Now showing items 1-20 of 10459

    • The anticancer activity of an air-stable Pd(I)-NHC (NHC = N-heterocyclic carbene) dimer.

      Scattolin, Thomas; Bortolamiol, Enrica; Palazzolo, Stefano; Caligiuri, Isabella; Perin, Tiziana; Canzonieri, Vincenzo; Demitri, Nicola; Rizzolio, Flavio; Cavallo, Luigi; Dereli, Busra; Mane, Manoj Vasisht; Nolan, Steven P.; Visentin, Fabiano (Chemical communications (Cambridge, England), Royal Society of Chemistry (RSC), 2020-09-14) [Article]
      A new dinuclear Pd(i) complex coordinating two bis(NHC) ligands revealed an unsuspected stability despite the unsaturation of the two metal centres. Even more surprisingly, the compound showed high and selective antiproliferative activity against different cancer cell lines and ovarian cancer tumoroids, and the mechanism of action was different from that of cisplatin.
    • Interplay between temperature and bandgap energies on the outdoor performance of perovskite/silicon tandem solar cells

      Aydin, Erkan; Allen, Thomas; de Bastiani, Michele; Xu, Lujia; Ávila, Jorge; Salvador, Michael; Van Kerschaver, Emmanuel; De Wolf, Stefaan (Nature Energy, Springer Science and Business Media LLC, 2020-09-14) [Article]
      Perovskite/silicon tandem solar cells promise power conversion efficiencies beyond the Shockley–Queisser limit of single-junction devices; however, their actual outdoor performance is yet to be investigated. Here we fabricate 25% efficient two-terminal monolithic perovskite/silicon tandem solar cells and test them outdoors in a hot and sunny climate. We find that the temperature dependence of both the silicon and perovskite bandgaps—which follow opposing trends—shifts the devices away from current matching for two-terminal tandems that are optimized at standard test conditions. Consequently, we argue that the optimal perovskite bandgap energy at standard test conditions is <1.68 eV for field performance at operational temperatures greater than 55 °C, which is lower compared with earlier findings. This implies that bromide-lean perovskites with narrower bandgaps at standard test conditions—and therefore better phase stability—hold great promise for the commercialization of perovskite/silicon tandem solar cells.
    • Two-Step Dopamine-to-Polydopamine Modification of Polyethersulfone Ultrafiltration Membrane for Enhancing Anti-Fouling and Ultraviolet Resistant Properties

      Mulyati, Sri; Muchtar, Syawaliah; Arahman, Nasrul; Syamsuddin, Yanna; Mat Nawi, Normi Izati; Yub Harun, Noorfidza; Bilad, Muhammad Roil; Firdaus, Yuliar; Takagi, Ryosuke; Matsuyama, Hideto (Polymers, MDPI AG, 2020-09-11) [Article]
      Polydopamine has been widely used as an additive to enhance membrane fouling resistance. This study reports the effects of two-step dopamine-to-polydopamine modification on the permeation, antifouling, and potential anti-UV properties of polyethersulfone (PES)-based ultrafiltration membranes. The modification was performed through a two-step mechanism: adding the dopamine additive followed by immersion into Tris-HCl solution to allow polymerization of dopamine into polydopamine (PDA). The results reveal that the step of treatment, the concentration of dopamine in the first step, and the duration of dipping in the Tris solution in the second step affect the properties of the resulting membranes. Higher dopamine loadings improve the pure water flux (PWF) by more than threefold (15 vs. 50 L/m2·h). The extended dipping period in the Tris alkaline buffer leads to an overgrowth of the PDA layer that partly covers the surface pores which lowers the PWF. The presence of dopamine or polydopamine enhances the hydrophilicity due to the enrichment of hydrophilic catechol moieties which leads to better anti-fouling. Moreover, the polydopamine film also improves the membrane resistance to UV irradiation by minimizing photodegradation’s occurrence.
    • Counterflow ignition and extinction of FACE gasoline fuels

      Alfazazi, Adamu; Mairinger, Gerald; Selim, Hatem; Seshadri, Kalyanasundaram; Sarathy, Mani (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-11) [Article]
      The demand for petroleum-derived gasoline in the transportation sector is on the rise. For better knowledge of gasoline combustion in practical combustion systems, this study presents experimental measurements and numerical prediction of autoignition temperatures and extinction limits of six FACE (fuels for advanced combustion engines) gasoline fuels in counterflow flames. Extinction limits were measured at atmospheric pressures while the experiments for autoignition temperatures were carried out at atmospheric and high pressures. For atmospheric pressure experiment, the fuel stream consists of the pre-vaporized fuel diluted with nitrogen, while a condensed fuel configuration is used for ignition experiment at higher chamber pressures. The oxidizer stream is pure air. Autoignition temperatures of the tested fuels are nearly the same at atmospheric pressure, while a huge difference is observed as the pressure is increased. Unlike the ignition temperatures at atmospheric pressures, minor difference exists in the extinction limits of the tested fuels. Simulations were carried out using a recently developed gasoline surrogate model. Both multi-component and n-heptane/isooctane mixtures were used as surrogates for the simulations. Overall, the n-heptane/isooctane surrogate mixtures are consistently more reactive as compared the multi-component surrogate mixtures. Transport weighted enthalpy and radical index analysis was used to explain the differences in extinction strain rates for the various fuels.
    • Development of a Eulerian Multi-Fluid Solver for Dense Spray Applications in OpenFOAM

      Keser, Robert; Ceschin, Alberto; Battistoni, Michele; Im, Hong G.; Jasak, Hrvoje (Energies, MDPI AG, 2020-09-11) [Article]
      The new generation of internal combustion engines is facing various research challenges which often include modern fuels and different operating modes. A robust modeling framework is essential for predicting the dynamic behavior of such complex phenomena. In this article, the implementation, verification, and validation of a Eulerian multi-fluid model for spray applications within the OpenFOAM toolbox are presented. Due to its open-source nature and broad-spectrum of available libraries and solvers, OpenFOAM is an ideal platform for academic research. The proposed work utilizes advanced interfacial momentum transfer models to capture the behavior of deforming droplets at a high phase fraction. Furthermore, the WAVE breakup model is employed for the transfer of mass from larger to smaller droplet classes. The work gives detailed instructions regarding the numerical implementation, with a dedicated section dealing with the implementation of the breakup model within the Eulerian multi-fluid formulation. During the verification analysis, the model proved to give stable and consistent results in terms of the selected number of droplet classes and the selected spatial and temporal resolution. In the validation section, the capability of the developed model to predict the dynamic behavior of non-evaporating sprays is presented. It was confirmed that the developed framework could be used as a stable foundation for future fuel spray modeling.
    • Spatial Variation of b-Values and Their Relationship with the Fault Blocks in the Western Part of the Tibetan Plateau and Its Surrounding Areas

      Hussain, Hamid; Shuangxi, Zhang; Usman, Muhammad; Abid, Muhammad (Entropy, MDPI AG, 2020-09-11) [Article]
      The Tibetan Plateau is considered to be one of the best natural laboratories for seismological research. This study sought to determine the spatial variations of b-values in the western part of the Tibetan Plateau, along with its surrounding areas, and the relation with the region’s fault blocks. The study region lies within 27–36.5° N, 78–89° E, and its fracture structure consists of strike-slip faults, as well as normal and thrust faults. A catalog record from 2009–2019 provided 4431 well-centered earthquakes that varied in magnitude from 0.1 to 8.2 M. The record was obtained from China’s seismological network, which is capable of recording low magnitudes to analyze b-values in the study area. The key findings of this study are as follows: (1) the range of earthquake depth in the region was 0–256 km, with the depth histogram showing a high frequency occurrence of shallow earthquakes in the area; (2) a time histogram showed that the major earthquakes occurred between 2014–2015, including the notable 2015 Gorkha earthquake (M = 8.2); (3) the b-value computed in the study area was 0.5 to 1.6, but in most of the study area, the b-value ranged from 0.6 to 0.9, which was a low to intermediate value, due to the presence of strike-slip faults in the central part of the study area and underthrusting in the region (south of the study area); and (4) a high b-value was found in the northwestern and eastern regions of the area, which proved that the area is prone to small earthquakes in the near future. The study also showed that the central and southern areas of the study region had low to intermediate b-values, meaning that it is prone to destructive and massive earthquakes with high magnitudes, such as the Gorkha earthquake (southern part of the study area). Low b-values revealed the degree of variation in rock properties, including large stress and strain, a fractured medium, a high deformation rate, and large faults. Small b-values were observed when the stress level was high in the investigated region, which might be used to predict a massive high-magnitude earthquake in the near future.
    • Impact and lifecycle of superfluid helium drops on a solid surface

      Wallace, Matthew L.; Mallin, David; Milgie, Michael; Aguirre-Pablo, Andres A.; Langley, Kenneth; Thoroddsen, Sigurdur T; Taborek, Peter (Physical Review Fluids, American Physical Society (APS), 2020-09-10) [Article]
      We have used high-speed video and interferometry to investigate the impact, spreading, and eventual contraction of superfluid 4 He drops on a sapphire substrate in a saturated atmosphere of helium vapor. We find that the short-term kinetic spreading of superfluid drops (time t<10 ms) is qualitatively similar to both normal helium and conventional fluids at room temperature. In contrast, the contraction phase of the superfluid drops is highly unusual. Superfluid drops survive for only a few seconds on the substrate due to superflow out of the drop into the surrounding helium film. The drop lifetime is strongly dependent on temperature and diverges at the superfluid transition temperature Tλ∼2.17 K. The contracting drops undergo a geometry-dependent two-phase contraction, which includes a toroidal phase where the radius decreases linearly in time and subsequently a spherical cap phase where the radius decreases with the square root of time. The receding contact angle is temperature dependent and becomes small near Tλ. We also observe that the superfluid outflow causes surprising edge effects, including the emergence of satellite droplets on the perimeter of the expanding drop, as well as ragged and frayed drop edges at lower temperatures.
    • SOILING LOSS RATE MEASUREMENTS OF PHOTOVOLTAIC MODULES IN A HOT AND HUMID DESERT ENVIRONMENT

      Abdullah, Marwan; Khayyat, Ahmad; Basaheeh, Ali; Kotsovos, Konstantinos; Ballard, Ian; AlSaggaf, Ahmed; Gereige, Issam; Theron, Ricardo (Journal of Solar Energy Engineering, ASME International, 2020-09-10) [Article]
      Abstract Power generation from renewable energy sources, in particular solar photovoltaics (PV), has become extremely attractive thanks to its very low levelized cost of electricity (LCoE). In desert-like environments, the energy yield is drastically reduced due to dust accumulation. While effective and affordable cleaning strategies can be implemented in large, MW-size PV power plants, soiling remains an economic and logistic challenge. In this paper, we analyze the soiling loss rates of PV modules for different tilt angles measured during a period of 15 months in the Western Region of Saudi Arabia. We observe a strong correlation between weather parameters like humidity and wind speed, and the mechanism of dust accumulation. Our measurements show that, for specific weather conditions, soiled modules undergo a partial cleaning process. As a consequence, and for the first time, the soiling loss rates are shown to have a clear dependence on the current soiling state of the modules, with clean modules soiling twice as fast as soiled ones. This dependency is key for predicting the correct cleaning frequency of a PV power plant. Finally, the results obtained for vertically mounted modules (90°), where dust accumulation is negligible, point to a favorable case for the use of bifacial PV modules.
    • Task feasibility of V shape electrothermal actuators

      Hussein, Hussein; Younis, Mohammad I.; Fariborzi, Hossein (Engineering Research Express, IOP Publishing, 2020-09-10) [Article]
      This paper investigates the modeling, design, and task feasibility of V shape electrothermal actuators. In the first part, the force-deflection governing expressions are derived and the actuator performance is analyzed based on these expressions. The influence of the actuator dimensions and properties on its performance is clarified. In the second part, the task feasibility is further investigated. A performance measure is proposed for evaluating the capacity of the actuator to accomplish required tasks depending on the application. This measure is used for finding a feasible configuration and optimized design. A case study is finally presented, showing very good agreement between analytical model and finite element simulations, and demonstrating the robustness of the task feasibility measure.
    • Exploring low temperature oxidation of 1-butene in jet-stirred reactors

      Chen, Bingjie; Ilies, Bogdan Dragos; Chen, Weiye; Xu, Qiang; Li, Yang; Xing, Lili; Yang, Jiuzhong; Wei, Lixia; Hansen, Nils; Pitsch, Heinz; Sarathy, Mani; Wang, Zhandong (Combustion and Flame, Elsevier BV, 2020-09-10) [Article]
      1-butene is an important intermediate in combustion of various hydrocarbon fuels and oxygenated biofuels (e.g., butanol). Understanding its oxidation chemistry can help improve ignition and combustion process in advanced engines and provide better emission control. This work addresses a discrepancy between experiments and simulations in 1-butene oxidation at low temperatures, wherein simulations with AramcoMech 3.0 model show greater fuel reactivity than experiments. To further explore 1-butene low temperature reaction pathways from 550 to 910 K, experiments were conducted in three jet-stirred reactors: two coupled to time-of-flight molecular beam mass spectrometers with synchrotron vacuum ultraviolet radiation as the photoionization source, and one coupled to gas chromatography mass spectrometer. Isomeric structure identification, comprehensive species datasets, and reactor cross examinations are provided by the combination of three experiments. The identified isomer-resolved species provide evidence of various 1-butene low temperature reaction pathways. For example, the identification of propanal confirms the Waddington reaction pathway. The kinetic model over-predicts fuel reactivity in the low temperature regime (550–700 K). Updating the rate coefficients of key reactions in the Waddington pathways, e.g., forward and reverse isomerization of hydroxyl-butyl-peroxide to butoxyl-peroxide and Waddington decomposition of butoxyl-peroxide reduces the discrepancies. The role of rate constant updates in each step of the Waddington pathway is evaluated and discussed to provide directions for future model development.
    • A Comprehensive Experimental Study on Mechanical Behavior, Microstructure and Transport Properties of 3D-printed Rock Analogs

      Song, Rui; Wang, Yao; Ishutov, Sergey; Zambrano-Narvaez, Gonzalo; Hodder, Kevin J.; Chalaturnyk, Rick J.; Sun, Shuyu; Liu, Jianjun; Gamage, Ranjith P. (Rock Mechanics and Rock Engineering, Springer Science and Business Media LLC, 2020-09-10) [Article]
      3D-printed (3DP) analogs of natural rocks have been used in laboratory tests concerning geomechanical and transport properties. Rock analogs manufactured by 3D printing can be used to manufacture batch of the samples with specified heterogeneity compared to natural rocks. Rock analogs were manufactured with silica sand (SS) and gypsum powder (GP) using binder jetting as well as with coated silica beads (CSB) using selective laser curing. The uniaxial and triaxial compressive tests were conducted to investigate the strength and deformation characteristics of 3DP rocks that were quantitatively compared with natural rocks. CSB and SS specimens experienced tensile failure, while the GP specimen has shown shear failure and shear-expansion behavior. The microstructural characteristics (e.g. grain shape, pore type, and bonding form) of the SS specimen were similar to a natural sandstone (Berea sandstone reported in the literature) with a relatively loose texture. In addition, 3DP rocks were more permeable than Berea sandstone (permeability of SS, CSB, and Berea sandstone was 12580.5 mD, 9840.5 mD, and 3950 mD, respectively). The effect of microscopic mechanical behavior on macroscopic strength and failure characteristics was investigated using scanning electronic microscopy. CSB and SS specimens could be suitable to simulate the transport behavior of the highly permeable sedimentary rocks. The GP specimen could be used to study the large deformation characteristics and creep failure mode of highly stressed soft rocks. Despite the early stage of 3DP rock analog studies, the potential applications could be expanded by controlling the physical properties (e.g. wettability and surface roughness).
    • NO and OH* emission characteristics of very-lean to stoichiometric ammonia–hydrogen–air swirl flames

      Zhu, Xuren; Khateeb, Abdulrahman A.; Guiberti, Thibault; Roberts, William L. (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-09) [Article]
      One of the main concerns regarding ammonia combustion is its tendency to yield high nitric oxide (NO) emissions. Burning ammonia under slightly rich conditions reduces the NO mole fraction to a low level, but the penalties are poor combustion efficiency and unburnt ammonia. As an alternative solution, this paper reports the experimental investigation of premixed swirl flames fueled with ammonia-hydrogen mixtures under very-lean to stoichiometric conditions. A gas analyzer was used to measure the NO mole fraction in the flame and post flame regions, and it was found that low NO emissions (as low as 100 ppm) in the exhaust were achieved under very lean conditions (ϕ ≈ 0.40). Low NO emission was also possible at higher equivalence ratios, e.g. ϕ = 0.65, for very large ammonia fuel fractions (XNH3 > 0.90). 1-D flame simulations were performed to elaborate on experimental findings and clarify the observations of the chemical kinetics. In addition, images of OH* chemiluminescence intensity were captured to identify the flame structure. It was found that, for some conditions, the OH* chemiluminescence intensity can be used as a proxy for the NO mole fraction. A monotonic relationship was discovered between OH* chemiluminescence intensities and NO mole fraction for a wide range of ammonia-hydrogen blends (0.40 < ϕ < 0.90 and 0.25 < XNH3 < 0.90), making it possible to use the low-cost OH* chemiluminescence technique to qualify NO emission of flames fueled with hydrogen-enriched ammonia blends.
    • Rapid Evolution of Plastic-degrading Enzymes Prevalent in the Global Ocean

      Alam, Intikhab; Gasol, Josep M; Arold, Stefan T.; Gojobori, Takashi; Kamau, Allan A; Aalismail, Nojood; Martin, Cecilia; Momin, Afaque Ahmad Imtiyaz; Acinas, Silvia G; Guzmán-Vega, Francisco J.; Agusti, Susana R; Jamil, Tahira; Duarte, Carlos M. (Cold Spring Harbor Laboratory, 2020-09-09) [Preprint]
      Estimates of marine plastic stocks, a major threat to marine life, are far lower than expected from exponentially-increasing litter inputs, suggesting important loss factors. These may involve microbial degradation, as the plastic-degrading polyethylene terephthalate enzyme (PETase) has been reported in marine microbial communities. An assessment of 416 metagenomes of planktonic communities across the global ocean identifies 68 oceanic PETase variants (oPETase) that evolved from ancestral enzymes degrading polycyclic aromatic hydrocarbons. Nearly 20 oPETases have predicted efficiencies comparable to those of laboratory-optimized PETases, suggesting strong selective pressures directing the evolution of these enzymes. We found oPETases in 90.1% of samples across all oceans and depths, particularly abundant at 1,000 m depth, with a strong dominance of Pseudomonadales containing putative highly-efficient oPETase variants in the dark ocean. Enzymatic degradation may be removing plastic from the marine environment while providing a carbon source for bathypelagic microbial communities.
    • Aberration-corrected STEM imaging of 2D materials: Artifacts and practical applications of threefold astigmatism

      Lopatin, Sergei; Aljarb, Areej; Roddatis, Vladimir; Meyer, Tobias; Wan, Yi; Fu, Jui-Han; Hedhili, Mohamed N.; Han, Yimo; Li, Lain-Jong; Tung, Vincent (Science Advances, American Association for the Advancement of Science (AAAS), 2020-09-09) [Article]
      High-resolution scanning transmission electron microscopy (HR-STEM) with spherical aberration correction enables researchers to peer into two-dimensional (2D) materials and correlate the material properties with those of single atoms. The maximum intensity of corrected electron beam is confined in the area having sub-angstrom size. Meanwhile, the residual threefold astigmatism of the electron probe implies a triangular shape distribution of the intensity, whereas its tails overlap and thus interact with several atomic species simultaneously. The result is the resonant modulation of contrast that interferes the determination of phase transition of 2D materials. Here, we theoretically reveal and experimentally determine the origin of resonant modulation of contrast and its unintended impact on violating the power-law dependence of contrast on coordination modes between transition metal and chalcogenide atoms. The finding illuminates the correlation between atomic contrast, spatially inequivalent chalcogenide orientation, and residual threefold astigmatism on determining the atomic structure of emerging 2D materials.
    • Mechanistic Insight into the Photoredox-Nickel-HAT Triple Catalyzed Arylation and Alkylation of α-Amino Csp3–H Bonds

      Maity, Bholanath; Zhu, Chen; Yue, Huifeng; Huang, Long; Harb, Moussab; Minenkov, Yury; Rueping, Magnus; Cavallo, Luigi (Journal of the American Chemical Society, American Chemical Society (ACS), 2020-09-09) [Article]
      We report here a comprehensive computational analysis of the mechanisms of the photoredox-nickel-HAT (HAT: hydrogen atom transfer) catalyzed arylation and alkylation of α-amino Csp3–H bonds developed by MacMillan and coworkers. Different alternatives for the three catalytic cycles were tested to identify unambiguously the operative reaction mechanism. Our analysis indicated that the IrIII photoredox catalyst, upon irradiation with visible light, can be either reduced or oxidized by the HAT and nickel catalysts, respectively, indicating that both reductive and oxidative quenching catalytic cycles can be operative, although the reductive cycle is favored. Our analysis of the HAT cycle indicated that activation of a α-amino Csp3‒H bond of the substrate is facile and selective relative to activation of a β-amino Csp3‒H bond. Finally, our analysis of the nickel cycle indicated that both arylation and alkylation of α-amino Csp3–H bonds occurs via the sequence of nickel oxidation states NiI-NiII-NiI-NiIII, and of elementary steps: radical addition-SET-oxidative addition-reductive elimination.
    • Extension of the Surface Organometallic Chemistry to Metal-Organic Framework: development of well-defined single site [(≡Zr-O-)W(=O)(CH2tBu)3] olefin metathesis catalyst.

      Thiam, Zeynabou; Abou-Hamad, Edy; Dereli, Busra; Liu, Lingmei; Emwas, Abdul-Hamid M.; Ahmad, Rafia; Jiang, Hao; Isah, Abdulrahman Adamu; Ndiaye, Papa Birame; Taoufik, Mostafa; Han, Yu; Cavallo, Luigi; Basset, Jean-Marie; Eddaoudi, Mohamed (Journal of the American Chemical Society, American Chemical Society (ACS), 2020-09-09) [Article]
      We report here the first step by step anchoring of a W(≡CtBu)(CH2 tBu)3 complex on a highly crystalline and mesoporous MOF, namely Zr-NU-1000, using Surface organometallic Chemistry (SOMC) concept and methodology. SOMC allowed us to selectively graft the complex on the Zr6 clusters and characterize the obtained single site material by using state of the art experimental methods including extensive solid-state NMR techniques and HAADF-STEM imaging. Further FT-IR spectroscopy revealed the presence of a W=O moiety arising from the in situ reaction of the W≡CtBu functionality with the coordinated water coming from the 8-connected hexanuclear Zr6 clusters. All the steps leading to the final grafted molecular complex have been identified by DFT. The obtained material was tested for gas phase and liquid phase olefin metathesis and exhibited higher catalytic activity than the corresponding catalysts synthesized by different grafting methods. This contribution establishes the importance of applying SOMC to MOF chemistry to get well defined single site catalyst on MOF inorganic secondary building units, in particular the in situ synthesis of W=O alkyl complexes from their W carbyne analogues.
    • Semi-transparent graphite films growth on Ni and their double-sided polymer-free transfer.

      Deokar, Geetanjali Baliram; Genovese, Alessandro; Surya, Sandeep Goud; Long, Chen; Salama, Khaled N.; Da Costa, Pedro M. F. J. (Scientific reports, Springer Science and Business Media LLC, 2020-09-08) [Article]
      Nanorange thickness graphite films (NGFs) are robust nanomaterials that can be produced via catalytic chemical vapour deposition but questions remain regarding their facile transfer and how surface topography may affect their application in next-generation devices. Here, we report the growth of NGFs (with an area of 55 cm2 and thickness of ~ 100 nm) on both sides of a polycrystalline Ni foil and their polymer-free transfer (front- and back-side, in areas up to 6 cm2). Due to the catalyst foil topography, the two carbon films differed in physical properties and other characteristics such as surface roughness. We demonstrate that the coarser back-side NGF is well-suited for NO2 sensing, whereas the smoother and more electrically conductive front-side NGF (2000 S/cm, sheet resistance - 50 Ω/sq) could be a viable conducting channel or counter electrode in solar cells (as it transmits 62% of visible light). Overall, the growth and transfer processes described could help realizing NGFs as an alternative carbon material for those technological applications where graphene and micrometer-thick graphite films are not an option.
    • Combustion behavior of ammonia blended with diethyl ether

      Issayev, Gani; Giri, Binod; Elbaz, Ayman M.; Shrestha, Krishna P.; Mauss, Fabian; Roberts, William L.; Farooq, Aamir (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-08) [Article]
      Ammonia (NH3) is recognized as a carbon-free hydrogen-carrier fuel with a high content of hydrogen atoms per unit volume. Recently, ammonia has received increasing attention as a promising alternative fuel for internal combustion engine and gas turbine applications. However, the viability of ammonia fueling future combustion devices has several barriers to overcome. To overcome the challenge of its low reactivity, it is proposed to blend it with a high-reactivity fuel. In this work, we have investigated the combustion characteristics of ammonia/diethyl ether (NH3/DEE) blends using a rapid compression machine (RCM) and a constant volume spherical reactor (CVSR). Ignition delay times (IDTs) of NH3/DEE blends were measured using the RCM over a temperature range of 620 to 942 K, pressures near 20 and 40 bar, equivalence ratios (Φ) of 1 and 0.5, and a range of mole fractions of DEE, χDEE, from 0.05 to 0.2 (DEE/NH3 = 5 – 20%). Laminar burning velocities of NH3/DEE premixed flames were measured using the CVSR at 298 K, 1 bar, Φ of 0.9 to 1.3, and χDEE from 0.1 to 0.4. Our results indicate that DEE promotes the reactivity of fuel blends resulting in significant shortening of the ignition delay times of ammonia under RCM conditions. IDTs expectedly exhibited strong dependence on pressure and equivalence ratio for a given blend. Laminar burning velocity was found to increase with increasing fraction of DEE. The burnt gas Markstein length increased with equivalence ratio for χDEE = 0.1 as seen in NH3-air flames, while the opposite evolution of Markstein length was observed with Φ for 0.1 < χDEE ≤ 0.4, as observed in isooctane-air flames. A detailed chemical kinetics model was assembled to analyze and understand the combustion characteristics of NH3/DEE blends.
    • On the oscillating flame characteristics in nonpremixed laminar coflow-jets: An experimental and numerical study

      Oh, Su Hyeon; Van, Kyu Ho; Jung, Ki Sung; Yoo, Chun Sang; Cha, Min Suk; Chung, Suk Ho; Park, Jeong (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-08) [Article]
      This study investigates the characteristics of oscillating lifted flames in laminar coflow-jets experimentally and numerically by varying both fuel density (by varying propane and n-butane mixtures) and coflow density (by diluting air with N2/He mixtures). Two different lifted flame oscillation behaviors are observed depending on these parameters: oscillating tribrachial lifted flame (OTLF) and oscillating mode-change lifted flame (OMLF), where a rapid increase in flame radius is observed. The regimes of the two flames are identified from experiments, which shows that OMLF occurs only when the effect of the negative buoyancy on the flow field by the fuel heavier than air becomes significant at low fuel jet velocity. OMLFs are also identified to distinguish OTLF regime from flame extinction, which implies that an OMLF can be extinguished when the positive buoyancy becomes weak, losing its stabilizing effect, or when the negative buoyancy becomes strong, further enhancing its destabilizing effect. Transient numerical simulations of both OTLF and OMLF reveal that the OMLF occurs by a strong toroidal vortex and a subsequent counterflow-like structure induced by relatively-strong negative buoyancy. Such a drastic flow redirection significantly changes the fuel concentration gradient such that the OMLF changes its mode from a tribrachial flame mode (decreasing edge speed with fuel concentration gradient) to the premixed flame-like transition mode when the fuel concentration gradient becomes very small (increasing edge speed with fuel concentration gradient). Again, a tribrachial flame mode is recovered during a rising period of flame edge and repeats an oscillation cycle.
    • Effects of Schmidt number on non-monotonic liftoff height behavior in laminar coflow-jet flames with diluted methane and ethylene

      Van, Kyu Ho; Oh, Su Hyeon; Cha, Min Suk; Yoo, Chun Sang; Park, Jeong; Chung, Suk Ho (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-08) [Article]
      Stabilization characteristics of laminar lifted jet flames in a coflow were investigated experimentally to elucidate the effect of Schmidt number in methane and ethylene fuels diluted with N 2 , He, and Ar. A non- monotonic (decreasing and then increasing) liftoff height ( H L ) behavior with jet velocity ( U 0 ) was observed previously for methane fuel diluted with N 2 . To further elucidate the fuel Schmidt number (Sc F ) effect in exhibiting such a non-monotonic (U-shaped) behavior, various diluents (N 2 , He, and Ar) were added to the fuel streams and methane and/or ethylene fuels were used. The result showed three flame types in terms of Sc F and fuel density; nozzle-attached flame, stationary lifted flame, and oscillating flame. Among sta- tionary lifted flames, two distinct H L behaviors with U 0 were observed; monotonic and non-monotonic H L behaviors. A critical Schmidt number (Sc F,cr1 ) existed over which monotonically increasing behavior was ob- served. A second critical Schmidt number (Sc F,cr2 ) also existed such that U-shaped behavior was observed for Sc F,cr2 < Sc F < Sc F,cr1 . An oscillating lifted flame was observed for Sc F < Sc F,cr2 . The oscillating and stationary lifted flames can be categorized in terms of the density differences among the fuel, air, and burnt gas. For the increasing H L cases (including the increasing regime in U-shaped behavior), H L behavior can be char- acterized in terms of Sc F , the density difference between fuel and air, Sc F,cr1 , and U 0 . While the decreasing H L regime in the U-shaped behavior can be characterized with Sc F and/or the Richardson number (defined based on the density difference between fuel and air). Oscillating flames were observed with the frequency range of 2.1–2.7 Hz by the repetitive action of positive (by burnt gas) and negative (when the fuel heavier than air) buoyancies.