Transport properties saturated liquids

Fairweather et al. [204] developed a microfluidic device and method to measure the capillary pressure as a function of fhe liquid water saturation for porous media wifh heferogeneous wetting properties during liquid and gas intrusions. In addition to being able to produce plots of capillary pressure as a function of liquid wafer safuration, their technique also allowed them to investigate both hydrophilic and hydrophobic pore volumes. This method is still in its early stages because the compression pressure and the temperatures were not controlled however, it can become a potential characterization technique that would permit further understanding of mass transport within the DL. 

Thermophysical and transport properties are presented for air. The results include physical constants, critical values, saturation pressure, density of liquid, enthalpy of vaporization, surface tension, heat capacity, viscosity, and thermal conductivity. Both gas and liquid are covered. The coefficients for property values as a function of temperature are provided in an easy to use tabular format. 

Many of the Vargaftik values also appear in Ohse, R. W., Handbook of Thermodynumie and Transport Froperties of Alkali Metals, Blackwell Sci. Pubs., Oxford, 1985 (1020 pp.). This source contains superheat data. Saturation and superheat tables and a diagram to 30 bar, 1650 K are given by Reynolds, W. C., Thermodynamic Froperties in S.L, Stanford Univ. pubk, 1979 (173 pp.). For aMollier diagram from 0.1 to 250 psia, 1300 to 2700°R, see Weatherford, W. D., J. C. Tyler, et ah, WADD-TR-61-96,1961. An extensive review of properties of the solid and the saturated liquid is given by Alcock, C. B., M. W. Chase, et ak, /. Fhys. Chem. Ref Data, 23, 3 (1994) 385-497. 

Fl, -, 2 8, Metier Diagram for Sodium Drawn from the Vargaftik et al. values in Ohse, R. W Handbook of Thermodynamic and Transport Froperties of Alkali Metals Blackwel Set Pubs. Oxford, UK, 1985. These values are identical with those of Vargaftik, N. B Handbook of Thermophysical Properties of Gases and Liquids, Moscow, 197 and the Hemisphere translation, p. 19. An apparent discontinuity exists between the superheat values and the saturation values not reproduced here. For a Mol her diagram in f.p.s. units from 0.1 to 150 psia, 1500 to 2700°R, see Fig. 3-36, p. 3-232 of the 6th edition of this handbook. An extensive review oi properties of the solid and the saturated liquid was given by Alcock, C. B., Chase, M. W et al., /. Phys. Chem. Ref Data, 23(3), 385-497,... 

Consider first the complete P and T behavior of the transport properties of an ordinary fluid, as shown schematically in Fig. 1. f Of particular significance is the decrease in these properties with increasing temperature, shown by the isobars in the upper left-hand portion of the diagram. This is typical classical liquid behavior. In the present paper only that portion of the behavior illustrated in Fig. 1 which is identified by the heavily dotted section of the saturation curve connecting the triple point and the critical point will be discussed. By utilizing the quantum mechanical law of corresponding states, it has been possible to extrapolate in a theoretically consistent manner the experimental data for the saturated liquid for the light elements between the triple point and the critical point, as well as predict entirely the transport properties of several other isotopic species. 

With regard to application, we will apply our tools to optimize a force field specific for fluorinated alcohols. Fluorinated alcohols are highly relevant in industrial applications (e.g., as solvents used in chemical separation processes). Their attractiveness is that they can be extracted firom the reaction medium and be reused, which makes them both environmentally friendly and economically attractive [90]. The challenge in optimizing such a force field arises frrom the lack of experimental data and lacks previously published parameters that can be used as an initial input [91-93]. The goal will be to fit both vapor-liquid equilibrium data (e.g., saturated liquid density, vapor pressure) and transport properties (e.g., diffusion coefficients) simultaneously and at different temperatures. Hence, not only parallelization over different substances but also over different ensembles and temperatures are required. 

The physical and solvent properties of water depend strongly on temperature and pressure. " Near the critical point T = 647.1 K, = 22.06 MPa), the isothermal compressibility of water may be 10" times higher than that of the saturated liquid at 25 °C, and isobaric specific heat capacity may increase to 1X10 kJ K kg whereas thermal conductivity can be as high as 0.8 W m K . The transport properties of hot compressed water fall between those of a gas and a liquid. At densities of ca. 700 kgm and lower, the diffusion coefficient D is proportional to the inverse of the density, like in gases. A change in transport properties has consequent effect on radical reactions, which are diffusion-controlled or partially diffusion-controlled (see section 15.4). 

Quantitative estimation of the effective material properties and constitutive closure relations is of paramount importance for high-fidelity macroscopic, voliune-averaged computational models deployed in the PEFC performance simulations. The microstractural heterogeneity (e g. morphology, pore connectivity, pore size distribution, anisotropy) inherent in the PEFC components (CL, GDL, MPL) poses a profound impact on the effective transport properties, such as effective diffusivity in the unsaturated and partially saturated (e g. pore blockage by liquid water)... 

Developments in liquid state theory and empirical studies of transport phenomena in liquids can be exploited by restricting the correlation to the region of saturated or compressed liquids away from the critical point (Brush 1962 Schwen Puhl 1988). The basis of one such class of correlations is the recognition that liquid transport properties, and in particular viscosity, can be correlated well in terms of the difference between the molar volume and a compact packing volume which is fluid-specific and a weak function of temperature. The simplest form of this type of correlation is the fluidity ( ) versus molar volume V = p ) approach described by Batschinski (1913) and Hildebrand (1977) for pure fluids based on experimental observations these variables are linearly related to a very good approximation. Hard-sphere theories such as that proposed by Enskog and several modifications of this approach have also been used for liquids. Current work in this area has evolved significantly and is described in detail in Chapters 5 and 10. 

For pure fluids, it is most common to represent the saturated vapor and saturated liquid transport properties as simple polynomial functions in temperature, although polynomials in density or pressure could also be used. Exponential expansions may be preferable in the case of viscosity (Bmsh 1962 Schwen Puhl 1988). For mixtures, the analogous correlation of transport properties along dew curves or bubble curves can be similarly regressed. In the case of thermal conductivity, it is necessary to add a divergent term to account for the steep curvature due to critical enhancement as the critical point is approached. Thus, a reasonable form for a transport property. 

Coefficients of the equadon of state and of the equation for transport properties are stored for each substance. Parameters of the critical point and coefficients of equations for calculadon of the ideal-gas functions, the saturated vapor pressure and the melting pressure are kept also. The thermal properties in the single-phase region and on the phase-equilibrium lines can be calculated on the basis of well-known relations with use of these coefficients. The system contains data for 30 reference substances monatomic and diatomic gases, air, water and steam, carbon dioxide, ammonia, paraffin hydrocarbons (up to octane), ethylene (ethene), propylene (propene), benzene and toluene. The system can calculate the thermophysical properties of poorly investigated gases and liquids and of multicomponent mixtures also on the basis of data for reference substances. 

The challenge for modeling the water balance in CCL is to link the composite, porous morphology properly with liquid water accumulation, transport phenomena, electrochemical kinetics, and performance. At the materials level, this task requires relations between composihon, porous structure, liquid water accumulation, and effective properhes. Relevant properties include proton conductivity, gas diffusivihes, liquid permeability, electrochemical source term, and vaporizahon source term. Discussions of functional relationships between effective properties and structure can be found in fhe liferafure. Because fhe liquid wafer saturation, 5,(2)/ is a spatially varying function at/o > 0, these effective properties also vary spatially in an operating cell, warranting a self-consistent solution for effective properties and performance. 

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