Lubricant thermal conductivity and heat capacity under high pressure

Abstract

The thermal conductivity λ and the heat capacity per unit volume, ρc_p, have been measured for a number of common lubricating oils. The oils tested were paraffinic and naphthenic mineral oils and a 50/50 blend of these. Poly-α-olefin, polyglycol, Santotrac, ester and rapeseed oils have also been tested. The measurements, using the transient hot-wire method, were carried out under isothermal conditions over a pressure range from atmospheric to 1.1 GPa and at two temperatures, 295 and 380K (22 and 107 °C respectively). The temperature had only a marginal effect on thermal conductivity; however, the thermal conductivity was doubled as the pressure was increased to 1 GPa. The heat capacity per unit volume was influenced by both the pressure and the temperature. Some of the lubricants solidified as the pressure increased and the transition from a fluid to a solid state could be detected in the measurements of ρc_p. The relationship between the thermodynamic properties and the pressure and temperature are described by two empirical equations. These equations can be used in thermal elastohydrodynamic analyses.