An empirical equation of state correlation is proposed for the Lennard-Jones model fluid. The equation in terms of the Helmholtz energy is based on a large molecular simulation data set and thermal virial coefficients. The underlying data set consists of directly simulated residual Helmholtz energy derivatives with respect to temperature and density in the canonical ensemble. Using these data introduces a new methodology for developing equations of state from molecular simulation. The correlation is valid for temperatures 0.5 < T/Tc < 7 and pressures up to p/pc = 500. Extensive comparisons to simulation data from the literature are made. The accuracy and extrapolation behavior are better than for existing equations of state.

Equation of State for the Lennard-Jones Fluid

The Lennard-Jones potential as well as its truncated and shifted (rc = 2.5σ) variant are applied to the noble gases neon, argon, krypton, and xenon. These models are comprehensively compared with the currently available experimental knowledge in terms of vapour pressure, saturated liquid density, as well as thermodynamic properties from the single phase fluid regions including density, speed of sound, and isobaric heat capacity data. The expectation that these potentials exhibit a more modest performance for neon as compared to argon, krypton, and xenon due to increasing quantum effects does not seem to hold for the investigated properties. On the other hand, the assumption that the truncated and shifted (rc = 2.5σ) variant of the Lennard-Jones potential may have shortcomings because the long range interactions are entirely neglected beyond the cut-off radius rc, are supported by the present findings for the properties from the single phase fluid regions. For vapour pressure and saturated liquid d...

How well does the Lennard-Jones potential represent the thermodynamic properties of noble gases?

Fundamental equation of state correlation for hexamethyldisiloxane based on experimental and molecular simulation data

The MolMod database is presented, which is openly accessible at http://molmod.boltzmann-zuse.de and contains intermolecular force fields for over 150 pure fluids at present. It was developed and is...

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MolMod – an open access database of force fields for molecular simulations of fluids

The Gruneisen parameter γG is widely used for studying thermal properties of solids at high pressure and also has received increasing interest in different applications of non-ideal fluid dynamics. Because there is a lack of systematic studies of the Gruneisen parameter in the entire fluid region, this study aims to fill this gap. Gruneisen parameter data from molecular modelling and simulation are reported for 28 pure fluids and are compared with results calculated from fundamental equations of state that are based on extensive experimental data sets. We show that the Gruneisen parameter follows a general density-temperature trend and characterize the fluid systems by specifying a span of minimum and maximum values of γG. Exceptions to this trend can be found for water.

Comparative study of the Grüneisen parameter for 28 pure fluids

Equations of state in terms of the Helmholtz energy are presented for octamethyltrisiloxane and decamethyltetrasiloxane. The restricted databases in the literature are augmented by speed of sound measurements, which are carried out by a pulse-echo method. The equations of state are valid in the fluid region up to approximately 600 K and 130 MPa and can be used to calculate all thermodynamic properties by combining the Helmholtz energy and its derivatives with respect to the natural variables. The accuracy of the equation is validated by comparison to experimental data and correct extrapolation behavior is ensured.

Speed of Sound Measurements and Fundamental Equations of State for Octamethyltrisiloxane and Decamethyltetrasiloxane

An equation of state for octamethylcyclotetrasiloxane is presented. The available experimental data from the literature are supplemented by new measurements on the speed of sound. Furthermore, a new force field is proposed, allowing the generation of a comprehensive thermodynamic data set by means of molecular simulation. Both the experimental and molecular simulation data are applied to develop a fundamental equation of state in terms of the Helmholtz energy. On the basis of the experimental data, the equation of state is valid from the triple-point temperature to T = 590 K for pressures up to p = 180 MPa and can be used to calculate any thermodynamic equilibrium state property. Its accuracy is assessed by a comprehensive analysis of the underlying experimental data. Finally, the range of validity was extended to Tmax = 1200 K and pmax = 520 MPa by means of molecular simulation data.

Thermodynamic Properties of Octamethylcyclotetrasiloxane

The pulse-echo technique determines the propagation time of acoustic wave bursts in a fluid over a known propagation distance. It is limited by the signal quality of the received echoes of the acoustic wave bursts, which degrades with decreasing density of the fluid due to acoustic impedance and attenuation effects. Signal sampling is significantly improved in this work by burst design and signal processing such that a wider range of thermodynamic states can be investigated. Applying a Fourier transformation based digital filter on acoustic wave signals increases their signal-to-noise ratio and enhances their time and amplitude resolutions, improving the overall measurement accuracy. In addition, burst design leads to technical advantages for determining the propagation time due to the associated conditioning of the echo. It is shown that the according operation procedure enlarges the measuring range of the pulse-echo technique for supercritical argon and nitrogen at 300 K down to 5 MPa, where it was limited to around 20 MPa before.

Burst design and signal processing for the speed of sound measurement of fluids with the pulse-echo technique

An apparatus for the measurement of the speed of sound based on the pulse-echo technique is presented. It operates up to a temperature of 480 K and a pressure of 125 MPa. After referencing and validating the apparatus with water, it is applied to liquid ammonia between 230 and 410 K up to a pressure of 124 MPa. Speed of sound data are presented with an uncertainty between 0.02% and 0.1%.

Frithjof H. Dubberke

Papers

Fundamental equation of state correlation for hexamethyldisiloxane based on experimental and molecular simulation data

Speed of Sound Measurements and Fundamental Equations of State for Octamethyltrisiloxane and Decamethyltetrasiloxane

Thermodynamic Properties of Octamethylcyclotetrasiloxane

Burst design and signal processing for the speed of sound measurement of fluids with the pulse-echo technique

Apparatus for the measurement of the speed of sound of ammonia up to high temperatures and pressures.