Non-uniform Field Conditions Research Articles

Multiphysics simulation of two-phase viscous fluid flow steered by electric field for jetting of microdroplets

In this research, we delve into the intricacies of viscous fluid flow with electric field coupling by employing the Finite Element Method (FEM) in tandem with the level set method. We generate a weak form for satisfying governing equations for electric field and fluid velocity while two phases are tracked by the level set function. The primary focus of this study is the complex interactions between free-falling jet and electric field, and the behavior of droplet encompassing deformation, fission, and fusion under the influence of electric field. The main contribution of this paper is given a new implement by using the P1/P1 scheme to directly solve the weak forms of coupled governing equations, which significantly improves calculation efficiency compared to the P2/P1 scheme, and we open source the code. This implement is verified by comparing with the experimental results of oil droplets deforming under an electric field. Computations are performed by FEniCS open-source packages. The phenomena documented underscore the multifaceted relationship between electrodynamic forces and fluid mechanics, accentuated distinctly under non-uniform electric field conditions.

Non-uniform flow characteristics and rotating instability of a transonic high-pressure compressor rotor with cavity bleed

The presence of bleed in an aero engine’s compressor can significantly impact its flow characteristics and contribute to rotating instability. This study focuses on the impact of a typical cavity bleed structure on the internal flow characteristics of a compressor, specifically its circumferential non-uniformity and aerodynamic stability. A numerical simulation involving multiple flow passages was conducted on the transonic high-pressure rotor of the E3 compressor, considering its typical bleed structure. The study delves deep into the non-uniform flow characteristics and the mechanisms behind their generation in the compressor flow field. Furthermore, the influence of bleed on the rotating instability of the compressor is explored by comparing changes in compressor instability and adiabatic efficiency under uniform and non-uniform flow field conditions. The findings indicate that the axial position of the cavity bleed structure plays a crucial role in influencing key parameters such as rotor stall margin, peak efficiency, and total pressure ratio under near-stall conditions. The circumferential non-uniformity, resulting from the presence of the cavity bleed, intensifies with higher bleed air flow rates. For the upstream bleed configuration applied to the rotor, with a total bleed rate of 5%, the maximum variation in absolute flow angle at the inlet of different rotor channels can reach up to 1°. Additionally, the maximum difference in inlet flow coefficient can reach 0.0392. These findings demonstrate that the non-uniformity caused by the typical bleed structure leads to a loss in stall margin for the rotor when compared to a uniform flow field scheme.

Monte Carlo simulation of ion kinetics in nitrogen and oxygen plasmas under non-uniform electric field conditions

The kinetics of N4+ and O− ions was numerically studied in nitrogen and oxygen plasmas in a highly non-uniform electric field. Mean ion energy and reaction rate coefficients in a background gas at pressures from 1 to 10 Torr were calculated through a Monte Carlo simulation. The ion characteristics followed the local reduced electric field at high pressures, whereas nonhydrodynamic effects leading to a nonlocal dependence of the mean ion energy and rate coefficients on the field were obtained at low pressures. As a result, the rates of N4+ ion dissociation, electron detachment from O− ions, and charge exchange in collisions between O− and O2 lagged the local field value. The non-local effect on the ion rate coefficients was more profound when the field decreased in space. We suggested a simplified method of describing ion rates in spatially varying electric fields on the basis of the Monte Carlo simulation of these rates for uniform electric field conditions and mean ion energy calculations in non-uniform fields. This method is similar to the local-mean-energy approximation utilized for describing electron swarm parameters in varying electric fields. The results of the simplified method were compared with the results of the direct Monte Carlo simulation.

Effect of Non-Uniformity of Electric Field on Breakdown Strength of Cryogenic Gaseous Insulation Media for Superconducting Power Applications

A new experimental set up for measuring the breakdown strength of gas media in weakly non-uniform electric field at cryogenic temperatures and high pressures is described. Measure of breakdown strength of helium gas at 77 K and 293 K in a non-uniform electric field with a field efficiency factor 62.5% are presented. The results suggest that the breakdown strength in weakly non-uniform electric field relate to that of uniform electric field by the field efficiency factor, η. This relationship holds good for both 293 K and 77 K data. This observation expands our previously reported systematics of dielectric strength in uniform electric field to the weakly non-uniform electric field conditions, which is important for designing HTS power applications. The established relationship eliminates the need for costly experiments for measuring the dielectric strength at a specific operating temperature and pressure and in the required non-uniformity of the electric field.

Study of treeing in epoxy-alumina nanocomposites using electroluminescence

Dielectric materials in electrical equipment are mostly subjected to non-uniform or divergent field conditions. Localized high electric field may give rise to space charge which may initiate material degradation, and eventually lead to dielectric breakdown. Thus, it is important to assess the performance of nanocomposites under divergent fields before deployment as insulation. Past research has shown a positive correlation between space charge and electroluminescence (EL). In this work, EL is used as a diagnostic tool to study space charge and field-related material degradation in epoxy-alumina nanocomposites. Various experiments are undertaken to detect early stages of material degradation through EL. These include endurance tests (below and above EL emission threshold), and characterization of EL from samples pre-stressed with DC and/or AC voltages. The effect of repeated square waves on material degradation is also analyzed. Visual inspection for physical damage is carried out using a high resolution camera. The EL signature is observed to deviate significantly from its initial value at the onset of material degradation, and may therefore serve as an aging marker. In addition, the EL studies show that nanocomposites exhibit a relatively high resistance to field-assisted material degradation. The experiments on pre-stressed samples indicate that charge kinetics play a crucial role in defect initiation and material degradation.

Research on the synergy of SF6/N2 mixture gas in low temperature environment

For high latitudes, due to the deficiencies of liquefaction of SF6 and extreme greenhouse effect, gas-insulated metal-enclosed switchgear (GIS), gas-insulated metal-enclosed transmission lines (GIL) and other electrical equipment have begun to use mixed gas of SF6 and N2 as the insulation medium. For this purpose, slightly non-uniform electric field distribution is generated using hemisphere-plane electrodes. Under the excitation of a negative DC high voltage, the breakdown voltage of the SF6/N2 mixture gas is measured at a temperature of 0 to -15°C and a gas pressure of 15 to 60 kPa. The voltage is used to calculate the synergistic coefficient of the mixed gas, and the effect of the temperature and pressure on the synergy of the mixed gas is analyzed. The results show that under slightly non-uniform electric field conditions, the synergy of the mixed gas decreases with decreasing temperature, and increases with increasing gas pressure. It follows that good insulating property of SF6/N2 gas mixture used in a low temperature environment can be achieved by selecting a reasonable temperature and gas pressure.

An Approach for the Dynamic Measurement of Ring Gear Strains of Planetary Gearboxes Using Fiber Bragg Gratings.

The strain of the ring gear can reflect the dynamic characteristics of planetary gearboxes directly, which makes it an ideal signal to monitor the health condition of the gearbox. To overcome the disadvantages of traditional methods, a new approach for the dynamic measurement of ring gear strains using fiber Bragg gratings (FBGs) is proposed in this paper. Firstly, the installation of FBGs is determined according to the analysis for the strain distribution of the ring gear. Secondly, the parameters of the FBG are determined in consideration of the accuracy and sensitivity of the measurement as well as the size of the ring gear. The strain measured by the FBG is then simulated under non-uniform strain field conditions. Thirdly, a dynamic measurement system is built and tested. Finally, the strains of the ring gear are measured in a planetary gearbox under normal and faulty conditions. The experimental results showed good agreement with the theoretical results in values, trends, and the fault features can be seen from the time domain of the measured strain signal, which proves that the proposed method is feasible for the measurement of the ring gear strains of planetary gearboxes.

Enhancement of Size Gradient of Imprinted Nanopattern by Plasma Etching under a Nonuniform Magnetic Field

We report a simple way to enhance the size gradient of an imprinted nanopattern through oxygen plasma etching under a nonuniform magnetic field. A sample substrate was placed next to a magnet, and then a nonuniform magnetic field condition was formed around the sample. Using oxygen plasma etching, a line pattern having an initial width of 273 nm was gradually modified from 248 nm at one end to 182 nm at the other end. Controlling the arrangement of the magnet and sample, we could induce a triangular shape size gradient. We verified that the gradually modified nanopatterns we produced are applicable to continual optical property control, showing a possibility to be utilized for optical components such as gratings and polarizers.

Sizing and control of trailing edge flaps on a smart rotor for maximum power generation in low fatigue wind regimes

In this paper an extension of the spectrum of applicability of rotors with active aerodynamic devices is presented. Besides the classical purpose of load alleviation, a secondary objective is established: power capture optimization. As a first step, wind speed regions that contribute little to fatigue damage have been identified. For these regions the turbine energy output can be increased by deflecting the trailing edge (TE) flap as a function of local, instantaneous speed ratios. For this purpose, sizing of TE flap configuration for maximum power generation is established using blade element momentum theory. The investigation then focuses on operation in non-uniform wind field conditions. Firstly, the deterministic fluctuation in local tip speed ratio due to wind shear was evaluated. The second effect is associated with delays in adapting the rotor speed to time varying inflow. The increase of power generation due to wind shear has been demonstrated with an increase of energy yield of 1%. Finally a control logic based on inflow wind speeds has been devised.

Impulse partial discharge and breakdown characteristics of rod–plane gaps in air and N2 gases

AbstractIn response to growing environmental concerns, we attempted to develop switchgear without using SF6 gas. In our research, we used compressed air and pure N2 as an electrical insulation gas, because of their low global warming potential. In this paper, we examined the impulse breakdown and impulse partial discharge characteristics under various conditions related to nonuniformity of the electric field. The experimental results show that the breakdown voltage (BDV) of air is higher than that of pure N2 gas under highly nonuniform field conditions in the rod–plane gap. On the other hand, the discharge inception voltage of air and N2 were almost the same. Furthermore, first partial discharge (PD), leader discharge, and its transition to the breakdown were successfully observed through the measurement of discharge current and light emissions under impulse voltage application. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 148(3): 36–43, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10277

Ion-migration Polarity Change and 3D Shape Evaluation in the WDT Method

In this paper, protection resistance was measured as a parameter for the 3D shape of the dendrite, which was produced on the copper, printed wired board by the WDT method. The measurement was occurred using the 3D shape measurement system. We measured the 3D shape of a dendrite in a nonuniform electric field with a changing polarity. Moreover, the polar effect of ion-migration was examined by the 3D shape electric field analysis using ANSYS. Consequently, the height of the accumulation things at anode was higher than the negative pole side under round shaped cathode condition. Similarly, the quantity of the accumulation thing is also found larger at anode. On the other hand, the dispersal of the accumulation thing is observed between the anode and the negative pole under a round shaped anode conditions. And it turns out that there is also smaller accumulation thing at the tip part of a round shaped electrode. Moreover, for a same protection resistance, the amounts of generating of the accumulation objects per unit electric charge were larger for round shaped anode conditions comparing to the cathode. It is seen from 3d shape electric field analysis, that horizontal part of the electric field vector at anode is greater than the cathode. At the same time, the field intensity at anode is also larger than cathode. For a round shaped anode in 3D shape electric field analysis, the strongest electric field vector was found between a round shape electrode tip part and flat electrode. Although in the round shape cathode, the electric field vector had become the strongest between the flat electrode and the round shape electrode tip part. The electric field intensity of a nearby flat electrode was strong for a round shape electrode facing the field. Since the action states of the accumulation thing after a short circuit is different depending on the polarity of electrode for a nonuniform electric field conditions, it appears that it will not take much time to occur a failure

Partial discharge and breakdown mechanisms in ultra-dilute SF6 and PFC gases mixed with N2 gas

Because of the high global warming potential of SF6 gas, research on alternative gases for electrical insulation with a lower environmental impact is essential. Gas mixtures composed of electronegative gases and N2 gas have the advantage of the reduction of the amount of SF6 gas and of utilizing the synergistic effect in electrical insulation performance. We investigated the partial discharge (PD) and breakdown (BD) characteristics of SF6/N2 and PFC (C3F8/N2 and C2F6/N2) gas mixtures under non-uniform electric field conditions, by changing the dilute content of electronegative gases. As a result, the synergistic effect in SF6/N2 gas mixtures was verified to be higher than that in PFC/N2 gas mixtures. The physical mechanism from PD inception to BD was discussed with consideration of the difference in electronegativity of SF6 and PFC gases. Furthermore, we found that PD inception and PD-to-BD mechanisms changed at a content of 10 ppm for SF6 due to the electron attachment activity of SF6 gas. The change in the PD and BD mechanisms in C3F8/N2 and C2F6/N2 gas mixtures appeared at 0.1% content for C3F8 and at 1% content for C2F6.

Explicit methods for the solution of the generalized Cauchy–Riemann equations and simulation of inviscid rotational flows

Different levels of approximations for inviscid rotational flows are discussed. The generalized Cauchy–Riemann equations, consisting of the continuity equation and the definition of the vorticity are considered. Artificial time dependent terms are added to integrate the system to steady state. The system is also augmented with artificial viscosity terms for numerical stability and to avoid odd/even decoupling of central differences of first-order derivatives. Additional artificial viscosity is needed to capture shock waves. Explicit integration of the augmented system by two methods is studied. Numerical results for inviscid flow with non-uniform far field conditions are presented. Both methods are suitable for computation on parallel machines.

Effects of chain length on prebreakdown phenomena in straight-chain saturated hydrocarbon liquids

Prebreakdown light emission, currents, and streamers in straight-chain saturated hydrocarbon liquids, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane, are investigated under nonuniform field conditions. The light inception voltage and the breakdown voltage increase with the chain length and the liquid's density. For a negative point, the breakdown voltage is much higher than the light inception voltage in all liquids but the difference decreases with the chain length. This voltage difference is not observed for a positive point except in n-pentane and n-hexane. The difference between the negative and positive breakdown voltages decreases with the chain length. Regarding the polarity effect, the negative breakdown voltage is much higher than the positive one in n-pentane, but there is no difference in n-decane. A negative density change streamer has a bushlike structure and propagates in stepped manner at an average speed between 70 and 160 m/s, while for a positive one, the structure is treelike having an average speed of a few kilometers per second. Only in n-pentane and in n-hexane, a slow positive bushlike streamer is observed whose average velocity is about 200 m/s, slightly faster than the negative bushlike one. © 1998 Scripta Technica. Electr Eng Jpn, 122(4): 8–16, 1998

Effects of chain length on prebreakdown phenomena in straight‐chain saturated hydrocarbon liquids

Prebreakdown light emission, currents, and streamers in straight-chain saturated hydrocarbon liquids, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane, are investigated under nonuniform field conditions. The light inception voltage and the breakdown voltage increase with the chain length and the liquid's density. For a negative point, the breakdown voltage is much higher than the light inception voltage in all liquids but the difference decreases with the chain length. This voltage difference is not observed for a positive point except in n-pentane and n-hexane. The difference between the negative and positive breakdown voltages decreases with the chain length. Regarding the polarity effect, the negative breakdown voltage is much higher than the positive one in n-pentane, but there is no difference in n-decane. A negative density change streamer has a bushlike structure and propagates in stepped manner at an average speed between 70 and 160 m/s, while for a positive one, the structure is treelike having an average speed of a few kilometers per second. Only in n-pentane and in n-hexane, a slow positive bushlike streamer is observed whose average velocity is about 200 m/s, slightly faster than the negative bushlike one. © 1998 Scripta Technica. Electr Eng Jpn, 122(4): 8–16, 1998

Discharge characteristics under non-uniform electric field in He, Ar and air at low pressures

We measured dc discharge inception voltage V/sub i/ for various electrode configurations in He, Ar and air in vacuum range from 10/sup 5/ to 10/sup -1/ Pa to determine discharge characteristics under a space vacuum environment. By quantitative consideration of the E/sub i//p distribution in the gap space at the discharge inception, the discharge inception mechanism under non-uniform electric field was investigated in vacuum. At the same time, we observed the discharge profile variation with residual gas pressure and quantitatively analyzed its characteristics, using an image processing technique. Based on the analysis, we found that the dependency of the discharge inception voltage and the length of the discharge path on the residual gas pressure under non-uniform field conditions agreed well with that previously obtained for uniform field conditions.

Electron rate coefficients in gases under non-uniform field and electron density conditions

The theory of perturbation is used to describe electron rate coefficients in gases under non-uniform field and electron-density conditions when the deviation from spatial uniformity is small. It is shown that the space-dependent rate coefficients can be represented as a sum of the local values and the product of new rate coefficients dependent on the local field and the gradients of the field and of the electron density. By solving the Boltzmann equation in a two-term approximation, the new rate coefficients are calculated for , , and Ar. The results of our calculations are compared with experimental data on the relative excitation rates in a steady-state Townsend discharge in a mixture and with results of previous Monte Carlo simulations of the electron swarm behaviour in non-uniform fields.

Breakdown behavior of SF/sub 6/ gas-insulated systems at low temperature

The controversy surrounding low-temperature SF/sub 6/ breakdown is addressed in detail. Earlier relevant studies are reviewed, some of the existing data is analyzed in a new light, and further theoretical considerations are presented. These discussions served to outline an experimental approach aimed at confirming or invalidating breakdown invariance at subnormal temperatures. Low-temperature dc breakdown of an SF/sub 6/ gas-insulated system was investigated experimentally for temperatures ranging from -50 to 24/spl deg/C, using associated pressure values that had been selected carefully to avoid phase transition of the gas-insulating medium. The context allowed experimentation under both uniform and nonuniform field conditions; the nonuniformity was due to the active role of the cathode-gas interface at high fields. The experiment was conducted for molecular densities ranging from 2.596 to 16.43/spl times/10/sup 19/ cm/sup -3/ (equivalent to pressures of 105 and 624 kPa, respectively, at 24/spl deg/C) and for gap lengths starting at 0.5 mm and extending to 7 mm. Data sets show consistency, low statistical scattering, and high reproducibility. Data analysis led to several major conclusions. At constant density, the breakdown of the SF/sub 6/-insulated system is temperature dependent, which is responsible for a decrease in the electric strength, by /spl sim/10%. This decrease occurs for uniform field conditions, the effect being small if not negligible for nonuniform field conditions, and is noted to appear at a threshold temperature (-25 to -30/spl deg/C), take a constant value, and be fairly independent of density.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Low-noise high-response prebreakdown current measurement system

The electrical prebreakdown currents in liquid and solid dielectrics under a highly nonuniform electric field condition have been measured successfully using an electro-optical current measuring system in conjunction with a 760-ns rectangular high-voltage pulse generator. The breakdown time lag was precisely measured from the current. The discharge propagation velocity was estimated from the linear relation to formative time lag and gap spacing to be 18 km/s in liquid nitrogen at +40-kV point voltage.

Electrical breakdown time delay and breakdown propagation velocity in polypropylene under a highly non-uniform field condition

The electrical breakdown phenomenon in polypropylene for point-to-plane gap configuration has been investigated using a photo-optical current-measuring technique and a 760 ns rectangular high-voltage pulse generator. The waveform of prebreakdown current in polyethylene is essentially the same as that in liquid nitrogen. A linear relation to formative time delay and gap spacing is obtained for gap lengths greater than some critical value, indicating a constant propagation velocity in this region. The velocity is deduced to be 1.7 km s-1 for a positive point at 60 kV. This value is coincident with that of polyethylene, and close to that of a longitudinal wave in polypropylene at 193 K.