627 research outputs found
Effect of Dynamic Surface Polarization on the Oxidative Stability of Solvents in Nonaqueous Li-O Batteries
Polarization-induced renormalization of the frontier energy levels of
interacting molecules and surfaces can cause significant shifts in the
excitation and transport behavior of electrons. This phenomenon is crucial in
determining the oxidative stability of nonaqeous electrolytes in high energy
density electrochemical systems such as the Li-O battery. On the basis of
partially self-consistent first-principles ScGW0 calculations, we
systematically study how the electronic energy levels of four commonly used
solvent molecules, namely dimethylsulfoxide (DMSO), dimethoxyethane (DME),
tetrahydrofuran (THF) and acetonitrile (ACN), renormalize when physisorbed on
the different stable surfaces of LiO, the main discharge product. Using
band level alignment arguments, we propose that the difference between the
solvent's highest occupied molecular orbital (HOMO) level and the surface's
valence band maximum (VBM) is a refined metric of oxidative stability. This
metric and a previously used descriptor, solvent's gas phase HOMO level, agree
quite well for physisorbed cases on pristine surfaces where ACN is oxidatively
most stable followed by DME, THF and DMSO. However, this effect is
intrinsically linked to the surface chemistry of solvent's interaction with the
surfaces states and defects, and depends strongly on their nature. We
conclusively show that the propensity of solvent molecules to oxidize will be
significantly higher on LiO surfaces with defects as compared to
pristine surfaces. This suggests that the oxidatively stability of solvent is
dynamic and is a strong function of surface electronic properties. Thus, while
gas phase HOMO levels could be used for preliminary solvent candidate
screening, a more refined picture of solvent stability requires mapping out the
solvent stability as a function of the state of the surface under the operating
conditions.Comment: 10 Pages, 8 Figure
Thermochemical properties of polycyclic aromatic hydrocarbons (PAH) from G3MP2B3 calculations
In this article, we present a new database of thermodynamic properties for polycyclic aromatic hydrocarbons (PAH). These large aromatic species are formed in very rich premixed flames and in diffusion flames as part of the gas-phase chemistry. PAH are commonly assumed to be the intermediates leading to soot formation. Therefore, accurate prediction of their thermodynamic properties is required for modeling soot formation. The present database consists of 46 species ranging from benzene (C_6H_6) to coronene (C_(24)H_(12)) and includes all the species usually present in chemical mechanisms for soot formation. Geometric molecular structures are optimized at the B3LYP/6-31++G(d,p) level of theory. Heat capacity, entropy, and energy content are calculated from these optimized structures. Corrections for hindered rotor are applied on the basis of torsional potentials obtained from second-order Møller-Plesset perturbation (MP2) and Dunning's consistent basis sets (cc-pVDZ). Enthalpies of formation are calculated using the mixed G3MP2//B3 method. Finally, a group correction is applied to account for systematic errors in the G3MP2//B3 computations. The thermodynamic properties for all species are available in NASA polynomial form at the following address: http://www.stanford.edu/group/pitsch/
Scalar mass conservation in turbulent mixture fraction based combustion models through consistent local flow parameters
Mixture fraction-based models are widely employed for predicting turbulent
non-premixed combustion processes due to their cost-effectiveness and
well-established subfilter closure. In these models, the transport of reactive
scalars in physical space is decomposed into two components: scalar transport
relative to mixture fraction and transport of mixture fraction in physical
space. Conventional flamelet models do not consider that these two processes
have to be formulated consistently, which can lead to scalar mass conservation
errors. In the context of multiphase flows, scalar transport in mixture
fraction space is governed by three conditional flow-dependent parameters: the
conditional scalar dissipation rate, the conditional scalar diffusion rate, and
the conditional spray source term. The evolution of mixture fraction in
physical space is typically modeled using the presumed Filtered Density
Function (FDF) approach. This paper introduces a novel formulation for the
conditional flow parameters that aligns with the presumed FDF approach, thereby
ensuring scalar mass conservation. The proposed model is applied to a
Large-Eddy Simulation (LES) of the inert ECN Spray A case, with a comparison
against a conventional flow parameter model that employs an inverse error
function shape for the scalar dissipation rate. The results indicate that the
conventional model produces similar conditional dissipation rates to the new
model in regions where combustion takes place. However, significant
discrepancies are observed in the conditional diffusion rate, highlighting the
susceptibility of the conventional model to scalar mass conservation errors for
non-unity Lewis number scalars
Время-амплитудный конвертор-хронотрон
Рассматривается структурная схема время-амплитудного преобразователя на основе многоканального хронотрона. Используется двойное преобразование: время - числовой код - амплитуда. Прилагаемая блок-схема преобразователя обеспечивает повышенное быстродействие устройства (~30 МГц)
In-situ temperature and major species measurements of sooting flames based on short-gated spontaneous Raman scattering
Spontaneous Raman spectroscopy (SRS) is a conventional in-situ laser
diagnostic method that has been widely used for measurements of temperature and
major species. However, SRS in sooting flames suffers from strong interference
including laser-induced fluorescence, laser-induced incandescence, and flame
luminosity, which is a long-lasting challenge. This work introduces a low-cost,
easy-to-implement, and calibration-free SRS thermometry in sooting flames based
on a 355-nm nanosecond laser beam. Several strategies were utilized to increase
the signal-to-noise ratio and suppress the interference: (1) nanosecond ICCD
gate width; (2) optimized ICCD gate delay; (3) specially designed focusing
shape of the laser beam; (4) ultraviolet polarizer filter. The temperature was
obtained by fitting the contour of Stokes-Raman spectra of N2 molecules, which
does not require additional calibration. Based on the measured temperature, the
mole fraction of major species can be obtained with calibration. This method
was used in the temperature and major species measurements of an ethylene-based
counterflow diffusion flame. The experimental results show an excellent
agreement with the simulation results, demonstrating the feasibility of
performing non-intrusive laser diagnostics of sooting and other particle-laden
flames accurately
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