Hypothesis Of Local Thermodynamic Equilibrium Research Articles

Molecular simulations of premelted films between C-S-H and ice: Implication for cryo-suction in cement-based materials

The premelted films between cement and ice observed at freezing temperatures play a role in the freeze-thaw damage of cement-based materials, as they play a role in cryo-suction. However, their properties are poorly understood. In this work, we unveil the dynamics and local structure of the premelted film using molecular simulations. Combining those results with a thermodynamic analytical derivation, we obtain a characteristic time of the cryo-suction process, which depends on temperature, the transport properties and thickness of the premelted film, and the pore geometry. Our calculations show that, down to about 230 K, cryo-suction occurs relatively fast at the pore scale and the hypothesis of local thermodynamic equilibrium used in most poromechanical models of freezing of cement-based materials is reasonable. A fragile-to-strong transition of water in the premelted films is witnessed at about 230 K. Below this temperature, the significant deceleration of water dynamics makes the cryo-suction significantly slower.

Method for Qualitative and Quantitative Analysis of Ancient Lead Enamel Using Laser Inducted Breakdown Spectroscopy

The method of laser induced breakdown spectroscopy (LIBS) was used in analysis of the archaeological enamel samples from the ancient settlement of Jochi Khan (XIV century). During the qualitative analysis, it was found that the elements of the enamel matrix are Si, Pb, K, Na, Mg, Ca, Al. The glaze color is due to the presence of copper and iron. Clustering by the k-means method revealed two groups of samples similar in composition of enamel, but differing in place of origin within the settlement. For the studied samples the semi-quantitative composition of glazes was established from the spectra of LIBS using a method based on the hypothesis of local thermodynamic equilibrium (LTE) and a chemometric approach (Projection to Latent Structures, PLS). Probabilistic deterministic design of experiment was used to search for pairs of element lines that are not subject to changes when the conditions for spectra registration vary, and to determine the coefficients in the equations based on LTR. Calculations using the PLC method were carried out in the “R” programming environment. The following content of the matrix elements was obtained in terms of the most stable oxides, %: SiO2 — 49–61, PbO — 23–31, MgO — 1.7–2.3, CaO — 4.6–6.9, Na2O — 4.1–5.3, K2O — 5.1–6.4, Al2O3 — 0.8–1.7, CuO — 0.32–0.4, Fe2O3 — 0.09–0.16. The results of both methods are generally consistent with each other. The inaccuracy between 4 parallel determinations were 15-24 % for LTE, and 9–14 % for PLS. Taking into account the data of semi-quantitative analysis, it was concluded that the division into two groups is based on a different ratio of copper and iron in enamel.

Modelación de una antorcha de plasma de arco no transferido

Las antorchas de plasma se usan para producir el plasma en la manufactura de materiales e industria energética, donde las antorchas de arco de corriente directa representan los componentes principales de los procesos con plasma térmico (plasma spraying, soldadura, corte de metales, tratamiento de residuos, producción de biogás, etc.). En una antorcha de plasma de arco no trasferido, se desarrolla un arco eléctrico aplicando una corriente directa entre el cátodo y el ánodo de la antorcha. Sucesivamente, el plasma se produce calentando, ionizando y expandiendo un gas alimentado a la antorcha antes de la región del cátodo. Los trabajos de laboratorio que usan técnicas de observación y de medición son de difícil aplicación, debido a las características físicas de estos fenómenos físicos. Así las técnicas computacionales pueden representar una herramienta valiosa para investigar los procesos con plasma, aunque el trabajo computacional es extremadamente desafiante, pues estos fenómenos son interdependientes y simultáneos. En este trabajo se propone un modelo computacional para simular una antorcha de plasma bidimensional de arco no trasferido en equilibrio termodinámico local. Se modelan condiciones de estado estacionario para las ecuaciones de conservación de mecánica de fluidos, trasferencia de calor y electromagnetismo en el software Comsol Multiphysics® 5.1. En la trasferencia de calor por radiación, el plasma se asume ópticamente fino y con un coeficiente de emisión neta. El flujo laminar de gas se modela con vórtice libre en la entrada para dos tipologías de antorchas axis-simétricas, respectivamente con argón y nitrógeno como gases de trabajo.

Evaluation of temperature history of a spherical nanosystem irradiated with various short-pulse laser sources.

Spatiotemporal thermal response and characteristics of net entropy production rate of a gold nanosphere (radius: 50-200 nm), subjected to a short-pulse, femtosecond laser is reported. In order to correctly illustrate the temperature history of laser-metal interaction(s) at picoseconds transient with a comprehensive single temperature definition in macroscale and to further understand how the thermophysical response of the single-phase lag (SPL) and dual-phase lag (DPL) frameworks (with various lag-ratios') differs, governing energy equations derived from these benchmark non-Fourier frameworks are numerically solved and thermodynamic assessment under both the classical irreversible thermodynamics (CIT) as well as extended irreversible thermodynamics (EIT) frameworks is subsequently carried out. Under the frameworks of SPL and DPL with small lag ratio, thermophysical anomalies such as temperature overshooting characterized by adverse temperature gradient is observed to violate the local thermodynamic equilibrium (LTE) hypothesis. The EIT framework, however, justifies the compatibility of overshooting of temperature with the second law of thermodynamics under a nonequilibrium paradigm. The DPL framework with higher lag ratio was however observed to remain free from temperature overshooting and finds suitable consistency with LTE hypothesis. In order to solve the dimensional non-Fourier governing energy equation with volumetric laser-irradiation source term(s), the lattice Boltzmann method (LBM) is extended and a three-time level, fully implicit, second order accurate finite difference method (FDM) is illustrated. For all situations under observation, the LBM scheme is featured to be computationally superior to remaining FDM schemes. With detailed prediction of maximum temperature rise and the corresponding peaking time by all the numerical schemes, effects of the change of radius of the gold nanosphere, the magnitude of fluence of laser, and laser irradiation with multiple pulses on thermal energy transport and lagging behavior (if any) are further elucidated at different radial locations of the gold nanosphere. Last, efforts are further made to address the thermophysical characteristics when effective thermal conductivity (with temporal and size effects) is considered instead of the usual bulk thermal conductivity.

Equilibrium and non-equilibrium thermodynamic analysis of high-order dual-phase-lag heat conduction

The compatibility of dual-phase-lag (DPL) heat conduction with the hypothesis of local thermodynamic equilibrium (LTE) and extended irreversible thermodynamic is investigated. The analysis is based on evaluating the entropy production rate (EPR) for a solid slab that is exposed to a sudden temperature gradient at its boundaries. In this regard, an analytical solution is presented for the first-order and second-order approximations of the DPL model. It is shown that by extending the non-Fourier Cattaneo–Vernotte (C–V) wave model to the high-order DPL model, the unphysical values of the slab temperature that exist in the solution of C–V wave model cannot be generally eliminated. However, a set of phase lag times for the DPL model exist based on which these unphysical values can be omitted by increasing the diffusion versus the wavelike features.

RELATIVISTIC VISCOELASTIC FLUID MECHANICS

We explain the relativistic theory of viscoelasticity which we have recently constructed on the basis of Onsager's linear nonequilibrium thermodynamics. This theory universally reduces to the standard relativistic Navier-Stokes fluid mechanics in the long time limit. Since effects of elasticity are taken into account, the dynamics at short time scales is modified from that given by the Navier-Stokes equations, so that acausal problems intrinsic to relativistic Navier-Stokes fluids are significantly remedied. We then present conformal higher-order viscoelastic fluid mechanics with strain allowed to take arbitrarily large values. We particularly show that a conformal second-order fluid with all possible parameters in the constitutive equations can be obtained without breaking the hypothesis of local thermodynamic equilibrium, if the conformal fluid is defined as the long time limit of a conformal second-order viscoelastic system.

Mean absorption coefficient of H2O–air–MgCl2/CaCl2/NaCl thermal plasmas

Under the local thermodynamic equilibrium hypothesis, the mean absorption coefficients (MACs) were calculated for H2O–air–MgCl2/CaCl2/NaCl thermal plasmas in a temperature range from 300 to 30 000 K and at atmospheric pressure. The MACs were computed under the hypothesis of isothermal plasmas which allows a good description of the radiation absorbed in cold regions. In this study, we took into account the absorption radiation resulting from the atomic continuum, molecular continuum, atomic lines and some molecular bands. Free–free transitions (bremsstrahlung) and free–bound (electron–ion recombination and electron attachment) or bound–free transitions in terms of absorption were considered for the calculation of atomic continuum. For bound–bound transitions, natural, resonance, van der Waals, Stark and Doppler effects were taken into account for the line broadenings while the escape factors were used to treat the self-absorption of the resonance lines. Molecular continuum was considered for the main molecules (H2, O2, N2, OH, NO, H2O, N2O, NO2, O3, NO3 and N2O5) whereas we studied only diatomic systems O2, N2, NO and for the absorption of molecular bands. The influence of the proportion of MgCl2, CaCl2 or NaCl in a water–air mixture was analysed as the effect of the strong self-absorbed resonance lines of the alkaline salts (Ca, Ca+, Na, Na+, Mg, Mg+, Cl and Cl+). Our results show that a low concentration of alkaline salts (less than 1% in molar proportions) in the plasma increased the MACs at low temperatures (T < 10 000 K) due to the resonance lines mainly localized in the near-UV and visible spectral regions in opposition to hydrogen, oxygen or nitrogen species for which 90% of them exist in ultraviolet. In addition to the atomic and molecular continuum, the absorption radiation of molecular bands is important at low temperatures.

Conformal higher-order viscoelastic fluid mechanics

We present a generally covariant formulation of conformal higher-order viscoelastic fluid mechanics with strain allowed to take arbitrarily large values. We give a general prescription to determine the dynamics of a relativistic viscoelastic fluid in a way consistent with the hypothesis of local thermodynamic equilibrium and the second law of thermodynamics. We then elaborately study the transient time scales at which the strain almost relaxes and becomes proportional to the gradients of velocity. We particularly show that a conformal second-order fluid with all possible parameters in the constitutive equations can be obtained without breaking the hypothesis of local thermodynamic equilibrium, if the conformal fluid is defined as the long time limit of a conformal second-order viscoelastic system. We also discuss how local thermodynamic equilibrium could be understood in the context of the fluid/gravity correspondence.

Identification of nitro-compounds with LIBS

Laser-induced breakdown spectroscopy (LIBS) is applied to characterize the organic compounds 4-nitroaniline and 4-nitrotoluene by measuring the intensity of atomic lines. The hypothesis of local thermodynamic equilibrium (LTE) is verified by comparing the measured value of electron density in the laser-induced plasma with its lower limit, for which LTE holds good. The present study infers that the O/N intensity ratio is a unique parameter to discriminate each nitro-compound from the explosives.

A numerical study of the supply mode effects on high-pressure mercury discharge lamp dynamic thermal behavior

This paper presents a numerical study of the dynamic behavior of high-pressure mercury discharge lamp as a function of supply mode. Bidimensional time-dependent equations have been solved by using a semi-implicit finite-element code. The model has been developed in the frame of the local thermodynamic equilibrium hypothesis. After validation by using experimental data from the literature, this model was applied to analyze the effects of some key parameters such as frequency for an ac arc current and the ratio cycle parameter for square arc-current wave form on the convective process. The results obtained have been used to analyze the dynamic thermal behavior of high-pressure mercury plasma (pressure equal to 0.3 MPa) working under currents widely different in frequency and wave form.

Laser-induced breakdown spectroscopy as an analytical tool for equivalence ratio measurement in methane–air premixed flames

Laser-Induced Breakdown Spectroscopy has been applied to laminar premixed methane–air flames in order to obtain, for the first time, quantitative measurements of atomic species densities in a reacting flow. The electron density and the plasma temperature were determined and it was shown that the hypothesis of local thermodynamic equilibrium is valid under the present experimental conditions. It was further shown, that the equivalence ratio ϕ of methane–air mixtures can be quantitatively determined from the ratio of the number densities N H/ N O, the latter being obtained from the ratio of the hydrogen (H α at 656 nm) and oxygen (O( I) at 777 nm) atomic lines intensities.

Theoretical modeling of microstructured liquids: a simple thermodynamic approach

A new method is presented for accounting for microstructural features of flowing complex fluids at the level of mesoscopic, or coarse-grained, models by ensuring compatibility with macroscopic and continuum thermodynamics and classical transport phenomena. In this method, the microscopic state of the liquid is described by variables that are local expectation values of microscopic features. The hypothesis of local thermodynamic equilibrium is extended to include information on the microscopic state, i.e., the energy of the liquid is assumed to depend on the entropy, specific volume, and microscopic variables. For compatibility with classical transport phenomena, the microscopic variables are taken to be extensive variables (per unit mass or volume), which obey convection-diffusion-generation equations. Restrictions on the constitutive laws of the diffusive fluxes and generation terms are derived by separating dissipation by transport (caused by gradients in the derivatives of the energy with respect to the state variables) and by relaxation (caused by non-equilibrated microscopic processes like polymer chain stretching and orientation), and by applying isotropy. When applied to unentangled, isothermal, non-diffusing polymer solutions, the equations developed according to the new method recover those developed by the Generalized Bracket [J. Non-Newtonian Fluid Mech. 23 (1987) 271; A.N. Beris, B.J. Edwards, Thermodynamics of Flowing Systems with Internal Microstructure, first ed., Oxford University Press, Oxford, 1994] and by the Matrix Model [J. Rheol. 38 (1994) 769]. Minor differences with published results obtained by the Generalized Bracket are found in the equations describing flow coupled to heat and mass transfer in polymer solutions. The new method is applied to entangled polymer solutions and melts in the general case where the rate of generation of entanglements depends nonlinearly on the rate of strain. A link is drawn between the mesoscopic transport equations of entanglements and conformation and the microscopic equation describing the configurational distribution of polymer segment stretch and orientation. Constraints are derived on the generation terms in the transport equations of entanglements and conformation, and the formula for the elastic stress is generalized to account for reversible formation and destruction of entanglements. A simplified version of the transport equation of conformation is presented which includes many previously published constitutive models, separates flow-induced polymer stretching and orientation, yet is simple enough to be useful for developing large-scale computer codes for modeling coupled fluid flow and transport phenomena in two- and three-dimensional domains with complex shapes and free surfaces.

Hyperbolic heat conduction and local equilibrium: a second law analysis

Abstract An analysis of the compatibility of Cattaneo-Vernotte's constitutive equation for the heat flux density vector with the hypothesis of local thermodynamic equilibrium is presented. This compatibility is checked by determining the entropy production rate per unit volume. In fact, within the scheme of local equilibrium, Clausius' inequality implies that the entropy production rate must be non-negative. The evaluation of the entropy production rate is performed for a solid slab which experiences a sudden change of the boundary temperature and for a semi-infinite solid with a time-varying boundary heat flux. A domain of non-stationary phenomena is found in which Cattaneo-Vernotte's equation is compatible with the assumption of local equilibrium and yields results definitely different from those predicted by Fourier's equation.

Non-fourier heat conduction in a plane slab with prescribed boundary heat flux

The non-stationary heat conduction in an infinitely wide plane slab with a prescribed boundary heat flux is studied. An arbitrary time dependent boundary heat flux is considered and a non-vanishing thermal relaxation time is assumed. The temperature and the heat flux density distributions are determined analytically by employing Cattaneo-Vernotte's constitutive equation for the heat flux density. It is proved that the temperature and the heat flux density distributions can be incompatible with the hypothesis of local thermodynamic equilibrium. A comparison with the solution which would be obtained by means of Fourier's law is performed by considering the limit of a vanishing thermal relaxation time.

鉄蒸気の混入したアルゴンの輸送特性

Compared with the conventional combustion, thermal plasma technology offers particular advantages for industrial heating and opens new applications. In order to support these applications theoretically, fundamental propeties of plasma are needed. So electrical and thermal conductivities of different compositions of the argon-iron mixture gases used in plasma torches have been calculated. The method we used was derived from Chapmann-Enskog's. Calculations were made for temperature between 2, 500-30, 000K and for pressure between 1-20atm assuming the hypothesis of local thermodynamic equilibrium. In the case of the pure argon, our results show good agreement with other experimental and calculated values. The effects of iron vapor and pressure on transport properties were clarified. Main results are as follows:(1) Admixture of iron vapor causes the transport properties to increase in lower temperatures and to decrease in higher temperatures; this results from the lower ionization potentials of iron atoms and ions compared with those of argon atoms and ions.(2) Increase of pressure causes the transport properties to decrease in lower temperatures and to increase in higher temperatures; this results from the upward shift of the temperature region in which the ionization reactions proceed efficiently.

Transport coefficients in arc plasma of SF6–N2 mixtures

We compute thermal and electrical conductivity coefficients as a function of temperature for different compositions of the N2–SF6 mixture. The method we used was derived from Chapman–Enskog’s. The composition of the plasma is calculated assuming the hypothesis of local thermodynamic equilibrium. Calculations were made for temperatures between 1000 and 15 000 K. We show that the densities of the compounds specifically due to the SF6–N2 mixture (i.e., NF, NS, N2F4, NF3, etc....) are negligible compared with the densities of the products due to SF6 alone and N2 alone. The characteristics of the transport coefficients of the mixture are therefore a combination (depending on the proportions in the mixture) of the characteristics of pure N2 and SF6. These theoretical values allow us to interpret the curves obtained experimentally by measuring, using laser interferometry at two wavelengths, the variation of electron number density in a decaying arc discharge in a SF6–N2 mixture. By substituting the values of the transport coefficients into an energy balance equation, we also determined, in the stationary state the profiles of the temperature and the electric field and the variation of the electron density with the discharge current.