Contemporary distribution transformers face modern technological and integration challenges due to modern electrification and load increase. Motivated by the need for improvement of transformer safety, lifetime, thermal and dielectric performance, we carry out research on transformer oil-based nanofluid with fullerene C60 nanoparticles as a potential replacement for conventional transformer oil. Unlike numerous studies based on numerical or experimental investigation of thermal or dielectric properties, we provide a comprehensive research on C60 nanofluid in academic laboratories and in industry, complemented by an analytical modeling. Large-scale nano-functionalization of transformer oil is performed unconventionally by dissolving C60 powder using an industrial oil treatment machine under controlled temperature and oil pumping conditions. The prepared nanofluid is applied in a three phase 250 kVA distribution transformer, on which temperature rise and high voltage tests are conducted. The nanofluid is subjected to experimental investigation of key physical properties, like density, viscosity, nanoparticle concentration, thermal conductivity, flash point, dielectric breakdown, dissipation factor and permittivity. An analytical model of a natural convection loop is employed to predict the effects of adding C60 nanoparticles into the oil on temperature and flow velocity within the distribution transformer. It is found that very small C60 concentration (0.004% w/V) determined by spectrophotometry has a low or negligible impact on viscosity, density and thermal conductivity, while significantly increases AC breakdown voltage (by 65%) and decreases flash point of the oil. The transformer filled with the C60 nanofluid met the requirements of the standard EN 60076-2:2011. Comparison of transformer temperature rise tests and analytical model predictions show a slight decrease in the transformer temperature due to enhanced coupling between the windings and cooling surfaces through improved oil circulation. The collected experimental, numerical and industrial results provide pros and cons of the large-scale C60 nanofluid preparation and future application in distribution transformers.

We investigate the scattering of two distinguishable particles with unequal masses and a mutual short-range interaction with the aim of quantifying the impact of a tunneling “projectile” particle on the quantum mechanical state of the “barrier” particle. We find that the states of the barrier particle after the tunneling or reflection of the projectile are displaced by a finite distance that is given by the derivative of the phase of the transmission or reflection amplitudes multiplied by factors dependent on particles' masses, respectively. We demonstrate these results on a numerical example with a resonant interaction between a projectile and barrier. Our work demonstrates physical implication of the concept of phase time delay in the form of finite displacements of particles that are, in principle, experimentally measurable.

This article introduces a simplified model for analyzing heat transfer in oil-filled transformers enhanced with magnetic nanofluids. The model integrates natural convection with thermomagnetic effects to predict the cooling performance of transformer oil channels and fins. The study employs a combination of theoretical formulations and Finite Element Method (FEM) simulations to evaluate the impact of magnetic field on heat transfer characteristics. The results indicate a notable contribution of thermomagnetic convection to the overall cooling efficiency, providing insights into the design and optimization of transformer cooling systems.

Abstract We apply our quasi-1D model to the temperature rise in oil-filled transformer foil windings with two symmetric partial ducts. We compare the results with a two-dimensional FEM simulation. To compare the results to industrial tests we try to find a relationship between the effective values for heat transfer coefficients of different foil windings. Finally, we compare the effectiveness of adding one vs two partial cooling ducts to the winding.

A quasi-one-dimensional approach to conduction heat transfer with convective boundary conditions is developed for bodies of variable geometry. The novelty of the method is demonstrated on the windings in oil-filled distribution transformers with partial cooling ducts. We obtain analytical solution for the windings average temperature rise and tangential temperature distribution and find them in an excellent agreement with the FEM simulation, numerically confirming the validity of the reduction from 2D to 1D. When the quasi-one-dimensional approach is applied to calculation of temperature rise for the low and high voltage windings in industrial setting we obtain 1% and 5% discrepancy with the industrial tests respectively. Hence, the method offers reliable estimates of the temperature rise, and temperature distribution in a chosen direction, in bodies of variable geometry.

A new transient technique of the thermal conductivity and diffusivity measurement for anisotropic materials is presented and validated. It is based on measuring the through-plane properties using the extended dynamic plane source (EDPS) method and in-plane conductivity employing the transient plane source (TPS) and modified dynamic plane source (MDPS) methods. The key advantage of this technique is that only one pair of specimens is required for measurements. While the EDPS method is implemented on real measurements, the TPS and MDPS are applied to the finite elements method (FEM) simulation of the experiment. The accuracy of the results is enhanced by the application of the FEM and is better than 1% for materials with through-plane conductivity of less than 2 W m−1 K−1 and a specimen thickness of 9 mm.

Abstract We compare the analytical results for a natural convection loop model with two-dimensional finite element method (FEM) simulations. We set the values for FEM simulations parameters to reflect oil circulation in a distribution transformer. We find that the calculated characteristics for the two approaches differ by less than 3%. This discrepancy is explained in terms of the variations in the fluid velocity profile.

The effect of heat source position on the heat transfer in an aluminium tube was investigated experimentally and analytically. The experimental data were obtained by measuring the temperature distribution along the tube wall for two different heat source locations. A theoretical model of the tube was developed and validated with the experimental data. A finite element method simulation was performed to analyze the temperature profile and to endorse assumptions for theoretical model. The results showed that the heat transfer rate depend on the heat source position.

We present a comprehensive analytical model for laminar flow in a vertically heated rectangular convection loop, with applications to oil cooling in transformer systems. Starting from an integral equation, we apply a quasi-one-dimensional approach to derive equations for both loop velocities and temperature distributions. In this model, we account for temperature-dependent dynamic viscosity and introduce cooling at the loop top and in the fin. The analytical predictions are rigorously validated against finite volume method (FVM) simulations across different geometries and heat flux conditions, demonstrating strong agreement within a 2% discrepancy range for most parameters. This model provides a viable analytical alternative to computationally intensive simulations, offering insights into natural convection dynamics within closed-loop configurations.

Defects in organic semiconductors can significantly affect the functionality of organic photonic devices. However, unlike inorganic materials, they are still little researched. This article investigates structural defect states in the bandgap of n-type small molecular semiconductor 2,2’-{[5,5’-(Naphthalene-2,6-diyl)bis(thiophene-5,2-diyl)]bis(2,2,2-trifluoroethan-1-yl-1-ylidene)}dimalononitrile (TNT-FEC). Using the energy-resolved electrochemical impedance spectroscopy (ER-EIS), we mapped the presence of defect states in this material, and by comparing measurements in the solution and the film, we identified the structural ones. Furthermore, we investigated the influence of the substrate on defect formation. Finally, using DFT and DFTB calculations, we analyzed the possibility of the conformers’ presence, which can influence the structural properties of thin films. The combination of experimental and theoretical approaches in the analysis of defect states has been shown as suitable for studying structural defects’ presence in the thin films of organic semiconductors.

We test performance of a quasi-one-dimensional approach to steady state heat transfer in broadening beams, characterized by variable cross-sectional area. The results obtained analytically and by solving the governing equation numerically in Python, for three different instances, are compared to FEM simulations. We demonstrate that the quasi-one-dimensional model gives accurate predictions for beam suitably averaged temperatures but the range of Biot numbers for which it can be accurately applied to is currently limited by a crucial parameter.

The paper addresses the influence of the heat source capacity and structure on the accuracy of the thermophysical parameters measurements within several transient methods. First, the New Plane Source (NPS) method is improved by taking into account the heat source capacity. The results of measurement are compared with those of Step Wise Transient (SWT) method and the relative differences for glass and polymethylmethacrylate (PMMA) are better than 0.5 %. The second part is devoted to the Transient Plane Source (TPS) method. The results of measurement are compared with those of SWT method. The relative differences for glass are less than 0.5 %, but for PMMA 6 %, which is explained by specimen anisotropy. Finally, the reliability of the analytical model of TPS method is supported by comparing it with the numerical one obtained by Finite Elements Method (FEM), where three models of material composition of the heat source are used. The temperature functions are determined from numerical responses obtained by FEM and analytical solution. The coefficients of variation of all four determinations of the thermal diffusivity and conductivity of glass and PMMA are less than 0.4 %.

Branching flow -- a phenomenon known for steady wave propagation in two-dimensional weak correlated random potential is also present in the time-dependent Schr\"odinger equation for a single particle in one dimension, moving in a fluctuating random potential. We explore the two-dimensional parameter space of this model using numerical simulations and identify its classical regions, where just one classical parameter is sufficient for its specification, and its quantum region, where such a simplification is not possible. We also identify region of the parameter space where known analytical results of a classical white-noise model are relevant. Qualitative behavior of quantum and classical particle dynamics is discussed in terms of branching time scale and a new time scale related to particle's kinetic energy.

We propose and test a simple analytical model of radial temperature profile for the VVER-440 fuel assembly sub-channel. The model includes 18 physical parameters, which affect the heat transport. Considering small changes in the input parameters of the model we evaluate their impact on temperature profile. The outcomes of this work include temperature profile in the representative thermo-hydraulic sub-channel and analysis of its sensitivity to changes in the input parameters. The analytical approach improves our understanding of themio-physical and material-related phenomena inside the fuel rod under operational conditions.

Identifying key parameters that can help control molecular ordering in the active layer of high-performance organic devices is an important topical issue. Finding a correlation between the spatial molecular structure of polymer backbones and the thin film's structural and optoelectronic properties is thus of utmost importance. In this paper, the influence of the backbone spatial structures of homopolymer PFO and copolymers F8BT and PFO-DBT with various stereoisomers varying by the up or down spatial orientation of individual chemical units in the comonomer on the film microstructure and electronic structure is studied. The computed energetics of the stereoisomers revealed at least two energetically favorable stereoisomers in both copolymers. These theoretical findings correlate with the thin film's crystallinity analyzed by the GIWAXS method, which showed ill packing in copolymers with stereoisomers. Thus, the molecular stereostructure was identified as a critical parameter that can help control molecular organization in active layers of organic photonic devices.

An effective one-dimensional model is presented that describes the temperature profile of a winding of an oil-filled distribution transformer with an arbitrary number of partial cooling ducts. An analytical solution of the model is applied to a specific example - a low voltage winding of a 400 kVA distribution transformer with one or two partial cooling ducts. Starting from the exact solution, a simple and practical formula for the temperature rise of similar windings has been derived that is suitable for transformer designers.

We study the polyfluorene crystal using computational modelling based on the density functional theory. We assume a geometry with two monomers per unit cell and periodicity along the polymer's backbone structure. The stable structure determined at these conditions is characterized by significant torsion angles between individual units. We calculate its optical spectra using the TDDFT method and find a gap of 2.73 eV which is in a very good agreement with the experimental value of 2.83 eV reported in our recent paper for a thermally annealed PFO film.