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In this article we use numerical simulations to study the effect of T1 dispersion on fluid polarization buildup in oil flow to characterize the sensitivity of both a conventional NMR concept (ROI located inside the polarization magnet) and a Earth's field NMR concept (ROI outside and downstream of the polarization magnet) to T1 dispersion of flowing samples. As a polarization field in both concepts we use a 90 cm long Halbach magnet. The T1 dispersion behavior of the oils is based on a set of crude oils that span a viscosity range of 0.7 cP up to 2·104 cP and T1 relaxation measurements for Larmor frequencies between 10 kHz and 20 MHz. Numerical simulations based on solving the Bloch-Torrey equation for the longitudinal magnetization component show that fluid polarization levels in a ROI of a Earth's field NMR system concept are much more strongly affected by T1 dispersion than in the conventional NMR system concept. As a result, we may conclude that the Earth's field NMR system design is less robust for measuring flowing samples that show strong T1 dispersion behavior. In comparison, the conventional NMR system design is relatively insensitive to the effect of T1 dispersion, as T1 dispersion effects were found to form a relatively small correction to the magnetization buildup. The conventional NMR system design consequently is the preferred implementation of a NMR system that operates on fluids with strong T1 dispersion behavior. We show that in the presence of T1 dispersion s = vT1(0)/Lm* may be used as a governing parameter for fluid polarization buildup, where T1(0) is the T1 relaxation time in the center of the polarization magnet, and we show how an modified analytical uniform field model can be used to describe fluid polarization for a uniform flow velocity distribution in the presence of T1 dispersion with an accuracy within 1% for the samples and field distribution considered in this study at industrially relevant flow velocities.

This paper presents the investigation on the breakdown characteristics and pre-breakdown streamer propagation of Palm Oil (PO) impregnated aged pressboard under positive lightning impulse voltages. The experimental work was carried out under a non-uniform field with needle-plane electrodes configuration. The streamer stopping length and breakdown voltage of 2 types of refined, bleached, and deodorized palm oil were examined in the presence of new and aged pressboards. The pressboard was placed in parallel to the needle-plane electrode at a gap distance of 50 mm. The lightning breakdown voltage was applied to the samples based on 1 shot per step rising voltage method under positive polarity as per IEC 60897. The presence of impregnated pressboard in both PO slightly increases the 50% positive lightning breakdown voltages. PO impregnated pressboards have lower 50% positive lightning breakdown voltages than MO. After subjected to ageing, the positive lightning breakdown voltages for PO and MO impregnated pressboards decrease. In the presence of aged pressboard, the streamers in PO generally propagate further than MO at the same voltage level.

A sample will undergo T1 relaxation at continuously changing applied magnetic field strengths when flowing from the Earth magnetic field outside an NMR instrument to the much stronger applied magnetic field inside an NMR instrument. The interaction between fluid flow, T1 relaxation and magnetic field distributions in the case of negligible T1 dispersion can be described by considering two length scales involved: the characteristic polarization length vT1, and the effective polarization magnet length Lm⁎. By comparing water flow experiments on a 0.9 m long Halbach magnet array with simulations using the Bloch-Torrey equation, we determined that Lm⁎ is given by the integral along flow direction x of the normalized axial polarization field distribution, p(x) = B0(x)/B0(xROI), in which xROI is the center of the polarization magnet. The fluid magnetization level Mz in a region-of-interest located at xROI is a function of the ratio s = vT1/Lm⁎ and can be generally expressed as Mz,ROI = M0⁎(s)[1–exp(-1/2 s)]. The effective equilibrium magnetization function M0⁎(s) was found to have both a sample independent contribution from the axial polarization field p(x) and a sample dependent contribution from the flow velocity profile at a given flow rate. In our experiments, the axial Halbach field was found to contribute with a weight of approximately 1/3 to M0⁎(s) and the remaining 2/3 wt is contributed by the flow velocity distribution. Based on this analysis, a set of general principles for magnetization build up modeling and online NMR instrument design has been derived.

SF 6 /N 2 mixtures is considered as a practical alternative proposal of SF 6 in GIS and GIL. Simulating particle transportations and chemical reactions during the positive streamer propagation in SF 6 /N 2 mixtures assists in the insulation performance optimization of SF 6 /N 2 mixtures. This paper presents a fluid dynamic model coupled with the chemical reaction system for the positive streamer propagation in SF 6 /N 2 mixtures. The Townsend coefficients of the 20%SF 6 /80%N 2 mixture derived from the proposed chemical reaction system are in accordance with the measurement results. Based on this model, the propagation of the positive streamer in a 1 mm needle-plate gap in the 20%SF 6 /80%N 2 mixture is simulated. Simulated results indicate that there exists a negative space charge region in the streamer channel. It can be attributed to the increasing attachment rates of SF 6 yielding SF 5 − and SF 6 −, and the reduction of the ionization rates of N 2 and SF 6 in the streamer channel. Besides, it is noted that the ionization activities of N 2 and SF 6 at the streamer head are equally important to support the continuous propagation of the positive streamer. The ionization of N 2 in the streamer channel serves as an important factor to supply electrons and avoid the accumulation of the negative space charge. [ABSTRACT FROM AUTHOR]

Sulphur hexafluoride (SF6) gas owns remarkable properties as insulation medium and current interrupter, which make it being widely used in gas-insulated equipment up to now. However, SF6 gas has a drawback that gives adverse effect to the environment since it is a strong greenhouse gas. As the effort to minimize the SF6 usage, this study was conducted to investigate the AC breakdown behavior of SF6/N2 gas mixtures with 10/90 ratio at low pressure levels (i.e. 0.11 MPa to 0.15 MPa) under non-uniform field (i.e. R0.5-plane and R6-plane electrodes configurations). The results of the study indicate that the breakdown voltage of SF6/N2 gas mixtures in non-uniform field increases linearly with the increase of gas pressure and electrodes gap distance. As a function of gap distance, a higher increasing rate of breakdown voltage values were achieved at lowest pressure of 0.11 MPa compared to other pressure levels. In addition, it is also found that a higher breakdown voltage values was obtained under R6-plane configuration. But, the difference in breakdown voltage values between R0.5-plane and R6-plane configuration is less significant as the gap distance is increased. It is also observed that the field efficiency factor of R6-plane is higher than R0.5-plane which indicates a more uniform field exists between the electrodes. [ABSTRACT FROM AUTHOR]

Abstract Droplets are a typical pollutant in industrial plants and their evaporation and movement characteristics are closely related to their surroundings. Because saturated moist air is emitted to the surrounding environment, the resulting flow field depends ultimately on the source temperature and ambient relative humidity. These two parameters in practical production processes typically change with the process. This study presents the numerical simulations of the dispersion process of fine water droplets in a non-uniform field, where the source temperature (298–353 K), ambient relative humidity (0–80%) and droplet size (20–80 μm) are selected as the control variables. Under the influence of non-uniform field quantities, the water vapor concentration at the droplet surface can no longer be maintained constant. Thus, it is necessary to modify it using user-defined functions. The results indicate that the influence of these three factors on the evaporation characteristics of droplets are not isolated, and it is impossible to discuss the influence of residual factors without any one of them. However, except for isothermal conditions, these three factors appear to be independent of each other in terms of movement characteristics. Our study can facilitate the improvement of fine control levels in an industrial building environment. Highlights • Droplets are a typical airborne pollutant in industrial plants. • The dispersion of droplets in a non-uniform field needs to be studied. • The water vapor concentration of droplet was corrected by a user-defined function. • The sensitivity parameters together determine the evaporation and movement. [ABSTRACT FROM AUTHOR]

Vegetable oils are considered nowadays as a potential alternative to Mineral Oil (MO) for dielectric insulation fluids in transformers due to its high biodegradability, non-toxicity and high fire resistance. This paper presents an experimental investigation on the streamer propagation and breakdown of Refined, Bleached and Deodorized Palm Oil (RBDPO) under both positive and negative lightning impulse voltages. The streamer shape, stopping length, velocity and breakdown voltage of 2 types of RBDPO were examined. Needle-plane electrodes configuration with 50 mm gap was used in this study and streamer images were captured by a high speed camera. It is observed that the negative streamer shape patterns are thicker than positive streamer for both RBDPO and MO. At the same applied voltage level and polarity, both RBDPO have longer streamer stopping lengths and the streamers propagate faster than MO. [ABSTRACT FROM AUTHOR]

The polarity reversal test is one of the routine tests for the evaluation of electric insulation in converter transformers. The polarity reversal time (PRT) has significant effect on the accumulation of interface charges in oil-impregnated pressboard insulation. IEC standard recommends that polarity reversal should be performed within 2 min so as to match with the dispersion rate of interface charges in uniform filed, but not necessarily in the non-uniform field. In order to investigate the influence of PRT on the dissipation rate of interface charges in non-uniform electric field, the density of interface charges was measured by an electrostatic capacitance probe in the needle-plate non-uniform field at the PRT of 2 ms, 4 ms and 2 min respectively. The results showed that the polarity of interface charges did not change with the applied voltage jump at the PRT of 2 ms and 4 ms, while the polarity of interface charges almost synchronously changed with the polarity reversal of applied voltage at the PRT of 2 min. For the PRT of 2 ms and 4 ms, the electric field nearby the needle electrode would be enhanced with applied electric field of the same direction, whilst the total electric field of insulation system would be weakened at the PRT of 2 min. It was inferred that polarity of interface charge might have not enough time to change in polarity reversal test within the PRT of 4 ms under non-uniform electric field, which was more close to the severe condition of covert transformer. The paper suggest that the value of PRT be reconsidered when the polarity reversal test is performed in non-uniform electric field. [ABSTRACT FROM AUTHOR]

Corona discharge is a self-sustained discharge which appears at electrodes with a small radius curvature in gas insulation. An almost invisible glow occurs just above the inception voltage. Corona phenomenon is mainly used in electro-technological processes to obtain space charge for electrostatic precipitation, separation of different particles, electrostatic liquid or solid coating, neutralization of space charge, etc. All of these processes rely on a strong nonhomogeneous electric field generated by a point - plate electrode system. When the critical value of the applied voltage is reached, the ionization processes near the point electrode start and give rise to the current between two electrodes. If the pointed electrode is positive, it is possible to observe an anomaly of the current - voltage (I-U) characteristic for the point-plate space. It means that while the voltage is raising the current density decreases in a narrow voltage area (2-3 kV). The anomaly was technically named as negative differential conductivity (dI/dU < 0). Unstable current can have a negative influence on electro-technological processes. The anomaly was detected for different shapes and materials of the electrode as well as for various temperatures and distances between electrodes. An oxidation layer, which appears on the metal electrode, also influences the ionization processes near the pointed electrode and causes a decrease of a current. In this paper measuring of the discharge activity in a point - plate electrode system is presented. Ionization of gas atoms and molecules in a high electric field and the following recombination of electrons and positive ions in the corona region can give rise to high-energy photons which produce new electrons in the field of discharge. Corona discharges are detected by DayCor Corona camera which can register UV emission generated by corona in a day light. The experiment was conducted with various shapes of the pointed electrode and distances between the high voltage and the grounded electrode under applied direct voltage with positive and negative polarity. [ABSTRACT FROM AUTHOR]

A calculation model for predicting the inception of partial discharge (PD) in a non-uniform air gap on the basis of theories of electron avalanche and streamer phenomena is proposed. ?Partial discharge inception voltages? (PDIVs), under an assumed absolute humidity of less than 20g/cm3, calculated using the proposed model were compared with experimentally measured ones and found to be almost equivalent. Furthermore, the relationship between negative ions and humidity was incorporated into the calculation model, and it was confirmed that the calculated and measured PDIVs tend to be the roughly the same under a wide humidity range under various gap length between electrodes. The proposed partial-discharge inception model based on theories mentioned above was therefore judged to be effective for predicting changes in PDIV when humidity changes. [ABSTRACT FROM PUBLISHER]

This paper presents a study of the lightning breakdown voltages of Palm Oil (PO) and Coconut Oil (CO) under a non-uniform field with consideration on the polarity effect at various gap distances. All tests were carried based on a needle-sphere electrode configuration and various gap distances ranging from 2 to 25 mm under positive and negative voltage polarities. Three different testing techniques were used in this study including rising-voltage, up-and-down and multiple-voltage methods. The PO used in this study was Refined, Bleached and Deodorised Palm Oil (RBDPO) Olein. Three different samples of RBDPO and one sample of CO were tested. The Weibull distribution was used as a statistical approach to determine the withstand voltages of all samples at 1% and 50% probabilities for each type of oil. Under positive voltage polarity, it was found that the 50% breakdown voltages of RBDPO and CO were comparable with Mineral Oil (MO) whereby the highest percentage of difference among all gap distances was less than 15%. RBDPO and CO have lower 50% breakdown voltages than MO under a negative lightning impulse for which the highest percentage of difference can be up to 40%. [ABSTRACT FROM AUTHOR]

Corona Discharge have been recorded in pin-to pin electrode at gas pressure gaps with high degree of non-uniformity electric field. For a wide range of experimental applicability the pin to pin electrode was used over pressure range from 1bar to 3bar and gap spacing from 0 to 5mm. High –pressure dc corona discharge experiments have been carried out in a 16cm long glass chamber with an inner diameter and outer diameter are 16.8cm, and 19cm respectively. Corona onset voltage and breakdown voltages were reached. The design considerations of the setup allow to use pin to pin electrode type geometries. Electronegative gases such as air, Freon (R22), were separately used for low temperature corona generation. The experiments have been carried out under ambient Laboratory conditions of temperature (19-24 oC), and humidity. The present experiment were studies the relation between the (V-P) and (V-d), on other hand, we study the relation between the current and applied voltage.

Townsend breakdown mechanism for direct current discharge in argon gas by considering the effects of the non-uniformity of electric field, the loss of electrons due to radial diffusion, and the applied axial magnetic field has been discussed and an analytical expression has been derived for the breakdown voltage. It was Shown that the non-uniformity of electric field, between two parallel disc electrodes with radius R and separation of d , significantly affects the electron multiplication for d ≥ 2.7 R and that effect was disappeared for the gas pressure higher than 0.35 Torr. The loss coefficient δ was used to quantify the effects of the electron radial diffusion and axial magnetic field and the PIC-MCC numerical simulations were used to determine it. The results show that increasing the gap size above the electrode radius leads to dramatic increase of the δ coefficient while applying an axial magnetic field can effectively decrease it. • Effect of the non-uniformity of electric field becomes significant for d ≥ 2.7 R. • The breakdown is inevitably impacted by the radial diffusion of electrons when d > R. • An axial magnetic field can effectively control the electron losses. • An analytical expression for the breakdown voltage was obtained by considering the above three factors. [ABSTRACT FROM AUTHOR]

The objective of this paper is to propose a breakdown mechanism of lightning impulse voltage in high voltage vacuum interrupters (VIs) with long contact gap by complementing the Cranberg "clump" hypothesis with an impulse voltage and a non-uniform field. The Cranberg "clump" hypothesis assumed that the vacuum breakdown was due to the loosely adhering microscopic particles under a DC voltage and a uniform field. The microscopic particles might be torn from an electrode surface at a critical voltage. We considered the microscopic particles located at the area with the maximum electric field had the highest possibility to be accelerated. Therefore, the transit process of the micro-particles to the opposing electrodes was under a non-uniform field distribution driven by an impulse voltage. The rise rate of the impulse voltage applied was also considered. As a result, it was found that the breakdown voltage of a vacuum gap depends upon a 0.76 power of the gap length for the plate-to-plate contacts. The diameter of the contacts was 60 mm. The contact radius of curvature was 6 mm. In the same conditions, our previous experimental results showed that the impulse breakdown voltage was in a range of 0.70 to 0.79 power of the contact gap. So the experimental results supported the Cranberg hypothesis in its extension to the impulse breakdown under non-uniform field in high-voltage VIs. Thereafter, this proposed breakdown mechanism is a complement to the Cranberg "clump" hypothesis in terms of impulse voltage breakdown under a non-uniform field in high voltage VIs. [ABSTRACT FROM AUTHOR]

Abstract: The flow of a ferrofluid in a stationary cylindrical container driven by a rotating magnetic field has received considerable attention since the inception of the field of ferrohydrodynamics. Much controversy has resulted regarding the existence, or lack thereof, of bulk flow under conditions of a rotating uniform magnetic field, which can be generated for example, using a two-pole stator winding. The original observations of flow at the interface showed counter-rotation of field and fluid, whereas recent observations of bulk flow using the ultrasound technique have shown co-rotation of field and fluid. Various theories have been advanced over the years to explain the observed phenomena, including the spin diffusion theory of Shliomis and the hypothesis that it is field non-uniformity, generated by non-ideal stator winding distributions, that actually drives the flow, as first proposed by Glazov. We have revisited this problem from an analytical perspective by solving the ferrohydrodynamic and magnetoquasistatic equations self-consistently for the case of ferrofluid in a cylindrical container, with and without an internal co-axial cylinder, and driven by the field generated by a multipole stator winding distribution. In such a winding increasing the number of poles results in increasingly non-uniform fields. It is shown that regardless of the number of poles in the stator winding the ferrohydrodynamic equations do not predict any flow in either geometry as long as the spin viscosity parameter is assumed to be zero. Velocity profiles are obtained for both geometries and arbitrary number of poles for the case of non-zero spin viscosity. It is shown that only for the case of a two-pole stator winding and ferrofluid constrained to the annular space between an inner and outer cylinder do the ferrohydrodynamic equations predict co-rotation of fluid and field close to the outer cylinder and counter-rotation of fluid and field close to the inner cylinder, in qualitative agreement with experimental measurements made using an oil based ferrofluid. [Copyright &y& Elsevier]

This paper describes discharge properties of N2 and CO2-based gas mixtures including a perfluorocarbon (PFC) gas such as CF4, C3F8 and c-C4F8 under non-uniform field. The mixture ratio between a base gas of N2 or CO2 and the additive PFC gas was fixed as 9:1; namely, 90%N2/10%PFC or 90%CO2/10%PFC gas mixture. The PFC gases have even smaller global warming potential (GWP) than SF6 gas and have good insulation properties as SF6 gas. Thus, PFC gas mixture is expected to be a SF6 substitute without highly pressurizing the gas over the conventional pressure of 0.5 to 0.6 MPa. in this study, in order to compare the partial discharge (PD) inception voltage VPD and breakdown voltage VB properties between N2 and CO2-based gas mixtures, as well as between the additive gas of PFC and SF6 gas, we investigated these properties of the gas mixtures with a needle to plane electrode under ac high voltage application. The gas pressure was changed from 0.1 to 0.6 MPa. As a result, it was found that VPD and VB characteristics of N2 and CO2-based gas mixtures differed considerably, especially the gas pressure dependence of VB (so-called the N shape characteristics). VB characteristics of N2-based gas mixture including c-C4F8 proved to be excellent within the test conditions over the wide gas pressure region, showing the maximum breakdown voltage. In terms of VPD properties, CO2-based gas mixture had an advantage over N2-based gas mixture due to higher VPD.Furthermore, we discussed the synergy effects of VPD and VB for N2 and CO2-based gas mixtures using the index Rn which was defined to quantify the degree of the effect. Rn for CO2-based gas mixture was higher than that of N2-based gas mixture. [ABSTRACT FROM AUTHOR]

Abstract: Computer simulation studies of the stability and transport properties of trapped non-neutral plasmas require the numerical realization of a three-dimensional plasma distribution. This paper presents a new numerical method for obtaining, without an explicit model for physical collisions in the code, a low noise three-dimensional computational equilibrium distribution. This requires both the loading of particles into an idealized distribution and the relaxation from that distribution toward an approximate numerical equilibrium. The equilibrium can then be modified through a slow change of system parameters, to generate other equilibria. In the present, work we apply this method to a UC Berkeley experiment on electron confinement in magnetic geometries appropriate for the ALPHA anti-hydrogen experiment, using the three-dimensional particle-in-cell code WARP. WARP’s guiding center mover and its option to switch between different solvers during a simulation are highly valuable because they speed up the simulations; they enable the practical use of the new technique for generating numerical equilibrium states of trapped non-neutral plasmas. [Copyright &y& Elsevier]

In this paper, a displaced dipole model for spheres in a non-uniform field is proposed. The displaced dipole moment and displacement can be approximately obtained from the external field values at the two poles of the sphere. The analytical expressions of the electric field are derived for a sphere in the non-uniform electric field. As the model does not have to assume the external field to be uniform in the vicinity of the spheres, it differs from the dipole-at-the-center approximation. In addition, it is simpler than the multipoles technique. Another result obtained is that the displaced dipole of two closely spaced spheres approaches the edge of the sphere when &epsiv;i/&epsiv;e&ges;4 and the displaced dipole model becomes ineffective. [Copyright &y& Elsevier]

• This paper first time measures and simulates the breakdown voltages of helium for a millimeter-scale needle-plate gap at high temperature and pressure (25–180 ℃, 0.01–7 MPa). • Two-dimensional drift-diffusion plasma model is used in COMSOL Multiphysics to carry out the simulation and the results are analyzed. • The effects of non-uniform property of the electric field are presented through the hump and corona stability. Comparisons of the breakdown voltages/field strength with the uniform breakdown are presented. • Time evolution of the current and particle number densities are presented corresponding to different pressures, indicating the key role that penning ionization plays in high-pressure helium discharge. Breakdown voltages of helium between needle-plane copper electrodes were measured to obtain the base data for insulation design of the electrical equipment working in high temperature and pressure helium gas in the High Temperature Gas-cooled Reactor (HTGR). The measurements were performed at a gap length between 1.02 mm and 5.30 mm, temperature and pressure varying from 0.01 to 7 MPa and 25–180 °C, respectively. Some abnormal "U-shapes" were found to appear especially at pressure over 3 MPa, which can be explained by the effects of space charge on the corona layer. COMSOL Multiphysics was used to simulate the discharge with a two-dimensional drift–diffusion model. The results agree well with the experimental data on the whole, but the errors rise when the pressure and gap length become very large. Evolutions of the current and particle number densities were also obtained. These breakdown voltages and characteristics of helium discharge is considered much valuable to the insulation design in HTGR. [ABSTRACT FROM AUTHOR]