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Heat exchange on a heat-stressed surface and the critical heat flow from it in the process of cooling this surface by a system of distributed impact submerged microjets of a dielectric liquid, depending on the velocity of these jets, were investigated experimentally for the case where the initial subcooling of the liquid in the jets to the saturation temperature comprises 40 ± 2°C. The experiments were conducted for 36 jets of the HFE-7100 dielectric liquid with an initial diameter of 174 μm, outflowing from the nozzles positioned at a distance of 1 mm from a cooling surface. It was established that the heat transfer coefficients of this surface and the critical heat flow from it are determined by the velocity of the jets, acting on the surface, and the initial subcooling of the cooling liquid, and that they are much larger than those in the case of pool boiling of the cooling liquid on the surface and in the case of its cooling by a single jet of a liquid with a small subcooling to the saturation temperature. [ABSTRACT FROM AUTHOR]

This study explores the impact of an adiabatic obstacle on the symmetry of temperature, flow, and electric charge fields during electrohydrodynamic (EHD) natural convection. The configuration studied involves a square, differentially heated cavity with an adiabatic obstacle subjected to a destabilizing thermal gradient and a potential difference between horizontal walls. A numerical analysis was performed using the finite volume method combined with Patankar's "blocked-off-regions" technique, employing an in-house FORTRAN code. The study covers a range of dimensionless electrical Rayleigh numbers (0 to 700) and thermal Rayleigh numbers (102 to 105), with various obstacle positions. Key findings indicate that while the obstacle reduces heat transfer, this can be counterbalanced by electric field effects, achieving up to 165% local heat transfer improvement and 100% average enhancement. Depending on the obstacle's position and size, convective transfer can increase by 27% or decrease by 21%. The study introduces five multiparametric mathematical correlations for rapid Nusselt number determination, applicable to numerous engineering scenarios. This work uniquely combines passive (adiabatic obstacle) and active (electric field) techniques to control heat transfer, providing new insights into the flow behaviour and charge distribution in electro-thermo-hydrodynamic systems. [ABSTRACT FROM AUTHOR]

The use of additive technology—in particular, selective laser melting (SLM)—in producing wire electrodes for electrical discharge machining is considered. By this means it is possible to produce electrodes of high precision and complex geometry, with distinctive properties. In addition, the results of topological optimization in electrode design are presented. Topological optimization permits the production of wire electrodes that are more efficient in the given operating conditions. It is shown that, on the basis of additive technology and optimized electrode geometry with internal supply channels for dielectric liquid, the removal of sludge from the machining area may be improved, with consequent stabilization of the machining process. [ABSTRACT FROM AUTHOR]

Exploring the behavior of viscoelastic dielectric liquids on an inclined stretching sheet involves a comprehensive mathematical analysis. Employing a Runge-Kutta-based shooting strategy, this study delves into the system's non-linear Ordinary Differential Equations (ODEs). The research investigates how physical parameters like the Prandtl number, dielectric interaction parameter, viscoelastic parameter, Grashof number, and angle of inclination influence both velocity and temperature. Through graphical representations, the study sheds light on the impact of these factors and compares its findings with existing data. This intriguing combination of dielectric liquid behaviour under varying inclinations holds significant potential applications in Mines, Materials, and Fuels. [ABSTRACT FROM AUTHOR]

Flow of viscoelastic dielectric liquid due to horizontal stretching sheet is studied. The problem is investigated with all mathematical backgrounds and is solved numerically using Runge–Kutta-based shooting strategy to illuminate the system incorporating nonlinear ordinary differential equations describing the system's equations. The impact of physical parameters Prandtl number, dielectric interaction parameter, and viscoelastic parameter on velocity and temperature is discussed and illustrated through graphs. For different values of non-dimensional parameters, local Nusselt number and skin friction are tabulated. The current findings using previously published works in a restricted number of cases are validated. [ABSTRACT FROM AUTHOR]

Accelerated thermal aging was conducted on Kraft papers impregnated with mineral insulating oil (MO) and palm insulating oil (PO), and the effect of aging time on the oils and Kraft papers was observed. Each sample consisted of insulating oil, dried Kraft paper, and weighed metal catalysts (copper, iron, zinc, and aluminum) in a bottle. Prior to aging, the bottles were left for 24 h at room temperature for impregnation to take place. The thermal aging experiments were carried out at 130 °C for 250, 500, and 750 h. The properties of the MO and PO (moisture content, acidity, and ultraviolet-visible absorption spectra) and the properties of the Kraft papers (tensile strength and colour) were determined. Results showed that the aged PO had higher moisture content compared with the aged MO. However, the Kraft papers impregnated with PO had better tensile strength after 750 h of aging, which may be attributed to the affinity of PO to moisture. This slows down the hydrolytic degradation mechanism. In terms of colour, the Kraft papers were darker than their original colour as the tensile strength decreased. To conclude, the Kraft paper impregnated with PO had higher tensile strength compared with those impregnated with MO. [ABSTRACT FROM AUTHOR]

The formation of metal nanoparticles in a dielectric liquid and their deposition on a solid surface are investigated. It is possible in principle to apply such coatings to a porous surface [ABSTRACT FROM AUTHOR]

• Heat transfer by 2.4 MHz waves in HFE-7100 fluid was studied. • Surface temperature significantly decreased using ultrasonic waves. • Acoustic streaming alters bubble behavior significantly. • Acoustic streaming reduces the size of nucleation sites. • 2.4 MHz waves induce 83 % maximum heat transfer enhancement. This research investigates the heat transfer mechanisms of nucleated bubbles on a vertical heating surface using 2.4 MHz ultrasonic waves. The experiments, conducted in a closed rectangular chamber filled with HFE-7100, a hydrofluoroether fluid, varied the surface heat flux from 4.91 kW/m2 to 12.06 kW/m2. The results revealed a notable reduction in temperature on the vertical heating surface due to the influence of 2.4 MHz ultrasound, leading to an average surface temperature decrease of up to 5.3 °C across the entire heat flux range. Analysis of acoustic streaming, flow patterns, and thermally nucleated bubbles using particle image velocimetry (PIV) and a high-speed camera demonstrated the pronounced impact of ultrasonic waves on heat transfer. Our findings highlight the crucial role of 2.4 MHz ultrasonic waves in influencing bubble behavior and enhancing heat transfer on the vertical heating surface. This enhancement is achieved by disturbing the near-wall flow, particularly at lower heat fluxes, with a peak velocity ratio of 5.27 times. This disturbance increases average velocities, indicating potential improvements in heat transfer. Consequently, our study showed a maximum heat transfer enhancement of 83 %. At higher heat fluxes, the interaction with the waves becomes more complex, increasing velocities but limiting streaming coverage in the lower region of the heating surface. The combined effect of 2.4 MHz ultrasound and nucleate boiling convection not only enhances near-wall heat transfer but also amplifies fluid mixing within the chamber. The examination of bubble evolution with and without 2.4 MHz ultrasonic waves underscored the role of acoustic streaming in sweeping bubbles, reducing the size of nucleation sites, and reducing their crowd density, particularly at lower heat fluxes. [ABSTRACT FROM AUTHOR]

Particle image velocimetry (PIV) method has emerged as a powerful technique for electrohydrodynamic (EHD) flow field measurements. Based on a series of parallel field measurements along the blade electrode, we reconstruct the three-dimensional (3D) structure of EHD flows produced by an actuator in the blade-plane geometry. A non-uniform distribution of injection intensity and velocity is observed along the electrode. With the aid of slice plots from different views of 3D Cartesian, it is understood that the applied voltage, electrode spacing and the electrode surface or tip roughness exert a remarkable effect upon the structure of EHD jets. The findings of this investigation will complement the full view of the 3D structure of the jet and be of broad use to guide the selection of appropriate positions along the blade for profile plane studies. [ABSTRACT FROM AUTHOR]

Liquid dielectrics are different from each other, but are used to perform the same tasks in high-voltage electrical equipment, especially transformers. In similar conditions, the insulation performance of transformer oils under different types of voltage will provide dielectric resistance. In this study, three different dielectric liquids applied in transformers, namely mineral oil, natural ester and synthetic ester, were tested. Tests under AC and negative DC voltage were performed at electrode gaps of 2.5 mm, 2 mm and 1 mm using disk and VDE type electrodes as per ASTM D1816-84A and ASTM D877-87 standards, respectively. In turn, the impulse voltage tests were performed under an electrode configuration suggested by the IEC 60897 standard. The current data of 500 ms prior to breakdown under AC electrical field stress was decomposed using the empirical mode decomposition (EMD) and variational mode decomposition (VMD) methods. These analyses were conducted before the full electrical breakdown. Although synthetic ester has the highest dielectric strength under AC and negative DC electrical field stress, mineral oil has been assessed to be the most resistant liquid dielectric at lightning impulse voltages. In addition, stabilization of mineral oil under AC and negative DC voltage was also seen to be good with the help of calculated standard deviation values. However, synthetic ester has a significant advantage, especially in terms of dielectric performance, over mineral oil in spite of the stability of mineral oil. This indicates that liquid dielectric selection for transformers must be carried out as a combined evaluation of multiple parameters. [ABSTRACT FROM AUTHOR]

Microchannel cooling systems are widespread due to their efficiency. In this work, boiling of dielectric liquid FC-72 in a slit microchannel with a height of 66 µm and a width of 10 mm was studied. Heating was carried out by a thin-film ITO heater in direct contact with the working liquid. The flow regimes at boiling in a wide range of liquid flow rates were investigated. It is shown that boiling regimes in a slit microchannel differ significantly from the flow regimes in circular and rectangular mini- and microchannels. The dependences of heat flux on the temperature head have been plotted. The dependence of the heat transfer coefficient on the heat flux was investigated for different regimes of the two-phase flow. [ABSTRACT FROM AUTHOR]

In this paper, the slicing of a conductive SiC ingot is performed by an oil and a water type WEDM. The machining characteristics by the both type WEDM are investigated. The survey items are volumetric removal rate, average machined groove width, machined surface roughness, and machined surface properties. Regarding the results of the investigation with a discharge current of 1A, the volumetric removal rate of the water type WEDM is about 1.1 times faster than that of the oil type WEDM. Comparing the average machined groove widths, it of the water type WEDM is about 30% wider than that of the oil type WEDM. As a result, it was found that the surface roughness of the machined surface of water type WEDM was about 3 times as rough as the surface roughness of the machined surface of oil type WEDM. The machined surface properties of the both type WEDM are completely different. Specifically, the molten recast layer is hardly seen on the machined surface of the oil type WEDM, but it is clear that the machined surface of the water type WEDM is covered with the molten recast layer. This result suggests that the EDM phenomenon differs between the oil and the water type WEDM. From the above results, it was clarified that the oil type WEDM gives better results than the water type WEDM for slicing of a conductive SiC ingot. [ABSTRACT FROM AUTHOR]

Insulating Liquids are widely used for their electrical and thermal properties in power apparatuses, particularly at the level of liquid-filled transformers. With the shift in engineering aspects towards sustainable development, it is important to find a sustainable solution with ecofriendly nature. Therefore, alternative (biodegradable) liquids are of high importance in the global transformer communities. In the present study, the alternative dielectric fluids (ester-based) feasibility for potential regeneration with Fuller’s earth is investigated. The experimental results are confined to the reclamation temperature as well as the ratio of Fuller's earth (the sorbent) and the liquid. A suitable laboratory treatment apparatus is designed and is adopted in this study. Promising measurements to comment on the effectiveness of the treatment have been performed at controlled treatment temperature and sorbent-liquid ratio with the ASTM 7150-13 as a reference norm.The results of this study allowed 80°C and 1 g/30 ml as affirmative conditions for the present experimental conditions. Diagnostic measurements include turbidity, particle counter, and UV spectrophotometry before and after treatments. It is inferred that fuller’s earth is not a promising sorbent for the reclamation of ester liquids.

• Synthetic ester insulating oil is a dielectric liquid with balanced properties. • A strategy to improve the lightning breakdown characteristics based on the design of molecular chain length. • Revealing the correlation between microscopic electron structural parameters and macroscopic discharge behavior. The high flash point and biodegradability of synthetic ester insulating oil offer transformers both operational stability and environmental protection. However, its poor lightning breakdown characteristics under long oil gaps and inhomogeneous electric field often lead to rapid streamer discharges, posing challenges and limits to the use of synthetic ester insulating oil and hinders the development of low-carbon, eco-friendly high voltage power transformers. To address this issue, we propose improving the lightning breakdown characteristics through molecular chain length design. Using density functional theory, time-dependent density functional theory, and wave function analysis, we investigate how microscopic factors like molecular polarity, discharge active sites, frontier orbitals, traps, ionization and electron affinity, and excitation processes influence macroscopic discharge characteristics. Our findings show that under an electric field, chain length is positively correlated with electron transfer, enhancing charge polarization. Chain length also correlates positively with dipole moment, facilitating the formation of space charge centers. While chain length minimally affects discharge active sites, the electric field can intensify their formation. Additionally, a longer chain length decreases the energy gap, increasing electron trap levels more than hole trap levels, thus capturing charges longer. Chain length is negatively correlated with ionization energy, promoting electron impact ionization, and has little effect on the excitation energy of S(0) → S(1), making excitation under high electric fields unlikely. [ABSTRACT FROM AUTHOR]

• A commercial CNT buckypaper is utilized to enhance boiling of dielectric liquids. • CHF and maximum HTC were increased by 74.2 % and 102 % compared to the flat surface. • Surface characteristics, wickability and bubble dynamics were investigated. • Predicted CHF based on a modified model agrees with the experimental result. • Low-cost and mass-producible buckypapers is promising for immersion cooling. To further develop the two-phase immersion cooling technology with dielectric liquids and accelerate their applications in industrial practices, a comparative experimental study on saturated pool boiling heat transfer of FC-72 on the carbon nanotube (CNT) buckypaper and the smooth indium tin oxide (ITO) film is performed at atmospheric pressure in the present work. The commercially available buckypaper is selected as the nanostructured reinforced surface because of its low-cost, highly-scalable and mass-producible advantages. It achieves enhancements of CHF and maximum HTC up to 74.2 % and 102 %, compared to the smooth ITO surface. The underlying mechanism is well elucidated from the perspectives of surface characteristics, capillary wicking capability and bubble dynamics. The interconnected nanoporous network structure of CNT buckypaper can significantly improve bubble dynamics behavior and induce a marked nanoscale capillary wicking effect. The outstanding wickability can provide additional liquid replenishment on the boiling surface, facilitating the rapid generation and departure of bubbles as well as delaying film boiling. Interestingly, in terms of dielectric fluid (FC-72), it is found that the markedly increased bubble departure frequency on the CNT buckypaper surface primarily arises from a substantial reduction in the bubble waiting period. The present work not only provides an in-depth insight into the enhanced pool boiling characteristics of dielectric liquids on nanostructured surfaces, but also opens opportunities for the industrial applications of state-of-the-art two-phase immersion cooling technology based on nanostructures. [Display omitted] [ABSTRACT FROM AUTHOR]

The dielectric fluid was normally used to electrically shield high-voltage applications. However, recent studies on the electrohydrodynamic technique using the dielectric fluid were attentively conducted to obtain heat transfer enhancement in keeping with its electrical-insulating feature. This study numerically investigates the electro-thermo-convection in a two-dimensional square enclosure with hot and cold vertical walls. The heat transfer rate in the free convection inside the enclosure can be significantly improved by the injection of space charges and working of the Coulomb force. In this study, we tested several electrode arrangements, including alternately arranged electrode pairs on walls, under the intense charge injection condition. Consequently, the vertical arrangements of the electrode pairs demonstrate a higher heat transfer rate than the horizontal arrangements. The heat transfer coefficient of the vertical electrode pairs at a Rayleigh number of 5000 and an electric Rayleigh number of 200 is larger than three times that of the horizontal electrode pairs. Using fluid with a Prandtl number of 10, the results of the stream lines, isotherms, and space charge distributions are summarized herein for various thermal and electrical buoyant conditions.

The article is dedicated to the issue of water influence on the main insulation of high voltage equipment and its technical condition in general. The article reviews modern methods of definition of moisture content of insulating liquids used in laboratories of public utilities. Their possibilities and limitations are provided. The attention is drawn to problems of metrological provision, performance technology and methods standardization. Prospective methods of moisture definition are provided. The article appraises the importance of relevant analysis results recovery in order to timely execute technical maintenance of high voltage equipment and its failure prevention.

Investigation of the natural convection in a dielectric fluid confined inside a vertical slot subject to a constant horizontal temperature gradient and a high-frequency electric field was revisited. The thermoelectric coupling produces a dielectrophoretic force which contains a term called electric buoyancy associated with an electric gravity. Similar to the Archimedean buoyancy, this force can induce thermo-electro-convective instability in a stable fluid flow. Including the feedback effect of the electric field on the flow, a three-dimensional linear stability analysis is performed to characterize the instability induced by this force. Different critical modes are found depending on the intensity of the electric field: stationary hydrodynamic modes, oscillatory thermal modes and stationary electric modes. The threshold of the electric modes is independent of the Prandtl number in contrast with the threshold of the hydrodynamic and thermal modes. The obtained results are compared with those of previous studies. [ABSTRACT FROM AUTHOR]

In this study, an improved streamer discharge model for natural ester is established by considering the effect of temperature on carrier mobility during discharge, the distribution of the ionization energy of the liquid molecules, and the fluctuation source of the carrier density related to the electric field gradient. Based on Poisson's equation and the continuity equation, the simulation models are improved by adding a fluctuation source term to the continuity equation. The source term consists of carrier density related to the electric field gradient during discharge. The effect of the temperature on the carrier mobility is studied experimentally and added to the simulation parameter settings. The results show that the improved model demonstrates similar branch patterns to those observed experimentally during the initiation of the streamer. The degree of fluctuation of carrier density related to the electric field quantifies the branching, pattern, and propagation characteristics of the streamer. [ABSTRACT FROM AUTHOR]

The paper deals with the study of effect of gravity modulation on double-diffusive convection in a dielectric liquid for the cases of rigid-rigid and free-free boundaries. Using a modified Venezian approach, expressions for the Rayleigh number and its correction are determined. Fourier–Galerkin expansion is employed for a weakly nonlinear stability analysis and this results in a fifth-order Lorenz system that retains the structure of the classical one in the limiting case. A local nonlinear stability analysis using the method of multiscales leads to the time-periodic Ginzburg–Landau equation from the time-periodic generalized Lorenz system and the numerical solution of this simpler equation helps in quantifying unsteady heat and mass transports. Influence of various non-dimensional parameters (Lewis number, solutal Rayleigh number, electrical Rayleigh number and Prandtl number), amplitude and frequency of gravity modulation on onset of convection and heat and mass transports is discussed. The study reveals that the influence of gravity modulation is to stabilize the system and enhance heat and mass transports. The results from free-free boundaries are qualitatively similar to that of rigid-rigid boundaries. Further, it is shown that in the case of free-free boundaries the heat and mass transports are less compared to those of rigid-rigid boundaries. [ABSTRACT FROM AUTHOR]

HFE-7000 and HFE-7100 have shown a promising importance in the electrohydrodynamic (EHD) domain. Their dielectric properties are investigated in a temperature range between −20 and 60 °C for relative permittivity and electric conductivity and between −20 and 80 °C for breakdown voltage. The aim is to electrically characterize these liquids and to develop experimental models to be used in numerical simulations. Mathematical models of electric properties are essential for understanding the physical phenomena that can affect the performance of EHD systems. The development of these models can then help design more reliable EHD devices and optimize their performance. [ABSTRACT FROM AUTHOR]

The linear Rayleigh-Bénard electro-convective stability of the Newtonian dielectric liquid is determined theoretically subject to the temperature modulation with time. A perturbation method is used to compute the critical Rayleigh number and the wave number. The critical Rayleigh number is calculated as a function of the frequency of modulation, the temperature-dependent variable viscosity, the electric field dependent variable viscosity, the Prandtl number, and the electric Rayleigh number. The effects of all three cases of modulations are established to delay or advance the onset of the convection process. In addition, how the effect of variable viscosity controls the onset of convection is studied. [ABSTRACT FROM AUTHOR]

• An experimental study of NOVEC-649 pool boiling on microporous surfaces. • 600% enhancement in heat transfer coefficient. • 55% enhancement in critical heat flux. • A heat transfer correlation was fitted. • A mechanistic model for critical heat flux was developed based on force analysis. Pool boiling performance of NOVEC-649 was experimentally studied on microporous surfaces prepared by an electrochemical deposition method. Microporous structures contribute to large surface roughness values and provide large quantities of cavities ranging from several hundreds of nanometers to several microns for bubble nucleation. The results show that a maximum enhancement of 600% in heat transfer coefficient and a maximum enhancement of 55% in critical heat flux are achieved on the deposited surfaces, compared with a smooth copper surface. Experimental heat transfer coefficients were compared with literature correlations, considering the effects of roughness and surface-liquid combination. Then a fitted Rohsenow correlation was discussed and developed to predict the present results. Experimental critical heat fluxes were compared with classical models. It was found that the critical heat flux on the smooth surface could be predicted by the lift-off model and the Kandlikar model, but these models cannot predict the critical heat fluxes on the deposited surfaces well. Following, the Kandlikar model was modified by further considering a wicking force and a roughness-factor-dependent surface tension force. The present modified CHF model was validated by comparing with present experimental data and the literature, with a deviation around ±30%. [ABSTRACT FROM AUTHOR]

This paper deals with flow electrification phenomenon for turbulent flows of liquids through pipes of a circular cross section. Two models are proposed. The first one is based on the comparison of the radial fluctuant velocity magnitude with the magnitude of the diffuse layer relaxation velocity: it assumes that in the region where the fluctuant velocity is greater than the relaxation velocity the diffuse layer is totally perturbed so that the charge is homogenized by the turbulence, whereas when the relaxation velocity is greater than the fluctuant velocity the diffuse layer is considered not perturbed at all. The charge transported by the flow is then calculated with this new charge profile. The second model consists in solving the system of the diffuse layer equations perturbed by the turbulence, taking into account the eddy diffusivity; then with the profile of the charge obtained in the cross section the charge transported is calculated. Finally, the predictions of the charge transported using each model are compared in a specific case and discussed. [ABSTRACT FROM AUTHOR]

The problem of electroconvection in an annular space between two elliptical concentric cylinders filled with a dielectric liquid and subjected to internal unipolar injection is considered. A finite volume method is used to solve the governing equations, including the Navier–Stokes ones and a simplified set of the Maxwell equations. For the first time the whole set of these coupled equations is solved, using the elliptical-cylindrical coordinates. We first validate numerical simulation in this field by comparing the results obtained with those available in the literature. The numerical solution of the electroconvection problem is then followed by a detailed analysis of the flow structure and electric charge distribution. It is shown that the multicellular convective pattern is observed. It is noticed that unipolar charge injection from the internal electrode significantly changes the topology of the fluid flow. Finally, the influence of various system parameters, such as the injection level and electric Rayleigh number, is also investigated. [ABSTRACT FROM AUTHOR]

In this study, results of polarisation and depolarisation currents measured on an insulating liquid are presented. The goal is to contribute to a better understanding of mechanisms occurring at the metal–liquid contact. Electrodes of different metallic nature are associated with the same mineral oil, the metal/liquid/metal structure is submitted to DC voltages and the transient current is recorded with a high sensitive electrometer. The current versus time feature shows that the conduction process is in good compliance with the metal work function rather than with its electric conductivity. The current versus field characteristics indicate a predominance of the ohmic behaviour for the low fields. The space charge limited conduction regime appears as soon as the electrical constraint exceeds ∼1 MV/m, which corresponds to 10% of the breakdown voltage. Schottky injection is suspected to occur from the metal into the liquid since the applied fields are uniform and non‐extremely intensified. [ABSTRACT FROM AUTHOR]

The current paper evaluates the thermal performance of immersion cooling for an Electric Vehicle (EV) battery module comprised of NCA-chemistry based cylindrical 21700 format Lithium-ion cells. Efficacy of immersion cooling in improving maximum cell temperature, cell’s temperature gradient, cell-to-cell temperature differential, and pressure drop in the module are investigated by direct comparison with a cold-plate-cooled battery module. Parametric analyses are performed at different module discharge C-rates and coolant flow rates to understand the sensitivity of each cooling strategy to important system performance parameters. The entire numerical analysis is performed using a validated 3D time-accurate Computational Fluid Dynamics (CFD) methodology in STAR-CCM+. Results demonstrate that immersion cooling due its higher thermal conductance leads to a lower maximum cell temperature and lower temperature gradients within the cells at high discharge rates. However, a higher rate of heat rejection and poor thermal properties of the dielectric liquid results in a much higher temperature non-uniformity across the module. At lower discharge rates, the two cooling methods show similar thermal performance. Additionally, owing to the lower viscosity and density of the considered dielectric liquid, an immersion-cooled battery module performs significantly better than the cold-plate-cooled module in terms of both coolant pressure drop.

Abstract A computational study has been performed to see the heat transfer intensification induced by electrically generated convection in a two co-focal elliptical cylinder layers for dielectric liquid. Temperature of the inner cylinder is higher than of outer one. Finite volume method is used to solve the governing equations by writing a homemade computer code. The study is performed for dimensionless electric Rayleigh number (0 ≤ T ≤ 700), Rayleigh numbers (102≤ Ra ≤105), eccentricity ratio (0.4 ≤ e ≤ 1). It is observed that the unipolar charge injection by the internal electrode changes significantly the topology of the fluid flow. Moreover, the computations highlight an important effect of the eccentricity value on the intensity and circulation cell number. Finally, it is found that heat transfer is improved with increasing dimensionless electric Rayleigh number. [ABSTRACT FROM AUTHOR]

This paper discusses the effect of doping of electro-insulating liquids with nanoparticle materials on the thermal properties of the obtained nanoliquids and heat transport in the transformer. Mineral oil, synthetic ester, and natural ester were used as base liquids. The effectiveness of doping base liquids with nanoparticles was supported by ultraviolet-visible (UV/VIS) measurements. In turn, Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) confirmed the absence of intermolecular interactions (i.e., hydrogen bonding). The influence of modification of electro-insulating liquids with fullerene C60 and titanium dioxide TiO2 nanoparticles on such thermal properties as thermal conductivity, specific heat, kinematic viscosity, density, and thermal expansion was investigated. Based on these properties and the theory of similarity, the cooling efficiency of the transformer filled with the analyzed nanofluids was determined. Nanofluids' cooling effectiveness was compared with the cooling effectiveness of the base liquids. This comparison was supported by an analysis of Grashof, Prandtl, and Nusselt numbers. It has been shown that the modification of electro-insulating liquids with nanoparticles widely used in order to improve their dielectric properties, such as C60 and TiO2, does not have a significant influence on their thermal properties. The addition of fullerene C60 caused an increase in kinematic viscosity, which was compensated by the increase in specific heat. In the case of TiO2, the addition of this nanoparticle resulted in an increase in kinematic viscosity and a decrease in specific heat, which were balanced out by the increase in thermal conductivity. In summary, the heat exchange-capacity of liquids did not change due to doping with nanoparticles. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we carry out a numerical study of the Coulomb-driven electro-convection flow in a dielectric liquid using the finite volume method (FVM). The system under study consists of an eccentric annulus layer subjected to strong unipolar injection from an inner cylinder. All equations are written in bi-cylindrical coordinate system and are expressed using the vorticity–stream function method. The behavior of the fluid for different system parameters is well studied. The taken parameters are in the range of eccentricity 0 ≤ e ≤ 80%, injection strength in the range of 0.5 ≤ C ≤ 10 and electric Rayleigh number in the range of 30 ≤ T ≤ 1600. The distributions of the electric charge density and stream function are carefully examined. The computations highlight a significant effect of the eccentricity value on the flow topography. Indeed, increasing the eccentricity value leads to a significant reduction in electro-plumes as well as circulation cells number. Furthermore, an intensification in cells rotation, of about 116%, was recorded when eccentricity reaches 80%. Depending on torque values e-T, we observe the existence of multicellular regime made of counter-rotating vortexes. [ABSTRACT FROM AUTHOR]

Propylene carbonate (PC) has a good application prospect in compact pulsed power source as the energy storage medium of pulse forming line. However, its insulation strength is susceptible to impurities, temperature, pressure, and other factors. Hence, it is of great significance to study the effect of temperature on breakdown characteristics of propylene carbonate. In this paper, the effect of temperature on breakdown characteristics of PC was carried out under microsecond pulse. First, pulsed breakdown setup was built. The temperature of PC varied from −50 °C to 50 °C. Breakdown voltages were captured with a capacitive voltage divider and streamer patterns are obtained by ultrahigh-speed cameras. Second, the experimental results are obtained. The breakdown strength of PC increases with increasing temperature. It is 239 kV/cm at −50 °C and rises to 407 kV/cm at 50 °C. At low-temperature breakdown, electron current is more concentrated and breakdown voltage is more stable. Moreover, the energy release for streamer at low temperature is more concentrated, but it is more dispersed for streamer at high temperature. At last, considering the CO2 solubility in PC, PC generates more bubbles and the increase of temperature benefits the release of bubbles at low temperature, resulting in lower breakdown strength. Because of the moisture in PC, breakdown strength of PC falls slightly near 0 °C. Thus, the experimental results are explained by use of stream theory and bubble breakdown theory. PC can adapt to wide-range temperatures, effectively expanding practical applications of the PC in pulsed power source. [ABSTRACT FROM AUTHOR]

In this paper the behaviour of the waves generated on insulating liquid-air interfaces and subjected to an electric field is experimentally studied. The experimental device is mainly composed of a tank filled with diesel oil and two parallel metallic electrodes; one of them immersed in the liquid and brought to a high potential and the other one placed above the liquid. The waves are created by a vibrator the frequency and amplitude of which are controlled. The electric charges present at the liquid/air interface come from injection and/or dissociation and/or conduction phenomena. A high speed camera records the shape of the waves and an associated image analysis system extracts from them all the information needed (frequency, amplitude, wavelength). This information is used to investigate the influence of the electric field and liquid conductivity on the wavelengths at different amplitudes and frequencies and to calculate the surface tension. The propagation of waves on electrically charged liquid/air interfaces is also studied. The measurements are carried out for different frequencies and amplitudes of the vibrator at various applied voltages with different electrical conductivities of the diesel oil. We found that the wavelength and the apparent surface tension decreased with the applied voltage. The amplitude of the vibrator practically has no influence on the wavelength and the surface tension. [ABSTRACT FROM AUTHOR]

This paper is an experimental study on electroconvective flows produced by an asymmetric electric field in a cavity. It is part of a scientific project aiming to develop electrohydrodynamic (EHD) flow actuator in order to enhance heat transfer and mixing efficiency in fluidic and microfluidic systems. In this study, the EHD flow is obtained by applying a dc voltage between two electrodes (cylinder/plane) immersed in a working liquid. The produced velocity fields strongly depend on the applied voltage. Whatever may be the applied voltage, the fluid goes from the cylinder toward the plate or the other way round. The velocity fields obtained by particle image velocimetry for different potentials are analyzed in both cases with the usual mechanisms (injection and conduction) at the origin of the motion of the liquid. A discussion on the transition between injection and conduction phenomena is finally presented. [ABSTRACT FROM AUTHOR]

Computational experiment was carried out using a neural network model that makes it possible to extract new knowledge from experimental data. Certain features and regularities of the heat exchange of chlorobenzene in an electric field have been identified. [ABSTRACT FROM AUTHOR]

This study presents the results of optical spectrometry based measurements performed in three commercial dielectric liquids: synthetic ester, natural ester, and mineral oil. The optical spectra collected concern the breakdown initiated by standard lightning impulse voltage of positive polarity in a point‐plane electrode system. The obtained results showed the differences between the spectra. The relationships between emission from atomic hydrogen (656.3 nm) and continuum concerning the wavelengths between 380 and 600 nm attributed to dissociation and fragmentation of molecules and recombination of molecular fragments were assessed as different for the distinctive liquids tested. The emission from atomic hydrogen was dominant in the case of mineral oil, however, for natural ester and first of all for synthetic ester, the light from the mentioned continuum had the highest intensity. Thus, this may be supposed that the differences in the spectra resulting from the different proportions of the physical processes responsible for the breakdown in a given liquid. It seems that in ester liquids the way for direct excitation and ionisation of liquid molecules is easier, so easier rotational and vibrational types of excitation of the high number density main molecules may take place giving a reflection in the optical spectra registered. [ABSTRACT FROM AUTHOR]

In this paper, the interaction between a Couette shear flow and an electroconvective motion induced by an unipolar injection between two-coaxial cylinders is numerically investigated. A flow is generated by two counter-rotating coaxial cylinders inducing a shear flow. Space charges are injected in the flow through a metallic electrode placed on the inner cylinder and brought to a given potential. Transient numerical simulations have been carried out to investigate the structure of the induced flow. The entire set of the coupled Navier-Stokes and EHD equations is solved using an efficient finite volume technique. The behaviour of the flow subjected to an applied voltage between the two electrodes is analyzed and time evolution of the charge density distributions is presented. The interaction between the convective motion induced by space charge injection and the mainstream flow, emphasizes the appearance of periodic counter-rotating electroconvective cells. The electroconvective cells are convected in the annular space by the azimuthal fluid velocity. From the stability point of view the bifurcation diagram is very similar to the one obtained in the case when Re  = 0. We observe a threshold value T c of the instability parameters T above which the electroconvective instability initiates. A non-linear criterion T f under which the electro-convective motion is suppressed is also found. When increasing the Reynolds number the flow induced by the two rotating cylinders has a sweeping effect on the charge density distribution. Consequently the instability parameter T must be drastically increased to allow the electroconvective instability to develop. For Re  = 10 a subcritical instability characterized by an hysteresis loop and therefore a linear and non-linear criteria, T c and T f respectively, are determined. While for Re  = 0, T c  = 122.42 and T f  = 86.5, for Re  = 10 we numerically found that T c  = 802 and T f  = 722. The magnitude of the linear and non-linear criteria are directly linked to the value of the Reynolds number. [ABSTRACT FROM AUTHOR]

The study aims to assess the role of the field-enhanced dissociation (or the Wien effect) in the electrohydrodynamic flow formation by comparing experimental data and the computer simulation results. The blade-plane electrode configuration was chosen for the investigation. The experiment used the particle image velocimetry and the dynamic current-voltage characteristic techniques. The conductivity of the working liquid—transformer oil—was varied by introducing a special admixture. The Wien effect yields velocity distributions and integral electric current characteristics that agree well with their experimental values and is therefore shown to be the predominant charge formation mechanism at higher values of conductivity. [ABSTRACT FROM AUTHOR]

This paper is devoted to the study of the generation and propagation modelling multichannel streamer in a dielectric liquid subjected to an electric field diverge. With Matlab-Simulink our model is proposed in this work in order to study the initiation and propagation of streamer in insulating liquids .The model suggested (geometry or system point-plan) allow us to model the generation and the propagation of the streamers which are near the infected zone. By 2D simulation, we achieved typical tree structures of discharge made up of streamers and which are in fact small cylindrical channels of 10 μm of ray in the dielectric liquids in geometry point-plan (distance inter electrode 20 mm).The obtained filamentary structures allow us to identify a number of parameters such as stopping length, injected currents and charges, electric field, propagation velocity and mobility of charge that can be used to characterize streamer phenomenon. [ABSTRACT FROM AUTHOR]