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     

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.     

Effect of chevron count and penetration on the acoustic characteristics of chevron nozzles

Experimental investigations have been carried out on chevron nozzles to assess the importance of chevron parameters such as the number of chevrons (chevron count) and chevron penetration. Acoustic measurements such as overall sound pressure level, spectra, directivity, acoustic power, and broadband shock noise have been made over a range of nozzle pressure ratio from sub-critical to underexpansion levels. Shadowgraph images of the shock-cell structure of jets from various chevron nozzles have also been captured for different nozzle pressure ratios. The results indicate that a higher chevron count with a lower level of penetration yields the maximum noise suppression for low and medium nozzle pressure ratios. Of all the geometries studied, chevron nozzle with eight lobes and 0° penetration angle gives the maximum noise reduction. Chevron nozzles are found to be free from screech unlike regular nozzles. Acoustic power index has been calculated to quantitatively evaluate the performance of the various chevron nozzles. Chevron count is the pertinent parameter for noise reduction at low nozzle pressure ratios, whereas at high nozzle pressure ratios, chevron penetration is crucial. The results illustrate that by careful selection of chevron parameters substantial noise reduction can be achieved.     

Scaling Laws for Gas-Blast Circuit-Breaker Arcs during the High Current Phase

A steady state nozzle arc model based on the boundary layer integral method is established and scaling laws are derived. For affinely related nozzles, the solution is uniquely determined by a nozzle coefficient N, which is related to the stagnation conditions, the arc current, and the dimensions of the nozzle. Tests have been performed on nozzle arcs in air using two geometricaly similar nozzles at three stagnation pressures. A good agreement between theory and experiment is obtained which indicates that circuit-breaker arcs can be scaled. To avoid nozzle clogging, the nominal current density at the throat (I/At) should not exceed the highest permissible nominal current density at the throat. For all affinely related nozzles, this upper limit of current density at the throat is proportional to ?p0?t, where p0 is the stagnation pressure and Zt the distance of the throat from the nozzle entrance. The overal arc voltage exhibits the precurrent-zero static behavior as indicated by Browne's composite arc model.     

Electrical and aerodynamic behavior of arc under shock conditions

Arc-shock interaction in a supersonic nozzle has been investigated for a current range from 200 to 1500 A and for three pressure ratios. The adverse pressure gradient associated with the shock causes flow separation and a broadening of arc cross section. Compared with the shock position in the absence of the arc, the shock center is moved toward upstream and to a region close to the wall. The shock is no longer plane. The center of the shock is not very sensitive to the current, but the shock strength reduces when the current is increased, V-I characteristics under shock conditions are slightly modified     

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.     

A Current-Zero Arc Model Based on Forced Convection

A simplified arc model based on the integral method is used to study the arc behavior in a supersonic nozzle. Emphasis is placed on the energy balance of the overall arc, which extends to the arc thennal boundary. Similarity rules for aerodynamic and electrical quantities are established, and a quantitative definition of current zero period is given. Computations have been done for two nozzle geometries. The nozzle geometry plays the role of shaping the arc, thereby affecting the axial electric field distribution. Performance curves in terms of the critical rate of rise of recovery voltage (rrrv)c and di/dt at current zero are established. It has been found that (rrrv)c can be seriously affected by the distortion of the current waveform near current zero due to arc-circuit interaction. When experimentally measured current waveform is used as an input, a good quantitative agreement is obtained for the Liverpool orifice arc [1] between theory and experimental results. A satisfactory agreement has also been achieved for the axial electric field distribution without adding a turbulence term into the energy equation. The limitations of the present arc model is also discussed in detail.     

保护气对切割弧特性影响的模拟研究

通过比较两种不同结构切割炬所产生的等离子体流场,发现保护气对等离子体的温度和速度分布影响很小.垂直保护气在切割炬喷口形成阻碍作用,造成切割炬内的压强有所升高,但是增加不大.两种结构保护气对切割弧的影响只是在炬喷口外的激波附近.加入保护气后激波的强度会减弱.相对于没有保护气的情况,保护气增加冷却作用,弧电压会略有升高.当改变保护气的成分时,发现弧柱区的氧气含量不受影响,所以保护气成分的改变不会影响到弧电压.计算发现轴线处氧气和周围气体的混合很少,在喷口下游10mm处,氧气的摩尔分数仍在90%以上. 关键词： 等离子体切割弧 保护气 数值模拟     

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.     

Effect of current connection to the anode nozzle on plasma torchefficiency

Experiments have been performed to demonstrate the influence of the location of the electric power connection to the anode nozzle on the efficiency of DC plasma torches. The DC plasma torch used in these experiments offers the flexibility to work with different anode geometries and the possibility of connecting the electrical power to the anode at two different locations. For each set of experiments, the controllable parameters such as total gas flow rate, gas composition, and electric current were kept constant, changing only the location of the electrical connection to the anode nozzle. The efficiency of the torch, derived from a conventional energy balance, shows a significant change as the location of the electrical connection to the anode nozzle is changed. The measured mean voltage as well as the amplitude of the voltage fluctuations were also affected by the location of the electrical connection to the anode nozzle. An explanation for the arc behavior is given, based on an analysis of the forces acting on the anode arc column and their influeuce on the variation of the arc column length. Experimental data are in good agreement with analytical predictions     

矩形喷口欠膨胀超声速射流对撞的实验研究

在不同喷口间距和射流压力下开展了矩形喷口欠膨胀超声速射流对撞实验并与自由射流进行了对比. 实验表明:超声速射流对撞的辐射噪声中存在四种不同的啸音模式, 且随喷口距离和射流压力的变化在不同模式间切换. 在射流压力大于0.5 MPa且喷口间距小于50 mm时, 射流对撞面在两个喷口外形成两道正激波之间, 啸音基频维持在3 kHz左右. 随喷口间距的增大或射流压力的降低, 射流对撞面在一侧喷口外的弓形激波与另一侧喷口外的正激波之间. 对撞面也有可能出现在两个弓形激波之间, 对应的啸音基频约为9 kHz, 但容易受扰动而回到喷口一侧或是在喷口之间大幅度振荡. 当射流压力小于0.36 MPa且喷口间距大于70 mm后, 对撞面在两个喷口之间大幅度振荡, 产生基频在1 kHz左右并随射流压力的降低和喷口间距的增大而降低的啸音. 关键词： 超声速射流 啸音 射流对撞 激波     

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.     

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.     

Acoustic disturbance from gas non-uniformities convected through a nozzle

The non-steady flow generated by convection of gas containing non-uniform temperature regions or “entropy spots” through a nozzle is examined analytically as a source of acoustic disturbance. The first portion of the investigation treats the “compact nozzle”, the case where all wave lengths are much longer than the nozzle. Strengths of transmitted and reflected one-dimensional waves are given for supersonic and subsonic nozzles and for one configuration of supersonic nozzle with normal shock at the outlet. In addition to a wave reflected from the nozzle inlet, the supersonic nozzle discharges two waves, one facing upstream and the other facing downstream. For reasonable values of the nozzle inlet Mach number, the pressure amplitude of each wave increases directly as the discharge Mach number.The acoustic perturbations from a supercritical nozzle of finite length, in which the undisturbed gas velocity increases linearly through the nozzle, are analyzed for several inlet and discharge Mach number values and over a wide frequency range. The results which agree with the compact analysis for low frequency, deviate considerably as the frequency rises, achieving pressure fluctuation levels of several times the compact values. It is shown that this result originates in a phase shift between the two waves emitted downstream and that the pressure fluctuations for moderate frequencies may be approximated from the compact analysis with an appropriate phase shift.In all cases, the pressure fluctuations caused by a 2% fluctuation in absolute inlet temperature are large enough to require consideration in acoustic analysis of nozzles or turbine blade channels.     

Control of spray spot in cold spray technology. Part 1. Gas dynamic aspects

This paper presents a study of supersonic jets formed by approaches that are new for cold spray technique: the main flow is swirled, the nozzles with permeable profiles and with exit slots on the supersonic section are engineered. The flow swirling achieved in the nozzle prechamber retains downstream to substrate surface. The system of vortices created within the permeable nozzles changes the shock wave features of the overexpanded jet and the geometry of the bow shock wave ahead of the substrate surface. These new features of flow may affect particle motion and particlesubstrate interaction under the conditions of cold spray process; this offers tools for obtaining the necessary shape of a spray spot.     

小功率等离子体射流的流特性

采用焓探针对小功率(5kW)热喷涂等离子体射流的焓、温度和速度进行了测量和计算。研究了气体成分、流量、电弧电压和电流对等离子流体的焓、温度和速度分布的影响。结果表明,对于单一氩气等离子体,当使用新喷嘴时,增大氩气流量能够使喷嘴内部电弧弧根向出口方向移动,从而增加等离子体射流的焓、温度和速度。对于Ar-N2等离子体,增加气体中氮气的含量,会提高等离子电弧电压,在同样的输入功率下,改变等离子电流和电压对等离子体的焓、温度和速度影响较小。对于Ar-N2等离子体,增加氢气含量会明显地提高等离子射流的速度和热传递。     

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.     

气动激光器喷管非平衡流2维数值仿真

将Anderson的两振型三温度弛豫模型和严海星整理的弛豫数据相结合,采用2维守恒型方程组对按照最小长度喷管型面设计方法设计的、面积比分别为50和20的气动激光器喷管非平衡流场进行了数值仿真。小信号增益计算结果在每个计算点都和J.S.Vamos等人针对这两种喷管的小信号增益测量试验结果符合很好.解决了传统的准1维非平衡流分析方法不能很好地和试验结果相符的问题.对气动激光器喷管性能设计提供了更精确的评估方法。     

Time-resolved imaging of anodic arc root behavior duringfluctuations of a DC plasma spraying torch

The fluctuating behavior of a Sulzer Metco F4 DC plasma gun has been investigated by simultaneous measurement of the time dependencies of the are voltage and of images from the nozzle interior. An end-on imaging arrangement using a mirror and a mask in the optical path from the are to the camera allows visualization of the anodic arc attachment by strongly attenuating the bright emission from the are column. With the torch operating in the restrike mode, sequences of images have been acquired in synchronization with several typical features of the are voltage fluctuations showing that the attachment nature changes during a restrike cycle. Multiple attachments which coexist at least during the 1 μs exposure time of the camera have been evidenced and are interpreted as a continuous process of creation/vanishing of successive arc roots with a smooth transfer of the current from one to the other. The anode wear is shown to have a strong effect on the root position over the anode periphery, with a preference for attachment in eroded regions. The effects of operation parameters such as current, gas flow and injector type on the attachment nature and position are also presented     

Investigation of the high-current arc in helium

The secondary and erosion characteristics of the electric-arc plasmatorch were studied experimentally. Formulas for arc voltage calculation depending on helium flow rate, diameter of the inner cavity of cylindrical copper anode, arc current, and distance from the nozzle to metal melt were derived.