Signe Kjelstrup Ratkje

On the molecular mechanism of thermal diffusion in liquids

A recently developed non-equilibrium molecular dynamics algorithm for heat conduction is used to compute the thermal conductivity, thermal diffusion factor, and heat of transfer in binary Lennard-Jones mixtures. An internal energy flux is established with local source and sink terms for kinetic energy. Simulations of isotope mixtures covering a range of densities and mass ratios show that the lighter component prefers the hot side of the system at stationary state. This implies a positive thermal diffusion factor in the definition we have adopted here. The molecular basis for the Soret effect is studied by analysing the energy flux through the system. In all cases we found that there is a difference in the relative contributions when we compare the hot and cold sides of the system. The contribution from the lighter component is predominantly flux of kinetic energy, and this contribution increases from the cold to the hot side. The contribution from the heavier component is predominantly energy transfer th...

CRITERIA FOR LOCAL EQUILIBRIUM IN A SYSTEM WITH TRANSPORT OF HEAT AND MASS

Nonequilibrium molecular dynamics is used to compute the coupled heat and mass transport in a binary isotope mixture of particles interacting with a Lennard-Jones/spline potential. Two different stationary states are studied, one with a fixed internal energy flux and zero mass flux, and the other with a fixed diffusive mass flux and zero temperature gradient. Computations are made for one overall temperature,T=2, and three overall number densities,n=0.1, 0.2, and 0.4. (All numerical values are given in reduced, Lennard-Jones units unless otherwise stated.) Temperature gradients are up to ∇T=0.09 and weight-fraction gradients up to ∇w1=0.007. The flux-force relationships are found to be linear over the entire range. All four transport coefficients (theL-matrix) are determined and the Onsager reciprocal relationship for the off-diagonal coefficients is verified. Four different criteria are used to analyze the concept of local equilibrium in the nonequilibrium system. The local temperature fluctuation is found to be δT≈0.03T and of the same order as the maximum temperature difference across the control volume, except near the cold boundary. A comparison of the local potential energy, enthalpy, and pressure with the corresponding equilibrium values at the same temperature, density, and composition also verifies that local equilibrium is established, except near the boundaries of the system. The velocity contribution to the BoltzmannH-function agrees with its Maxwellian (equilibrium) value within 1%, except near the boundaries, where the deviation is up to 4%. Our results do not support the Eyring-type transport theory involving jumps across energy barriers; we find that its estimates for the heat and mass fluxes are wrong by at least one order of magnitude.

Water transport in cation exchange membranes

Abstract The time variation in emf caused by pressure differences has been used to find transference coefficients of water as well as water permeabilities in ion exchange membranes. Previous experimental methods have been corrected and facilitated. The cation exchange membrane CR61 AZL 386 was investigated. Equilibrium solutions contained 10−4 kmol-m−3

Transported Entropy in Zirconia with 3 to 12 Mole Percent Yttria

Seebeck coefficients were measured for the fuel cell (T[sub 1],p)Pt/O[sub 2](g)/YSZ/O[sub 2](g)/Pt(T[sub 2],p). The electrolyte was yttria-stabilized zirconia (YSZ) and the content of yttria in the electrolyte varied between 3 and 12 mole percent (m/o). Thus tetragonal and cubic crystal structures were investigated. Different sintering temperatures and times were used in sample preparation to produce different grain sizes in the electrolyte. The reference temperature is T[sub 1] = 1,000 C and T[sub 2] has been varied up to 1,200 C. Two different oxygen pressures were used. The transported entropy of oxygen ions calculated from the Seebeck coefficient at 1,273 K, is constant and equal to 42 [plus minus] 2 J/KF for all samples. The heat of transport q* is irrelevant to the Seebeck coefficient, as q* is related to diffusion, which does not take place. The Thomson coefficient of the material is estimated to be 18 J/KF. This result is used to compare values for the transported entropy of oxygen ions calculated from literature reports. Common expressions from thermoelectric theory are rejected on theoretical as well as experimental grounds.

Ionic transport across corneal endothelium

Abstract. The active potential difference across bovine corneal endothelium was measured in vitro at different temperatures, pH, osmolality and salt compositions. The measurements were made using either identical or different solutions on each side of the membrane. The experimental results are consistent with a model in which sodium is actively transported into the intercellular cleft. We propose that Na+ re‐enters the cell electroneutrally by coupled co‐transport with carbonate, derived from bicarbonate in the solution.

Modelling the Freeze Concentration Process by Irreversible Thermodynamics

Abstract Ice growth from a subcooled falling film of aqueous solutions has been studied theoretically as a method of freeze concentration. Coupled heat and mass flux equations from irreversible thermodynamics have been used to describe the ice growth under approximately stationary state conditions. An equation for the ice growth rate has been derived. The temperature difference dependence that we derive is supported by experimental results reported in the literature. Conditions for controlled ice growth are discussed and some general perspectives of the application of irreversible thermodynamics are pointed out.

Thermoelectric power relevant for the solid-polymer-electrolyte fuel cell

The Nation ® 117 membrane was investigated by measuring the emf as a function of the temperature difference between the electrodes in the following cell: Ag(s, T~,) l AgCl(s, T~)I HClfaq, c, Tj) l Nafion® l 171HCI(a q, c, T2) [ AgCI(s, T2) ] Ag(s, T2) The reversible entropy transferred due to the membrane, was SHM twSw - 13.4 + 0.2 J K- mol- J at 25.0°C, where S*M is the transported entropy of protons in the membrane and Sw is the partial molar entropy of water in the bulk solution. The membrane transference coefficient of water, measured by the streaming potential technique, was tw = 2.6 ± 0.2. The results were used to calculate the reversible heat effects at the electrodes in the solid-polymer-electrolyte fuel cell (SPEFC). We concluded that water management in the fuel cell had a strong impact on the heat effects. The total operation of the SPEFC was discussed in view of the new results. The stack method presented here, is simple and accurate for finding the Seebeck coefficient in electrochemical cells with ion-exchange membranes.

DENBIGH REVISITED: REDUCING LOST WORK IN CHEMICAL PROCESSES

A new way of calculating lost work in a reacting, diffusing mixture is presented. We show that a fast chemical reaction can be treated within the framework of coupled transport equations for heat and mass, after elimination of dependent components. Two approaches to reduction of lost work follow from the equations: reduction of driving forces and utilization of coupling. When Ficks and Fouriers laws apply, losses can be minimized, but not avoided, by reduction of driving forces. When transport processes are coupled, as expressed by the theory of irreversible thermodynamics, loss reductions can be obtained also through the coupling coefficients. Maximum reduction is obtained for strict coupling, i.e. unique flux relationships. Coupling implies physical interaction and is discussed for isothermal and non-isothermal systems. The relation between irreversible thermodynamics and exergy analysis is also discussed.

Membrane transference numbers from a new emf method

Abstract A new method for determination of the transference numbers of two ions in ion exchange membrances has been established. The method uses emf measurements in a cell with a membrane stack, and it avoids problems of concentration-polarization, diffusion and water transport. Stack thickness is about 10 mm. The method needs less time and is more precise than a corresponding Hittorf method. The method has been developed for a cation exchange membrane, with cation sites M−, in equilibrium with aqueous solutions of two electrolytes, first HCl and KCl, next HCl and NaCl. Transference numbers for both systems are reported with a precision of ±1%. The results imply that the ratios of the ionic mobilities, ui/uH+, for i=K+ and Na+ in the membrane are constant as the mole fractions vary.

Entropy production by heat, mass, charge transfer and specific chemical reactions

Abstract By using only well-defined measurable thermodynamic quantities, the entropy production by heat, mass and charge transfer in a discontinuous and a continuous system is derived. The derivation follows the principle that thermodynamic derivations should in general be carried out as far as possible without the introduction of any kind of model concerning structural units (ions)[1–3]. This is in contrast to the conventional thermodynamic treatment of electrolytes. The present description has fluxes and forces that depend on the choice of electrodes. It is, however, shown that the local dissipated energy does not have this dependency. The components of the system are in accordance with the Gibbs phase rule. Corrections in electric forces due to the use of different electrodes at different temperature or pressure are discussed. Chemical reactions with zero affinity are treated, and the complete coupling between mass transport and a chemical reaction is discussed.