Thermodynamic and Transport properties of fluids: towards a single LJ-SAFT like molecular model valid for n-alkanes ?

International audienceApart from the low density conditions, there is still a lack of a molecular based approach able to provide both equilibrium and transport properties using the same molecular parameters in all fluid regimes (gas, liquid and supercritical). This is even more critical when dealing with poly-atomic fluids. This lack is largely due to the fact that a comprehensive theory is still not available for evaluating the transport properties in dense fluids in terms of a realistic molecular model, i.e. molecular structure and interaction potentials. However, from the equilibrium properties side, the combination of the Lennard-Jones Chain (LJC) molecular model with the Statistical Associating Fluid Theory (SAFT) is able to represent very well the thermodynamic properties of a great variety of fluids [1]. Furthermore, combined with Density Functional or Density Gradient Theory, these approaches are capable to yield very good results on interfacial properties for the same systems [2]. Additionally, based on Molecular Dynamics simulations results, has been proposed recently semi-empirical accurate correlation to describe viscosity [3] and thermal conductivity [4] of the LJC fluid model over a wide range of thermodynamic states. So, in this work, we have used the LJC molecular model combined with LJ-SAFT and transport properties correlation on normal alkanes (methane, n-butane, n-heptane and n-decane) along the vapour/liquid coexistence line. The idea was to test if, with a single set of molecular parameters for each n-alkane, this model was able to provide a reasonable estimate of both thermodynamic and transport properties. Good results have been obtained for methane (monomer) and n-butane (dimer) except for thermal conductivity in the gas state. For n-heptane (trimer) and n-decane (quadrimer) it has been found that both viscosity and thermal conductivity are not always well estimated using this approach. It will be shown that these trends are fully related to the bad modelling of the internal degrees of freedom when using the LJC model. Furthermore using molecular dynamics simulation it will be shown that, for viscosity, a single additional "rigidity" parameter allows to strongly improve all the results. [1] S. P. Tan, H. Adidharma, M. Radosz, Ind. Eng. Chem. Res., 47 (2008), 8063-8082. [2] P. Paricaud, A. Galindo, G. Jackson, Fluid Phase Equilib 194-197 (2002), 87-96. [3] G. Galliero, C. Boned, Phys. Rev. E, 79 (2009) 021201. [4] G. Galliero, C. Boned, Phys. Rev. E, 80 (2009) 061202

Molecular dynamics simulation of thermodiffusion and mass diffusion in structureless and atomistic micropores

International audienceIn this work, we have studied the effect of surface roughness on thermodiffusion in simple "isotopic" mixtures confined in a slit nanopore. To do so, we have performed non-equilibrium molecular dynamics simulations of Lennard-Jones binary equimolar mixtures confined in structureless (in which the interaction with the fluid is described by a Lennard-Jones 9-3 potential) and atomistic walls for various widths, from 5 to 35 times the size of a molecule, in the NP//T ensemble. For that purpose, a new algorithm is proposed in atomistic pore. Different super-critical conditions have been explored, ranging from low to moderate densities. In addition to the thermal diffusion factor, we have also estimated the mass diffusion and thermodiffusion coefficients separately. The results show that the two types of walls lead to noticeably different results. The thermal diffusion factor tends to increase in atomistic wall and slightly decrease in structureless wall when the pore width is decreasing, this being related to the average density behaviour. More precisely, both mass and thermodiffusion coefficients are weakly affected by the pore width for structureless walls, whereas both quantities largely decrease (up to 70% and 55% respectively compared to bulk fluid) when pore size decreases in the case of a rough solid surface because of the friction on the walls

Reference Correlation of the Viscosity of Squalane from 273 to 373 K at 0.1 MPa

International audienceThe paper presents a new reference correlation for the viscosity of squalane at 0.1 MPa. The correlation should be valuable as it is the first to cover a moderately high viscosity range, from 3 to 118 mPa s. It is based on new viscosity measurements carried out for this work, as well as other critically evaluated experimental viscosity data from the literature. The correlation is valid from 273 to 373 K at 0.1 MPa. The average absolute percentage deviation of the fit is 0.67, and the expanded uncertainty, with a coverage factor k = 2, is 1.5%

Conception of a compact flow boiling loop for the International Space Station- First results in parabolic flights

International audienceThe design of a pipe flow boiling experiment for the International Space Station is proposed, taking into account typical weight, power consumption and size constraints. The effect of singularities such as elbows upstream the test section is investigated. Velocity profiles downstream two elbows, measured by Particle Image Velocimetry are in good agreement with numerical simulation and allow to determine a specific distance (decay length) downstream the elbows for which the velocity profile recover its axisymmetry. From these results a breadboard is designed and tested in parabolic flights. Care has been taken to generate boiling downstream the decay length. Two-phase bubbly flow is observed with 2 perpendicular high-speed cameras in the test section and a symmetry of the bubble distribution in the pipe is verified for different gravity conditions when the bubbles are created after the decay length

Dynamics viscosity of the Lennard-Jones Chains fluid

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Microscopie sous haute pression pour la mesure de température de fusion finissante de systèmes paraffiniques

Lors de leur exploitation et de leur transport, les fluides pétroliers subissent de fortes variations de température et de pression susceptibles d'entraîner la formation de dépôts solides. Pour bien comprendre leur comportement, il est indispensable de disposer de données expérimentales fiables permettant de décrire correctement leur diagramme de phase afin d'élaborer des modèles thermodynamiques prédictifs. Au cours de ce travail, un microscope haute pression a été développé pour combler les insuffisances des techniques existantes dans le domaine des équilibres de phase liquide solide ; il permet de mesurer la température de fusion finissante de divers types de systèmes dans la gamme 0,1 100 MPa pour des températures comprises entre 243 et 373 K. Dans un premier temps, ce dispositif a été mis à contribution pour étudier la variation de la température de fusion de corps purs rencontrés au sein des fluides pétroliers (alcanes linéaires, alkylcyclohexanes et alkylbenzènes) en fonction de la pression et du nombre d'atomes de carbone. Cette étude a révélé une dépendance entre la pente des courbes de liquidus dans un diagramme (T, P) et la structure cristalline du solide considéré. Dans un deuxième temps, les températures de fusion finissante de plusieurs mélanges synthétiques ont été étudiées en fonction de la pression. Une modélisation des résultats a été proposée pour chaque système en tenant compte de l'influence de la structure cristalline sur la pente des courbes de liquidus. Enfin, le dispositif de microscopie sous haute pression a permis l'étude de fluides de gisement et une discussion sur la taille des cristaux qui apparaissent dans de tels systèmes est proposée.During their exploitation and transport, petroleum fluids undergo high variations in temperature and pressure that are likely to bring about solid deposits. In order to understand the behaviour of these fluids, it is necessary to have reliable experimental data which describes their phase diagram accurately, with the aim of creating predictive thermodynamic models. In this work, a high pressure microscopy device was developed to make up for the inadequacies of the existing techniques in the field of solid liquid equilibria; it allows the measuring of the solid disappearance temperature of miscellaneous systems in the 0.1 100 MPa pressure range, for temperatures between 243 and 373 K. At first, this device was used to study the influence of pressure and of the number of carbon atoms on the melting temperature of pure compounds encountered in petroleum fluids (normal alkanes, alkylcyclohexanes and alkylbenzenes). This study revealed a relationship between the slopes of the liquidus curves in a (T, P) diagram and the crystalline structure of the melting solid phase. Then solid disappearance temperatures of several synthetic mixtures were studied according to pressure. A modelling of the results was proposed for each system by taking the influence of the crystalline structure on the slopes of the liquidus curves into account. The high pressure microscope permitted the study of reservoir fluids and eventually a discussion about the size of the crystals that appear in such systems is proposed.PAU-BU Sciences (644452103) / SudocSudocFranceF