The DC Arc in a Supersonic Nozzle Flow

The boundary layer integral method at its second level of approximation has been used to study the DC arc in a supersonic nozzle flow. It is shown that with the inclusion of the arc momentum balance, the critical point of the flow is, generally, not the sonic point of the external flow. The speed, at which a disturbance propagates relative to the external flow, is in general supersonic and is dependent on the arc conditions. The arc model is capable of predicting the axial electric field, the arc size and the axial pressure distribution as a function of current. For affinely related nozzles, the solution is determined by a parameter N, which is related to zt, the stagnation condition and the nominal current density at the throat (I/At). Numerical results are given for a particular nozzle shape although the method of analysis is general. Practical implications as regards nozzle design for a gas blast circuit breaker are briefly discussed.     

The Dynamic Behavior of Nozzle Arcs

The dynamic behavior of ac arcs in a supersonic nozzle has been investigated. The close coupling between the arc and its external flow determines the arc characteristics as well as the flow transients within the nozzle in a gas-blast circuit breaker. For affinely related nozzles, the dynamic behavior of the system is found to be fully determined by two nondimensional parameters: the nozzle coefficient N [17] and the nondimensional frequency ? which is the product of the angular frequency of the current and the time scale of the system. Of these two parameters, ? controls the qualitative behavior of the arc and its external flow. When ? is of the order of 10-2 or less, a quasi-steady period of arcing exists. Arc modeling can then be conveniently divided into a quasi-steady-state (hence dc) phase and a current-zero period.     

Nature of convection-stabilized DC arcs in dual-flow nozzlegeometry. II. Optical diagnostics and theory

For pt.I see ibid., vol.18, no.1, p.91-101, 1990. A supersonic flow field with a 5.5-cm-long and ≈2.2-mm-thick cylindrical arc plasma column was observed with a four-mirror Schlieren optical system in dual-flow nozzle geometries. For both the orifice-type nozzle and the two dimensional convergent-divergent nozzle, the arc current was varied from 45 to 110 A. The optical cold-flow-plasma boundary displayed a sharp and laminar character in both nozzles, and a sharply defined, almost-constant-diameter, quiet arc is observed between the nozzles. Downstream of the nozzle throat the arc expands and assumes a conical shape. In this region, the fringe formation inside the arc is still clear, which is an indication of the laminar nature of the plasma. However, the arc boundary is not as distinct. A cooler arc is observed downstream of the nozzle throat. Using the experimentally determined axial static pressure and cold-flow mass flux rate distributions of pt.I and the channel-flow model with constant arc temperature, the energy integral was solved for the arc radius as a function of the axial distance. From this, the arc electric field strength, voltage, resistance, and power were determined, and the total heat transfer was related to the arc power. Good agreement between the calculated values and experimental data was observed     

Nature of convection-stabilized DC arcs in dual-flow nozzlegeometry. I. The cold flow field and DC arc characteristics

An experimental investigation of the steady-state low current air arcs in a dual-flow nozzle system is presented. The cold flow field with no arc was determined for various nozzle geometries, i.e. two- and three-dimensional and orifice nozzles, and nozzle pressure ratios. Supersonic flow separation and oblique and detached shock waves were observed in the flow field. Using a finite-element computer program, the Mach number contours were determined in the flow field for various nozzle-gap spacings and pressure ratios. In addition, the DC arc voltage and current measurements were made for an electrode gap spacing of ≈5.5 cm and current levels of I≈25, 50, and 100 A for the three nozzle geometries. The arc voltage and arc power increased rapidly as the flow speed increased from zero to sonic velocity at the nozzle throat. The shock waves in the converging-diverging nozzles resulted in a decrease in the overall resistance by about 15%     

Experimental Investigation on Arc Phenomena in SF6 Puffer Circuit Breakers

A detailed observation of an arc in a model puffer-type SF6 gas circuit breaker in the current range between 10 and 50 kA (rms) has been carried out. It was found that the arc column remained stable on the center axis during the high-current region, then became turbulent near current zero. It was found that the time interval during which the turbulent arc was observed decreased with increasing values of the peak current. These phenomena indicated that the thermal effects of high-current arcs remain even at current zero. It also was observed that the arc diameter at the nozzle throat outlet was smaller than that at the throat (29 mm), even at a current as high as 70 kA (instantaneous), and that the boundary of gas flow at a downstream region had a very large diameter when the arcs were present. However, around current zero the boundary diameter became as small as that without arc.     

Untersuchungen über die thermische Belastbarkeit und die Form von Düsen zur Führung von Hochstromplasmen

The electrical and thermal limitings of nozzle are calculated from electrical dates of argon arcs for guiding and constricting high current plasmas. The electrical field strength in nozzles resulted from the arc characteristics. Hence it follows the maximum nozzle length as a function of nozzle diameter and amperage. From the arc power converted in the nozzle the power density resulted at the nozzle wall. By use of known material functions of nozzle material the needed wall thicknes of nozzle and quantities and velocities of cooling water are calculable. The experimental verification of the results gives the following relation between the minimum nozzle diameter and amperage for argon arcs using the best nozzle cooling d [cm] = 10^?3 I [A]. With superposed gas flow values are riched up to d [cm] = 0,5 · 10^?3 I [A]. The attainable current density is 6 · 10³ A cm^?2 without superposed gas flow and 6 · 10⁴ A cm^?2 with superposed gas flow. New nozzle typs for plasma technological uses ensue from the results of investigations.     

Ablation Controlled Arcs

An experimental and theoretical study on ablation controlled arcs in cylindrical tubes is presented. Measuring techniques for stagnation pressure, electric field strength, mass ablation rate, and arc cross section are described with which a comprehensive set of experimental data is obtained for blackened PTFE as a reference material. These data are interpreted with an isothermal two-zone model that consistently accounts for the balance of mass and axial momentum and yields simple scaling laws for the arc characteristics. Consistent agreement with the experimental data is found for an arc temperature TA = 19 000 ± 2000 K, a vapor layer temperature Tv = 3400 ± 200 K, and a transparently radiated fraction of the arc power of v = 0.32 ± 0.03. The vapor temperature can be explained with a photoablation mechanism. The ablation arc model allows quantifying of the phenomena related to nozzle clogging in gas-blast circuit breakers, namely flow blocking and reverse flow heating. How these phenomena determine the pressure rise in self-blast circuit breakers is shown.     

Unipolarbögen

Unipolar arcs have achieved considerable importance as a plentiful source of contaminations in tokamak plasmas. A definition of unipolar and plasma induced discharges is given, and the main properties of the space charge sheath between plasma and wall as well as the existence conditions of unipolar arcs (necessary voltage drop, current balance) are considered. From a simple model that includes the superposition of two completely different plasmas — the tokamak plasma and the cathode spot plasma of the arc — the radial dependencies of the current densities and voltages between plasma and wall and of the plasma parameters are derived. It is shown, that — in typical cases — the anode region is a ring-shaped area situated at distances from the arc cathode spot r > b with 0.3 cm ? b ? 1 cm, and that the density decrease of the arc plasma n ～ r^?μ necessitates an exponent 2 ? μ ? 3 because of the current balance condition (arc current = backflow current). Moreover some thermodynamic aspects (unipolar arcs as a kind of dissipative structures), the ignition problem, the effects of magnetic fields, the problems of diagnostics and the possibilities of simulations are shortly discussed.     

任意长度喷管型面设计方法在气动激光器喷管设计中的应用

 将一种新的任意长度喷管型面设计（ALN）方法应用于气动激光器喷管型面设计,在相同的喷管出口设计马赫数、喷管喉部高度及喷管扩张段长度条件下,设计了4条ALN方法喷管型面,并和一条最短长度（MLN）喷管型面作对比,采用2维气动激光器增益场仿真方法对5个喷管的小信号增益场进行仿真。计算结果表明:ALN方法可以有效地实现增益场分布的控制,选择合理的设计参数得到的喷管长度比MLN喷管更短,小信号增益更大。     

超音速喷嘴叶栅造型设计及数值分析

分析了超音速喷嘴内部的流动机理并研究了超音速喷嘴的设计方法,对某液体火箭发动机用氢涡轮超音速喷嘴叶栅进行了改型设计。设计采用尖边喉部和光滑过渡喉部两种方法,其中光滑过渡喉部喷嘴型线设计采用具有局部修改能力的三次非均匀B样条曲线。对超音速喷嘴叶栅复杂内流的全三维数值模拟结果表明:改型叶栅流动性能明显改善,效率提高,光滑过渡喉部喷嘴性能优于尖边喉部喷嘴。     

Results of radius scaling experiments and analysis of neon K-shellradiation data from an inductively driven Z-pinch

The K-shell radiated energy (yield) from neon Z-pinch implosions with annular, gas-puff nozzle radii of 1, 1.75, and 2.5 cm was measured for implosion times from 50 to 300 ns while systematically keeping the implosion kinetic energy nearly constant. The implosions were driven by the Hawk inductive-storage generator at the 0.65-MA level. Initial neutral-neon density distributions from the nozzles were determined with laser interferometry. Measured yields are compared with predictions from zero-dimensional (0-D) scaling models of ideal. One-dimensional (1-D) pinch behavior to both benchmark the scaling models, and to determine their utility for predicting K-shell yields for argon implosions of 200 to >300 ns driven by corresponding currents of 4 to 9 MA, such as envisioned for the DECADE QUAD. For all three nozzles, the 0-D models correctly predict the Z-pinch mass for maximum yield. For the 1and 1.75-cm radius nozzles, the scaling models accurately match the measured yields if the ratio of initial to final radius (compression ratio) is assumed to be 8:1. For the 2.5-cm radius nozzle, the measured yields are only one-third of the predictions. Analysis of K-shell spectral measurements suggest that as much as 70% (50%) of the imploded mass is radiating in the K-shell for the 1-cm (1.75-cm) radius nozzle. That fraction is only 10% for the 2.5-cm radius nozzle. The 0-D scaling models are useful for predicting 1-D-like K-shell radiation yields (better than a factor-of-two accuracy) when a nominal (≈10:1) compression ratio is assumed. However, the compression ratio assumed in the models is only an “effective” quantity, so that further interpretations based on the 0-D analysis require additional justification. The lower-than-predicted yield for the 2.5-cm radius nozzle is associated with larger radius and not with longer implosion time, and is probably a result of two-dimensional effects     

Study of operating conditions of plasma spray torch using lowelectric power levels

The scaling-down of plasma spray torches is connected with problems of anodic arc attachment fluctuations. It Is known that thermally constricted arcs in molecular gases generate unstable anodic arc roots in the nozzle of the torch. Moreover, using a usual plasma spray torch at reduced electrical power level in an argon-hydrogen gas mixture at a pressure of about 120 mbar, the arc is strongly influenced by hydrodynamic forces. Especially, its long time behavior shown by the arc voltage course is very variable and nonreproducible. It is the purpose of this work to optimize the nozzle geometry and the gas flow rate in order to obtain stabilized operating conditions of the plasma torch for an arc current of 100 A. It will be shown that the design of the entrance region and the diameter of the anodic nozzle influence the are column motion as well as the level and the frequency of the voltage fluctuations. With a particularly modified anodic nozzle, stable and reproducible operating conditions of the plasma torch, which is used to synthesize diamond layers, will be obtained     

The Local Electrical Properties of Gas Blast Arcs near Current Zero

Results are reported of experiments performed on a model air-blast circuit breaker to determine the distribution of voltage along the axis of the arc gap during the current zero period, following half sinusoidal current pulses of frequency 85 Hz. The air-blast was sustained by a reservoir pressure of 6.89 × 105 N/m2. Measurements were made following peak currents of 3 and 8 kA, with copper and carbon upstream cathodes, different separations of upstream cathode from nozzle inlet and different conditions downstream of the nozzle throat. The results have enabled the conductance decays at various axial positions to be determined and these have been related to the improved circuit breaking performance observed by other authors when the nozzle and arc gap geometries are optimized. Although a detailed theoretical analysis awaits the measurement of other fundamental plasma properties, an approximate evaluation of the role played by various basic processes has been made.     

Radiatively Induced Nozzle Ablation in High-Power Circuit Interrupters

Scientific investigation of nozzle ablation during arcing in gas-blast circuit breakers have hitherto been based solely upon pressure measurements along the length of the nozzle. Although such measurements have provided some insight into the ablation effects, a more detailed understanding requires the measurement of additional parameters. For this reason investigations have been undertaken of the nozzle ablation process using (in addition to pressure distribution) current, voltage, radiative emission, and absorption measurements. In particular, new measurements are presented of total and spectrally resolved radiation emission from high-current SF6 arcs burning through nozzles made from PTFE and copper. The arcs were sustained by alternating current waveforms with a frequency of 78 Hz and peak currents in the range 33-62 kA. Nominal upstream and downstream pressures of 8.5 and 1 bar absolute were used. By analyzing the complex interactions involving pressure transients and radiative transfer, an approximate indication has been obtained not only of the total amount of nozzle material ablated but also of the amount of ablated material present in the nozzle at a given instant and the average temperature of this ablated material. Insight has been gained into factors governing the different ablation behavior of copper and PTFE.     

An Experimental Study of Circuit-Breaker Arcs in Nozzles under Clogged Conditions

This paper describes an experimental investigation of airblast arcs in nozzles under clogged conditions. From a suitable computer model of the air supply system and by the use of approximately flat current pulses of sufficiently long duration of nearly 18 ms, the clogging current as well as the reversed rate of flow energy from the nozzle to the upstream chamber under overclogged conditions have been determined. Theoretical scaling law for the clogging current [2] has been verified for upstream pressures in the range of 5-13 bar using three-nozzle geometries.     

Mass-spectroscopic study of the influence of nozzle material on high-pressure SF6 arcs

The interrupting capability of a gas-blast high-voltage circuit breaker (CB) is mainly determined by the self-induced pressure rise caused by the thermal arc energy, the composition of the arc plasma and the chemical reactions occuring during and after current interruption. We have studied the nozzle materials boron nitride (BN), quartz (SiO₂), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene (ETFE), polyethylene (PE) and epoxy resin (ER) with respect to their influence on these processes with the aid of a model circuit breaker (MCB). Direct measurements of the arc-induced pressure rise reveal that the portion of the arc energy available for the pressure rise varies greatly (20%–65%) with the properties of the nozzle material. Nozzle erosion is significantly higher for materials with high values (e.g. polymers). Therefore, the lifetime of polymer nozzles is considerably shorter than that of ceramic nozzles. We have investigated the influence of the nozzle material on the decomposition products formed in the arc discharge of our MCB by studying the composition and time dependence of these products. The MCB was directly attached to the time-of-flight mass spectrometer (TOFMS) with the aid of a molecular-beam sampling system, which allowed real-time measurements of the arced gas during and after current interruption, thus providing information on the ablation mechanism and on the reaction kinetics of vaporised nozzle material with dissociated SF₆. The most abundant long-lived reaction products are SF₄, SOF₂, C₂H₂, CO, and CS₂. Their formation rates have been determined as functions of the nozzle material. With respect to quantities and properties of decomposition products, ceramics are superior to polymers since they form only small concentrations of corrosive and toxic products.     

Instabilities and Statistical Behavior of Low Current Cold Cathode Arcs

This paper presents a study of instabilities and of spontaneous extinctions which occur in low current dc arcs between metal electrodes. A theoretical model for the behavior of emitting sites on the cathode surface is proposed. This model explains experimental results obtained in arc duration measurements and arc noise correlation. The model yields figures for site life time and elementary current of emitting sites. These values are compared with those given by other authors. The proposed model can be applied to explain arc duration and related erosion occuring in telecommunication relays and circuit breakers.     

The Dependence of the Air Blast Arc Characteristics on Electrode Position

The effect of the position of the upstream electrode on the characteristics of the air blast arc in a converging-diverging nozzle is investigated using a simple one-dimensional model. It is found that altering the upstream electrode position affects the pressure gradient between the electrode and the nozzle throat. The pressure gradient in this region determines to a large extent the arc area contraction rate as the arc current decreases to zero and, therefore, the interruption capability of the arc after current zero. Moreover, the upstream electrode position affects the arc diameter in the steady and transient states throughout the whole arc length. It is found that radial turbulent cooling need not be incorporated into the model to explain the experimental results. However, turbulent radial cooling, if present, is found to enhance the interruption capability of the arc by a large factor.     

Flow visualization and pressure measurement in micronozzles

Micro devices have been widely used in aerospace engineering for years. Engineers are interested in applications of micro devices such as microjets, micro actuators, and micronozzles. The small size nozzles can be used for attitude adjustment and propulsion of micro-satellites or mini-spacecraft. In this paper, convergent-divergent micronozzles have been investigated at supersonic speed with various total pressures and Reynolds numbers. The throat of the micronozzle is 250 micron wide and the nozzle is designed as de Laval type. For the measurements, the Reynolds number at the throat varies from 1200 to 11000 and total pressure varies from 6 psia to 55 psia. Experimental results are obtained with pressure-sensitive paint for pressure measurement and schlieren imaging for flow visualization. Flow visualization is a challenge for conventional techniques due to the small length scales and small depth of the density gradient. A modified schlieren technique is used to increase the sensitivity by taking the ratio of wind-on and wind-off images. Pressure-sensitive paint is also used to obtain global pressure measurement of the flow field and to compare with the schlieren results.     

Cathode processes in free burning and stabilized by axial magneticfield vacuum arcs

Collective behavior of the cathode spots (CS) has been investigated in free burning and stabilized by axial magnetic field (AMF) vacuum arcs. Experiments carried out proved previously discovered phenomenon of CS group formation in free burning arc to be a general phenomenon for a short high-current vacuum arc. The dependency of CS group size in the developed are on arc current for different contact materials has been analyzed. Application of AMF with even relatively low intensity strongly affects on cathode processes. In short arcs, it hinders formation of the CS group and consequently reduces thermal stress applied to the electrodes. It has been revealed that high current vacuum arc under the action of AMF can exist only at current densities exceeding certain minimal value that depends on AMF intensity, contact gap, and does not depend on current itself. The dependency of this minimal (or normal) current density on AMF intensity has been studied for short and long vacuum arcs. A qualitative model of the cathode spot dynamics has also been proposed