Heat Transfer Characteristic of Al2O3 Nanofluid with Naphthenic Transformers Oil as Base Fluid

Power transformers are static electrical devices with the highest costs compared to other components in the power grid system. Damage to these transformers can lead to significant financial losses, emphasizing the need for high efficiency. One method to enhance transformer oil quality is by adding nanoparticles. Al₂O₃ is commonly used in transformer oil research due to its ability to increase thermal conductivity and breakdown voltage. In this study, Al₂O₃ is incorporated at a concentration of 0.01 wt% and testing equipment is designed to replicate power transformer conditions using an ONAN cooling system. The experimental setup in this study comprised a reservoir made of SUS 304 material, with a thickness of 2 mm and internal dimensions of 300 x 200 x 150 mm. In the center of the reservoir, there is a 650-watt cylindrical heater that simulates a transformer winding. Type K thermocouples monitored temperature changes recorded every 2 seconds over 3 hours. The research results indicate that the most significant temperature increase occurred during the first phase for all samples, with the highest rate recorded at 2.75 °C per minute for a 200-watt input power. Heat transfer coefficient increases with the increasing power in both uninhibited naphthenic mineral oil and nano insulating liquid Al₂O₃ 0.01 wt%. The heat transfer coefficient of the nano insulating liquid Al₂O₃ 0.01 wt% tends to be greater than that of the base fluid, and the same trend is observed in the Nusselt number. What differs is that the value of Ra decreases with the addition of Al₂O₃. The decrease in Ra is an indication that conduction becomes more dominant than convection. Therefore, the use of a small concentration of Al₂O₃ is recommended to minimize the impact of the decrease in Ra. Overall, Al₂O₃ has great potential for use as a cooling oil in power transformers at low concentrations, but further research is needed on other factors to ensure its suitability for industrial applications.