Heat Transfer Performance of Oil-Based Nanofluids in Electric Transformers

Natural convection takes place in transformer by its heat dissipating medium - transformer oil which helps in regulating transformer operating temperature. Degradation of transformer oil causes transformer dielectric breakdown because of conductible bubble gas formation. This is because of the low thermal conductivity of transformer oil which results in poor heat transfer performance. Selected naphtha based transformer oil is used as base fluid in this research project. Carbon nanotubes (CNT), graphite, and diamond nanoparticles with various concentrations (0.25 to 2 vol%) are used as dispersant in oil-based nanofluids. Nanoparticles with high thermal conductivity, when mixed with base fluids, can improve the overall heat transfer characteristics of the base fluid. This can help to improve the oil degradation problem in transformer. Computational Fluid Dynamics (CFD) simulation tool – ANSYS Fluent 15.0 is used to perform 3D simulation to visualize the heat transfer performance inside the transformer based on the designed transformer model geometry. Slice model had been developed with defined heat flux as boundary conditions at winding and core area. Specific heat capacity and viscosity of the base fluid (transformer oil) and nanofluids (transformer oil with nanoparticles) are defined as function of temperature while density, thermal conductivity, and thermal expansion coefficient are set as constant for Fluent solver. Results show that CNT, graphite, and diamond nanofluids have better heat transfer coefficient than transformer oil. It is found that CNT and graphite based nanofluids show lower temperature at winding area than transformer oil alone. CNT based nanofluids at 2.0 vol% showed the highest value of overall heat transfer coefficient i.e. 239.36 W/m2.K with lowest winding temperature i.e. 78.63°C. Heat transfer performance of CNT based nanofluids are found to be better than graphite and diamond based nanofluids which can be recommended as a new kind of synthetic fluid specific for transformer usage.