C. A. Nieto de Castro

The theory of the Taylor dispersion technique for liquid diffusivity measurements

This paper presents a complete analysis of the theory of an instrument to measure the diffusion coefficients in liquid mixtures based upon the phenomenon of Taylor dispersion. The analysis demonstrates that it is possible to design an instrument that operates very nearly in accordance with the simplest mathematical description of the dispersion of a solute pulse in a fluid in laminar flow within a straight, circular cross-section tube. The small departures of a practical instrument from the ideal are evaluated as corrections by means of a general perturbation treatment that allows them to be examined one at a time. The corrections considered include the effects of the finite volume of the injection pulse, the finite volume of the concentration monitor, the coiling of the tube, and the nonuniformity and noncircularity of the cross section, as well as the variation of the fluid properties with composition. All the equations necessary for the design of an instrument of this type, and for the evaluation of experimental data free from significant systematic errors, are presented.

Standard Reference Data for the Thermal Conductivity of Liquids

The available experimental liquid‐phase thermal conductivity data for water, toluene, and n‐heptane have been examined with the intention of establishing standard reference values along the saturation line. The quality of available data is such that for toluene and water new standard reference values can be proposed with confidence limits better than ±1.0% for most of the normal liquid range. For n‐heptane there are insufficient reliable experimental data for the system to be treated as a primary reference standard, so a lower quality correlation has been developed which yields a set of secondary reference data with confidence limits of ±1.5% for most of the normal liquid range.

An apparatus to measure the thermal conductivity of liquids

The paper describes an apparatus for the measurement of the thermal conductivity of non-conducting liquids under their saturation vapour pressure. The instrument, which is based upon the transient hot wire principle, has been designed so that the measuring element conforms as closely as possible to an infinite line source of heat in an infinite fluid. Under these conditions the thermal conductivity of the liquid can be determined from the slope of a plot of the temperature rise of the heating element against the logarithm of time. The measurement system has been arranged so as to provide as many as 60 points on this plot for any particular thermodynamic state of the fluid under investigation. The reproducibility of the instrument is of the order of 0.03% and the precision of the measurements is estimated as +or-0.1%. Owing to a lack of a suitable theory for the effects of radiative heat transfer, the accuracy of the thermal conductivity values cannot be defined unequivocally, but a reasoned upper bound is +or-0.3%. Preliminary results are presented for n-heptane at three temperatures in the range 20 to 30 degrees C.

Viscosity of Toluene and Benzene Under High Pressure

The viscosity of toluene and benzene was measured at the temperatures of 298.15, 323.15, 348.15, and 373.15 K (ITS-90) from atmospheric pressure up to 200 MPa, by a torsionally vibrating quartz-crystal viscometer, with an estimated accuracy of 0.5%. The experimental data for each temperature were fitted with a Tait-like equation, by a non linear iterative program based on the Marquardt Levenberg method. A comparison between present data and available data from other authors, whenever possible, was made in terms of the dispersion of each data set with respect to the Tait-like equation.

Thermal conductivity of toluene in the temperature range 35–90°C at pressures up to 600 MPa

New, absolute measurements of the thermal conductivity of liquid toluene are reported. The measurements extend over the temperature range 35–90°C and the pressure range 0.8–600 MPa. A new analytic evaluation of the contribution of radiation in an absorbing emitting fluid to the measurement process is presented. This analysis indicates that the thermal conductivity determined in a transient hot-wire instrument is the radiation-free value. As a consequence it is possible to assert that the overall uncertainty in the experimental data is one of ± 0.3%. A comparison of the data with the results of independent measurements by the same technique shows that the various sets of data are consistent within their mutual uncertainty.

Thermal conductivity of molten alkali halides from equilibrium molecular dynamics simulations

The thermal conductivity of molten sodium chloride and potassium chloride has been computed through equilibrium molecular dynamics Green-Kubo simulations in the microcanonical ensemble (N,V,E). In order to access the temperature dependence of the thermal conductivity coefficient of these materials, the simulations were performed at five different state points. The form of the microscopic energy flux for ionic systems whose Coulombic interactions are calculated through the Ewald method is discussed in detail and an efficient formula is used by analogy with the methods used to evaluate the stress tensor in Coulombic systems. The results show that the Born-Mayer-Huggins-Tosi-Fumi potential predicts a weak negative temperature dependence for the thermal conductivity of NaCl and KCl. The simulation results are in agreement with part of the experimental data available in the literature with simulation values generally overpredicting the thermal conductivity by 10%-20%.

Benzene: A Further Liquid Thermal Conductivity Standard

The available experimental liquid‐phase thermal conductivity data for benzene have been examined with the intention of establishing a further liquid thermal conductivity standard along the saturation line. The quality of the available data is such that new standard reference values can be proposed with confidence limits better than ±1% for most of the normal liquid range.

The thermal conductivity and heat capacity of fluid nitrogen

Abstract This paper presents new absolute measurements of the thermal conductivity and the thermal diffusivity of nitrogen made with a transient hot wire instrument. The instrument measures the thermal conductivity with an uncertainty less than ±1% and the thermal diffusivity with an uncertainty of ±5% except at the fluid critical point. The data cover the region from 80 to 300KK pressures to 70 MPa. The data consist of 8 supercritical isotherms, 3 vapor isotherms, and 4 liquid isotherms. A surface fit is developed for our nitrogen thermal conductivity data from 80 to 300 K at pressures to 70 MPa. The data are compared with a recent theory for the first density coefficient of thermal conductivity and a new mode-coupling theory for the thermal conductivity critical enhancement. These data illustrate that it is necessary to study a fluid over a wide range of temperatures and densities in order to characterize the thermal conductivity surface. Isobaric heat capacity results were determined from the simultaneously measured values of thermal conductivity and thermal diffusivity, using the density calculated from an equation of state. The heat capacities obtained by this technique are compared to the heat capacities predicted by a recent equation of state developed specifically for nitrogen.

Mutual diffusivity of a mixture of n-hexane and nitrobenzene near its consolute point

It is demonstrated that the Taylor dispersion method can be used to measure the mutual diffusivity of liquid mixtures near a critical mixing point. For this purpose we have measured the mutual diffusivity of a liquid mixture of n-hexane and nitrobenzene at the critical composition at temperatures from 16 K down to 1 K above the critical temperature. The results are in agreement with the theoretically predicted behavior of the diffusivity near a critical point of mixing.

The thermal conductivity of n-hexane, n-heptane, and n-decane by the transient hot-wire method

New absolute measurements of the thermal conductivity of liquid n-hexane, n-heptane, and n-decane are reported. The measurements have been carried out in the temperature range 300–370 K at atmospheric pressure in a transient hotwire instrument. The accuracy of the measurements is estimated to be ±0.5%. The density dependence of the thermal conductivity of n-hexane and n-heptane is found to be well described by a universal equation for the hydrocarbons based on a rough hard-sphere model. The measurements of the three hydrocarbons studied are also employed to generate more accurate effective core volumes, which are the only parameters characteristic of the fluid required for the application of the proposed universal scheme.