Heat Transfer Studies in Thermally Conducting and Electrically Insulating Nano-Oils in a Natural Circulation Loop

Abstract

Mineral oil (MO), a dielectric insulating fluid, is commonly used as coolant and lubricant in various applications, such as in high voltage power transmission systems and machinery. The mode of heat transfer in most of these systems is natural convection. Prolonged operation at higher temperatures leads to the degradation of the dielectric coolant, which leads to diverse problems, such as shortage or breakdown of these devices and apparatuses. Increasing the heat transfer capability of the insulating fluid will minimize the energy consumption of the system, prolonging its useful life. It is proposed to improve the heat transfer performance of insulating fluid by the addition of hexagonal boron nitride (h-BN), which is synthesized and finally obtained in 2D-nanosheets through wet exfoliation technique, without affecting its electrical insulating property. h-BN was reported to have superb effect on thermal conductivity of MO (∼ 80% increase at 0.10wt.%) on addition at very low filler fraction [1], thermal stability of up to 800°C [2], and good electrical insulating properties due to its nature (electron band gap of approx. 4.5eV). The present work reports the application of nano-oil (MO + h-BN 2D-nanosheets) for enhanced heat dissipation. A rectangular thermosyphon loop was modeled as the thermal system in transformer. The aspect ratio of the loop and the positions of the heater and cooler were chosen according to the stability criterion so that the flow remains stable and unidirectional throughout the experiment. The effect on heat removal by varying the concentration of h-BN 2D-nanosheets (h-BNNS) in MO was measured and discussed.