Initiation Of Breakdown Research Articles

Multi-fracture initiation sequence and breakdown pressure in horizontal wells during TDPF: A visualization experimental investigation based on PMMA

Temporary plugging and diverting fracturing (TDPF) is an effective stimulation method to achieve multiple uniform fracture propagation in unconventional reservoirs. The plugging mechanism of diverters and the feasibility of diverted fractures have been investigated well in previous studies. However, few studies aim at the relationship between the multi-fracture morphology and injection pressure curve during TDPF because multiple fractures can not be directly observed during the fracturing process. This study established a visual true tri-axial fracturing physical experiment based on transparent polymethyl methacrylate (PMMA) to investigate the multi-fracture initiation sequence and breakdown pressure in the horizontal wellbore during TDPF. The critical fracture parameters, including completion method, injection flow rate, injection fluid type, fracture density, perforation depth and stress difference coefficient, were analyzed in detail based on twelve PMMA samples. The results showed that all experiments just produced one hydraulic fracture in the initial fracturing stage, but sequentially created multiple fractures after multiple diversion stages. Meanwhile, due to the effect of stress shadow, the break-down pressure of the first and second diverted fracture was 1.36 times and 1.67 times of the initial fracture respectively. In addition to the transverse fractures produced in the diversion stages, longitudinal fractures were also created along the horizontal wellbore, which decreased the multiple fracture initiation effectiveness. Optimized parameters, i.e. perforation completion method, high injection flowrate, moderate fracture density, uniform perforation depth and high-viscosity carrier fluid with diverters, were recommended to obtain more transverse fractures during TPDF. In addition, our results also revealed three fracture initiation sequences of multiple fractures in the horizontal well during TPDF. This study described the relationship between the visual multi-fracture geometries and breakdown pressure in horizontal wells during TPDF, which provides new insights into the propagation mechanism of multiple fractures and guidance for fracturing design in the oilfield.

Experimental study of axial spark location effects on transient flame/flow dynamics during ignition in a kerosene-fueled gas turbine model combustor

Effects of axial spark location on the transient flame and flow field dynamics during successful ignition events in a kerosene-fueled gas turbine model combustor were investigated using high-speed flame OH* chemiluminescence (CL) imaging, direct photography, and planar particle image velocimetry (PIV) techniques. The combustor was operated with single nozzle and lean premixed pre-vaporized kerosene/air mixture at atmospheric pressure. Three different axial locations, which located respectively in the outer recirculation zone (ORZ), jet and close to inner recirculation zone (IRZ) regions were considered. Experimental results suggested that the axial location of the ignitor played an important role in determining the transient evolutions of the flame and flow field in phases of flame kernel initiation, flame growth and flame breakdown, while minor effects on the flame recovery and final stabilization. More specifically, during the flame kernel initiation and growth phases, when the mixture was ignited in the ORZ, the flame kernel propagated to the nozzle first and then expanded further downstream due to the recirculation flow. However, as the ignitor was in the jet and close to IRZ regions, the flame kernels were seen to travel only downstream and upstream, respectively. As for the phase of flame breakdown, images captured from both front and back views indicated that effects of the swirling flow on the flame dynamics were weakened as the ignition location was away from the nozzle. PIV measurements implied that the spark location had significant effects on the ignition evolutions of large-scale coherent flow structures, including ORZ, jet, shear layers and IRZ. Finally, temporal profiles of the flame OH* CL signals and PIV measurements demonstrated that longest time was needed for the formation of flame kernel and flame growth if the mixture was ignited close to IRZ.

Influence of low dose rate gamma radiation on breakdown voltage and electrical breakdown time delay of working gas Geiger-Muller chamber

This paper presents the experimental data on the dynamic breakdown voltage and electrical breakdown time delay of commercial Geiger-Muller chamber manufactured by PHILIPS. The distributions of these data were determined. Static breakdown voltage was estimated based on the mean value of dynamic breakdown voltage as the function of voltage increase rate, for the cases when gamma radiation was not presented and in its presence. In order to separate the mechanisms that predominantly influence the initiation of breakdown for different values of relaxation time, the dependences of mean value of electrical breakdown time delay as the function of relaxation time was analyzed. It has been shown that during the relaxation times up to 3 ms positive ions plays a dominant role in initiating the breakdown while for relaxation times from 3 ms to 30 s initiation of the breakdown is caused by electrically neutral active particles. The effect of gamma radiation on electrical breakdown time delay was observed for relaxation time greater than 7 s.

On the initiation and evolution of dielectric breakdown in auto-magnetizing liner experiments

Auto-magnetizing (AutoMag) liners are cylindrical tubes composed of discrete metallic helices encapsulated in insulating material; when driven with a ∼2 MA, ∼100-ns prepulse on the 20 MA, 100-ns rise time Z accelerator, AutoMag targets produced >150 T internal axial magnetic fields [Shipley et al., Phys. Plasmas 26, 052705 (2019)]. Once the current rise rate of the pulsed power driver reaches sufficient magnitude, the induced electric fields in the liner cause dielectric breakdown of the insulator material and, with sufficient current, the cylindrical target radially implodes. The dielectric breakdown process of the insulating material in AutoMag liners has been studied in experiments on the 500–900 kA, ∼100-ns rise time Mykonos accelerator. Multi-frame gated imaging enabled the first time-resolved observations of photoemission from dynamically evolving plasma distributions during the breakdown process in AutoMag targets. Using magnetohydrodynamic simulations, we calculate the induced electric field distribution and provide a detailed comparison to the experimental data. We find that breakdown in AutoMag targets does not primarily depend on the induced electric field in the gaps between conductive helices as previously thought. Finally, to better control the dielectric breakdown time, a 12–32 mJ, 170 ps ultraviolet (λ = 266 nm) laser was implemented to irradiate the outer surface of AutoMag targets to promote breakdown in a controlled manner at a lower internal axial field. The laser had an observable effect on the time of breakdown and subsequent plasma evolution, indicating that pulsed UV lasers can be used to control breakdown timing in AutoMag.

Breakdown Initiation and Electrical Strength of a Vacuum Insulating System in the Environment of Selected Noble Gases at AC Voltage

This paper presents the results of testing the electrical strength of an insulating system in a vacuum obtained from three noble gases: argon, neon, helium, and air. The breakdown voltages were measured for contact gaps of 1 mm and 2 mm. A difference was observed in the pressure range where the electrical strength was kept constant. The chamber filled with helium residual gases lost its insulating properties at the highest pressure among the tested gases (2.00 × 100 Pa at contact gap d = 2 mm), while the chamber filled with argon gas lost its insulating properties at the lowest pressure among the tested gases (2.00 × 10−1 Pa at contact gap d = 2 mm). After a decrease in electrical strength, an intense glow discharge was observed. A theoretical description related to the initiation of an electrical breakdown in vacuum insulating systems is also presented. The situation in which the discharge chamber with a contact system was filled with the mentioned gases was analyzed. The mean free paths of the electrons and molecules as well as the velocities and energies of the electrons accelerated by the voltage applied to electrodes were calculated. The obtained results were related to the measurement parameters and analyzed in terms of the discharge development. The results of the research suggest alternatives for the further development of vacuum-extinguishing chambers used in environmentally-friendly electrical switchgear by increasing the rated operating pressure, maintaining the required electrical strength values, and thus facilitating the operation due to greater certainty in regard tomaintaining the integrity of such a vacuum interrupter.

PaNAC089 is a membrane-tethered transcription factor (MTTF) that modulates flowering, chlorophyll breakdown and trichome initiation.

Flowering and senescence are essential developmental stages of green plants, which are governed by complex molecular regulatory networks. However, the connection between flowering regulation and senescence regulation in London plane tree (Platanus acerifolia ) remains unknown. In this study, we identified a gene PaNAC089 from London plane tree, which encodes a membrane-tethered transcription factor (MTTF) belonging to the NAC (NAM, ATAF1/2, CUC2) transcription factor family. We investigated the functions of PaNAC089 in the regulation of flowering and senescence through the analysis of expression profiles and transgenic phenotypes. Heterologous overexpression of ΔPaNAC089 delayed flowering and inhibited chlorophyll breakdown to produce dark green rosette leaves in Arabidopsis . In addition, the trichome density of rosette leaves was decreased in transgenic lines. In ΔPaNAC089 overexpression plants, a series of functional genes with inhibited expression were identified by quantitative real-time polymerase chain reaction (qRT-PCR), including genes that regulate flowering, chlorophyll decomposition, and trichome initiation. Furthermore, Δ PaNAC089 directly binds to the promoter of CONSTANS (CO ) and NON-YELLOWING2 (NYE2 ) in the yeast one-hybrid assay. Consistent with this, luciferase (LUC) transient expression assays also showed that Δ PaNAC089 could inhibit the activity of NYE2 . To summarise, our data suggests that PaNAC089 is an MTTF that modulates flowering, chlorophyll breakdown and trichome initiation.

Corrosion of Chromia-Forming and Alumina-Forming Ferritic Stainless Steels under Dual Atmosphere Exposure Conditions

The surface morphology and chemistry of oxide scales formed on select chromia-forming and alumina-forming ferritic steels have been studied after exposure to a dual atmosphere of hydrogen and air. Localized Fe-rich oxide nodules with surface whiskers/platelets form at the onset of corrosion. The initiation and growth of localized nodules and breakdown of passivation are attributed to the presence of hydrogen, inclusion of iron oxide in the passivating scale, and subsequent growth of iron-rich oxide due to the establishment of redox (H2-H2O) atmosphere and modification of oxide defect chemistry.

Effect of shock vector/polarization vector configuration on the generation of ultrahigh voltage by adiabatically compressed ferroelectric materials

Compact autonomous megawatt-power systems based on shock depolarization of ferroelectric materials are capable of producing kiloampere currents and ultrahigh-voltage pulses with amplitudes exceeding 100 kV. Herein, we report the results of experimental investigations of the generation of ultrahigh voltage by poled Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 and Pb0.99(Zr0.52Ti0.48)0.98Nb0.01O3 ferroelectrics subjected to shock loading at different shock vector/polarization vector configurations. Our experiments demonstrated that under loading perpendicular to the polarization vector (transverse stress mode) the ferroelectrics are capable of generating high voltages exceeding 400 kV, while the loading parallel to the polarization vector (longitudinal stress mode) causes a distortion of the depolarization process in ferroelectrics of large thicknesses, resulting in inefficient generation of ultrahigh voltage. It was shown that for transverse semi-planar shock waves, the presence of the longitudinal component of stress due to non-perfect planarity of the shock front can cause a complex electric field distribution in the shock front area, resulting in energy losses in ferroelectrics operating in the ultrahigh-voltage mode. The important finding is that a cylindrical, radially expanding shock wave results in no significant distortion of the depolarization process and energy losses during ultrahigh-voltage generation by transversely shock-compressed ferroelectrics. The experimental results indicate that the voltage amplitude generated by transversely shock-compressed ferroelectrics is directly proportional to the ferroelectric thickness in the range from 6 to 230 mm. We found that over the full range of investigated thicknesses the breakdown-field-on-thickness dependence of shocked ferroelectrics is described by a power law and the mechanism of initiation of electric breakdown does not significantly change with ferroelectric thickness.

Dynamics and energy coupling of gas puff Z-pinches on a fast linear transformer driver

Gas puff Z-pinch experiments with annular Ar and Ne gas shells have been conducted on the Compact Experimental System for Z-pinch and Ablation Research (CESZAR) linear transformer driver (LTD) with 500 kA current and 160 ns rise time. Here, we present results from the first systematic gas puff Z-pinch experiments using a fast (≤200 ns) LTD as a driver, in which we show that 7% of the stored energy in the capacitors is coupled to plasma kinetic energy as estimated via self-emission and laser schlieren images. 0D and 1D simulations—which do not allow instability growth and thus reach greater maximum average velocities—using initial conditions inferred from experimental implosion trajectories predict coupling in excess of 10% of the stored energy. The Ar and Ne implosions were comparably massed and thus achieved similar maximum kinetic energies, though the Ne pinches were more stable and the x-ray pulses were longer and produced higher yield: 2–5 ns and 0.21–0.52 J (0.15–0.37 J/cm) of Ar K-shell and 12–25 ns and 2.2–3.9 J (1.6–2.6 J/cm) of Ne K-shell, respectively. The difference in stability is most likely attributed to variations in initial conditions such as density distribution and gas breakdown initiation.

Electron generation and multiplication at the initial stage of nanosecond breakdown in water

Electrical breakdown of liquid dielectrics under nanosecond pulsed high voltage has been investigated extensively in the last decade. Prior studies have focused on either experimental characterization of the breakdown process and discharge plasma or formulation/verification of the electrostrictive cavitation mechanism of the breakdown initiation. There remain knowledge gaps toward a clear physical picture of how the first plasma is generated in a region saturated by nanoscale cavities created by electrostrictive forces in inhomogeneous fields at the nanosecond timescale. Initial plasma results from the multiplication of primary electrons that gain energy collisionlessly in the cavities to cause collisional ionization of water molecules on the cavity walls. This paper quantitatively discusses the possible sources of primary electrons that seed the plasma discharge. Electron detachment from hydroxide is shown to be the most probable and sustainable electron source. Using numerical modeling, this study demonstrates the plausibility of an electron multiplication mechanism involving two neighboring cavities. The drift of hydrated electrons from one cavity to the next is the rate-limiting step and sets the minimum electric field requirement. This work will inform subsequent experimental studies and have implications in various applications such as plasma sources in biomedical applications, cavitation study, and insulation of pulsed power equipment.

Through-plane high thermal conducting networks via incorporation of graphene nanoplatelets in nanocomposite film under electric field and avoiding breakdown voltage

Electric field inducement technology is one of the most efficient and straightforward way to direct and form a percolation network of conducting fillers in a polymer without necessitating chemical functionalization. However, extended application is very restricted due to the initiation of electrical breakdown of conducting fillers in polymers. Herein, thermal conductive polymer nanocomposites are fabricated using graphene nanoplatelets (GNP) as a conducting filler under application of electric field that avoids electric breakdown. An electrode surface is coated with fuluoacrylate of nano-to-micro-dimensional thickness, which is applied as insulating electrodes to inhibit electric spark under application of electric current. Two different kinds of electric fields; direct current (DC) and switching DC (SWDC), were applied to envisage the enhancement of filler assembly. Analysis showed that an insulating layer with approximately 0.35–18 μm of thickness was successfully coated on the electrodes, which enabled increased formation of through-plane thermal conducting networks as well as enhanced thermal conductivity compared to composites with random filler distribution. The intensity of the available electric field could be increased over 30 times without breakdown with the insulating layer. Moreover, the field-induced GNP assembly generated by SWDC improved the thermal conductivity of the composite of over 112% compared to composite with random GNP distribution. This technique has the potential to control the linear assembly of conducting fillers an under electric field without electric breakdown.

Case Report: Investigation of the Time Evolution of Optical Breakdown Plasma During Irradiation of Aqueous Solutions of Fe Nanoparticles

The time sweeps of images of optical breakdown plasma flashes under irradiation of aqueous solutions of Fe nanoparticles with Nd: YAG laser radiation (λ = 1,064 nm, τ = 10 ns, ν = 10 kHz, ε = 2 mJ) are studied. It is shown that the sizes of plasma flares, the plasma glow intensity, and the time to reach the maximum intensity are concentrated around certain values. So, for example, the values of 30 μm are most typical for the maximum size of flares. The maximum luminescence is usually reached in 3–4 ns. It is found that at initial stage of optical breakdown process the water-vapor boundary moves at typical speed of 11 km/s and have maximum and minimum values at 15 and 4 km/s respectively. It is assumed in this work that such a distribution may be associated with the initiation of breakdown on nanoparticles located in different places of the laser radiation waist.

Photothermal therapy with silver nanoplates in HeLa cells studied by in situ fluorescence microscopy.

Photothermal therapy (PTT) is a noninvasive treatment for cancer relying on the incorporation of NIR-light absorbing nanomaterials into cells, which upon illumination release heat causing thermally induced cell death. We prove that irradiation of aqueous suspensions of poly(vinylpyrrolidone)-coated silver nanoplates (PVPAgNP) or PVPAgNP in HeLa cells with red or NIR lasers causes a sizeable photothermal effect, which in cells can be visualized with the temperature sensing fluorophore Rhodamine B (RhB) using spinning disk confocal fluorescence microscopy or fluorescence lifetime imaging. Upon red-light irradiation of cells that were incubated with both, RhB and PVPAgNP at concentrations with no adverse effects on cell viability, a substantial heat release is detected. Initiation of cell death by photothermal effect is observed by positive signals of fluorescent markers for early and late apoptosis. Surprisingly, a new nanomaterial-assisted cell killing mode is operating when PVPAgNP-loaded HeLa cells are excited with moderate powers of fs-pulsed NIR light. Small roundish areas are generated with bright and fast (<1 ns) decaying emission, which expand fast and destroy the whole cell in seconds. This characteristic emission is assigned to efficient optical breakdown initiation around the strongly absorbing PVPAgNP leading to plasma formation that spreads fast through the cell.

The alterations of critical pore water pressure and micro-cracking morphology with near-wellbore fractures in hydraulic fracturing of shale reservoirs

The crack initiation and propagation in a laminated shale formation have been widely investigated, but the alterations of critical pore water pressure (including fracture initiation pressure and breakdown pressure) and the micro-cracking morphology with near-wellbore fracture geometry are still unclear. This paper numerically investigates the alterations of critical pore water pressure and the morphology of the hydraulic fractures in shale formation with different near-wellbore fractures. First, a hydro-fracturing model is developed for a laminated shale formation within the framework of PFC2D. Then, this model is validated with the Blanton’s criterion, pressure history and fracture geometry. Finally, the effects of length, shape, and symmetry properties of near-wellbore fractures on the critical pore water pressure and micro-cracking morphology are comparatively investigated. The critical pore water pressure, micro-cracking morphology, aperture and permeability, and the fractal dimension of hydraulic fractures are quantitatively analyzed at three initial near-wellbore fractures. Simulation results indicate that the length and the distribution of near-wellbore fracture branches play a crucial role in pore water pressure variation and micro-crack propagation during hydraulic fracturing. The critical pore water pressure is sensitive to the initial near-wellbore fracture length especially when the initial fracture length exceeds a critical fracture length. The shape of initial near-wellbore fracture could change the distribution of micro cracks and induce large pore water pressure. The symmetrical radial fracture (θ=0°) more likely causes a hydraulic fracture with high permeability while the X-shape fracture (θ=30°) contributes greatly to the complexity of the fracture network and stimulated reservoir volume. These findings on the pore water pressure variation and the micro-cracking morphology are helpful in the optimization design of hydraulic fracturing treatment.

Thunderstorm High‐Frequency Radio Bursts With Weak Low‐Frequency Radiation

AbstractBrief bursts of high‐frequency (HF) and very high frequency (VHF) radio emissions unaccompanied by strong low‐frequency radiation have been observed during initiation and propagation of lightning or thunderstorm electrical breakdown without leading to fully fledged lightning. This paper investigates a physical mechanism to generate such radio bursts by electrical discharge activity inside a thundercloud. When a discharge consists of many high‐frequency emission sources, such as streamers, that generate currents in random directions, its radiation spectrum peaks in the HF and VHF bands, and the spectral magnitudes in low frequencies are much smaller or even negligible. Combined with recent observational findings, the present study suggests that lightning initiation may begin with a short burst of many randomly occurring small‐scale discharges in a localized thundercloud region.

A Multigap Multiaperture Pseudospark Switch and Its Performance Analysis for High-Voltage Applications

In this article, the development and switching characterization of a coaxial three-gap demountable multigap multiaperture pseudospark switch (MGMA-PSS) prototype is presented. The investigated MGMA-PSS geometry has three gaps which are separated by two cavity drift space regions. A high dielectric constant ferroelectricbased single plasma trigger unit, located within the hollow cathode of the first gap, is used for effective discharge initiation and subsequent synchronous breakdown of the three gaps of the MGMA-PSS. To guarantee long lifetime with high currents and high hold-off voltages and optimized plasma coupling, kidney-shaped ring slot electrodes with baffles are used. Each of the gaps is designed for a minimum voltage hold-off up to 30 kV, with a total voltage hold-off for the MGMA-PSS of approximately 70 kV. Studies of switching behavior are carried out at different operating conditions, such as operating voltages (5-40 kV), hydrogen gas pressures (10-40 Pa), various trigger configurations,and change of circuit parameters [charging capacitance (36, 200, and 380 nF), load resistance(2.7 and 8.3 Ω)]. The short time-scale analysis of the breakdown voltage waveform shows steps in the voltage fall at low gas pressure (<; 20 Pa) and low hold-off voltages (<; 15 kV), which indicates a too-long time delay in plasma formation and coupling within the drift regions. This effect could not be observed at higher voltages (≥15 kV) and gas pressure (≥20 Pa). The overall performance of the threegap prototype PSS has been analyzed in terms of hold-off voltage, fall time, current rise time, peak current, delay time, and related parameters.

Role of air bubbles in the breakdown of flowing transformer oil

In this paper, a liquid circulation system is constructed whereby gas bubble dynamics and electrical signal evolution preceding the oil gap breakdown are registered in stationary and flowing transformer oil. Based on a number of repetitive tests, two types of breakdown are classified: one takes place after bubble growth and originates in the bubble or the mixture of liquid and gas when the applied voltage is in the negative half-cycle; the other follows streamers resulting from the protrusion of bubble tip in the positive half-cycle. For each type of breakdown, partial discharge (PD) pulse is captured preceding the breakdown. As the oil flow rate varies, bubble dynamics change but the characteristics of each breakdown remain. In addition, it is found that oil breakdown voltage is improved as the flow rate increases. In terms of PD occurrence and bubble growth, the role of gas bubble in oil breakdown initiation, as well as the effect of oil flow on breakdown, is discussed.

Experimental and numerical studies on hydraulic fracturing characteristics with different injection flow rates in granite geothermal reservoir

AbstractGeothermal energy has been widely proposed as a potential renewable energy to replace traditional fossil fuel energy. Hot dry rock (HDR) reservoirs were the main geothermal energy resources and usually consist of low‐permeability hard granite without fluid. Developing HDR requires water cyclically flowing between injection and production wells to extract heat energy. Hydraulic fracturing, as a key reservoir stimulation technology, can create the path of fluid cyclically flowing. However, few studies have investigated hydraulic induced artificial fractures in HDR geothermal formations. This paper investigated HDR geothermal reservoir stimulation characteristics and fracture patterns under different injection flow rates. Numerical simulations were conducted to model the laboratory tests. Results showed they were in good agreement, and this indicated the possibility of numerical simulation to predict hydraulic fracturing behavior under different injection flow rates. With the increase of injection flow rate, the fracture initiation pressures and breakdown pressures increased, and the propagation times and postfracturing pressures decreased. The fracture geometries were observed and analyzed, mean injection power was proposed, and results showed that it could be used to roughly estimate the fracture total lengths. Moreover, the fracture permeabilities based on the pressure data were calculated. These results can provide some reasonable advice for implementing reservoir stimulations on application to field‐scale HDR operation.

The ionization waves propagation in a long capillary dielectric tube in the absence of surface charge

The initiation of the electrical breakdown in a narrow tube filled with a gas under reduced pressure requires the voltage higher compared to that for a much broader tube. Besides, at the fixed applied voltage formally sufficient for breakdown, the probability for the breakdown happening in a narrow tube is far from 100%. This probability can be increased appreciably if to organize the surface ionization waves preceding the volume breakdown in a narrow tube. The breakdown development in a long capillary dielectric tube induced by the surface ionization waves was investigated in this work. The experiments were carried out with the use of a quartz capillary tube with a high aspect ratio of length to diameter. The tube was filled with helium at pressures of 1 - 30 Torr. The pre-posited surface charge on the tube wall was absent. It has been found the completed high-current breakdown between two electrodes located at the ends of the long capillary tube develops with a high probability with the presence of surface ionization waves within the tube.

Investigating a Plasma Lens with the Initiation of Electron-Beam Discharge

Currently, work is underway to create laser accelerators with beam focusing by plasma lenses. For effective focusing, it is necessary to investigate at which initiation of breakdown a discharge forms with a more uniform distribution of current density. Our studies have shown that the plasma distribution is uniform over a longer period of time, including the region of maximum current, if the discharge is initiated by an electron beam.