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Lubrication with a refrigerant : An industrial challenge investigated through multiscale modeling based on fluid/surface chemistry

Abstract : In large refrigeration systems, using the refrigerant as lubricant instead of oil can help to simplify the design, lighten the systems, and reduce their environmental impact. However, the very low viscosity of refrigerants leads to ultra-thin films separating the surfaces, with a thickness comparable to surface roughness. Nevertheless, experiments with the R1233zd refrigerant suggests that lubrication is still possible in that situation thanks to an adsorbed layer formed on iron oxide surfaces. Experimental in situ analysis area is very difficult because of high confinement and high pressure. That is why a multiscale numerical approach is developed here, to explore the impact of chemical reactions and physical processes at the refrigerant-surface interface on large-scale lubrication properties. Density functional theory is used to quantify the adsorption of a refrigerant molecule on an iron oxide surface at the quantum level. Binding energies ranging from -0.92 eV to -0.22 eV are measured and related to different adsorption cases. These results are then used to parametrize an interfacial force field, whose predictions of interfacial molecular structure differs from those obtained using potentials based on standard mixing rules. Large-scale molecular dynamics simulations involving this parametrized force field confirm the existence of a strongly adsorbed layer of R1233zd molecules on iron oxide surface. With atomically smooth surfaces, and a refrigerant film thickness as small as 2 nm, the adsorbed refrigerant layers resists pressures as high as 4 GPa and sliding velocities as high as 100 m/s. A minimum value of 5 refrigerant molecules per nm² is necessary to the formation of two adsorbed layers at 0.5 GPa. Moreover, sliding simulations with a rough surface reveal total film breakdown for ca. 13 GPa.
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Stéphane Tromp. Lubrication with a refrigerant : An industrial challenge investigated through multiscale modeling based on fluid/surface chemistry. Mechanics [physics.med-ph]. Université de Lyon, 2018. English. ⟨NNT : 2018LYSEI060⟩. ⟨tel-02067116⟩

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