• Energy Research

Progress in electrical engineering puts a greater demand on the cooling and insulating properties of liquid media, such as transformer oils. To enhance their performance, researchers develop various nanofluids based on transformer oils. In this study, we focus on novel commercial transformer oil and a magnetic nanofluid containing iron oxide nanoparticles. Three key properties are experimentally investigated in this paper. Thermal conductivity was studied by a transient plane source method dependent on the magnetic volume fraction and external magnetic field. It is shown that the classical effective medium theory, such as the Maxwell model, fails to explain the obtained results. We highlight the importance of the magnetic field distribution and the location of the thermal conductivity sensor in the analysis of the anisotropic thermal conductivity. Dielectric permittivity of the magnetic nanofluid, dependent on electric field frequency and magnetic volume fraction, was measured by an LCR meter. The measurements were carried out in thin sample cells yielding unusual magneto-dielectric anisotropy, which was dependent on the magnetic volume fraction. Finally, the viscosity of the studied magnetic fluid was experimentally studied by means of a rheometer with a magneto-rheological device. The measurements proved the magneto-viscous effect, which intensifies with increasing magnetic volume fraction.

Abstract In electrical engineering, the heat transfer can be enhanced by changing the thermophysical properties of insulating oils. In this paper, a single-phase power transformer with a nominal power of 5 kVA is subjected to a temperature rise test with three different transformer liquids. The first test is carried out with a novel gas-to-liquid transformer oil applied as a cooling and insulating medium. The other tests are conducted with ferrofluids based on this oil and MnZn ferrite nanoparticles of a low and a high nanoparticle concentration. The ferrofluids are characterized by magnetization curves, magnetic susceptibility and temperature-dependent magnetization measurements. The nanoparticle size distribution is determined from dynamic light scattering and the magnetization data. From the temperature rise profiles of the transformer at various inner locations, it has been found that the low-concentrated ferrofluid significantly reduces the transformer temperature rise. The enhanced cooling performance is ascribed to the thermomagnetic and natural convection, and increased thermal conductivity. The application of the ferrofluid with the high nanoparticle concentration resulted in a remarkable increase of the transformer temperature rise. The deteriorative cooling effect is attributed to the hindered natural and thermomagnetic convection due to the high ferrofluid magnetization and strong magnetic interaction of the ferrofluid with the magnetic field near the transformer core.

We report on the investigation of a transformer oil-based magnetic nanofluid exposed to an electric field by means of synchrotron small angle X-ray scattering. Two types of small angle X-ray scattering experiments were carried out. In the first one, the electric field up to 6 kV / cm was generated in the nanofluid between two immersed electrodes. The other experiment focused on the nanofluid in an external electric field up to 10 kV / cm, when the electrodes were not in a direct electrical contact with the nanofluid. In the available range (0.02–4.5 nm$^{−1}$) of scattering vector $q$, the non-contact mode has no effect on the scattering intensity. The contact mode yielded noticeable low-$q$ intensity variations. In comparison to small angle neutron scattering, the small angle X-rayscattering study did not prove the proportional increase in the low $q$ scattering intensity with increasing electric field, but rather stochastic variations. The observed intensity variations reflect the local structural nanofluid changes caused by the induced electrohydrodynamics. The electrical conductivity and relaxation processes are pointed out as favorable conditions for electrohydrodynamics in the magnetic nanofluid. Acta physica Polonica / A 137(5), 942 - 944 (2020). doi:10.12693/APhysPolA.137.942 Published by Acad. Inst., Warsaw