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Molecular Simulations of the Vapor–Liquid Phase Interfaces of Pure Water Modeled with the SPC/E and the TIP4P/2005 Molecular Models
Vinš, Václav ; Celný, David ; Planková, Barbora ; Němec, Tomáš ; Duška, Michal ; Hrubý, Jan
In our previous study [Planková et al., EPJWeb. Conf. 92, 02071 (2015)], several molecular simulations of vapor-liquid phase interfaces for pure water were performed using the DL POLY Classic software. The TIP4P/2005 molecular model was successfully used for the modeling of the density profile and the thickness of phase interfaces together with the temperature dependence of the surface tension. In the current study, the extended simple point charge (SPC/E) model for water together with TIP4P/2005 were employed for the investigation of vapor-liquid phase interfaces over a wide temperature range from 250 K to 600 K. Results of the new simulations are in a good agreement with most of the literature data. TIP4P/2005 provides better results for the saturated liquid density with its maximum close to 275 K, while SPC/E predicts slightly better saturated vapor density. Both models give qualitatively correct representation for the surface tension of water.
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Mathematical modeling of planar and spherical vapor–liquid phase interfaces for multicomponent fluids
Celný, David ; Vinš, Václav ; Planková, Barbora ; Hrubý, Jan
Methods for accurate modeling of phase interfaces are important for understanding natural processes and application in technology. In particular, prediction of the non-equilibrium phase transition requires the knowledge of the strongly curved phase interfaces of microscopic droplets. In our work, we focus on the spherical vapor–liquid interfaces for binary mixtures. We developed a computational method able to determine the density and concentration profiles. The fundamentals of our approach lie in the gradient theory, allowing to transcribe the functional formulation into a system of Euler-Langrange equations. System is then modified into a shape suitable for iterative computation. For this task, we employ the Newton-Raphson and the shooting methods ensuring a good convergence speed. For the thermodynamic properties, the PC–SAFT EoS is used. We determined the density and concentration profiles of the binary mixture C O 2 & C 9 H 20 for spherical phase interfaces at various saturation factors.
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An apparatus with a horizontal capillary tube intended for measurement of the surface tension of supercooled liquids
Vinš, Václav ; Hošek, Jan ; Hykl, Jiří ; Hrubý, Jan
New experimental apparatus for measurement of the surface tension of liquids under the metastable supercooled state has been designed and assembled in the study. The measuring technique is similar to the method employed by P.T. Hacker [NACA TN 2510, 1951]. A short liquid thread of the liquid sample was located in a horizontal capillary tube partly placed in a temperature-controlled chamber. One end of the capillary tube was connected to a setup with inert gas which allowed for precise tuning of the gas overpressure. The open end of the capillary tube was precisely grinded and polished in order to assure its planarity and perpendicularity. The liquid meniscus at the open end was illuminated by a laser beam and observed by a digital camera. Application of an increasing overpressure of the inert gas at the inner meniscus of the liquid thread caused variation of the outer meniscus such that it gradually changed from concave to flat and subsequently convex shape. The surface tension at the temperature of the inner meniscus could be evaluated from the overpressure corresponding to exactly planar outer meniscus.
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Density gradient theory combined with the PC-SAFT equation of state used for modeling the surface tension of associating systems
Vinš, Václav ; Planková, Barbora ; Hrubý, Jan ; Celný, D.
The density gradient theory (GT) combined with a SAFT-type (Statistical Associating Fluid Theory) equation of state has been used for modeling the surface tension of associating fluids represented by a series of six alkanols ranging from methanol to 1-pentanol. The effect of nonzero dipole moment of the selected alkanols on the predicted surface tension was investigated in this study. Results of the GT + non-polar Perturbed Chain (PC) SAFT equation of state were compared to predictions of GT combined with the PC-polar-SAFT, i.e. PCP-SAFT, equation. Both GT + PC-SAFT and GT + PCP-SAFT give reasonable prediction of the surface tension for pure alkanols. Results of both models are comparable as no significant difference in the modeled saturation properties and in the predicted surface tension using GT was found. Consideration of dipolar molecules of selected alkanols using PCP-SAFT had only minor effect on the predicted properties compared to the non-polar PC-SAFT model.
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Phase equilibria of carbon dioxide and methane gas-hydrates predicted with the modified analytical S-L-V equation of state
Vinš, Václav ; Jäger, A. ; Hrubý, Jan ; Span, R.
Gas-hydrates (clathrates) are non-stoichiometric crystallized solutions of gas molecules in the metastable water lattice. Two or more components are associated without ordinary chemical union but through complete enclosure of gas molecules in a framework of water molecules linked together by hydrogen bonds. The clathrates are important in the following applications: the pipeline blockage in natural gas industry, potential energy source in the form of natural hydrates present in ocean bottom, and the CO2 separation and storage. In this study, we have modified an analytical solid-liquid-vapor equation of state (EoS) [A. Yokozeki, Fluid Phase Equil. 222–223 (2004)] to improve its ability for modeling the phase equilibria of clathrates. The EoS can predict the formation conditions for CO2- and CH4-hydrates. It will be used as an initial estimate for a more complicated hydrate model based on the fundamental EoSs for fluid phases.
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Droplet and Bubble Nucleation Modeled by Density Gradient Theory - Cubic Equation of State versus SAFT Model
Vinš, Václav ; Hrubý, Jan ; Planková, Barbora
The study presents some preliminary results of the density gradient theory (GT) combined with two different equations of state (EoS): the classical cubic equation by van der Waals and a recent approach based on the statistical associating fluid theory (SAFT), namely its perturbed-chain (PC) modification. It has been shown that PC-SAFT is a promising tool for accurate modeling of nucleation using the GT. Besides the basic case of a planar phase interface, the spherical interface was analyzed to model a critical cluster occurring either for nucleation of droplets (condensation) or bubbles (boiling, cavitation).
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Experimental Investigation of Throttling Process Affected by Gas Impurities
Vinš, Václav ; Vacek, V.
The throttling process in a small-diameter tube – capillary tube – was experimentally investigated in this study. A special testing capillary tube equipped with precise temperature and pressure sensors was used to describe the effect of gas impurities, i.e. non-condensing gases (nitrogen in our case), on the flow of throttled refrigerant R218. Total pressure at the onset of vaporization was increased by partial pressure of contaminating gas. Therefore, the two-phase flow of gas-contaminated refrigerant started to be generated notably earlier than in the case of pure refrigerant flow. Mass flow rate of refrigerant delivered through capillary tube decreased even by 20 % in some cases. The experimental data were compared with a numerical simulation of pure refrigerant flow in capillary tube.
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Modeling of Throttling Process inside Capillary Tube
Vinš, Václav ; Vacek, V.
Presented study is part of larger project focused on a design and testing measurements of unique cooling systems ntended to particle detectors built at CERN laboratory. Existing numerical model of a throttling process inside a capillary tube was extended by a solution of two metastable regions – superheated liquid flow and a two-phase metastable flow. The underpressure of vaporization was determined on the base of heterogeneous nucleation theory. The correlation was modified by using our experimental data measured on special testing capillary tube. Detailed analysis of the experimental data pointed on a negative effect of gas impurities on the throttling process.
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