• Energy Research

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.

Within the frequency range 10–1–105 Hz under the temperatures 293–295 K, the dielectric properties of a planar-oriented nematic liquid crystal 6CB with the embedded of 0.1% superionic conductor Ag7GeS5I nanoparticles have been investigated. It has been shown that for the whole temperature range, the obtained frequency dependences of the components ε' and ε" composing the complex dielectric function can be separated into 3 sections. The dispersion of ε' and ε" for the lowest frequencies (less than 102 Hz) is described by the Debye equation and is caused by the rotation of the dipole moments of LC molecules under the action of electric field within the angles corresponding to the fluctuations of the order parameter in a thin near-electrode layer. It has been shown that the temperature dependence of the value of inverse relaxation time for such a process is described by straight lines in the Arrhenius coordinates within each mesophase. The activation energies for these dependences have been estimated for each mesophase. It should be noted that within the middle range of frequencies (102–104 Hz), for each temperature, one can separate a section where the magnitude of the conductivity does not depend on the frequency. The conductivity in these sections is equal to the conductivity of LC with the nanoparticles. It has been found that both the value of inverse relaxation time and the value of conductivity change according to the Arrhenius law on the temperature. The activation energies for the temperature dependence of conductivity and the temperature dependence of inverse relaxation time have been estimated, and it has been shown that they are close (for the nematic phase) and equal (for the isotropic phase). In the highest frequency section of the dielectric spectrum (104–105 Hz), the conductivity of the mixture 6CB + 0.1 wt.% Ag7GeS5I changes according to the power law of the frequency. It has been suggested that a sharp increase in the conductivity of 6CB with nanoparticles of the superionic conductor Ag7GeS5I at the concentration 0.1 wt.% was caused by the sharp increase in the electronic component of conductivity through Ag7GeS5I nanoparticles.

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 This article describes influence of strong (ionizing) electric field on sprayability of magnetic fluid containing colloid particles with size in the range from 10 to 20 nm of magnetite Fe 3 O 4 . Magnetic fluids can be based for example on both transformer oil and physiological solution for application in medical using (in human medical science research), that supports a fluid colloidal system. Further component of magnetic fluid is surfactant. It is acting as surface-active substance that prevents from nanometric dimension particle settlement. Magnetic fluid gets off nozzle with diameter in range 0.3–1.0 mm from container in surroundings of ionizing (i.e. strong) electric field ( E > 10 7 V m −1 ). As a consequence of action of electric field it gives out suppression surface tension in fluid what leads onwards to decomposition of magnetic fluid ligament at the end of nozzle. The diameter of nozzle oneself respects basic theoretical calculations in regards of fluid concentration and thereinbefore its selected size. Magnetic fluid in dependency on its used liquid base has weak-polar till polar orientation polarization character. It gives out sprayability in non-homogeneous electric field E in combination with magnetic field of intensity H . Orientation of vectors E and Ĥ , resp. induction of magnetic field B is defined by parallel or vertical direction. Results are confronted with measurements realized explicitly only at action of electric field (variable B = 0). In the case of magnetic field applications with permanent magnet together with electric non-homogeneous field it gives out unconventional dynamics of electrical charging particles of macroscopic dimension. Orientation particle track is influenced by orientation of field vector combinations. This phenomenon develops magneto-dielectric anisotropy, which oneself manifests behaviour of electrophysical quantities characterizing examination system. In consideration of technology utilization of this method it is very important to respect applied magnetic fluid concentration. Electrical characteristics were examined for volume concentration of magnetite particles in the range from 0.125% to 18%. Nevertheless efficiency optimization of given media suggests to boundary concentration of magnetic fluid of 4.0%, when it is in the regions of weak polar till polar material. Electrophysical research refers to exploitation of applied magnetic layer technology on dielectric insulating substances with inorganic origin as well as thin layer technology coating plastic foils created from macromolecular organic substance.

Copper-enriched (Cu0.75Ag0.25)7SiS5I-based ceramics were prepared from the micro- and nanopowders by pressing and sintering under developed technological conditions. Structural studies at different process step stages of ceramic samples preparation were performed using the XRD technique and microstructural analysis. The frequency and temperature dependences of the total electrical conductivity for (Cu0.75Ag0.25)7SiS5I-based ceramics were investigated by the impedance measurements. From the Nyquist plots, by using the electrode equivalent circuits the ionic and electronic components of the total electrical conductivity were determined. It has been shown that both ionic and electronic conductivity nonlinearly depend on the average crystallites size of (Cu0.75Ag0.25)7SiS5I-based ceramics.

Magnetic nanofluids (widely known as ferrofluids or magnetic liquids) have a unique property - they are responsive to the application of a magnetic field, which allows for the possibility of controlling the flow and the convective heat transfer. This paper presents the characteristic thermo-physical, magnetic and dielectric properties of a transformer oil based magnetic nanofluid, specially prepared for use as a cooling and insulating medium in a power transformer.

Within the temperature range 288–343 K and frequencies 10–1–106 Hz, the dielectric properties of planar and homeotropic oriented nematic liquid crystal 6CHBT with the impurity of 0–3 wt.% Ni-TMTAA-TCNQ molecules have been investigated. It has been shown that at the lowest frequencies, the dispersion of components of the complex dielectric permittivity in the case of a planar oriented liquid crystal is caused by oscillations of molecular dipoles within the range of angles corresponding to fluctuations of the order parameter under the action of electric field. The values of relaxation times have been estimated, and it has been shown that with the increase in the impurity concentration, the value of the relaxation time decreases. The temperature dependences of the values inverse to the relaxation time have been analyzed. It has been shown that for the middle frequency range the obtained dielectric spectra characterize the bulk properties of the samples, in particular the value of the electrical conductivity. It has been found that with increasing the molecular concentration, the activation energy for the temperature dependence of the electrical conductivity increases. This dependence for the whole range of studied temperatures correlates with the temperature dependence of the values inverse to the relaxation time. It has been shown that at the concentration of the impurity up to 1 wt.%, the conductivity of the samples both in homeotropic and planar orientations of molecules varies according to the power law of the impurity concentration. It has been estimated the value of the exponent for the concentration dependence of conductivity, and it has been shown that it is the same for various molecular orientations.