Recent Submissions

  • Analysis of Fuel Properties on Combustion Characteristics in a Narrow-throat Pre-chamber Engine

    Hlaing, Ponnya; Marquez, Manuel Echeverri; Burgos, Paula; Cenker, Emre; Ben Houidi, Moez; Johansson, Bengt (The Society of Automotive Engineers, 2021-04-14) [Conference Paper]
    In this study, the authors investigated the effect of fuel properties on the combustion characteristics by employing methane, methanol, ethanol, and primary reference fuels (PRFs) as the main chamber fuel while using methane for the pre-chamber. Global excess air ratios (λ) from 1.6 to lean limit were tested, while 13% of total fuel energy supplied to the engine was delivered via the pre-chamber. The gaseous methane was injected into the pre-chamber at the gas exchange top-dead-center (TDC). Port fuel injection was tested with both open and closed inlet valves. The pre-chamber assembly was designed to fit into the diesel injector pocket of the base engine, which resulted in a narrow throat diameter of 3.3 mm. The combustion stability limit was set at 5% of the coefficient of variation of gross IMEP, and the knock intensity limit was set at 10 bar. GT-Power software was used to estimate the composition of pre-chamber species and was used in heat release analysis of the two chambers. It was found that the rich limit was controlled by engine knock. Hence a higher reactivity fuel (lower octane) had to be operated leaner. However, with the increasing reactivity, the lean limit was also extended, while the peak efficiency was also obtained with a leaner mixture. With PRF 90, the lean limit was at global-λ = 3.0, while the limit was 2.3 with methane. The alcohol fuels exhibited a different behavior from the methane and the PRFs. Ethanol has the same lean limit as PRF100, but methanol could be operated up to global-λ = 3.2. The pre-chamber combustion did not change much with the different fuels in the main chamber, so the combustion stability trends must be related to the transition from burning jets to ignition of the main chamber charge and its subsequent combustion.
  • Development of a simplified n-heptane/methane model for high-pressure direct-injection natural gas marine engines

    Li, Jingrui; Liu, Haifeng; Liu, Xinlei; Ye, Ying; Wang, Hu; Wang, Xinyan; Zhao, Hua; Yao, Mingfa (Frontiers in Energy, Springer Science and Business Media LLC, 2021-01-15) [Article]
    High-pressure direct-injection (HPDI) of natural gas is one of the most promising solutions for future ship engines, in which the combustion process is mainly controlled by the chemical kinetics. However, the employment of detailed chemical models for the multi-dimensional combustion simulation is significantly expensive due to the large scale of the marine engine. In the present paper, a reduced n-heptane/methane model consisting of 35-step reactions was constructed using multiple reduction approaches. Then this model was further reduced to include only 27 reactions by utilizing the HyChem (Hybrid Chemistry) method. An overall good agreement with the experimentally measured ignition delay data of both n-heptane and methane for these two reduced models was achieved and reasonable predictions for the measured laminar flame speeds were obtained for the 35-step model. But the 27-step model cannot predict the laminar flame speed very well. In addition, these two reduced models were both able to reproduce the experimentally measured in-cylinder pressure and heat release rate profiles for a HPDI natural gas marine engine, the highest error of predicted combustion phase being 6.5%. However, the engine-out CO emission was over-predicted and the highest error of predicted NOx emission was less than 12.9%. The predicted distributions of temperature and equivalence ratio by the 35-step and 27-step models are similar to those of the 334-step model. However, the predicted distributions of OH and CH2O are significantly different from those of the 334-step model. In short, the reduced chemical kinetic models developed provide a high-efficient and dependable method to simulate the characteristics of combustion and emissions in HPDI natural gas marine engines.
  • Laminar Burning Velocities of Formic Acid and Formic Acid/Hydrogen Flames: An Experimental and Modeling Study

    Osipova, K.N.; Sarathy, Mani; Korobeinichev, Oleg P.; Shmakov, A.G. (Energy & Fuels, American Chemical Society (ACS), 2021-01-05) [Article]
    Laminar flame speed of formic acid and formic acid/hydrogen (4/1) flames was studied both experimentally and numerically. Experiments with flames of pure formic acid were performed at temperatures of 373 and 423 K, while for formic acid/hydrogen flames the temperature value was 368 K. All of the experiments were performed under atmospheric pressure and at an equivalence ratio ranging from 0.5 to 1.5. To measure the laminar flame speed, the heat flux balance technique was applied. Three detailed chemical-kinetic mechanisms were tested on experimental data. Experiments showed that addition of 20% of hydrogen increases the laminar burning velocity of formic acid, for example, at around 1.5 for stoichiometric flames. The comparison of experimental and numerical data showed that all models tend to overestimate laminar burning velocities of studied flames, especially in the case of rich flames. The obtained results indicate that further improvement of existing chemical-kinetic models of formic acid oxidation is highly required.
  • Experimental and numerical study of polycyclic aromatic hydrocarbon formation in ethylene laminar co-flow diffusion flames

    Jin, Hanfeng; Guo, Junjun; Li, Tianyu; Zhou, Zhongyue; Im, Hong G.; Farooq, Aamir (Fuel, Elsevier, 2021) [Article]
    Recent literature kinetic studies revealed the importance of new mechanisms for polycyclic aromatic hydrocarbon (PAH) and soot inception beyond hydrogen–abstraction–acetylene–addition (HACA) and hydrogen–abstraction–vinylacetylene–addition (HAVA) mechanisms in the combustion of ethylene and other hydrocarbons. Co-flow diffusion flame is a canonical flame used to investigate the interaction between fluid dynamics and PAH chemistry. In this study, supersonic molecular beam sampling technique was utilized for the first time with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) to measure laminar co-flow diffusion flame at atmospheric pressure. We report quantitative measurement of precursor radicals as well as critical intermediates and odd carbon number PAH species. A custom-designed computational code, based on OpenFOAM and Cantera, was adopted to simulate laminar co-flow diffusion flames with literature kinetic model. Chemical kinetic analyses show that addition reactions of odd carbon number species provide considerable contribution to PAH formation processes beside HACA and HAVA mechanisms. Reasonable mass growth reactions are postulated for aromatic species with odd carbon numbers, such as ethynyl-indene, fluorene, benzo-indene, which need further investigations. Reactions of resonantly stabilized radicals followed by ring expansion are shown to be critical for both odd and even carbon number aromatics, and are suggested to be included in future PAH models.
  • On the distillation of waste tire pyrolysis oil: A structural characterization of the derived fractions

    Campuzano, Felipe; Abdul Jameel, Abdul Gani; Zhang, Wen; Emwas, Abdul-Hamid M.; Agudelo, Andrés F.; Martínez, Juan Daniel; Sarathy, Mani (Fuel, Elsevier BV, 2020-12-31) [Article]
    Tire pyrolysis oil (TPO) is a complex mixture of hydrocarbons spanning a wide boiling point range. Due to its complexity, direct implementation of TPO to combustion applications has been challenging. Distillation is a simple method for grouping similar compounds, based on their volatility, thereby facilitating further upgrading and use. In this work, TPO was distilled at atmospheric pressure into different fractions (light, low-middle, high-middle, and heavy), and the structural characteristics of each fraction were explored. Therefore, advanced analytical techniques such as GC–MS, APPI FT-ICR MS and 1H and 13C NMR were utilized. For the light fraction, the GC–MS revealed a significant presence of benzene, toluene, and xylene, as well as limonene. From the APPI FT-ICR MS results, the low-middle, high-middle, and heavy fractions were classified into a number of molecular classes. Among these, pure hydrocarbons (HC), hydrocarbons containing one sulfur atom (S1), hydrocarbons containing two oxygen atoms (O2), etc. Here, HC and S1 were found to be the most abundant molecular classes in all fractions. Finally, a structural analysis of the functional groups present in each TPO fraction was conducted by 1H and 13C NMR. Average molecular parameters (AMPs), such as the number of aromatic, naphthenic, and olefinic carbons/hydrogens, were determined. In addition, derived AMPs, such as the aromaticity factor (fa), C/H paraffinic, C/H aromatic, etc., were calculated. Fractionation by distillation resulted in concentration of both the sulfur and aromatic compounds in the heaviest fraction. In this manner, effective application and upgrading strategies could be individually designed for each fraction.
  • Soot formation in turbulent flames of ethylene/hydrogen/ammonia

    Boyette, Wesley; Steinmetz, Scott A.; Guiberti, Thibault; Dunn, Matthew J.; Roberts, William L.; Masri, Assaad R. (Combustion and Flame, Elsevier BV, 2020-12-29) [Article]
    This paper presents an experimental study of turbulent non-premixed jet flames of ethylene/nitrogen where the nitrogen is substituted with different proportions of hydrogen and/or ammonia. The focus is largely on the effects of hydrogen and ammonia on soot production in turbulent flames. A combination of pointwise, laser-induced fluorescence in the visible and UV bands (LIF-UV–visible), and laser-induced incandescence (LII) is used to measure soot precursors and soot along the flame centerline. All signals are collected at a repetition rate of 10 Hz with fast photomultiplier tubes to resolve the time decay. In a separate experiment, joint imaging of LIF-OH[sbnd]CH is also performed at a repetition rate of 10 kHz. Hydrogen substitution is found to increase the production of soot, whereas ammonia substitution inhibits soot formation. The peak mean soot volume fraction is almost a factor of 3 lower in the 25% ammonia case in comparison to the 25% nitrogen case. The mean signal decay time constant decreases with ammonia substitution, implying the formation of smaller soot nanoparticles. The mean signal decay time constant remains unaffected with hydrogen substitution. Measured peaks in LIF-CH and LIF-OH are reduced with ammonia substitution but only in regions upstream of where soot is formed. Further downstream in the sooting region, neither OH nor CH appear to be affected by the substitution of N2 with H2 or NH3.
  • Exergy loss characteristics of DME/air and ethanol/air mixtures with temperature and concentration fluctuations under HCCI/SCCI conditions: A DNS study

    Zhang, Jiabo; Luong, Minh Bau; Pérez, Francisco E.Hernández; Han, Dong; Im, Hong G.; Huang, Zhen (Combustion and Flame, Elsevier BV, 2020-12-29) [Article]
    The exergy loss characteristics of combustion processes under homogeneous-charge compression ignition (HCCI) and stratified-charge compression ignition (SCCI) conditions are numerically investigated by analyzing two-dimensional (2-D) direct numerical simulation (DNS) data. Two fuels, dimethyl ether and ethanol, together with the initial conditions of different mean temperatures, and levels of temperature and concentration fluctuations relevant to HCCI/SCCI conditions were investigated. It is found that the prevalent deflagration mode significantly decreases the maximum exergy loss rates and spreads out the exergy loss rate for all the cases regardless of fuel types, temperature regimes, and temperature and/or concentration fluctuations. The primary irreversible sources of exergy loss are also identified. The chemical reaction is found to be the primary contributor to the total exergy loss, followed by heat conduction and mass diffusion, regardless of the fluctuation levels. It is also found that the relative change of exergy loss due to chemical reactions, ELchemrel, correlates strongly with the heat release fraction by deflagration. The maximum ELchemrel is found to be less than 10%. Chemical pathway analysis reveals that the exergy loss induced by low-temperature reactions, represented by the decomposition of hydroperoxy–alkylperoxy and the H-abstraction reactions of the fuel molecule, is much lower under the SCCI conditions than that under the HCCI conditions. Generally, the dominant reactions contributing to the exergy loss in the high-temperature regime are nearly identical for the HCCI and SCCI combustion. Key reactions, including the H2O2 loop reactions, the reactions of the H2–O2 mechanism, and the conversion reaction of CO to CO2, CO+OH=CO2+H, are found to contribute more than 50% of the total exergy loss. Due to locally higher reactivities by temperature and concentration fluctuations inducing deflagration dominance, these reactions occur at a relatively higher temperature (1600 K–1900 K) compared with the homogeneous zero-dimensional cases (∼1400 K), resulting in a net reduction in exergy loss.
  • An RCT of acute health effects in COPD-patients after passive vape exposure from e-cigarettes

    Rosenkilde Laursen, Karin; Bønløkke, Jakob Hjort; Bendstrup, Elisabeth; Bilde, Merete; Glasius, Marianne; Heitmann Gutzke, Vibeke; Puthukkadan Moosakutty, Shamjad; Olin, Anna-Carin; Ravn, Peter; Østergaard, Kirsten; Sigsgaard, Torben (European Clinical Respiratory Journal, Informa UK Limited, 2020-12-24) [Article]
    Background: E-cigarette use has been shown to have short-term acute effects among active users but less is known of the acute passive effects, particularly among individuals with existing respiratory diseases. Objective: To investigate local and systemic effects of short-term passive vape exposure among patients with mild or moderate chronic obstructive pulmonary disease (COPD). Methods: In a double-blinded crossover study 16 non-smoking COPD-patients (mean age 68) were randomly exposed for 4 h to passive vape (median PM2.5: 18 µg/m3 (range: 8–333)) and clean air (PM2.5 < 6 µg/m3) separated by 14 days. Particles were measured using an ultrafine particle counter (P-TRAK) and a scanning mobility particle sizer (SMPS). Health effects including Surfactant Protein-A (SP-A) and albumin in exhaled air, spirometry, FeNO, and plasma proteins were evaluated before, right after, and 24 hours after exposure. Participants reported symptoms throughout exposure sessions. Data were analyzed using mixed models. Results: SP-A in exhaled air was negatively affected by exposure to vape and several plasma proteins increased significantly. Throat irritation was more pronounced during passive vape exposure, while FVC and FEV1 decreased, however, not significantly. Conclusions: SP-A in exhaled air and some plasma proteins were affected by passive vape in patients with COPD indicating inflammation, showing that passive vape exposure is potentially harmful.
  • Prediction of mean radical concentrations in lean hydrogen-air turbulent flames at different Karlovitz numbers adopting a newly extended flamelet-based presumed PDF

    Lipatnikov, A. N.; Sabelnikov, V. A.; Hernandez Perez, Francisco; Song, W.; Im, Hong G. (Combustion and Flame, Elsevier BV, 2020-12-23) [Article]
    A recent analysis (Lipatnikov et al., 2020) of complex-chemistry direct numerical simulation (DNS) data obtained from lean hydrogen-air flames associated with corrugated-flame (case A), thin-reaction-zone (case B), and broken-reaction-zone (case C) regimes of turbulent burning has shown that the flamelet concept (i) can predict mean concentrations of various species in those flames if the probability density function (PDF) for the fuel-based combustion progress variable c is extracted from the DNS data, but (ii) poorly performs for the mean rate W¯c of product creation. These results suggest applying the concept to evaluation of mean species concentration (but not the mean rate) in combination with another closure relation for W¯c whose predictive capabilities are better. This proposal is developed in the present paper whose focus is placed on studying a new flamelet-based presumed PDF P(c) for predictions of mean concentration of radicals in engineering computational fluid dynamics (CFD) applications. Analysis of the DNS data shows that (i) the flamelet PDF performs well at intermediate values of c in cases A and B, but should be truncated at small and large c, (ii) modeling P(c) in the radical recombination zone (i.e., at large c) is of importance for predicting mean concentrations of H,O, and OH. Accordingly, the flamelet PDF is truncated and combined with a uniform P(c) at large c. Moreover, the mean rate W¯c extracted from the DNS data is used to calibrate the PDF (the rate is considered to be given by another model). Assessment of the approach against the DNS data shows that it well predicts mean density, temperature, and concentrations of reactants, product, and the aforementioned radicals in cases A and B. In case C, the approach performs worse for OandOH at large c¯ and moderately underestimates the mean concentration of H in the entire flame brush.
  • Analysis of Thermally Induced Breakup of Ultrasonically Emulsified Heavy Fuel Oil using Dynamic Mode Decomposition

    Guida, Paolo; Saxena, Saumitra; Roberts, William L. (International Journal of Heat and Mass Transfer, Elsevier BV, 2020-12-16) [Article]
    Clean and efficient processing of heavy fuels is a major challenge for several combustion driven prime movers like internal combustion engines, used in marine or power generation sectors. Emulsification was recognized in the past as practical technology for heavy fuels combustion since it engenders an enabling phenomenon called micro-explosion that proceeds during the spray process. Micro-explosions allow finer secondary break-up, leading to improved mixing, and subsequent cleaner and fuller burning. However, the translation of this technology to real applications is still not fully exploited due to lack of basic understanding and characterization of the evaporation process which includes both micro-explosions and puffing. Ultrasonically induced cavitation is a promising technology for the production of water-in-oil emulsions at industrial scale. Fundamental research performed in the field of liquid fuels gasification and combustion mostly regards ideal or simple mixtures and not all the considerations made in these cases apply for real fuels. In this work, we investigated the evaporation characteristics of ultrasonically produced heavy fuel oil (HFO) emulsions with a set of newly developed methodologies. We characterized the emulsions by using a state-of-the-art microscopy technique, the Cryogenic Scanning Electron Microscopy, Cryo- SEM and obtained accurate droplet size distribution up to nano-scale. We tested the fuel emulsion in a suspended droplet experiment and reconstructed the interface from the obtained images. The normalized squared diameter profile is not representative of the complex physics involved in heavy fuel evaporation; therefore, it was substituted with the normalized distance of the interface from the centroid of the droplet. By using this procedure, it is possible to highlight both evaporation and stochastic events like puffing and ejections. A dimensionality reduction algorithm, the dynamic mode decomposition (DMD), was then performed on the evolving interface to highlight the main modes describing the emulsion system and the dynamics. The overall objective was to develop a strategy for optimizing emulsions for improved combustion performance. From the experimental data, it was observed that a water concentration of 5% by mass decreases the vaporization time of the mixture and that the presence of water favors puffing and ejections with different intensity depending on the percentage of water enhancing the volatilization of the fuel.
  • Calcium Looping: On the Positive Influence of SO2 and the Negative Influence of H2O on CO2 Capture by Metamorphosed Limestone-Derived Sorbents

    Homsy, Sally Louis; Moreno, Joseba; Dikhtiarenko, Alla; Gascon, Jorge; Dibble, Robert W. (ACS Omega, American Chemical Society (ACS), 2020-12-07) [Article]
    The CO2 capture performance of sorbents derived from three distinct limestones, including a metamorphosed limestone, is studied under conditions relevant for calcium looping CO2 capture from power plant flue gas. The combined and individual influence of flue gas H2O and SO2 content, the influence of textural changes caused by sequential calcination/carbonation cycles, and the impact of CaSO4 accumulation on the sorbents’ capture performance were examined using bubbling fluidized bed reactor systems. The metamorphosed limestone-derived sorbents exhibit atypical capture behavior: flue gas H2O negatively influences CO2 capture performance, while limited sulfation can positively influence CO2 capture, with space time significantly impacting CO2 and SO2 co-capture performance. The morphological characteristics influencing sorbents’ capture behavior were examined using imaging and material characterization tools, and a detailed discussion is presented. This insight into the morphology responsible for metamorphosed limestone-derived sorbent’s anomalous capture behavior can guide future sorbent selection and design efforts.
  • First aromatic ring formation by the radical-chain reaction of vinylacetylene and propargyl

    Jin, Hanfeng; Xing, Lili; Liu, Dapeng; Hao, Junyu; Yang, Jiuzhong; Farooq, Aamir (Combustion and Flame, Elsevier BV, 2020-12-01) [Article]
    Recent investigations illustrated that clustering of hydrocarbons by radical-chain reaction (CHRCR) mechanism provides key mechanistic steps for the rapid synthesis of polycyclic aromatic hydrocarbons (PAHs) and soot. Resonance-stabilized radicals (RSRs) play critical roles in this mechanism, and non-benzene first-ring species have attracted considerable attention as precursors of larger aromatic hydrocarbons. C7H7 RSRs, such as benzyl, tropyl, vinyl-cyclopentadienyl, are particularly stable and are thus quite important in the growth of PAHs. The addition of vinylacetylene to propargyl radical, a prototypical CHRCR reaction, provides a facile route to C7H7 RSRs. We have directly investigated the reaction of propargyl and vinylacetylene in isomer-resolved elementary experiments by synchrotron vacuum ultra-violet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). In good agreement with theoretical predictions, vinyl-cyclopentadienyl is found to be the major product of vinylacetylene and propargyl reaction while benzyl is minor. This work demonstrates a feasible CHRCR pathway, not proceeding through benzene, for PAH formation.
  • Current status of the high-temperature kinetic models of silane: Part I. Pyrolysis

    Chatelain, Karl P.; He, Yizhuo; Alharbi, Reham; Mével, Rémy; Petersen, Eric L.; Lacoste, Deanna (Combustion and Flame, Elsevier BV, 2020-12) [Article]
    The present work compares the performance of seven reaction models with respect to a large experimental dataset relevant to the high-temperature pyrolysis of both silane (SiH) and disilane (SiH). Their performances were established based on different validation criteria that account for the shape and the amplitude of the validation profile. Then, the model performances were quantified with a global error, which accounts for the experimental uncertainties. The most satisfactory model has a global error as low as 3.1 (i.e., meaning 3.1 times higher than the experimental uncertainty) and the highest fraction (74%) of criteria with a low error (), while most of the models have large discrepancies with the validation dataset, global error near 8 and up to 110 for the less accurate model. The origins of these discrepancies are identified with reaction pathway and sensitivity analyses. Among the seven tested model, three main decomposition pathways are evidenced, including one more specific to the models presenting the lowest errors. Based on the global error values, the ability to reproduce all the experimental conditions, and the model analyses, the reaction pathways relevant to the high-temperature pyrolysis of silane and disilane are determined. In addition, the present study provides experimental and numerical guidance for the future developments of silicon hydride reaction models. The limited performance of most of the oldest reaction models may have a significant impact on our current understanding of the pyrolysis and oxidation kinetics of silane.
  • Current status of the high-temperature kinetic models of silane: Part II. Oxidation

    Chatelain, Karl P.; He, Yizhuo; Javoy, Sandra; Mével, Rémy; Petersen, Eric L.; Lacoste, Deanna (Combustion and Flame, Elsevier BV, 2020-12) [Article]
    The present study is the second part of our work on the current status of high-temperature kinetic models of silane. Except Slakman’s model, all the models tested in the first part of the study, restricted to the pyrolysis dataset, are now compared against a large validation dataset (230 conditions) for silane oxidation. This large validation dataset is composed of both new and literature data, mainly representative of the highly-diluted and high-temperature oxidation of silane with different oxidizers ( and NO) and diluents (Ar and ) over an extensive range of temperature ( = [801– 2955 K]) and pressure ( = [50 – 629 kPa]) conditions. The new experimental data are limited to --Ar mixtures, obtained in a double-diaphragm shock tube equipped with an Atomic Resonance Absorption Spectroscopy (ARAS) detection technique. Experimental results present the temporal evolution of the total absorption signal, considering the absorption of O, Si, and . The performance of the models is assessed based on the same five validation criteria and objective function calculation, as presented in the first part of the study. The model of Chatelain and of Mével present good performances with a global error of 2.4, i.e. meaning an average error of 2.4 fold above the experimental uncertainty, and high fraction (70 %) of criteria predicted within two times the experimental uncertainty. Although the reference reaction models performed better on the oxidation dataset compared to the pyrolysis dataset (part I), their global error is still 50 to 125 % higher than the two most accurate reaction models. Rate of production and sensitivity analyses revealed that the origin of the discrepancy of the least performing models can be attributed to the reaction pathways consuming/producing Si and O atoms and to some kinetic rates that must be updated.
  • A theoretical study of the Ḣ- and HOȮ-assisted propen-2-ol tautomerizations: Reactive systems to evaluate collision efficiency definitions on chemically activated reactions using SS-QRRK theory

    Grajales Gonzalez, Edwing; Monge Palacios, Manuel; Sarathy, Mani (Combustion and Flame, Elsevier BV, 2020-12-01) [Article]
    In combustion, enols can undergo keto-enol tautomerizations, which are intermediate steps in the formation of pollutant species. In this work, we performed a theoretical kinetic study of the step-wise propen-2-ol tautomerization catalyzed by hydrogen and hydroperoxyl radicals. Ab initio calculations at the CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ level were run, and rate constants were calculated using the multistructural torsional variational transition state theory with small-curvature tunneling corrections. Hydrogen and hydroperoxyl radicals can induce a step-wise mechanism toward keto formation with a lower barrier than that of unimolecular tautomerization. The potential energy surface comprising these reactions is complex, involving different intermediates that are connected by different types of pathways. The hydrogen-assisted tautomerization consists of two steps where the formation of an intermediate radical takes place as a result of the addition of the hydrogen atom to the double bond of propen-2-ol. The high-pressure limit rate constants of the reactions of this intermediate radical toward propen-2-ol and acetone exhibit an Arrhenius behavior, in agreement with previous works. In the hydroperoxyl-assisted tautomerization, the acetone formation has two routes involving an overall of four steps. The route with the highest energy barrier becomes prominent above 800 K due to multistructural anharmonicity effects, which must be included for an accurate kinetic description of the titled reactions. Calculations of pressure-dependent rate constants showed that the original system-specific quantum Rice-Ramsperger-Kassel theory, together with the modified strong collision model (SS-QRRK/MSC), significantly underpredict the bimolecular stabilization rate constants for the hydrogen-assisted tautomerization above 1200 K by factors of up to three orders of magnitude when compared with the benchmark Rice-Ramsperger-Kassel-Markus/master equation method. To solve this problem, we tested two alternative definitions of the collision efficiency parameter by using an improved implementation of the SS-QRRK/MSC approach developed by us for chemically activated reactions. One of these definitions, provided by Gilbert et al. (1983), corrected the bimolecular stabilization rate constant behavior and yielded a maximum deviation factor of only 4.5 at 2000 K and 100 atm. For the hydroperoxyl-assisted tautomerization, pressure effects are negligible because the stabilization of the energized adduct cannot compete with the reaction leading to the final product for most of the physical conditions studied. Our calculated rate constants can be used to perform more accurate kinetic modeling of alcohols. Besides, the implementation of the SS-QRRK theory with the collision efficiency of Gilbert et al. (1983) proposed in this work is useful for computing pressure-dependent rate constants of chemically activated reactions, including all possible refinements (multi-dimensional tunneling, multistructural anharmonicity, etc.) considered in high-pressure limit calculations.
  • Direct numerical simulations of turbulent reacting flows with shock waves and stiff chemistry using many-core/GPU acceleration

    Desai, Swapnil; Kim, Yu Jeong; Song, Wonsik; Luong, Minh Bau; Hernandez Perez, Francisco; Sankaran, Ramanan; Im, Hong G. (Computers and Fluids, Elsevier BV, 2020-11-30) [Article]
    Compressible reacting flows may display sharp spatial variation related to shocks, contact discontinuities or reactive zones embedded within relatively smooth regions. The presence of such phenomena emphasizes the relevance of shock-capturing schemes such as the weighted essentially non-oscillatory (WENO) scheme as an essential ingredient of the numerical solver. However, these schemes are complex and have more computational cost than the simple high-order compact or non-compact schemes. In this paper, we present the implementation of a seventh-order, minimally-dissipative mapped WENO (WENO7M) scheme in a newly developed direct numerical simulation (DNS) code called KAUST Adaptive Reactive Flows Solver (KARFS). In order to make efficient use of the computer resources and reduce the solution time, without compromising the resolution requirement, the WENO routines are accelerated via graphics processing unit (GPU) computation. The performance characteristics and scalability of the code are studied using different grid sizes and block decomposition. The performance portability of KARFS is demonstrated on a variety of architectures including NVIDIA Tesla P100 GPUs and NVIDIA Kepler K20X GPUs. In addition, the capability and potential of the newly implemented WENO7M scheme in KARFS to perform DNS of compressible flows is also demonstrated with model problems involving shocks, isotropic turbulence, detonations and flame propagation into a stratified mixture with complex chemical kinetics.
  • Implicitly coupled phase fraction equations for polydisperse flows

    Keser, Robert; Ceschin, Alberto; Battistoni, Michele; Im, Hong G.; Jasak, Hrvoje (International Journal for Numerical Methods in Fluids, Wiley, 2020-11-28) [Article]
    This work presents the implementation, verification and the validation of an incompressible Eulerian multi-fluid model for polydisperse flows. The proposed model uses a novel monolithic, i.e. implicitly coupled phase continuity equation for an arbitrary number of fluids, where the breakup source and sink terms are handled implicitly in the block-system. The implemented model is tested for an upward bubbly flow inside a large vertical pipe. The selected flow conditions exhibit both breakup and coalescence. The grid refinement study is conducted on four structured grids with varying levels of refinement. In the validation section, the numerical results are compared to the TOPFLOW experimental measurements. The last presented test examines the performance of the novel implicitly coupled phase continuity equation to the corresponding segregated formulation and the standard segregated formulation. The performance is evaluated by comparing the conservation error over the non-linear iterations. The presented model exhibits good agreement with the experimental measurements and gives stable results on various grids with different levels of refinement. Moreover, the implicit coupling reduces the conservation error during the calculation.
  • High-speed filtered Rayleigh scattering thermometry in premixed flames through narrow channels

    Krishna, Yedhu; Mahuthannan, Ariff Magdoom; Luo, Xinguang; Lacoste, Deanna; Magnotti, Gaetano (Combustion and Flame, Elsevier BV, 2020-11-20) [Article]
    High-speed filtered Rayleigh scattering at 10 kHz repetition rate for time-resolved temperature measurements in premixed flames propagating in mm-high channels is demonstrated for the first time. The instrument relies on a pulse burst laser with a 200 ns temporal width, and a CCD camera operated in subframe burst gate mode to achieve high signal to noise ratio despite the limited optical access. A 10-inch-long iodine cell acts as a molecular filter and the laser, which is seeded using an external cavity diode laser and operated in a temporally stretched mode, is wavelength-tuned to the peak of a strong iodine absorption line. A CCD camera, operated in the sub-frame burst gating mode with the help of an external slit configuration, is used as the detector to achieve improved camera noise performance. Filtered Rayleigh scattering temperature measurements in premixed flat flames stabilized over a McKenna burner indicate an instrument precision of 3%. Temperature measurements in the products region are within 3% of measurements obtained with conventional Rayleigh scattering. The system's capabilities are demonstrated through time-resolved temperature measurements in a premixed methane-air flame propagating in a 1.5 mm-high rectangular channel designed to study flame quenching in flame arrestors. Surface scattering from the optical windows and the channel surfaces is successfully suppressed and time-resolved temperature profiles are obtained for both quenching and no-quenching events.
  • Investigating Water Injection in Single-Cylinder Gasoline Spark-Ignited Engines at Fixed Speed

    Singh, Eshan; Hlaing, Ponnya; Dibble, Robert W. (Energy & Fuels, American Chemical Society (ACS), 2020-11-17) [Article]
    Increasingly stringent emission norms have always brought forth innovative measures to improve engine efficiency. Spark-ignited engines have been limited in efficiency, traditionally by knock, and more recently by preignition too. Water injection has recently regained interest as a knock suppressant. The current work explored water injection via port and direct injection at a fixed engine speed of 2000 rpm and varying engine loads. The data presented in this work emphasize that the gains from using water injection are best realized at a specific injection timing (neither too early nor too late), and the effectiveness of water in suppressing knock decreases rapidly with increasing water mass injected. In general, direct water injection offers a more significant knock reduction because of better utilization of the charge cooling effect than port water injection. Engine-out emission confirms a reduction in NOx and CO, while the HC emissions increased when using water injection. No preignition events were observed at the engine load up to 27 bar. Chemical kinetics simulations confirm the role of water in suppressing reactivity under the operating conditions considered in the current study.
  • A numerical investigation of isobaric combustion strategy in a compression ignition engine

    Liu, Xinlei; Aljabri, Hammam; Mohan, Balaji; Babayev, Rafig; Badra, Jihad; Johansson, Bengt; Im, Hong G. (International Journal of Engine Research, SAGE Publications, 2020-11-16) [Article]
    Three-dimensional computational fluid dynamic simulations were conducted to study the means to achieve isobaric combustion mode in a compression ignition engine, which is intended to be used in the high-efficiency double compression-expansion engine (DCEE) concept. Compared to the conventional diesel combustion mode, the isobaric combustion mode generated a significantly lower peak combustion pressure, which was beneficial for the high load extension. For both combustion modes, the ignition was triggered downstream of the nozzle, with the heat release dominated by HCO + O2 = CO+HO2, while the injection-combustion duration for the isobaric combustion mode was significantly longer. The effects of swirl ratio, spray angle, and piston geometries on the isobaric combustion at various engine loads were also investigated. The higher swirl ratio resulted in a higher heat transfer loss and thus lower thermal efficiency. Due to the higher air utilization rates and lower heat transfer losses, cases with spray angles of 140° and 150° generated the higher thermal efficiencies. The piston bowl geometry was found to have a significant impact on the mixing and combustion processes, especially at high engine load conditions. For the conditions under study, the original piston geometry with a swirl ratio of 0 and a spray angle of 140° demonstrated the highest thermal efficiency for the isobaric combustion mode. The results of this work will provide guidance in the practical design and implementation of the DCEE concept.

View more