Recent Submissions

  • Theoretical and experimental investigations of the crossover phenomenon in micromachined arch resonator: part II—simultaneous 1:1 and 2:1 internal resonances

    Hajjaj, Amal; Alfosail, Feras; Jaber, Nizar; Ilyas, Saad; Younis, Mohammad I. (Nonlinear Dynamics, Springer Science and Business Media LLC, 2019-09-27) [Article]
    We investigate in a silicon micromachined arch beam the activation of a one-to-one internal resonance between the first symmetric and first antisymmetric modes simultaneously with the activation of a two-to-one internal resonance between these modes and the second symmetric mode. The arch is excited electrically, using an antisymmetric partial electrode to activate both modes of vibrations, and tuned electrothermally via Joule’s heating. Theoretically, we explore the dynamics of the beam using the Galerkin and multiple timescales methods. The simulation results are shown to have good agreement with the experimental data. The results show the merging of both modes at crossing, after which the first antisymmetric mode exchanges the nonlinear behavior with the first symmetric mode. The nonlinear behavior of the arch beam is demonstrated and analyzed experimentally and theoretically as experiencing the simultaneous 2:1 and 1:1 internal resonances.
  • Flame spread over twin electrical wires with applied DC electric fields

    Park, Sun Ho; Kang, Min Seong; Cha, Min Suk; Park, Jeong; Chung, Suk-Ho (Combustion and Flame, Elsevier BV, 2019-09-26) [Article]
    The effect of DC electric field on the characteristics of flame spread over polyethylene-insulated twin electrical wires was studied by varying wire gap (S) and voltage (VDC). Under an applied electric field, the flame spread rate (FSR), flame width, leaning direction of the interacting twin flames varied substantially with varying the voltage and wire gap. The flame spread rate was initially larger for the wire with negative voltage (spreading flame with negative charge; SF−) than the wire with positive voltage (SF+), but the two eventually became the same in the developed region when a quasi-steady state was reached. The FSR behavior could be classified into two regimes; twin flame spread (regime I) and single flame spread (regime II) after the extinction of SF+. Under regime I, three sub-regimes were identified depending on the wire gap and voltage. For the twin flame spread, the flame spread rate initially decreased with increasing voltage as the flame leaned toward the burnt wire. As the two flames interacted, the flame spread rate increased because of the ionic wind effect, and eventually decreased because of the loss of molten PE mass and the electrospray phenomenon. In regime II after the extinction of SF+, the single flame spread showed a transient behavior since the influences of electric field from burnt and unburned wire sections of SF+ wire varied with flame spread. When the voltage was increased even further, SF– was extinguished by streamer generation and, at excessive voltages, an electrical short occurred. The flame spread rates for twin flame spread were best correlated with the electric field intensity in the form of |VDC|0.91/S0.72.
  • 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.
  • Blowout of non-premixed turbulent jet flames with coflow under microgravity condition

    Wang, Qiang; Hu, Longhua; Wang, Shaoming; Wang, Shuangfeng; Chung, Suk-Ho; Fujita, Osamu (Combustion and Flame, Elsevier BV, 2019-09-13) [Article]
    The blowout behavior of non-premixed turbulent coflow jet flames under microgravity environment was studied experimentally by utilizing a 3.6 s drop tower. Variations of flames leading to liftoff as well as blowout were examined by varying the coflow velocity and compared with those obtained under the normal gravity condition. A modeling work was conducted to incorporate the effects of the gravity (buoyancy) and coflow velocity on blowout behavior. Major findings include: (1) the flame length in microgravity was longer than that in normal gravity and decreased with increasing coflow velocity. The flame in microgravity showed more intense yellow luminosity with larger sooting zone; (2) the flame liftoff height increased with increasing coflow velocity in both gravity levels. The flame base was closer to the burner in microgravity as compared with that in normal gravity; (3) the blowout velocity in microgravity was appreciably larger than that obtained in normal gravity; and (4) a physical model based on Damköhler number was developed by using similarity solutions to characterize the differences in the blowout limits considering both the coflow and gravity (buoyancy) effects. The proposed model can successfully predict the experimental data. This work provided new data and basic scaling analysis for blowout limit of non-premixed turbulent jet flames considering both the coflow and gravity (buoyancy) effects.
  • 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.
  • Gliding on a layer of air: impact of a large-viscosity drop on a liquid film

    Langley, Kenneth; Thoroddsen, Sigurdur T (Journal of Fluid Mechanics, Cambridge University Press (CUP), 2019-09-06) [Article]
    In this paper we contrast the early impact stage of a highly viscous drop onto a liquid versus a solid substrate. Water drops impacting at low velocities can rebound from a solid surface without contact. This dynamic is mediated through lubrication of a thin air layer between the liquid and solid. Drops can also rebound from a liquid surface, but only for low Weber numbers. Impacts at higher velocities in both cases lead to circular contacts which entrap an air disc under the centre of the drop. Increasing the drop viscosity produces extended air films for impacts on a smooth solid surface even for much larger velocities. These air films eventually break through random wetting contacts with the solid. Herein we use high-speed interferometry to study the extent and thickness profile of the air film for a large-viscosity drop impacting onto a viscous film of the same liquid. We demonstrate a unified scaling of the centreline height of the air film for impacts on both solid and liquid, when using the effective impact velocity. On the other hand, we show that the large-viscosity liquid film promotes air films of larger extent. Furthermore, the rupture behaviour becomes fundamentally different, with the air film between the two compliant surfaces being more stable, lacking the random wetting patches seen on the solid. We map the parameter range where these air films occur and explore the transition from gliding to ring contact at the edge of the drop dimple. After the air film ruptures, the initial contraction occurs very rapidly and for viscosities greater than 100 cSt the retraction velocity of the air film is s ∼0.3 m s−1 , independent of the liquid viscosity and impact velocity, in sharp contrast with theoretical predictions.
  • High-speed Rayleigh–Raman measurements with subframe burst gating

    Krishna, Yedhu; Tang, Hao-Ling; Elbaz, Ayman M.; Magnotti, Gaetano (Optics Letters, OSA - The Optical, 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]
  • Enstrophy transfers in helical turbulence

    Sadhukhan, Shubhadeep; Samuel, Roshan; Plunian, Franck; Stepanov, Rodion; Samtaney, Ravi; Verma, Mahendra Kumar (Physical Review Fluids, American Physical Society (APS), 2019-08-28) [Article]
    In this paper we study the enstrophy transfers in helical turbulence using direct numerical simulation. We observe that the helicity injection does not have significant effects on the inertial-range energy and helicity spectra (∼k-5/3) and fluxes (constants). We also calculate the separate contributions to enstrophy transfers via velocity-to-vorticity and vorticity-to-vorticity channels. There are four different enstrophy fluxes associated with the former channel or vorticity stretching, and one flux associated with the latter channel or vorticity advection. In the inertial range, the fluxes due to vorticity stretching are larger than that due to advection. These transfers too are insensitive to helicity injection.
  • 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, 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, 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.
  • Numerical study of variable camber continuous trailing edge flap at off-design conditions

    Raheem, Mohammed Abdul; Edi, Prasetyo; Pasha, Amjad A.; Rahman, Mustafa M.; Juhany, Khalid A. (Energies, MDPI, 2019-08-20) [Article]
    Numerical simulations are performed to study the outboard airfoil of advanced technology regional aircraft (ATRA) wings with five different variable camber continuous trailing edge flap (VCCTEF) configurations. The computational study aims to improve the aerodynamic efficiency of the airfoil under cruise conditions. The design of outboard airfoil complies with the hybrid laminar flow control design criteria. This work is unique in terms of analysis of the effects of VCCTEF on the ATRA wing’s outboard airfoil during the off-design condition. The Reynolds–Averaged Navier–Stokes equations coupled with the Spalart-Allmaras turbulence model are employed to perform the simulations for the baseline case and VCCTEF configurations. The current computational study is performed at an altitude of 10 km with a cruise Mach number of 0.77 and a Reynolds number of 2.16 × 107. Amongst all five configurations of VCCTEF airfoils studied, a flap having a parabolic profile (VCCTEF 123) configuration shows the maximum airfoil efficiency and resulted in an increase of 6.3% as compared to the baseline airfoil.
  • 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.
  • A numerical study of the pyrolysis effect on autoignited laminar lifted dimethyl ether jet flames in heated coflow air

    Jung, Ki Sung; Jung, Ba Reum; Kang, Sang Hun; Chung, Suk-Ho; Yoo, Chun Sang (Combustion and Flame, Elsevier, 2019-08-09) [Article]
    The liftoff, autoignition, and stabilization characteristics of autoignited laminar lifted dimethyl ether (DME) jet flames in heated coflow air are numerically investigated by varying the fuel jet velocity, U0. The detailed numerical simulations are performed using the laminarSMOKE code with a 55-species detailed kinetic mechanism of DME oxidation. An unusual U-shaped liftoff height, HL, behavior under MILD combustion condition is observed from the simulations, which is qualitatively consistent with previous experimental results. From additional numerical simulations with modified mass diffusivity of hydrogen, it is verified that the decreasing HL trend of the lifted flames under relatively-low U0 conditions is mainly attributed to the fast diffusion of hydrogen generated from the DME pyrolysis. The species transport and displacement speed analyses verify that the differential diffusion effect renders the lifted flames to be leaner at the center of the jet, ultimately leading to the change of their stabilization mechanism from the autoignition to the autoignition-assisted flame propagation mode with increasing U0. The chemical explosive mode analysis (CEMA) identifies important variables and reactions contributing to the autoignition of the DME jet flames, through which the fast diffusion rates of small species are found to cause the deviation of 2-D autoignition characteristics from that of 0-D homogeneous ignition. The effects of DME pyrolysis on the characteristics of the autoignited laminar DME jet flames are further investigated by varying the fuel tube length, Lres. HL shows a non-monotonic behavior with increasing Lres because the flame structure changes from a MILD combustion to a tribrachial edge flame and to an attached flame while the stabilization mechanism also changes from the autoignition to the autoignition-assisted flame propagation mode as the degree of the DME pyrolysis increases.
  • 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
  • Copolymer-enabled stretchable conductive polymer fibers

    Tian, Guoqiang; Zhou, Jian; Xin, Yangyang; Tao, Ran; Jin, Gang; Lubineau, Gilles (Polymer, Elsevier BV, 2019-08) [Article]
    Next-generation stretchable electronics, such as wearable electronics and implantable sensors, require stretchable conductive fibers. Despite their great popularity for wearable electronics, conducting polymers do not sustain deformation very well because of their rigid conjugated backbone. Here, we report the production of stretchable conductive poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT/PSS)- conjugated polymer fibers, using optimal wet-spinning, followed by hot drawing. We engineer the fibers by introducing a copolymer, polyethylene-block-poly (ethylene glycol) (PBP), that modifies both the electrical and mechanical properties of the raw PEDOT/PSS. We then systematically investigate the effects of the PBP fraction (fs) on the properties of the PEDOT/PSS by analyzing the changes in the conductivity, morphology, stretchability, and conformation of the PEDOT chains. We find that the conductivity of PEDOT/PSS increases from 311 ± 8 S/cm to 415 ± 12 S/cm (133% increase), when fs = 0.4, and that the strain of the fibers, at failure, is as high as (ε = 36%) for fs = 0.7, eq. 3x the value of as-spun PEDOT/PSS fibers. Raman and XRD analyses show that the conformational changes from benzoid to quinoid structures, in the PEDOT chains, significantly enhance the conductivity of the fibers. This conformational change facilitate the switch from a coil structure of PEDOT/PSS into a linear or an extended-coil conformation that increases interchain interaction.
  • 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.
  • Multifrequency excitation of an inclined marine riser under internal resonances

    Alfosail, Feras; Younis, Mohammad I. (Nonlinear Dynamics, Springer Science and Business Media LLC, 2019-07-26) [Article]
    We study the multifrequency excitation of an inclined marine riser under two-to-one and three-to-one internal resonances. The riser model accounts for the initial static deflection, self-weight, and mid-plane stretching nonlinearity. By tuning the initial applied tension and configuration angles of the riser, the ratio between its first and third natural frequencies approaches two. In another case, the ratio between its first and fifth natural frequencies approaches three. As recently revealed by experimental observations, a riser can experience multifrequency vortex-induced vibrations. Hence here, the excitation frequencies are tuned such that one frequency is near the first primary resonance, while the other frequency is near the second primary resonance. The multiple-timescale perturbation method is used to analyze the nonlinear motion of the riser considering the internal resonances. Frequency response results of the perturbation method are compared to a Galerkin solution, which show good agreement. The perturbation results in the two-to-one internal resonance case demonstrate that increasing the forcing amplitude at the second primary resonance suppresses the energy exchange due to internal resonance and reduces the likelihood of Hopf bifurcations, while an opposite trend is observed in the three-to-one internal resonance case. Then, the dynamic solutions of the modulation equations of the perturbation method are analyzed using the Floquet theory to examine the post-Hopf bifurcation response. The limit cycle responses in both internal resonance cases exhibit several period doubling bifurcations possibly leading to quasi-periodic and other complex motions, which can endanger the life of the riser.

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