Transport coefficients of liquid CF4 and SF6 computed by molecular dynamics using polycenter Lennard-Jones potentials

For several liquid states of CF₄ and SF₄, the shear and the bulk viscosity as well as the thermal conductivity were determined by equilibrium molecular dynamics (MD) calculations. Lennard-Jones four- and six-center pair potentials were applied, and the method of constraints was chosen for the MD. The computed Green-Kubo integrands show a steep time decay, and no particular longtime behavior occurs. The molecule number dependence of the results is found to be small, and 3×10⁵ integration steps allow an accuracy of about 10% for the shear viscosity and the thermal conductivity coefficient. Comparison with experimental data shows a fair agreement for CF₄, while for SF₆ the transport coefficients fall below the experimental ones by about 30%.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Correlation and prediction of dense fluid transport coefficients. III. n-alkane mixtures

Viscosity and thermal conductivity coefficients for binary, ternary, and quaternary n-alkane mixtures are predicted over extended ranges of temperature and pressure, in excellent agreement with experiment, by extension of a method recently described for the correlation of n-alkane transport coefficients. The outstanding advantage of this approach is that there are no adjustable parameters. Furthermore, in contrast with other mixture viscosity equations, this scheme does not require experimental viscosity coefficient data for the pure components under the same conditions of temperature and pressure.     

Nonequilibrium molecular dynamics of liquid crystals

During the last 15 years, noneyuilibrium molecular dynamics (NEMD) has been successfully applied to study transport phenomena in fluids that are isotropic at equilibrium. A natural extension is therefore to study liquid crystals, which are anisotropic al equilibrium. The lower symmetry of these systems means that the linear transport coefficients are considerably more complicated than in an isotropic system. Part of the reason for this is that there are crosscouplings between tensors of different rank and parity. Such couplings arc symmetry-forbidden in isotropic phases. In this paper. we review some of fundamental theoretical results we have derived concerning the rheology of liquid crystals. report NEMD simulations of thermal conductivity and shear viscosity of liquid crystals, and present NEMD simulations of shear cessation phenomena. All of the NEMD results are presented for a model liquid crystal fluid which is a modification of the Gay-Borne fluid. The results obtained are in qualitative agreement with experimental measurements on liquid crystal systems.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Phonon-Impuriton Relaxation and Transport in 3He-4He Superfluid Solutions

No Heading The measurements of the effective thermal conductivity coefficient of a superfluid³He-⁴He mixture with initial concentration of 9.8% ³He are carried out in the temperature range 70 – 500 mK. The results obtained and the other experimental data available on effective thermal conductivity, shear viscosity, and spin diffusion are interpreted within the kinetic theory of phonon - impuriton system of superfluid solutions. It is shown that all experimental data can be explained by taking into account the following relaxation processes: longitudinal phonon relaxation, phonon scattering by impuriton, phonon absorption and emission by impuriton, and impuriton - impuriton interaction. The last process is characterized by different relaxation times for each transport phenomenon: transfer of mass, momentum, and nucleus spin. The impuriton - impuriton and phonon - impuriton relaxation times are estimated and the hierarchy of relaxation times in such system is established. The phonon longitudinal relaxation time in phonon diffusion process should be taken into account. The phonon and impuriton contributions to thermal conductivity, viscosity, and mass diffusion are estimated.PACS numbers: 66.20.+d, 67.60–g, 64.75+g, 67.40.Pm, 66.60+a, 66.30.Xj     

Spin-independent transport parameters for superfluid3He-B

The scalar kinetic equation for Bogoliubov quasiparticles in the B phase of superfluid³He is discussed and the collision integral is represented in a compact form. For the cases of shear and second viscosity and diffusive thermal conductivity the problem is reduced to solving one-dimensional integral equations. The quasiparticle interaction enters via weighted angular averages of the normal state scattering amplitude. The effect of strong coupling renormalization of the gap function is accounted for. The transport coefficients are exactly related to relaxation parameters that describe how the system tends toward local equilibrium. For low temperatures the transport parameters are evaluated exactly, including corrections of orderT/T _c. The results are compared with those of a previous paper in which an approximate form of the collision operator was used, as well as with results of a variational approach and with recent experimental data.     

Transport in simple dense fluids

A transport theory for Lennard-Jones (LJ) fluids is described. The underlying mean-field kinetic theory models the LJ potential by adding a hard-sphere core to the attractive tail of the LJ potential. The transport coefficients discussed here — shear viscosity, thermal conductivity, and self-diffusion coefficient — exhibit Enskog-like forms, but now the radial distribution function (rdf) bears explicit dependence on the LJ tail as well as on the hard-sphere core. The hardsphere diameter is determined according to the well-known WCA method used in equilibrium statistical mechanics to mimic the LJ fluid. Hence the transport theory employs no adjustable parameters. Numerical results are compared to simulation and experimental results for many states, including saturated liquid, triple point, and dense gas. In general, a quantitatively accurate transport theory is obtained for the states considered. This represents improvement, both numerically and conceptually, over an earlier theory.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Comparison of the Results of Prediction of Transport Coefficients of Sodium Vapor with Experimental Data

The interatomic interaction potentials derived from new precision spectroscopic data are used to calculate averaged collision cross sections of two sodium atoms. The coefficients of viscosity and thermal conductivity of sodium vapor at temperatures from 700 to 2500 K and pressures from those corresponding to monatomic rarefied gas to the saturation curve (but not exceeding 1 MPa) are predicted. The prediction results are compared with the available experimental data about the transport coefficients of sodium vapor. Possible reasons for observed systematic discrepancy are discussed.     

Prediction of the viscosity and thermal conductivity coefficients of mixtures

A corresponding states procedure to predict the viscosity and thermal conductivity coefficients of a pure fluid or mixture is discussed. We show the transport properties of a fluid or mixture can be calculated to within experimental error given only corresponding values for a reference fluid and equation of state data. With methane, as the reference fluid, we consider nitrogen, ethane, propane, butane, carbon dioxide, and mixtures of these fluids. LNG is also included. It is shown that the conventional corresponding states approach is not sufficient to predict correctly the transport properties. The effect of internal degrees of freedom on the thermal conductivity coefficient and the enhancement in the critical region for this coefficient is discussed briefly.     

MgO nanofluids: higher thermal conductivity and lower viscosity among ethylene glycol-based nanofluids containing oxide nanoparticles

Five kinds of oxides, including MgO, TiO₂, ZnO, Al₂O₃ and SiO₂ nanoparticles were selected as additives and ethylene glycol (EG) was used as base fluid to prepare stable nanofluids. Thermal transport property investigation demonstrated substantial increments in the thermal conductivity and viscosity of all these nanofluids with oxide nanoparticle addition in EG. Among all the studied nanofluids, MgO–EG nanofluid was found to have superior features, with the highest thermal conductivity and lowest viscosity. The thermal conductivity enhancement ratio of MgO–EG nanofluid increases nonlinearly with the volume fraction of nanoparticles. In the experimental temperature range of 10–60°C, thermal conductivity enhancement ratio of MgO–EG nanofluids appears to have a weak dependence on the temperature. Viscosity measurements showed that MgO–EG nanofluids demonstrated Newtonian rheological behaviour, and the viscosity significantly decreases with the temperature. The thermal conductivity and viscosity increments of the nanofluids are much higher than the corresponding values predicted by the existing classical models for the solid–liquid mixture.     

Equations for Thermal Conductivity of Natural Refrigerants

Experimental results for the thermal conductivity of ammonia, propane, butane, isobutane, and propylene are reviewed, with special attention given to the liquid phase. New equations for the thermal conductivity of these five substances applicable for practical use over wide ranges of temperature and pressure including the critical region are proposed based on the experimental data. The present equations as well as the existing equations are compared with the experimental data. Compared with existing equations for ammonia, isobutane, and propylene, which are not reliable in the liquid phase, the behavior of the thermal conductivity for these substances is much improved using the present equations.     

The Simulation of Liquid Mercury by Diffraction Data and the Inference of Interparticle Potential

Structural diffraction data in a wide temperature range are used for the construction of models of liquid mercury with the aid of the Schommers algorithm and for the inference of effective pair interparticle potentials. The potentials are characterized by a steeply rising repulsive branch at short interatomic distances and by a relatively weak oscillating branch at long interatomic distances. No regular variation of the potentials with increasing temperature was observed. The predicted coefficients of self-diffusion in liquid mercury are in adequate agreement with the experimental data obtained at low temperatures. The Stokes–;Einstein relation is valid, and used to predict the viscosity of mercury under close-to-critical conditions. The degree of friability of the structure of liquid mercury increases with temperature. At 1803 K, large pores due to fluctuations in density are observed in the model of liquid mercury.     

Analysis of the Thermal Conductivity Coefficients for Gas Mixtures: Application of the Modified Enskog Theory

The method for the calculation of transport coefficients for mixtures of high-density simple gases, which is a generalization of the modified Enskog theory for pure (individual) gases, is used in a simplified form suggested by the author. The simplification is derived using the perturbation method. Computer codes are compiled for performing an analysis of the coefficients of viscosity and thermal conductivity for multicomponent mixtures of high-density gases. The results of calculation of the thermal conductivity coefficient of a nitrogen–carbon dioxide mixture are compared with relevant experimental data: the deviation does not exceed 7% in the experimentally treated region of states.     

Self-Diffusion and Binary Maxwell–Stefan Diffusion Coefficients of Quadrupolar Real Fluids from Molecular Simulation

Self- and binary Maxwell–Stefan (MS) diffusion coefficients were determined by equilibrium molecular dynamics simulations with the Green–Kubo method. This study covers self-diffusion coefficients at liquid states for eight pure fluids, i.e., F₂, N₂, CO₂, CS₂, C₂H₆, C₂H₄, C₂H₂, and SF₆ as well as MS diffusion coefficients for three binary mixtures N₂+CO₂, N₂+C₂H₆, and CO₂+C₂H₆. The fluids were modeled by the two-center Lennard–Jones plus point-quadrupole pair potential, with parameters taken from previous work of our group which were determined solely on the basis of vapor–liquid equilibrium data. Self-diffusion coefficients are predicted with a statistical uncertainty less than 1%, and they agree within 2–28% with the experimental data. The correction of the simulation data due to the finite size of the system increases the value of the self-diffusion coefficient typically by 10%. If this correction is considered, better agreement with the experimental data can be expected for most of the studied fluids. MS diffusion coefficients for three binary mixtures were also predicted; their statistical uncertainty is about 10%. These results were used to test three empirical equations to estimate MS diffusion coefficients in binary mixtures, i.e., the equations of Caldwell and Babb, of Darken, and of Vignes. The equations of Caldwell and Babb and of Vignes show qualitatively different behavior of the MS diffusion coefficient than that observed in the simulations. In agreement with previous work, the best results are obtained in all cases with the equation of Darken.     

Prediction of transport properties of R22-DMF refrigerant-absorbent combinations

The present work aims to evaluate the transport properties of R22-DMF solutions; one of the most promising combinations for absorption refrigeration. A number of methods have been used to estimate the thermal conductivity, viscosity and surface tension. The selection of suitable methods has been made by computing the properties of ammonia-water mixtures and comparing them with available experimental data. Other thermophysical properties, i.e. thermal diffusivity, specific heat and liquid density, have been predicted using standard, well established methods over a wide range of temperature and composition. Correlations have been developed to express each property as a function of composition and temperature. The properties are also presented in a suitable graphical form.     

A New Approach to the Evaluation of Transport Properties of Azeotropic and Quasi-Azeotropic Refrigerant Mixtures

Azeotropic and quasi-azeotropic mixtures of organic compounds could become the most effective candidates as replacement fluids in refrigeration devices and heat pumps. Following the development of effective prediction formulas for several families of pure organic compounds, in this paper the evaluation of transport properties of liquid mixtures is approached from a rather different point of view. Azeotropic and near-azeotropic mixtures are treated as pure compounds rather than as a combination of several pure substances. This is now possible, having developed a single, specialized formula for both the liquid thermal conductivity and the dynamic viscosity. The prediction method requires the knowledge of only a few equilibrium properties of the mixture to be analyzed and thus is a simple and powerful tool for the evaluation of alternative refrigerants. Each formula has been tested against experimental data and shows deviations below those required for engineering purposes. Important results have also been achieved on applying the same equations to quasi-azeotropic mixtures with deviations comparable to those for azeotropic mixtures.     

Transport Properties in Gases at High Temperature and Low Pressure: Comparison of Kinetic Theory with Direct Simulation Monte Carlo

Expressions for the transport coefficients obtained from the Gross-Jackson and the Chapman–Enskog methods are used to derive explicit relations incorporating the internal energy of the molecules for pure polyatomic gases and for binary mixtures of gases. Various coefficients such as the binary diffusion, thermal conductivity, and the viscosity coefficients and the thermal diffusion factor are calculated and a comparison with the direct simulation Monte Carlo (DSMC) method is carried out. The results show that the contribution of the internal energy is important and cannot be neglected.     

Prediction of Fluid Mixture Transport Properties by Molecular Dynamics

Equilibrium molecular dynamics simulations of mixtures of n-decane with methane, ethane, and carbon dioxide and of the mixture carbon dioxide–ethane were performed using the anisotropic united atoms model for n-decane and one-and two-center Lennard–Jones models for the light components. The Green–Kubo relations were used to calculate the viscosity, thermal conductivity, and inter- and intradiffusion. Viscosities are predicted with a maximum deviation of 30% at low gas concentrations and less than 10% deviation at high gas concentrations. The viscosity and thermal conductivity are less sensitive to the cross interactions than the diffusion coefficients, which exhibit deviations between models and with experiments of up to 60%.     

Experimental investigation on the influence of high temperature on viscosity,thermal conductivity and absorbance of ammonia–water nanofluids

To select the optimal ammonia–water nanofluids and apply to ammonia–water absorption refrigeration systems (AARS), this paper investigated the influence of heating on viscosity, thermal conductivity and absorbance of binary nanofluids. The hysteresis phenomenon was observed after heating at high temperature which is rarely reported in the literature. Experimental results show that most of nanofluids' thermal conductivity increased by about 3–12% after heating. However, their viscosities increased by as much as 15% to 25% except the γ-TiO₂ ammonia–water nanofluid, which was reduced by 2% to 7%. This study also shows that the trend of viscosity is consistent with the absorbance. Due to fact that the thermal conductivity of γ-TiO₂/NH₃–H₂O mixture increased after heating, while the viscosity decreased, even if the concentration of the base liquid is 12.5% or 25%, therefore it is the optimal choice for practical research in AARS at present.     

Calculation of the transport properties of dilute gas mixtures on the basis of model potentials. Mercury–argon system

Interatomic potentials V(Hg–Hg), V(Ar–Ar), and V(Hg–Ar) of atoms in their ground electronic state are analyzed for the technically important mercury–argon dilute gas mixture. The collision integrals are calculated for these potentials, and the transport properties of mercury, argon, and their mixture, such as, viscosity, thermal conductivity, and self-diffusion and mutual diffusion coefficients of compounds, are determined using the molecular-kinetic theory relationships. Detailed tables of properties on five isotherms within a range of 300–2000 K and mixture concentrations of 0.001–0.999 are given. Tables of the properties also contain thermal diffusion factors and Prandtl (Pr) and Schmidt (Sc) numbers. Some specific features in the behavior of properties depending on the composition are considered.