Stabilization and characterization of a nanofluid based on a eutectic mixture of diphenyl and diphenyl oxide and carbon nanoparticles under high temperature conditions

Synthetic thermal oils are used as heat transfer fluids (HTFs) in different applications, due to their higher working temperature. In this way, one of the applications of interest is the use of thermal oils in Concentrated Solar Power (CSP) plants with Parabolic Trough technology. Nowadays, the HTF known commercially as Therminol VP1 (Solutia Inc.) is being used in CSP plants. This fluid is composed of a eutectic mixture of diphenyl (C12H10) and diphenyl oxide (C12H10O), and it is used as an HTF with a maximum working temperature of 400 °C. However, one of the drawbacks of Therminol VP1 is its low thermal conductivity. In recent years it has been demonstrated that the addition of nanoparticles can improve the thermal properties of HTFs, and they are then called nanofluids. The key factor of nanofluids is their high stability over time. However, at high temperatures it is necessary to add chemically compatible surfactants that do not degrade and endow the nanofluids with stability through steric repulsion even under high temperature conditions. In this work, a carbon black/Therminol VP1 nanofluid was synthesized and stabilized using diphenyl sulfone as a stabilizer. Stability tests after thermal cycling at 400 °C showed the higher performance of this additive compared to others commonly used in the literature. Thermal conductivity, heat capacity, and viscosity of the nanofluids at 3 vol% and 5 vol% were characterized from 50 °C to 350 °C. Finally, the Mouromsteff number was calculated to determine the heat transfer enhancement provided by the use of nanofluids.