. Reactivity comparison of DEE/EtOH and dimethyl ether/ethanol (DME/EtOH) blends showed that the oxidation of DEE blends is controlled by acetaldehyde whereas formaldehyde controls the oxidation of DME blends.
• #### QCL-Based Dual-Comb Spectrometer for Multi-Species Measurements at High Temperatures and High Pressures

(Sensors, MDPI AG, 2020-06-29) [Article]
Rapid multi-species sensing is an overarching goal in time-resolved studies of chemical kinetics. Most current laser sources cannot achieve this goal due to their narrow spectral coverage and/or slow wavelength scanning. In this work, a novel mid-IR dual-comb spectrometer is utilized for chemical kinetic investigations. The spectrometer is based on two quantum cascade laser frequency combs and provides rapid (4 µs) measurements over a wide spectral range (~1175–1235 cm−1). Here, the spectrometer was applied to make time-resolved absorption measurements of methane, acetone, propene, and propyne at high temperatures (>1000 K) and high pressures (>5 bar) in a shock tube. Such a spectrometer will be of high value in chemical kinetic studies of future fuels.
• #### Cool flame chemistry of diesel surrogate compounds: n-Decane, 2-methylnonane, 2,7-dimethyloctane, and n-butylcyclohexane

(Combustion and Flame, Elsevier BV, 2020-06-23) [Article]
Elucidating the formation of combustion intermediates is crucial to validate reaction pathways, develop reaction mechanisms and examine kinetic modeling predictions. While high-temperature pyrolysis and oxidation intermediates of alkanes have been thoroughly studied, comprehensive analysis of cool flame intermediates from alkane autoxidation is lacking and challenging due to the complexity of intermediate species produced. In this work, jet-stirred reactor autoxidation of four C10 alkanes: n-decane, 2-methylnonane, 2,7-dimethyloctane, and n-butylcyclohexane, as model compounds of diesel fuel, was investigated from 500 to 630 K using synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry (SVUV-PIMS). Around 100 intermediates were detected for each fuel. The classes of molecular structures present during the autoxidation of the representative paraffinic functional groups in transport fuels, i.e., n-alkanes, branched alkanes, and cycloalkanes were established and were found to be similar from the oxidation of various alkanes. A theoretical approach was applied to estimate the photoionization cross sections of the intermediates with the same carbon skeleton as the reactants, e.g., alkene, alkenyl keto, cyclic ether, dione, keto-hydroperoxide, diketo-hydroperoxide, and keto-dihydroperoxide. These species are indicators of the first, second, and third O2 addition reactions for the four C10 hydrocarbons, as well as bimolecular reactions involving keto-hydroperoxides. Chemical kinetic models for the oxidation of these four fuels were examined by comparison against mole fraction of the reactants and final products obtained in additional experiments using gas chromatography analysis, as well as the detailed species pool and mole fractions of aforementioned seven types of intermediates measured by SVUV-PIMS. This works reveals that the models in the literature need to be improved, not only the prediction of the fuel reactivity and final products, but also the reaction network to predict the formation of many previous undetected intermediates.
• #### A Systematic Theoretical Kinetics Analysis for the Waddington Mechanism in the Low-Temperature Oxidation of Butene and Butanol Isomers

(The Journal of Physical Chemistry A, American Chemical Society (ACS), 2020-06-23) [Article]
The Waddington mechanism, or the Waddington-type reaction pathway, is crucial for low-temperature oxidation of both alkenes and alcohols. In this study, the Waddington mechanism in the oxidation chemistry of butene and butanol isomers was systematically investigated. Fundamental quantum chemical calculations were conducted for the rate constants and thermodynamic properties of the reactions and species in this mechanism. Calculations were performed using two different ab initio solvers: Gaussian 09 and Orca 4.0.0, and two different kinetic solvers: PAPR and MultiWell, comprehensively. Temperature- and pressure-dependent rate constants were performed based on the transition state theory, associated with the Rice Ramsperger Kassel Marcus and master equation theories. Temperature-dependent thermochemistry (enthalpies of formation, entropy, and heat capacity) of all major species was also conducted, based on the statistical thermodynamics. Of the two types of reaction, dissociation reactions were significantly faster than isomerization reactions, while the rate constants of both reactions converged toward higher temperatures. In comparison, between two ab initio solvers, the barrier height difference among all isomerization and dissociation reactions was about 2 and 0.5 kcal/mol, respectively, resulting in less than 50%, and a factor of 2−10 differences for the predicted rate coefficients of the two reaction types, respectively. Comparing the two kinetic solvers, the rate constants of the isomerization reactions showed less than a 32% difference, while the rate of one dissociation reaction (P1 ↔ WDT12) exhibited 1−2 orders of magnitude discrepancy. Compared with results from the literature, both reaction rate coefficients (R4 and R5 reaction systems) and species’ thermochemistry (all closed shell molecules and open shell radicals R4 and R5) showed good agreement with the corresponding values obtained from the literature. All calculated results can be directly used for the chemical kinetic model development of butene and butanol isomer oxidation.
• #### PAH formation from jet stirred reactor pyrolysis of gasoline surrogates

(Combustion and Flame, Elsevier BV, 2020-06-20) [Article]
Soot particles and their precursor polycyclic aromatic hydrocarbon (PAH) species, formed during combustion, are responsible for particulate emissions in gasoline direct injection (GDI) engines. To better understand the effects of fuel composition on formation of soot in GDI engines, the pyrolysis of several gasoline surrogates was studied in a jet-stirred reactor across a broad temperature range at atmospheric pressure and 1 s residence time. Fuel and intermediate species, including aromatics up to naphthalene, were measured using gas chromatography (GC). PAH concentrations from pyrolysis of surrogate fuels were compared to gain insight into the effects of fuel composition on PAH formation. In addition, synergistic effects were observed in pyrolysis experiments of binary blends. A detailed kinetic model, recently developed at Lawrence Livermore National Laboratory (LLNL), successfully captured the effects of blending and the concentration of major PAHs. Major reaction pathways are discussed, as well as the role of important intermediate species, such as acetylene, and resonantly stabilized radicals such as allyl, propargyl, cyclopentadienyl, and benzyl in the formation of PAH.
• #### Effects of soot volume fraction on local gas heating and particle sizing using laser induced incandescence

(Journal of Aerosol Science, Elsevier BV, 2020-06-19) [Article]
Several decades of developing laser induced incandescence (LII) as a combustion diagnostic have led to great advances in understanding soot formation. The bulk of diagnostic development has focused on time-resolved LII (TiRe-LII) where heating of soot particles using a laser, and the subsequent particle cooling, can be measured and matched to a model to determine soot primary particle size. These models vary significantly in both complexity and accuracy. This work focuses on the effect of local gas heating during the soot cooling process. Multi-color pyrometry is performed 5 μs following laser heating on flames ranging in soot volume fraction (SVF) of 10 ppm up to 43 ppm with laser fluences of 0.09 J/cm2, 0.07 J/cm2, and 0.05 J/cm2. Both laser fluence and SVF had significant effects on the local gas heating as measured by the soot temperature. Simulations were performed implementing local gas heating and showed prediction errors depend on pressure and soot volume fraction. As SVF increases, the errors in primary particle size predictions show a nearly linear rate of increase that can become significant at higher SVF values.
• #### Multi-stage heat release in lean combustion: Insights from coupled tangential stretching rate (TSR) and computational singular perturbation (CSP) analysis

(Combustion and Flame, Elsevier BV, 2020-06-17) [Article]
There is a growing interest in leaner burning internal combustion engines as an enabler for higher thermodynamic efficiency. The extension of knock-limited compression ratio and the increase in specific heat ratio with lean combustion are key factors for boosting efficiency. Under lean burning conditions, there is emerging evidence that certain fuels exhibit unusual heat release characteristics. It has been reported that fuel/air mixtures undergo three-stage heat release or delayed high temperature heat release: starting with an initial low temperature heat release, similar to the one observed in two stage ignition, followed by an intermediate stage where thermal runaway is inhibited, and then advances to a relatively slow third stage of combustion. The focus of this study is to examine the conditions under which various fuels exhibit three stage ignition or delayed high temperature heat release. The auto-ignition of hydrocarbons/air mixtures is simulated in a closed adiabatic homogenous batch reactor where the charge is allowed to auto-ignite at constant volume vessel under predefined initial temperature and pressure. The simulations cover pressures of 10–60 bar, temperatures of 600 K–900 K, and fuel to air ratio from stoichiometry (equivalence ratio) of 0.3–1.0. Tangential stretching rate (TSR) and the computational singular perturbation Slow Importance Indices for temperature are used to identify important reactions contributing to the temperature growth rate at critical time instants of the auto-ignition process. Overall, three-stage ignition or delayed high temperature heat release is found to be present for most fuels under lean fuel/air mixtures, high pressures, and low temperature conditions. The radical termination reactions of H, OH, and HO2 during the high temperature heat release are leading factors for the distinct separation of heat release stages.