Recent Submissions

  • Chemical kinetic study of triptane (2,2,3-trimethylbutane) as an anti-knock additive

    Atef, Nour; Issayev, Gani; Mohamed, Samah; Najjar, Ahmed; Wang, Zhandong; Wang, Jui-Yang; Farooq, Aamir; Sarathy, Mani (Combustion and Flame, Elsevier BV, 2019-09-19) [Article]
    2,2,3-Trimethylbutane (i.e., triptane) is a potential gasoline octane booster with a research octane number (RON) of 112. Recent studies showed that it can be catalytically produced with high selectivity from methanol (CH3OH) and dimethyl ether (DME), which presents a promising route for utilizing biomass derivatives as transportation fuels. Understanding the ignition properties of triptane at engine relevant conditions is crucial for its further evaluation. In this work, a detailed kinetic model for triptane combustion is developed and validated. The rate rules for the low-temperature oxidation reactions are evaluated based on quantum chemistry calculations from literature, and thermochemical properties of all the species are assessed based on new thermodynamic group values with careful treatment of gauche interactions. In addition, alternative isomerization pathways for peroxy-alkylhydroperoxide species (ȮOQOOH) are incorporated in the model. The model is validated against new ignition delay data from facilities at King Abdullah University of Science and Technology (KAUST): rapid compression machine (RCM) experiments at pressures of 20 and 40 bar, equivalence ratios of 0.5 and 1 and across a temperature range of 620 to 1015 K, and shock tube experiments at 2 and 5 bar, 0.5 and 1 equivalence ratio and over 1000–1400 K. Moreover, the model prediction of various species is compared against species profiles from jet stirred reactor experiments at three equivalence ratios (0.5, 1 and 2) at atmospheric pressure. Finally, triptane is compared with its less branched isomers, n-heptane and 2-methylhexane, to evaluate the effect of branching on fuel reactivity and importance of alternative isomerization pathway.
  • In Situ Injection Rate Measurement to Study Single and Split Injections in a Heavy-Duty Diesel Engine

    Aljohani, Bassam S. E.; Ben Houidi, Moez; Babayev, Rafig; Aljohani, Khalid; Johansson, Bengt (SAE International, 2019-09-12) [Conference Paper]
    The split injection strategy holds a potential for high pressure combustion engines. One advantage of such strategy is the capability to control the heat release rate, which also implies the use of multiple split-injections with relatively short dwell intervals. Most injection rate measurement techniques require installment of the injector on a dedicated test rig. However, these techniques fail to accurately reproduce real-engine operating conditions. Using the spray impingement method, this paper investigates the injection rate of a high flow-rate solenoid injector while being operated on the engine. The aim is to have an experimental configuration as similar as possible to the real engine in terms of the acoustics and the fuel temperature within the injection system. The assumption of spray force proportional to the spray momentum is used to measure the injection rate. The spray momentum is measured while the injector is mounted on the Volvo D13 engine and connected to the in-series fuel rail and pump. A high-natural-frequency piezoelectric pressure transducer is mounted perpendicularly at 4 mm from one of the nozzle holes. The injector and sensor are contained within a specially designed collector for the injected fuel, which is maintained at atmospheric pressure and temperature. Experiments with single injection are conducted varying the Duration of Injection (DOI) from 400 up to 2000 µs. The tests with split double-injections are conducted with fixed DOI of 500 µs while the dwell time are varied from 100 up to 1000 µs. All tests are performed at the rail pressures of 500, 1000, 1500 and 2000 bar while the engine is operated at 1200 rpm. Results show that the injection rate shape of single injections is highly dependent on the rail pressure profile. With double split-injections, the rate of the second injection as well as the total fuel mass injected increases when the dwell time is shortened. Short dwell intervals boost the fuel quantities as a result of the altered needle response. Long dwell time between two equally-long injections generate similar injection rates. The injector hydraulic delay was more pronounced when dwell time was kept long enough. Overall, higher injection pressure advances the effective start of injection while retarding the effective end of injection.
  • Comparison of Electrical Breakdowns Produced by a Nanosecond High-Voltage Pulse Applied to Metallic and Composite Material Electrodes

    Reguig, Abdeldjalil; Ramljak, Belikse; Chatelain, Karl P.; Damazo, Jason S.; Kwon, Eddie; Lacoste, Deanna (IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers (IEEE), 2019-09-11) [Article]
    In this article, the effect of electrode material on the electrical breakdown, produced by a 500-ns duration high-voltage pulse in dry air at atmospheric pressure, is investigated. The configuration chosen is a pin-to-plane geometry with a gap distance of 2 mm. Both polarities of the high-voltage pulse have been investigated for three different pin electrodes. The reference pin is a copper wire of 50 mm length, while the two other pins are made of a highly resistive composite material of 240 kΩ /m, with two different lengths of 50 and 500 mm. The plane electrode is a tungsten plate of 3 cm diameter. The discharges obtained for the highly resistive wires (HRWs) can be categorized as resistive barrier discharges. Both electrical and optical characteristics of the discharges are presented and discussed. The current, voltage, and energy deposition are first analyzed. Then, the time-resolved phase-locked images of the discharges are presented, showing the propagation of the discharge filaments in the gap. The experimental results demonstrate a strong influence of the electrode material on the discharge characteristics, regardless of the polarity of the applied voltage. The main finding is that, for the same applied high-voltage pulse, the use of highly resistive materials significantly reduces the energy deposition into the discharge and the light emission from the discharge.
  • Flame–spray interaction and combustion features in split-injection spray flames under diesel engine-like conditions

    Zhao, Wanhui; Wei, Haiqiao; Jia, Ming; Lu, Zhen; Luo, Kai H.; Chen, Rui; Zhou, Lei (Combustion and Flame, Elsevier BV, 2019-09-05) [Article]
    In compression ignition engines, split-injection strategy has shown great benefits in reducing pollutant emissions and improving combustion efficiency. Spray–flame interaction involving in split injections is significantly complex, which affects the ignition process and even pollutant emissions. Therefore, the objective of this study is to investigate how the flame–spray interaction affects the subsequent ignition process and combustion features in split injections under diesel engine-like conditions. In this work, large eddy simulation coupled with a 54-species mechanism for split injections of n-dodecane is performed to study the effect of injection duration and dwell times (DTs) on spray–flame interactions and the ignition mechanism. The numerical model gives a reasonable agreement with the experiments in terms of the vapor penetration length, ignition delay times, mixture fraction distributions and the flame structures. The present study revealed that combustion for split injections is a multi-stage process and the ignition processes for the first and second injections are controlled by different mechanisms, namely autoignition for the first injection, and the accelerating ignition for the second injection due to the intermediate species and heating effect formed in the first injection. Moreover, the increase in dwell time between individual injections reduces the subsequently promoting ignition effect for the second injection and thus weakens the interacting process between the two injections. Consumption of the fuel in the first injection leads to a temperature increase and production of different species, which in turn accelerates the ignition of the second injection. Finally, the competition between the local flow timescale and chemical timescale is investigated based on the chemical explosive mode analysis (CEMA) methods. A balance between reaction and mixing processes dominates the combustion of the quasi-steady spray in the second injection with a short DT. However, the flame is controlled by autoignition when a longer DT is used.
  • High-speed Rayleigh–Raman measurements with subframe burst gating

    Krishna, Yedhu; Tang, Hao-Ling; Elbaz, Ayman M.; Magnotti, Gaetano (Optics Letters, OSA - The Optical Societycustserv@osa.org, 2019-09-01) [Article]
    A 10-kHz one-dimensional Rayleigh–CH4 Raman instrument capable of achieving highly precise measurement of temperature and methane mole fraction is demonstrated. The system uses a pulse-burst laser as the light source and back-illuminated electron-multiplied CCD cameras as the detectors. The cameras are operated in the subframe burst gating mode, to combine a high sampling rate, low noise, and a slow readout. The improved precision of this configuration is demonstrated by measuring temperature and methane mole fractions in ambient temperature gas mixtures and in a non-premixed inverse diffusion flame.
  • On the universality of ignition delay times of distillate fuels at high temperatures: A statistical approach

    KHALED, Fethi; Farooq, Aamir (Combustion and Flame, Elsevier BV, 2019-08-31) [Article]
    Ignition delay times (IDTs) of fuels provide very important macro-information about the fuel reactivity and autoignition behavior. IDTs constitute a key metric for fuel/engine co-optimization studies. Chemical kinetic modeling pursuits rely on experimental IDTs as their primary validation target. There have been extensive works in literature on measuring, calculating, modeling and correlating IDTs of a wide range of hydrocarbons, oxygenates, mixtures of pure components and real fuels. Recently, some studies employed a simplified ignition model at high temperatures, comprising of a fast fuel decomposition step and a rate-determining small molecule oxidation step. This description suggests that high-temperature IDT is mainly controlled by the ignition of fuel fragments and is rather weakly dependent on the initial fuel composition. In this work, we study the validity of the hypothesis that IDT of multi-component fuels is weakly dependent on fuel composition under specific thermodynamic conditions. If so, high-temperature IDTs of practical fuels may be described by a universal Arrhenius type correlation. By combining experimental observations and chemical kinetic simulations, we determine the ranges of key parameters (temperature, pressure, equivalence ratio, composition) under which a universal IDT assumption is valid. We conclude that, for fairly random composition and within a P-T-ϕ constraint, IDTs of gasolines and jet fuels may be predicted with a high degree of certainty by the following modified Arrhenius expressions (P = 10–80 bar, P0 = 1 bar, ϕ = 0.5–2, fuel/air mixtures, units are ms, bar, K, mol, kcal): τgasoline=6.76*10−7( [Formula presented] )−1.01φ1.13− [Formula presented] exp( [Formula presented] ), forT> [Formula presented] τjetfuel=4.46*10−7( [Formula presented] )−1.21φ2.04− [Formula presented] *exp( [Formula presented] ), forT> [Formula presented]
  • Evolution of oxygenated polycyclic aromatic hydrocarbon chemistry at flame temperatures

    Liu, Peng; Chen, Bingjie; Li, Zepeng; Bennett, Anthony; Sioud, Salim; Sarathy, Mani; Roberts, William L. (Combustion and Flame, Elsevier Inc.usjcs@elsevier.com, 2019-08-24) [Article]
    Oxygenated polycyclic aromatic hydrocarbons (OPAH) have received increasing attention due to their toxic effect on human health. This study comprehensively investigates the evolution of OPAH chemistry at flame temperatures. Jet-stirred reactor (JSR) experiments with benzene/phenol/C2H2/N2 and benzene/C2H2/O2/N2 revealed that OPAH with oxygenated heterocycle can be formed by the addition of C2H2 at 1400 K. To further clarify the evolution of OPAH chemistry in soot systems, OPAH formation and decomposition reaction pathways and kinetic parameters have been theoretically investigated. The potential energy surfaces of 1-naphtholate and 2-naphtholate growth, and thermal decomposition reactions, were calculated by combining the density functional theory B3LYP/6–311+G(d,p) and CCSD(T)/cc-pvdz methods. The reaction rate coefficients in the temperature range of 800–2500 K and pressure range of 0.1–100 atm were calculated using RRKM theory by solving the master equations. The potential energy surface of C2H2+1-naphtholate and C2H2+2-naphtholate growth reactions showed that the O atom could be locked in a naphthofuran molecule with the formation of a C[sbnd]O[sbnd]C oxygenated heterocycle; and the reaction rates were determined by adding the C2H2 elementary step with the energy barrier of 26.0 and 19.9 kcal/mol, respectively. Thermal decomposition reactions of 1-naphtholate and 2-naphtholate yielded an indenyl radical and CO. The thermal decomposition reaction rates were significantly sensitive to the zig-zag site structure next to the C[dbnd]O bond. The decomposition rate of 1-naphtholate at 1500 K, with a zig-zag site near the C[dbnd]O bond, was 14.8 times lower than that of 2-naphtholate with no zig-zag site near the C[dbnd]O bond. Rate comparison results indicate that the C[dbnd]O functional group rapidly converts to a C[sbnd]O[sbnd]C functional group with the addition of C2H2. The formation, growth and thermal decomposition reactions of 1-naphtholate and 2-naphtholate were added to a detailed PAH mechanism to check the effect of OPAH reactions on PAH formation chemistry. The concentration profile of naphthalene predicted by the updated PAH mechanism was lower than current PAH mechanism predictions by 29%, indicating that the OPAH reactions had a significant effect on PAH formation chemistry, and should be included in the PAH mechanism. However, due to the relatively low concentration of OPAH compared to PAH, it is possible to ignore the correlation between OPAH and soot nucleation at flame temperatures; therefore an OPAH evolution pathway (PAH → incipient soot → OPAH formation on soot particle → selective thermal decomposition of OPAH), is proposed to explain the high content of OPAH molecules (e.g., 9,10-anthraquinone, benz(a)anthracene-7,12-dione, and benzanthrone) adsorbed on the soot particle.
  • Implicitly Coupled Phase Fraction Equations for the Eulerian Multi-Fluid Model

    Keser, Robert; Vukčević, Vuko; Battistoni, Michele; Im, Hong G.; Jasak, Hrvoje (Computers & Fluids, Elsevier BV, 2019-08-23) [Article]
    In this work, the implementation, verification and validation of an implicitly coupled solution procedure for the phase fraction equations in the Eulerian multi-fluid model are presented. The model is implemented within the foam-extendtoolbox, a community-driven fork of OpenFOAM. The implicitly coupled system for an arbitrary number of phases is based on the modified formulation of the phase fraction equation. This formulation takes advantage of the mixture divergence-free velocity and the cross-coupling with the remaining phase fraction equations via the non-linear relative velocity term. The linearised and implicitly coupled phase-fraction equations are solved simultaneously within a single block matrix. The model is tested for a bubbly air-water upward flow which forms a mixing layer inside a square duct. In the first test, the mesh verification analysis is performed on structured grids with different levels of refinement. The second test investigates the influence of the number of bubble phases on the flow solution for the same flow conditions. In the third test, the implemented model is validated against experimental data from the literature. The last test compares the performance of the implemented implicitly coupled solution procedure for the phase fraction equations against the standard segregated implementation. The proposed method shows good agreement with experimental data, and has proven to be consistent both in terms of the number of phases and grid refinement. Furthermore, the method improved the convergence of the solution for flows at higher bubble phase fraction
  • The influence of chemical composition on ignition delay times of gasoline fractions

    Naser, Nimal; Abdul Jameel, Abdul Gani; Emwas, Abdul-Hamid M.; Singh, Eshan; Chung, Suk-Ho; Sarathy, Mani (Combustion and Flame, Elsevier Inc.usjcs@elsevier.com, 2019-08-22) [Article]
    Tailoring fuel properties to maximize the efficiency of internal combustion engines is a way towards achieving cleaner combustion systems. In this work, the ignition properties along with the chemical composition (expressed as functional groups) of various light distillate (e.g., gasoline) cuts were analyzed to better understand the properties of full boiling range fuels. Various distillation cuts were obtained with a spinning band distillation system, which were then tested in an ignition quality tester (IQT) to obtain their global chemical reactivity (i.e., ignition delay time (IDT)). The distillates were further analyzed with 1H nuclear magnetic resonance (NMR) spectroscopy to identify and quantify various functional groups present in them. Various gasolines of research grade with specific target properties set forth by the Coordinating Research Council (CRC) that are known as FACE (fuels for advanced combustion engines) gasolines were distilled. When fuels with low aromatic content were distilled, the higher boiling point (BP) range (i.e., higher molecular weight) fractions exhibited lower IDT. However, distilled fractions of fuels with high aromatic content showed an initial decrease in IDT with increasing BP, followed by drastic increase in IDT primarily due to increasing aromatic groups. This study provides an understanding of the contribution of various volatile fractions to the IDTs of a multicomponent fuel, which is of relevance to fuel stratification utilized in gasoline compression ignition (GCI) engines to tailor heat release rates.
  • Study on fluorescence spectroscopy of PAHs with different molecular structures using laser-induced fluorescence (LIF) measurement and TD-DFT calculation.

    Zhang, Yiran; Liu, Peng; Li, Youping; Zhan, Reggie; Huang, Zhen; Lin, He (Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy, Elsevier BV, 2019-08-19) [Article]
    Laser-induced fluorescence (LIF) is an effective technique for non-intrusive and on-line measurement of PAHs in sooting flames but it is still need further investigation due to the complexity of PAH fluorescence characteristics. Therefore, in-depth investigations on the fluorescence spectroscopy of PAHs with different molecular structures are relevant. In this study, we investigated the fluorescence spectrum characteristics of 13 gas-phase PAHs using LIF measurement and time-dependent density functional theory (TD-DFT) calculation. The experimental results showed that the fluorescence emission wavelengths increased with more aromatic (benzenoid) rings, but this relationship no longer existed when the PAH molecules contain the five-membered ring structures. The TD-DFT calculation showed that the fluorescence emission wavelength ranges of PAHs with different molecular structures were dominantly determined by the electronic structures of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and their energy gaps. It was found that the saturated aliphatic branched chains (methyl and ethyl) only slightly influenced the LIF spectra, while the unsaturated aliphatic branched chains (ethenyl and ethynyl) caused remarkable redshifts. The TD-DFT results indicated that the aliphatic branched chains changed the electric structures of HOMO and LUMO of the core aromatic rings, and then influence the fluorescence emission wavelength ranges.
  • Detection of low Temperature heat release (LTHR) in the standard Cooperative Fuel Research (CFR) engine in both SI and HCCI combustion modes

    Waqas, Muhammad; Hoth, Alexander; Kolodziej, Christopher P.; Rockstroh, Toby; Gonzalez, Jorge Pulpeiro; Johansson, Bengt (Fuel, Elsevier BV, 2019-08-14) [Article]
    To this date, extensive research has been conducted to understand the low-temperature auto-ignition chemistry of gasoline. The detection of low-temperature chemical reactions under Spark Ignition (SI) combustion cannot be detected, as they are hidden by the flame propagation. Alternatively, Homogeneous Charge Compression Ignition (HCCI) combustion has a two-stage combustion involving low and high-temperature heat release (LTHR and HTHR respectively). Both Knocking SI and HCCI combustion involve auto-ignition and are governed by fuel characteristics and the pressure-temperature (P-T) history. Therefore, HCCI combustion might provide an alternative to understand the knocking behavior and LTHR in modern SI engines. A standard Cooperative Fuel Research (CFR) engine was operated at lean HCCI conditions (lambda 3), as well as SI conditions at stoichiometry. For SI combustion, the CFR engine was operated with RON-like conditions, but at late spark timing to induce LTHR prior to flame propagation. Three RON 90 binary fuel blends were investigated, being composed of n-heptane with isooctane, toluene, or ethanol. This work demonstrated that the CFR engine under stoichiometric SI with late spark timing and HCCI combustion mode can help to detect LTHR which is not possible in the standard RON test. The intake pressure and temperature sweeps showed similar effects on LTHR for both combustion modes. The linking of auto-ignition behavior of SI and HCCI was dependent primarily on intake valve closing (IVC) conditions. The high exhaust temperature in SI lead to high IVC temperatures. In order to match the IVC temperatures and to overlap the P-T trajectories, the intake temperature for HCCI was increased.
  • Comb-calibrated sub-Doppler spectroscopy with an external-cavity quantum cascade laser at 77 μm

    Alsaif, Bidoor; Gatti, Davide; Lamperti, Marco; Laporta, Paolo; Farooq, Aamir; Marangoni, Marco (Optics Express, The Optical Society, 2019-08-05) [Article]
    We study the frequency noise and the referencing to a near-infrared frequency comb of a widely tunable external-cavity quantum-cascade-laser that shows a relatively narrow free-running emission linewidth of 1.7 MHz. The frequency locking of the laser to the comb further narrows its linewidth to 690 kHz and enables sub-Doppler spectroscopy on an N2O transition of the ν1 band near 7.7 μm with sub-MHz resolution and absolute frequency calibration. The combined uncertainty on the measured transition center is estimated to be less than 50 kHz
  • Experimental and analytical study on liquid and vapor penetration of high-reactivity gasoline using a high-pressure gasoline multi-hole injector

    Du, Jianguo; Mohan, Balaji; Sim, Jaeheon; Fang, Tiegang; Roberts, William L. (Applied Thermal Engineering, Elsevier BV, 2019-07-29) [Article]
    Spray penetration length is an important parameter which is of great interest to both experimentalists and modelers. As it affects engine efficiency and emissions, measurement and prediction of spray penetration can significantly benefit engine optimization under various operating conditions. In this study, penetration length was investigated in a pre-burn constant volume combustion chamber using a gasoline multi-hole injector with high reactivity gasoline-like fuel designed explicitly for gasoline compression ignition (GCI) engines. Diffused back illumination (DBI) and shadowgraph were implemented for liquid and vapor phase penetration measurements, respectively. Different pre-burn gas mixtures are compared to investigate the influence of ambient gas properties on gasoline spray penetration under evaporating conditions. The liquid penetration under the gas composition of higher molecular weight tends to be longer. However, the vapor penetration showed insignificant effect under different gas compositions. Ambient gas temperature and gas composition were found to be an essential parameter for liquid phase penetration. Pressure difference was found to affect the vapor penetration length while its influence on liquid phase steady state penetration length at high ambient gas temperature is marginal. Statistical analysis was performed for both liquid and vapor phase penetration lengths, and a prediction model was developed with good agreement to the data under all test conditions.
  • Topological and chemical characteristics of turbulent flames at MILD conditions

    Manias, Dimitris M.; Tingas, Efstathios Al; Minamoto, Yuki; Im, Hong G. (Combustion and Flame, Elsevier BV, 2019-07-10) [Article]
    Dominant physical processes that characterize the combustion of a lean methane/air mixture, diluted with exhaust gas recirculation (EGR), under turbulent MILD premixed conditions are identified using the combined approach of Computational Singular Perturbation (CSP) and Tangential Stretching Rate (TSR). TSR is a measure to combine the time scale and amplitude of all active modes and serves as a rational metric for the true dynamical characteristics of the system, especially in turbulent reacting flows in which reaction and turbulent transport processes compete. Applied to the MILD conditions where the flame structures exhibit nearly distributed combustion modes, the TSR metric was found to be an excellent diagnostic tool to depict the regions of important activities. In particular, the analysis of turbulent DNS data revealed that the system's dynamics is mostly dissipative in nature, as the chemically explosive modes are largely suppressed by the dissipative action of transport. On the other hand, the convective transport associated with turbulent eddies play a key role in bringing the explosive nature into the system. In the turbulent MILD conditions under study, the flame structure appears nearly in the distributed combustion regime, such that the conventional statistics conditioned over the progress variable becomes inappropriate, but TSR serves as an automated and systematic way to depict the topology of such complex flames. In addition, further analysis of the CSP modes revealed a strong competition between explosive and dissipative modes, the former favored by hydrogen-related reactions and the convection of CH4, and the latter by carbon-related processes. This competition results in a much smaller region of explosive dynamics in contrast to the widespread existence of explosive modes.
  • Valorization of spent coffee grounds into biofuels and value-added products: Pathway towards integrated bio-refinery

    Atabani, A. E.; Al-Muhtaseb, Ala'a H.; Kumar, Gopalakrishnan; Saratale, Ganesh Dattatraya; Aslam, Muhammad; Khan, Hassnain Abbas; Said, Zafar; Mahmoud, Eyas (Fuel, Elsevier Ltd, 2019-06-26) [Article]
    Coffee is the second largest traded commodity after petroleum and the second most popular beverage after water. This big industry is believed to generate huge amount of waste with spent coffee grounds (SCGs) represents one of the main by-products. Recycling of such waste to fuels and value-added products through bio-refineries is a promising way to solve the problem of many countries that face daily challenges and heavy cost in waste disposal. This review aims to shadow the light on SCGs recycling potential in which over 230 published papers on SCGs recycling topic were gathered and discussed. Various opportunities to produce biofuels such as biodiesel, biogas, bioethanol, bio-oil and fuel pellets besides value-added products such as bioactive compounds, adsorbents, polymers, nanocomposites, and compost were discussed. Moreover, the potential of membrane technology related to various processes of biorefining, separation and purification in the proposed SCG-integrated biorefinery are presented. Based on the presented review, it is obvious that recycling of SCGs offers many worthwhile options to policymakers that can contribute towards huge financial saving on taxpayers of running and maintaining landfills besides saving the environment from harmful emissions. In conclusion, this review emphasizes that SCG-integrated biorefineries to produce different types of biofuels and value-added products are a very promising approach that shall be economically more scrutinized in the foreseen future.
  • Autoignition Characteristics of Ethers Blended with Low Cetane Distillates

    Nicolle, André; Naser, Nimal; Javed, Tamour; Rankovic, Nicolas; Sarathy, Mani (Energy & Fuels, American Chemical Society (ACS), 2019-06-25) [Article]
    The introduction of high cetane components has enabled the use of low cetane base gasoline in compression ignition engines. This study provides an understanding of the autoignition characteristics of various ethers blended with light distillates. The spontaneous ignition of mixtures was herein studied both experimentally [ignition quality tester (IQT)] and computationally, allowing the impacts of distillate composition, ether structure, and reaction progress on key ignition pathways to be determined. Various multicomponent base fuel surrogates were formulated to closely match actual fuel composition, thereby accurately simulating the interplay between distillates and oxygenates. Despite its lower cetane number, di-n-butyl ether (DNBE) was found to promote a more vigorous ignition than diethylether. However, OH radical scavenging by p-xylene counteracts the DNBE effect. Two preignition phases may be distinguished, namely, oxidation initiation by ether and subsequent chemical runaway involving simultaneously fuel and ether. According to the present kinetic mechanism, direct cross-reactions between ether radicals and light distillate components have little impact on the ignition delay time under the IQT operating conditions. As ignition progress increases, ether contribution to OH production decreases and oxidation paths related to aliphatic and cyclic alkanes become dominant. In the case of polyoxymethylene methyl ethers, the extra production of formaldehyde during the ignition phase does not impair the overall reactivity. The respective effects of OME1 and OME3 on ignition may be explained by the emergence of a new OH production path from OME3 oxidation products, while methyl formate production from OME1 acts as an OH radical sink. Even though locally lean zones of the IQT reactor may favor specifically neopentane oxidation at the expense of n-hexane, the new OH production path remains active over a wide range of conditions. Overall, the present detailed model qualitatively captures the nonlinear impact of various ethers on autoignition over the 15–30 DCN range, which makes it attractive for optimizing low cetane fuel formulation.
  • Large Eddy Simulation on the Effects of Pressure on Syngas/Air Turbulent Nonpremixed Jet Flames

    Ciottoli, Pietro P.; Lee, Bok Jik; Lapenna, Pasquale E.; Malpica Galassi, Riccardo; Hernandez Perez, Francisco; Martelli, Emanuele; Valorani, Mauro; Im, Hong G. (Combustion Science and Technology, Informa UK Limited, 2019-06-24) [Article]
    The influence of increasing pressure on nonpremixed syngas/air turbulent jet flames is numerically investigated using large eddy simulations in conjunction with a steady laminar flamelet approach. The applicability of the steady flamelet approach is assessed through an extensive parametric study of laminar counterflow flames and tangential stretching rate analysis on target flame structures at different pressures. Two sets of large eddy simulations, exploring pressure values up to 10 atm, are carried out. The first one (series A) is characterized by a constant jet Reynolds number, while the second one (series B) is characterized by a constant jet inlet velocity. Both campaigns show narrower flame brushes and reduced radical concentrations with increasing pressure. While for series A the flame length is not sensitive to pressure, a longer flame brush is noticed for series B, being mainly caused by the increased mass flow rate. The sensitivity of the local flame behavior to pressure, such as the OH layer thickness and position, is compared to the available experimental results, showing similar trends with a satisfactory agreement.
  • The effect of fuel on high velocity evaporating fuel sprays: Large-Eddy simulation of Spray A with various fuels

    Kaario, Ossi Tapani; Vuorinen, Ville; Kahila, Heikki; Im, Hong G.; Larmi, Martti (International Journal of Engine Research, SAGE Publications, 2019-06-19) [Article]
    Lagrangian particle tracking and Large-Eddy simulation were used to assess the effect of different fuels on spray characteristics. In such a two-way coupled modeling scenario, spray momentum accelerates the gaseous phase into an intense, multiphase jet near the nozzle. To assess fuel property effects on liquid spray formation, the non-reacting Engine Combustion Network Spray A baseline condition was chosen as the reference case. The validated Spray A case was modified by replacing n-dodecane with diesel, methanol, dimethyl ether, or propane assuming 150 MPa injection pressure. The model features and performance for various fuels in the under-resolved near-nozzle region are discussed. The main findings of the paper are as follows. (1) We show that, in addition to the well-known liquid penetration (Formula presented.), and vapor penetration (Formula presented.), for all the investigated fuels, the modeled multiphase jets exhibit also a third length scale (Formula presented.), with discussed correspondence to a potential core part common to single phase jets. (2) As a characteristic feature of the present model, (Formula presented.) is noted to correlate linearly with (Formula presented.) and (Formula presented.) for all the fuels. (3) A separate sensitivity test on density variation indicated that the liquid density had a relatively minor role on (Formula presented.). (4) Significant dependency between fuel oxygen content and the equivalence ratio (Formula presented.) distribution was observed. (5) Repeated simulations indicated injection-to-injection variations below 2% for (Formula presented.) and 4% for (Formula presented.). In the absence of experimental and fully resolved numerical near-nozzle velocity data, the exact details of (Formula presented.) remain as an open question. In contrast, fuel property effects on spray development have been consistently explained herein.
  • An experimental apparatus to measure soot morphology at high pressures using multi-angle light scattering

    Amin, Hafiz; Roberts, William L. (Measurement Science and Technology, Institute of Physics Publishing, 2019-06-14) [Article]
    In this work, a novel experimental setup is described which is designed and built specifically to study soot morphology using light scattering and extinction techniques at elevated pressures. The experimental setup consists of a counterflow burner housed inside a pressure vessel. A unique feature of this pressure vessel is the four curved optical windows which can provide the required optical access for light scattering measurements in order to infer the morphological parameters of soot. Using this setup, N 2-diluted ethylene and air counterflow flames are stabilized from 3 to 5 atm. Global strain rate (a) of 30 s-1 is maintained at all conditions and all the flames studied are soot formation (SF) flames. Light scattering by soot is measured between 15° to 165° at different locations along the axis of the burner. Ratio of total scattering to absorption (ρ sa), path averaged soot volume fraction (fv), mean primary particle size (d p), mean radius of gyration of aggregates (R gm) and fractal dimension (D f) are calculated from multi-angle light scattering and extinction data using Rayleigh-Debye-Gans theory for fractal aggregates (RDG-FA). ρ sa, fv, d p, and R gm increase as the pressure is raised. The scattering contribution in these measurements vary from 1.3% to 16% of absorption which suggests that wide angle optical access is essential for accurate measurements of f v. D f equal to 1.27 is measured near the flame at 3 atm which increases as the particles are convected away from the flame and D f increases to 1.98 at 5 atm.
  • Velocity and mass diffusivity effects on the linear and nonlinear phenomena of the Burke-Schumann flame with acoustic excitation

    Kim, Taesung; Ahn, Myunggeun; Lim, Daehong; Yoon, Youngbin (Journal of Mechanical Science and Technology, Springer Science and Business Media LLC, 2019-06-03) [Article]
    The dynamics of the Burke-Schumann flame in terms of the Péclet number variation were investigated. The effect of the Péclet number on the flame shape and heat release perturbation in the theoretical study was experimentally confirmed. This number changed through alterations in the average velocity and fuel composition. In addition, the nonlinear effects were reported. These effects were made by different magnitude of velocity oscillation of harmonic frequencies. The effect on the 2nd harmonic frequency is larger when the 1st harmonic flame transfer function showed the lower value. Therefore, the nonlinear characteristics are shown within the range of this study. Also, the specific forcing frequency that makes a non-oscillated flame phenomenon is shown. This frequency makes the very low heat release perturbations and 2nd harmonic oscillations.

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