Cynthia D. Holcomb

A critical study of the simulation of the liquid-vapour interface of a Lennard-Jones fluid

Despite the fact that the surface tension for a Lennard-Jones fluid has been simulated many times in the past, there is some considerable disagreement between the results. This paper calculates the surface tension and density profiles for the liquid-vapour interface of a Lennard-Jones fluid using molecular dynamics (MD) simulation techniques for a variety of system sizes, film thicknesses, interfacial areas, interatomic potential cut-offs, and temperatures. The results are compared with previous work in order to resolve some of the discrepancies of the past work. Combining this work with some reliable results from the past, the minimum system size, film thickness, and equilibration time necessary for the accurate description of the surface tension was determined. Using simulation results calculated for computationally-economic values of the potential cut-off, the surface tension was extrapolated to the full potential value using a tail correction and the results compared to simulations performed with long...

Tail corrections to the surface tension of a Lennard-Jones liquid-vapour interface

The Kirkwood-Buff formula for surface tension is used to derive an expression for the tail correction to the surface tension. This expression reduces to the expression for the tail correction, given by Chapela, G. A., Saville, G., Thompson, G., and Rowlinson, J. S. (1977, J. chem. Soc. Faraday Trans II, 8, 1133), when the interface is sharp but differs from it near the critical point. The difference appears to be the result of a mistake in the algebra by Chapela et al. In an example we show that, for a comparison with the surface tension of real fluids, both the tail correction to the surface tension, and the influence of the cut-off radius on the phase diagram are important.

A theoretically-based calibration and evaluation procedure for vibrating-tube densimeters

Abstract A calibration procedure for vibrating-tube densimeters is developed which properly accounts for the effects of pressure and temperature on the Youngs modulus and internal volume of the vibrating-tube. The calibration equation is based on the theoretical dependence of the Youngs modulus, the compressibility, and the thermal expansion coefficient of the tube material on temperature and pressure. Experience shows that the vibration period of the evacuated tube can shift a small amount over time as the stresses in the tube and welds age. Therefore, the calibration equation is formulated relative to a vacuum reference period to adjust for these shifts. A first-order approximation of our theoretically-based equation is also derived. The calibration procedure is accomplished in two parts. First, the evacuated tube is calibrated to characterize the elastic modulus and linear thermal expansion coefficient of the tube as a function of temperature. Second, the change of the internal volume of the tube with temperature and pressure is characterized using two or more well-characterized calibration fluids. A procedure for choosing calibration fluids, temperatures, and pressures for the calibration points is developed. The densimeters are thoroughly tested with a variety of gases and liquids to show the validity of the equation over the calibration range. Finally, the magnitude of the temperature and pressure corrections are shown using propane+i–butane as a test system.

Coexisting densities and vapor pressures of refrigerants R-22, R-134a, and R-124 at 300–395 K

Abstract The results of the investigation are presented in two parts. Part I, given in this paper, presents the experimentally measured coexisting densities and vapor pressures for the refrigerants R-22 (chlorodifluoromethane), R-134a (1,1,1,2-tetrafluoroethane), and R-124 (1-chloro-1,2,2,2-tetrafluoroethane) from 300 K to near their respective critical points. In addition, compressed liquid and supercritical densities were measured for R-22 and compared to literature values. The compressed R-22 densities agreed within experimental error with those of Kohlen et al. (Kohlen R., Kratzke, H. and Muller, S., 1985. Thermodynamic properties of saturated and compressed liquid difluorochloromethane. J. Chem. Thermodyn., 17: 1141-1151). Considerable discrepancies were found in the literature for R-134a and R-124 coexisting densities and vapor pressures. For both R-134a and the R-124, at least one set of data from the literature agreed with our results. The analysis of the measurements to determine critical densities which are internally consistent with our experimental measurements is presented as Part II (Van Poolen, L.J., Niesen, V.G., Holcomb, C.D. and Outcalt, S.L., 1994. Critical densities from coexisting density data: application to refrigerants R-22, R-134a, and R-124. Fluid Phase Equilibria, in press) in a separate paper.

Coexisting densities, vapor pressures and critical densities of refrigerants R-32 and R-152a, at 300-385 K

Abstract Holcomb, C.D., Niesen, V.G., Van Poolen, L.J. and Outcalt, S.L., 1993. Coexisting densities, vapor pressures and critical densities of refrigerants R-32 and R-152a at 300-385 K. Fluid Phase Equilibria , 91: 145-157. Experimental measurements for the vapor pressures and coexisting densities are presented for the refrigerants R-32 (difluoromethane) and R-152a (1,1-difluoroethane) from 300 K to near thier respective critical points. In addition, the coexisting density measurements have been analyzed to determine an internally consistent critical density using the critical liquid volume fraction method. Experimental results have been correlated and are in good agreement with existing literature values for each compound.

Critical temperature and density from liquid-vapor coexistence data: application to refrigerants R32, R124, and R152a☆

Abstract Liquid-vapor coexisting density and temperature data within 3 to 20 K of the critical point at approximately 1 K intervals are used to obtain the critical temperature (Tc) and the critical density (ϱc) of the refrigerants R32, R124, and R152a. The difference between the saturated liquid density and the saturated vapor density is used to obtain Tc which is, in turn, used in an equation for the rectilinear diameter to obtain ϱc. For the determination of Tc, coexisting densities and temperature are allowed to vary as normally distributed variables about a mean, the reported experimental value, with a standard deviation equal to one-third the reported experimental uncertainty. For the determination of ϱc, Tc is similarly allowed to vary based on its previously determined value and uncertainty. Equally probable data subsets are chosen randomly from the complete data set. Consistent results for Tc and ϱc, obtained as averages, and their uncertainties expressed at the 2σ level based on deviations from these averages, are achieved when the data selection-data fitting procedure is repeated two to three hundred times. The critical values obtained agree with those in the literature within the estimated uncertainties. This method yields critical properties consistent with experimental coexisting properties using data removed from the critical point and can complement various methods of determining Tc and ϱc that include the direct observation of the critical point.

Near-saturation (P,ρ,T) and vapor-pressure measurements of NH3, and liquid-phase isothermal (P,ρ,T) and bubble-point-pressure measurements of NH3+H2O mixtures

Abstract Near-saturation pressure, density, and temperature ( P , ρ , T ) and vapor-pressure measurements for NH 3 are reported over a temperature range from 279 to 392 K. Liquid-phase isothermal ( P , ρ , T ) and bubble-point-pressure measurements for two standard mixtures of NH 3 +H 2 O ( x NH 3 =0.8360 and 0.9057 mole fraction) are reported over a temperature range from 280 to 379 K and at pressures to 7.7 MPa. These data are compared to literature data and correlations and agree within ±3% for bubble-point pressures, ±0.005 g/cm 3 for liquid densities, and ±0.0011 g/cm 3 for vapor densities. A consistent data set for equation-of-state optimization at high concentrations of NH 3 is proposed.

Coexisting densities and vapor pressures for R 143 from 314 to 401 K with new critical point property estimates

Abstract The coexisting densities and vapor pressures of R 143 ( 1,1,2-trifluoroethane) have been measured at NIST from 314 to 401 K. These results were compared with available R143 data. The critical point was not determined experimentally because of the temperature limitations of the apparatus. However, a revised estimate of the critical point based on the new experimental data was obtained by combining a new method of critical temperature estimation with a previously reported method of critical density determination. These data and the data of others were represented by both the Carnahan-Starling-DeSantis equation of state and the Deiters equation of state. The Camahan-Starling-DeSantis equation of state better represented the data and is easier to use for the calculation of the thermodynamic properties of R143.

Forum 2000: Fluid Properties for New Technologies, Connecting Virtual Design with Physical Reality

Forum 2000 was held at the 14th Symposium for Thermophysical Properties, with all symposium attendees invited. The forum addressed the present needs and priorities for thermophysical properties measurements and the challenges facing the experimental community. Seven distinguished panelists presented brief overviews of issues related to a wide variety of subjects, and three discussion periods were held. Topics included whether simulation can replace experiment, properties needs for new miniaturization techniques, real problems such as nuclear waste cleanup, data needs for electrolyte systems and new generations of electric power plants, and data needs for unconventional materials such as molten metals and soft solids.

Critical temperatures, pressures, and densities for the mixtures CO2-C3H8, CO2-nC4H10, C2H6-C3H8, and C3H8-nC4H10

Abstract A simple method is developed to estimate mixture critical temperatures (Tc), pressures (Pc), and densities (ρc) as a function of overall composition (X) from near critical region experimental coexistence data. This three-step method is applied to four mixtures, CO2–C3H8, CO2–nC4H10, C2H6–C3H8, and C3H8–nC4H10. Isothermal liquid–vapor coexistence data, which includes temperature, vapor pressure, coexisting densities (ρl and ρv), and coexisting compositions for the more volatile component (x1v and x1l) are used. In the first step, the difference of the saturated liquid and vapor densities (ρl−ρv) is fitted to an empirical function in ((Pc−P)/Pc) to obtain Pc. Then P/Pc and ((ρl+ρv)/2ρc) are simultaneously fitted to functions of a polynomial in (X1−(x1v+x1l)/2) yielding estimates of ρc and X1. Finally, the discrete estimated critical data points are fitted with an equation to provide a continuous representation of the critical lines. The method is successfully tested for the mixtures, CO2–C3H8 and CO2–nC4H10, for which there is a reasonable amount of isothermal data. The procedure is then applied to the mixtures, C2H6–C3H8 and C3H8–nC4H10, for which there are sparse data. For all four mixtures, the critical temperature line, Tc vs. X1, matches literature values within ±0.5%. The critical pressure line, Pc vs. X1, and critical density line, ρc vs. X1, match literature values, in general, within ±2%.