Velisa Vesovic

The Transport Properties of Carbon Dioxide

The paper contains new, representative equations for the viscosity and thermal conductivity of carbon dioxide. The equations are based in part upon a body of experimental data that have been critically assessed for internal consistency and for agreement with theory whenever possible. In the case of the low‐density thermal conductivity at high temperatures, all available data are shown to be inconsistent with theoretical expectation and have therefore been abandoned in favor of a theoretical prediction. Similarly, the liquid‐phase thermal conductivity has been predicted owing to the small extent and poor quality of the experimental information. In the same phase the inconsistencies between the various literature reports of viscosity measurements cannot be resolved and new measurements are necessary. In the critical region the experimentally observed enhancements of both transport properties are well represented by theoretically based equations containing just one adjustable parameter. The complete correlations cover the temperature range 200 K≤T<1500 K for viscosity and 200 K≤T≤1000 K for thermal conductivity, and pressures up to 100 MPa. The uncertainties associated with the correlation vary according to the thermodynamic state from ±0.3% for the viscosity of the dilute gas near room temperature to ±5% for the thermal conductivity in the liquid phase. Tables of the viscosity and thermal conductivity generated by the representative equations are provided to assist with the confirmation of computer implementations of the calculation procedure.

The Viscosity of Carbon Dioxide

When representative equations for the viscosity of carbon dioxide were published in 1990, it was recognized that, owing to inconsistencies among the available experimental liquid viscosity data which could not be resolved, new measurements were necessary. Since then, two new sets of measurements have been performed and it is appropriate to revise the published equations in order to improve their performance in the liquid region. In the previous work, the excess viscosity was represented by two separate equations, one for the gas phase and the other, a provisional one, for the liquid phase. Both equations were joined by a blending function. In the present work, the excess viscosity for the whole thermodynamic surface is represented by one equation. The resulting overall viscosity representation for carbon dioxide covers the temperature range 200 K⩽T⩽1500 K and densities up to 1400 kg m−3. In terms of pressure, the viscosity representation is valid up to 300 MPa for temperatures below 1000 K, whereas for hi...

Measurement of the current distribution along a single flow channel of a solid polymer fuel cell

We present a method of performing high spatial and time-resolution, non-intrusive and dynamic current measurements along the length of a single flow channel in a solid polymer fuel cell. Current profiles at different cell polarisations and reactant flow rates are examined along with the dynamic response of the fuel cell upon introduction of reactant gases.

Localized impedance measurements along a single channel of a solid polymer fuel cell

A method is presented, for the first time, for measuring the localized electrochemical impedance spectroscopy response over a frequency range of 0.1 Hz to 10 kHz as a function of position in a solid polymer fuel cell. The highly idealized fuel cell on which the measurements were performed is composed of a single linear flow channel. Measurements have been made at both 0.8 and 0.6 V. A distribution of impedance characteristics is seen along the channel with evidence of mass transport effects that are not evident from localized dc measurements. The membrane conductivity does not vary with position at both potentials, as is expected from the fact that reactant gases are fully humidified. A time constant characteristic of convective transport within the flow channel dominates the low frequency response at low potentials. This response is caused by consumption of reactant upstream of the point at which the measurement is made.

Calculation of the transport properties of carbon dioxide. I. Shear viscosity, viscomagnetic effects, and self-diffusion

Transport properties of pure carbon dioxide have been calculated from the intermolecular potential using the classical trajectory approach. Results are reported for shear viscosity, viscomagnetic coefficients, and self-diffusion in the dilute-gas limit and in the temperature range of 200–1500 K for the three recently proposed carbon dioxide potential energy hypersurfaces. Agreement with the measurements is, in general, within the experimental error. The calculations indicate that the corrections in the second-order approximation and those due to the angular-momentum polarization for the viscosity are small, <1% in the temperature range considered. The very good agreement of the calculated values for the Bukowski et al. potential energy hypersurface (1999) with the experimental viscosity data is consistent with the rigid-rotor assumption made in the calculations being reasonable for the three properties considered.

Calculation of the transport properties of carbon dioxide. II. Thermal conductivity and thermomagnetic effects

The transport properties of pure carbon dioxide have been calculated from the intermolecular potential using the classical trajectory method. Results are reported in the dilute-gas limit for thermal conductivity and thermomagnetic coefficients for temperatures ranging from 200 K to 1000 K. Three recent carbon dioxide potential energy hypersurfaces have been investigated. Since thermal conductivity is influenced by vibrational degrees of freedom, not included in the rigid-rotor classical trajectory calculation, a correction for vibration has also been employed. The calculations indicate that the second-order thermal conductivity corrections due to the angular momentum polarization (< 2%) and velocity polarization (< 1%) are both small. Thermal conductivity values calculated using the potential energy hypersurface by Bukowski et al. (1999) are in good agreement with the available experimental data. They underestimate the best experimental data at room temperature by 1% and in the range up to 470 K by 1%-3%, depending on the data source. Outside this range the calculated values, we believe, may be more reliable than the currently available experimental data. Our results are consistent with measurements of the thermomagnetic effect at 300 K only when the vibrational degrees of freedom are considered fully. This excellent agreement for these properties indicates that particularly the potential surface of Bukowski et al. provides a realistic description of the anisotropy of the surface.

Quantum mechanical calculations of effective collision cross-sections for He-N2 interaction

For the He-N2 system quantum mechanical calculations of the effective cross-sections that govern both the viscomagnetic effect and the collision broadening of the depolarized Rayleigh (DPR) line in mixtures have been performed. The results provide a set of ‘benchmark’ cross-sections for atom-heavy rigid rotor systems interacting through a realistic anisotropic intermolecular potential. In addition, comparisons have been made with equivalent calculations using the classical trajectory approach. The accuracy of the classical calculations has been tested, both at the macroscopic level in the temperature range from 70 K to 500 K and at an intermediate level as a function of the total energy. The results indicate that the classical trajectory calculations are in good agreement with the present quantal calculations. For all cross-sections the difference between classical and quantal results becomes progressively smaller with increase in total energy or temperature. This indicates that a hybrid scheme, which emp...

Calculation of the transport and relaxation properties of methane. I. Shear viscosity, viscomagnetic effects, and self-diffusion.

Transport properties of pure methane gas have been calculated in the rigid-rotor approximation using the recently proposed intermolecular potential energy hypersurface [R. Hellmann et al., J. Chem. Phys. 128, 214303 (2008)] and the classical-trajectory method. Results are reported in the dilute-gas limit for shear viscosity, viscomagnetic coefficients, and self-diffusion in the temperature range of 80-1500 K. Compared with the best measurements, the calculated viscosity values are about 0.5% too high at room temperature, although the temperature dependence of the calculated values is in very good agreement with experiment between 210 and 390 K. For the shear viscosity, the calculations indicate that the corrections in the second-order approximation and those due to the angular-momentum polarization are small, less than 0.7%, in the temperature range considered. The very good agreement of the calculated values with the experimental viscosity data suggests that the rigid-rotor approximation should be very reasonable for the three properties considered. In general, the agreement for the other measured properties is within the experimental error.

Second-order corrections for transport properties of pure diatomic gases

Second-order kinetic theory expressions involving either internal angular momentum or translational and rotational energies have been investigated for two potential energy surfaces for N2 at temperatures between 77 and 1000 K. The largest correction is for the volume viscosity, where it is about 15% at 300 K and 20% at 600 K. For the thermal conductivity the angular-momentum related correction is approximately 1% and varies weakly with temperature, while the rotational and translational-energy-related correction increases with temperature, reaching approximately 1% at 1000 K. For the shear viscosity both corrections are no more than 0·5%. The Mason-Monchick approximation provides useful estimates of the energy-related corrections for the thermal conductivity and shear viscosity. Results calculated using the ab initio potential energy surface of Van der Avoird et al. agree better with experiment than those using the site-site potential of MacRury et al. for the shear viscosity and the thermal conductivity ...

Calculation of the transport and relaxation properties of dilute water vapor

Transport properties of dilute water vapor have been calculated in the rigid-rotor approximation using four different potential energy hypersurfaces and the classical-trajectory method. Results are reported for shear viscosity, self-diffusion, thermal conductivity, and volume viscosity in the dilute-gas limit for the temperature range of 250-2500 K. Of these four surfaces the CC-pol surface of Bukowski et al. [J. Chem. Phys. 128, 094314 (2008)] is in best accord with the available measurements. Very good agreement is found with the most accurate results for viscosity in the whole temperature range of the experiments. For thermal conductivity the deviations of the calculated values from the experimental data increase systematically with increasing temperature to around 5% at 1100 K. For both self-diffusion and volume viscosity, the much more limited number of available measurements are generally consistent with the calculated values, apart from the lower temperature isotopically labeled diffusion measurements.