Recent Submissions

  • Synthesis of Copper and Copper Oxide Nanomaterials by Pulsed Electric Field in Water with Various Electrical Conductivities

    Hamdan, Ahmad; Glad, Xavier; Cha, Min Suk (Nanomaterials, MDPI AG, 2020-07-10) [Article]
    <jats:p>Nanomaterial synthesis is a hot research subject that has been extensively studied in the last two decades. Recently, plasmas in liquid systems have been proposed as an efficient means of synthesizing various types of nanomaterials. The formation processes implicate many physical and chemical phenomena that take place at the electrode surface, as well as in the plasma volume, which renders it difficult to fully understand the underlying mechanisms. In this study, we assess the effect of electric field on nanomaterial synthesis in a system composed of two copper electrodes immersed in water, in the absence of an electrical discharge. The obtained results indicate that various nanostructures, including copper nanoparticles, copper oxide nanowires, and/or hollow nanoparticles, may be produced, depending on the electrical conductivity of the solution (adjusted by adding highly diluted HCl to deionized water). The materials synthesized herein are collected and characterized, and a formation mechanism is proposed. Overall, our results provide insight into the physical and chemical phenomena underlying nanomaterial synthesis in plasmas in liquid.</jats:p>
  • Autoignition of diethyl ether and a diethyl ether/ethanol blend

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

    Xu, Yangguang; Lubineau, Gilles; Liao, Guojiang; He, Qianyun; Xing, Tao (Materials Research Express, IOP Publishing, 2020-07-01) [Article]
    The rate-dependent effect of viscoelasticity plays a critical role in the hardening mechanisms of impact-hardening polymers (IHP) when forcefully impacted. In this study, we used dynamic mechanical analysis (DMA) to characterize the rate-dependent viscoelasticity of an IHP under oscillatory shear. We found that the storage modulus increased by three orders of magnitude within the experimental range when the oscillatory frequency varied from 0.1 to 100 rad/s. To further understand the real strain rate effect of IHP, we introduced the Havriliak-Negami (H–N) model to predict the dynamic viscoelastic behaviors of the IHP for a wider frequency range (from zero to infinity) than that applied in the DMA experiments. Based on the H–N model results, we defined a parameter to describe the rate-dependent effect of the IHP, which was not dependent on the frequency range and reflected the intrinsic material properties of IHP. We used the time-temperature superposition principle (TTSP), which extended the experimental range from 0.1 rad s−1 down to 0.005 rad s−1, to verify the accuracy of the rate-dependent viscoelasticity predicted by the H–N model. Finally, we outlined the influence of temperature on the dynamic viscoelastic behaviors of IHP and discussed the phase transition mechanism induced by temperature and the oscillatory frequency. The results presented here not only provide a method (i.e., by combining experimental results with the H–N model results) to characterize the real rate-dependent viscoelasticity of IHP but are also valuable to further our understanding of the impact-hardening mechanisms of IHP.
  • QCL-Based Dual-Comb Spectrometer for Multi-Species Measurements at High Temperatures and High Pressures

    Zhang, Guangle; Horvath, Raphael; Liu, Dapeng; Geiser, Markus; Farooq, Aamir (Sensors, MDPI AG, 2020-06-29) [Article]
    <jats:p>Rapid multi-species sensing is an overarching goal in time-resolved studies of chemical kinetics. Most current laser sources cannot achieve this goal due to their narrow spectral coverage and/or slow wavelength scanning. In this work, a novel mid-IR dual-comb spectrometer is utilized for chemical kinetic investigations. The spectrometer is based on two quantum cascade laser frequency combs and provides rapid (4 µs) measurements over a wide spectral range (~1175–1235 cm−1). Here, the spectrometer was applied to make time-resolved absorption measurements of methane, acetone, propene, and propyne at high temperatures (>1000 K) and high pressures (>5 bar) in a shock tube. Such a spectrometer will be of high value in chemical kinetic studies of future fuels.</jats:p>
  • PAH formation from jet stirred reactor pyrolysis of gasoline surrogates

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

    Xiang, Ke; Yang, Zhonghua; Wu, Shiqiang; Gao, Wei; Li, Dan; Li, Qiong (Physics of Fluids, AIP Publishing, 2020-06-19) [Article]
    In recent years, increasing attention has been paid to the ecological role of groyne fields as habitats for aquatic vegetation; however, knowledge on interactions between vegetation and recirculating flow is still lacking, especially vegetation effects on large-scale coherent structures in the mixing layer, which control the mass exchange between the side-cavity and the main channel. In this paper, the hydrodynamics of the mixing layer in straight open channels without sediments in the flow, with consecutive groyne fields, of different vegetation densities, is investigated both experimentally through particle image velocimetry and numerically through large eddy simulation. The results show that the presence of plants rearranges the circulation systems in the groyne field, namely, from double gyres to a single gyre. With an increase in the vegetation density, the exchange coefficient between the cavity and the main channel gradually decreases. Note that the exchange rate is calculated from a newly proposed exchange layer, which is located away from the groyne tip. Based on the analysis of the Kelvin−Helmholtz eddies along the shear layer, a phenomenological model is proposed for the evolution of coherent structures and the variations in flow hydrodynamics associated with these eddies. Compared to the non-vegetation case, the presence of vegetation could suppress the evolution of coherent eddies in the mixing layer, with a consequent effect on the flow hydrodynamics around the interface.
  • Effects of soot volume fraction on local gas heating and particle sizing using laser induced incandescence

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

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

    Mohapatra, Chinmoy K; Schmidt, David P; Sforozo, Brandon A; Matusik, Katarzyna E; Yue, Zongyu; Powell, Christopher F; Som, Sibendu; Mohan, Balaji; Im, Hong G.; Badra, Jihad; Bode, Mathis; Pitsch, Heinz; Papoulias, Dimitrios; Neroorkar, Kshitij; Muzaferija, Samir; Martí-Aldaraví, Pedro; Martínez, María (International Journal of Engine Research, SAGE Publications, 2020-06-11) [Article]
    <jats:p> The internal details of fuel injectors have a profound impact on the emissions from gasoline direct injection engines. However, the impact of injector design features is not currently understood, due to the difficulty in observing and modeling internal injector flows. Gasoline direct injection flows involve moving geometry, flash boiling, and high levels of turbulent two-phase mixing. In order to better simulate these injectors, five different modeling approaches have been employed to study the engine combustion network Spray G injector. These simulation results have been compared to experimental measurements obtained, among other techniques, with X-ray diagnostics, allowing the predictions to be evaluated and critiqued. The ability of the models to predict mass flow rate through the injector is confirmed, but other features of the predictions vary in their accuracy. The prediction of plume width and fuel mass distribution varies widely, with volume-of-fluid tending to overly concentrate the fuel. All the simulations, however, seem to struggle with predicting fuel dispersion and by inference, jet velocity. This shortcoming of the predictions suggests a need to improve Eulerian modeling of dense fuel jets. </jats:p>
  • Large deformation near a crack tip in a fiber-reinforced neo-Hookean sheet

    Liu, Yin; Moran, Brian (Journal of the Mechanics and Physics of Solids, Elsevier BV, 2020-06-11) [Article]
    The asymptotic fields at the tip of a crack in a fiber-reinforced neo-Hookean sheet are derived. The investigation is carried out for the case of a strain energy function for a fiber-reinforced hyperelastic material motivated by composite mechanics (Guo et al., 2006, 2007ab), where the fibers are also neo-Hookean. The resulting asymptotic deformation and stress fields depend qualitatively and quantitatively on the degree of fiber reinforcement. For suitable choice of parameters, the strain energy potential for the material reduces to that of a pure neo-Hookean material and the corresponding asymptotic fields to those obtained by Knowles and Sternberg (1983). The result obtained may prove useful in providing a framework for future exploration in modeling and assessing the mechanical behavior near a slit or tear in soft biological tissue reinforced by collagen fibers and in other applications of fiber-reinforced soft materials.
  • LES/PDF modeling of swirl-stabilized non-premixed methane/air flames with local extinction and re-ignition

    Yu, S.; Liu, X.; Bai, X. S.; Elbaz, Ayman M.; Roberts, William L. (Combustion and Flame, Elsevier BV, 2020-06-08) [Article]
    Turbulent non-premixed flames with local extinction and re-ignition exhibit multiple combustion modes including ignition waves, diffusion flames, partially premixed flames, and ignition-assisted partially premixed flames. The mechanisms of local extinction and re-ignition are not well understood and numerical modeling of multi-mode combustion is a challenging task. In this work, a specially designed swirl-burner was used to study local extinction and re-ignition of non-premixed turbulent methane/air flames. High speed Particle Image Velocimetry (PIV) and laser induced fluorescence of OH radicals (OH-PLIF) measurements along with Large Eddy Simulation (LES) were carried out to investigate the mechanisms of extinction and re-ignition processes in the burner. LES is based on a transported probability density function model within the framework of Eulerian Stochastic Fields (PDF-ESF). It is found that local extinction occurs when the scalar dissipation rate around the stoichiometric mixture fraction is high. The characteristic time scale for local extinction and re-ignition in the present flames is an order of magnitude longer than the characteristic time scale of diffusion/extinction of laminar flamelets. There are two mechanisms for flame hole re-ignition in the present flames. First, under low degree of local extinction conditions (i.e., for small flame holes surrounded by flames) the flame hole re-ignition is due to the mechanism of turbulent flame folding. Second, under high degree of extinction conditions (i.e., with large regions of extinction and lifted flames), re-ignition of the locally extinguished flame is due to the mechanism of ignition assisted partially premixed flame propagation. The results show that the PDF-ESF model is capable of simulating the quenching and re-ignition process found in the experiments.
  • Droplet impacts onto soft solids entrap more air

    Langley, Kenneth; Castrejón-Pita, Alfonso A.; Thoroddsen, Sigurdur T (Soft Matter, Royal Society of Chemistry (RSC), 2020-06-08) [Article]
    <p>A liquid drop impacting onto a soft solid will entrap more air in the central air disc than an equivalent drop impacting onto a rigid surface.</p>
  • Quenching distance of laminar methane-air flames at cryogenic temperatures and implications for flame arrester design

    Guiberti, Thibault; Belhi, Memdouh; Roberts, William L.; Lacoste, Deanna; Damazo, Jason S.; Kwon, Eddie (Applications in Energy and Combustion Science, Elsevier BV, 2020-06-08) [Article]
    Understanding flame quenching is needed to develop efficient flame arresters. Here, the quenching distance of methane-air laminar flames is measured at atmospheric pressure for temperatures of the quenching surface down to the cryogenic, Tw = 138 K to 293 K, for two configurations: head-on and tube quenching. Fuels or flammable mixtures in contact with surfaces at temperatures below 293 K are, for example, representative of aircraft during cruise, cryogenic rocket engines, fuel distribution pipes at high altitude, or cryogenic storage of liquified natural gas and hydrogen. The experimental methods are first validated for Tw = 293 K by comparing measured quenching distances to that available in the literature. Then, quenching distances are measured for Tw = 138 K to 293 K. The quenching distance increases when temperature decreases. In the head-on quenching configuration, the quenching distance is almost multiplied by two, from δq = 0.17 mm for Tw = 290 K to δq = 0.32 mm for Tw = 175 K. In the tube quenching configuration, the quenching diameter increases by 40%, from 2.5 mm for Tw = 293 K to 3.5 mm for Tw = 138 K. Experiments conducted in tubes demonstrate that reducing the wall temperature allows quenching with larger tube diameters, yielding lower pressure drops in tubes, which is of practical interest.
  • Magnetohydrodynamic Richtmyer–Meshkov instability under an arbitrarily oriented magnetic field

    Shen, Naijian; Wheatley, Vincent; Pullin, D. I.; Samtaney, Ravi (Physics of Plasmas, AIP Publishing, 2020-06-05) [Article]
    The effect of an initially uniform magnetic field of arbitrary orientation on the Richtmyer–Meshkov instability in Hallmagnetohydrodynamics (MHD) and ideal MHD is considered. Attention is restricted to the case where the initial density interface has a single-mode sinusoidal perturbation in amplitude and is accelerated by a shock traveling perpendicular to the interface. An incompressible Hall-MHD model for this flow is developed by solving the relevant impulse-driven linearized initial value problem. The ideal MHD theory is naturally obtained by taking the limit of vanishing ion skin depth. It is shown that the out-of-plane magnetic field component normal to both the impulse and the interface perturbation does not affect the evolution of the flow. For all field orientations other than strictly out-ofplane, the growth of interface perturbations is suppressed. However, the suppression is most effective for near tangential fields but becomes less effective with increasing ion skin depth and Larmor radius. The modeled suppression mechanism is transport of vorticity along magnetic field lines via Alfven fronts in ideal MHD, and via a dispersive wave system in Hall-MHD. Oscillation of the interface growth rate is caused by a continuous phase change of the induced velocities at the interface due to vorticity transport parallel to the perturbation direction in ideal MHD, while it can also result from interfacial vorticity production associated with the ion cyclotron effect in Hall-MHD with a finite Larmor radius. The limiting flow behavior of a large ion-skin-depth is explored. To assess the accuracy and appropriateness of the incompressible model, its ideal MHD predictions are compared to the results of the corresponding shock-driven nonlinear compressible simulations.
  • High-pressure direct injection of methanol and pilot diesel: A non-premixed dual-fuel engine concept

    Dong, Yabin; Kaario, Ossi Tapani; Hassan, Ghulam; Ranta, Olli; Larmi, Martti; Johansson, Bengt (Fuel, Elsevier BV, 2020-06-05) [Article]
    In order to reduce the climate impacts, methanol produced from carbon-neutral methods plays an important role. Due to its oxygen content and high latent heat, methanol combustion can achieve low soot and NOx emissions. In the present study, direct injection (DI) of methanol is investigated in a non-premixed dual-fuel (DF) setup with diesel pilot. The present DF engine study is carried out via a specially-designed new cylinder head operating with a centrally located methanol injector and with an off-centered diesel pilot injector. The target is to inject methanol close to top dead center (TDC) in a similar fashion as in standard diesel combustion enabling robust operation with high efficiency. The ignition of the DI methanol is achieved with an almost simultaneously injected diesel pilot. The experiments were conducted in a single-cylinder heavy-duty research engine at a constant engine speed of 1500 rpm with a compression ratio of 16.5. The indicated mean effective pressure (IMEP) varied between 4.2 and 13.8 bar while the methanol substitution ratio was swept between 45 and 95%. In addition, the diesel pilot and methanol injection timings were varied for optimum efficiency and emissions. The introduced non-premixed DF concept using methanol as the main fuel showed robust ignition characteristics, stable combustion, and low CO and HC emissions. The results indicate that increasing both the load and the methanol substitution ratio can increase the thermal efficiency and the stability of combustion (lower COV) together with decreased CO and HC emissions.
  • A comprehensive study of spray and combustion characteristics of a prototype injector for gasoline compression ignition (GCI) application

    Du, Jianguo; Mohan, Balaji; Sim, Jaeheon; Fang, Tiegang; Chang, Junseok; Roberts, William L. (Fuel, Elsevier BV, 2020-06-04) [Article]
    In this study, the spray and combustion characteristics of high reactivity gasoline (HRG) fuel of RON 77 were tested and compared with E10 certification fuel under the gasoline compression ignition (GCI) engine conditions using a high-pressure multi-hole GCI engine injector. A comprehensive characterization in terms of the rate of injection, spray morphology under flash boiling conditions, penetration lengths under both nonevaporative and evaporative conditions, and ignition delay at reactive conditions was performed. It was found that both the high reactivity gasoline and E10 certification fuel exhibit very similar characteristics. The ignition delay times were found to be very similar between both the fuels tested under ambient temperatures higher than 800 K. This work further serves as an extensive database to validate and calibrate the spray models, combustion models and reaction mechanisms for computational fluid dynamics (CFD) driven development of GCI engines.
  • Effects of Turbulence and Temperature Fluctuations on Knock Development in an Ethanol/Air Mixture

    Luong, Minh Bau; Desai, Swapnil; Hernandez Perez, Francisco; Sankaran, Ramanan; Johansson, Bengt; Im, Hong G. (Flow, Turbulence and Combustion, Springer Science and Business Media LLC, 2020-05-29) [Article]
    The effects of turbulence on knock development and intensity for a thermally inhomogeneous stoichiometric ethanol/air mixture at a representative end-gas autoignition condition in internal combustion engines are investigated using direct numerical simulations with a skeletal reaction mechanism. Two- and three-dimensional simulations are performed by varying the most energetic length scale of temperature, lT, and its relative ratio with the most energetic length scale of turbulence, lT/ le, together with two different levels of the turbulent velocity fluctuation, u′. It is found that lT/le and the ratio of ignition delay time to eddy-turnover time, τig/ τt, are the key parameters that control the detonation development. An increase in either lT or le enhances the detonation propensity by allowing a longer run-up distance for the detonation development. The characteristic length scale of the temperature field, lT, is significantly modified by high turbulence intensity achieved by a large le and u′. The intense turbulence mixing effectively distributes the initial temperature field to broader scales to support the developing detonation waves, thereby increasing the likelihood of the detonation formation. On the contrary, high turbulence intensity with a short mixing time scale, achieved by a small le and a large u′, reduces the super-knock intensity attributed to the finer broken-up structures of detonation waves. Either τig/ τt less than unity or le= lT even with a large u′ is found to have no significant effect on super-knock mitigation. Finally, high turbulent intensity may induce high-pressure spikes comparable to the von Neumann spike. Increased temperature and pressure by combustion heating, noticeably after the peak of heat release rate, significantly enhance the collision and interaction of multiple emerging autoignition fronts near the ending combustion process, resulting in localized high-pressure spikes.
  • Investigating the effects of C3 and C4 alcohol blending on ignition quality of gasoline fuels

    Angikath Shamsudheen, Fabiyan; Naser, Nimal; Sarathy, Mani (Energy & Fuels, American Chemical Society (ACS), 2020-05-28) [Article]
    The study of the ignition quality of alcohol blends with petroleum fuels is a subject of practical interest. It is well known that the ignition delay time (IDT), as well as octane number (ON), increases when gasoline fuels are blended with ethanol. This study focuses on the impact on inverse ignition delay time (IDT-1) when alcohols, such as n-propanol and n-butanol, are blended with gasoline fuels. A non-linear decrease in the IDT-1 of the blends was observed. Predicting the extent of non-linearity in blends is complicated because it involves unknown inter-molecular interactions between base fuel components and the blended components. The purpose of this study is to establish the dependence of base fuel composition (in terms of functional groups) on observed non-linearity. Gasoline fuel contains hundreds of compounds (predominantly hydrocarbons), making it a challenge to understand observed non-linearity when they blend with other components. In this study, the IDT of primary reference fuels (PRF, a binary mixture of iso-octane and n-heptane) and FACE gasolines (fuels for advanced combustion engines) blended with two alcohols (n-propanol and n-butanol) were obtained with an ignition quality tester (IQT) following ASTM D6890 standards. A mole-based Gaussian fit was used to model the blending effects of alcohol with gasoline. The synergistic effect of the different mixtures tested in this study was investigated by analyzing the Gaussian parameters. A multiple linear regression model was formulated to provide information about the impact of the structural composition (functional group) on the synergistic blending effects of gasoline-alcohol mixtures. Constant volume homogenous batch reactor simulations were also conducted, using Chemkin-Pro for alcohols blended with a FACE J surrogate mixture to provide kinetic information about the blending effects observed in the IQT measurements.
  • Theoretical and experimental investigation of mode localization in electrostatically and mechanically coupled microbeam resonators

    Ilyas, Saad; Younis, Mohammad I. (International Journal of Non-Linear Mechanics, Elsevier BV, 2020-05-23) [Article]
    The phenomenon of mode localization is explored theoretically and experimentally on two mechanically or electrostatically coupled beam resonators. Lumped parameter models are used to simulate the response of the systems. The eigenvalue problems are solved for both case studies under different stiffness perturbations and coupling strengths. The influence of the side electrode bias on the veering points is also explored. The dynamics of the systems are studied and compared using their frequency response curves under different perturbation and damping scenarios. The effect of damping for different elements of the coupled system is studied and proposed to improve sensitivity in high damping environments. It is observed that the exploitation of mode localization depends primarily on the choice of the resonator of the coupled system to be under direct excitation, its stiffness to be perturbed, and its response to be monitored. The revealed dynamic behaviors show great potential for applications in sensing and mechanical computing. The theoretical findings are validated using experimental case studies of two silicon doubly-clamped mechanically coupled microbeams and two electrostatically coupled microcantilevers. Electrothermal voltage is applied to the mechanically coupled resonators to introduce the stiffness perturbation in order to investigate mode localization. The theoretical and experimental results show good qualitative agreement.
  • Screening gas-phase chemical kinetic models: Collision limit compliance and ultrafast timescales

    Yalamanchi, Kiran K.; Tingas, Alexandros; Im, Hong G.; Sarathy, Mani (International Journal of Chemical Kinetics, Wiley, 2020-05-22) [Article]
    Detailed gas-phase chemical kinetic models are widely used in combustion research, and many new mechanisms for different fuels and reacting conditions are developed each year. Recent works have highlighted the need for error checking when preparing such models, but a useful community tool to perform such analysis is missing. In this work, we present a simple online tool to screen chemical kinetic mechanisms for bimolecular reactions exceeding collision limits. The tool is implemented on a user-friendly website, cloudflame.kaust.edu.sa, and checks three different classes of bimolecular reactions; (ie, pressure independent, pressure-dependent falloff, and pressure-dependent PLOG). In addition, two other online modules are provided to check thermodynamic properties and transport parameters to help kinetic model developers determine the sources of errors for reactions that are not collision limit compliant. Furthermore, issues related to unphysically fast timescales can remain an issue even if all bimolecular reactions are within collision limits. Therefore, we also present a procedure to screen ultrafast reaction timescales using computational singular perturbation. For demonstration purposes only, three versions of the rigorously developed AramcoMech are screened for collision limit compliance and ultrafast timescales, and recommendations are made for improving the models. Larger models for biodiesel surrogates, tetrahydropyran, and gasoline surrogates are also analyzed for exemplary purposes. Numerical simulations with updated kinetic parameters are presented to show improvements in wall-clock time when resolving ultrafast timescales.

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