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1.
Outside of the fields where flow visualization is traditionally applied, there exist many processes where fluid phenomena are critical. Here, we survey flow visualization work with a focus on two thermal metal cutting processes. These two processes – plasma-arc cutting and gas assisted laser cutting – account for a large fraction of the means by which steel is cut in our world. Plasma-arc cutting utilizes an electric arc transferred between a cathode and the steel being cut to produce a high temperature gas jet that melts and removes metal. In gas assisted laser cutting, the assist jet is often high-pressure supersonic nitrogen for stainless steel, or near-atmospheric pressure, low-speed oxygen for carbon steel. Visualization of these millimeter-range diameter jets helps to understand the different roles that the assist gas has in these cutting processes, particularly with how the jets interact with the metal being cut. We describe experimental techniques for visualization of the arc jet and gas assist jet, as well as the liquid metal flows being removed from the cut and the gas flow in the torch itself. These visualizations overcome the small physical scales of the process, the bright illumination from the arc itself, and harsh high-temperature environment. The results lend perspective and understanding of the physical phenomena important to process control. 相似文献
2.
A study was made to evaluate the electrothermal efficiency of a DC arc plasma torch and temperature and thermal conductivity
of plasma jet in the torch. The torch was operated at power levels from 4 to 20 kW in non-transferred arc mode. The effect
of nitrogen in combination with argon as plasma gas on the above properties was investigated. Calculations were made from
experimental data. The electrothermal efficiency increased significantly with increase in nitrogen content. The plasma jet
temperature and thermal conductivity exhibited a decrease with increase in nitrogen content. The experiment was done at different
total gas flow rates. The results are explained on the basis of dissociation energy of nitrogen molecules and plasma jet energy
loss to the cathode, anode and the walls of the torch 相似文献
3.
4.
Breiter M. Nutsch G. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1997,25(5):872-876
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 相似文献
5.
基于可压缩的全Naiver-Stokes方程,利用PHOENICS程序对由会聚 辐射阳极形状等离子体炬产生的超声速等离子体射流进行了数值模拟.考虑了等离子体的黏性、可压缩性以及变物性对等离子体射流特性影响.研究了超声速等离子体射流的流场结构特性以及不同环境压力对等离子体射流产生激波结构的影响.结果表明,超声速等离子体射流在喷口附近形成的周期性激波结构是其和环境气体相互作用的结果.
关键词:
等离子体炬
超声速等离子体射流
PHOENICS 相似文献
6.
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10mm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 相似文献
7.
介绍了一种新型大气压直流双阳极等离子喷枪,并对其电特性参数和发射光谱进行了测量.通过对氩等离子体射流的电信号进行时域和频域分析,研究了载气流量和弧电流的变化对射流脉动的影响,结果表明氩等离子体电弧的伏安特性呈上升趋势,射流脉动属于接管模式,电源特性中的交流分量引起的电压波动是影响氩等离子体射流脉动的主要因素. 通过光谱法测量了氩等离子体射流在弧室内和弧室出口的发射光谱,利用玻尔兹曼曲线斜率法计算了射流的激发温度,根据Ar I谱线的斯塔克展宽得到了射流的电子密度,并对等离子体射流满足局域热力学平衡(LTE)
关键词:
等离子喷枪
射流脉动
激发温度
局域热力学平衡 相似文献
8.
《等离子体物理论文集》2017,57(2):58-66
Plasma jets from conventional non‐transferred arc plasma devices are usually operated in turbulent flows at atmospheric pressure. In this paper, a novel non‐transferred arc plasma device with multiple cathodes is introduced to produce long, laminar plasma jets at atmospheric pressure. A pure helium atmosphere is used to produce a laminar plasma jet with a maximum length of >60 cm. The influence of gas components, arc currents, anode nozzle diameter, and gas flow rate on the jet characteristics is experimentally studied. The results reveal that the length of the plasma jet increases with increasing helium content and arc current but decreases with increasing nozzle diameter. As the gas flow rate increases, the length of the plasma jet initially increases and then decreases. Accordingly, the plasma jet is transformed from a laminar state to a transitional state and finally to a turbulent state. Furthermore, the anode arc root behaviours corresponding to different plasma jet flows are studied. In conclusion, the multiple stationary arc roots that exist on the anode just inside the nozzle entrance are favourable for the generation of a laminar plasma jet in this device. 相似文献
9.
介绍一种结构设计简单、操作运行方便的新型毫米量级大气压冷等离子体射流发生技术.这种射流可以在大气压条件下,利用多种工作气体(如Ar,He,N2),通过毛细管介质阻挡放电(DBD)的方式实现.使用频率为33kHz,峰值电压为1—12kV的双向脉冲电源,利用Ar,He,N2等工作气体,在毛细管内形成了稳定的冷等离子体射流.放电区域的光辐射空间分布利用商用CCD摄像机记录,从中研究放电形态和空间分布,观察到了在DBD区域的流动气体放电和在毛细管出口处形成的等离子体射流
关键词:
冷等离子体射流
毛细管介质阻挡放电
射流射程
射流激发温度 相似文献
10.
F. Quintero J. Pou J.L. Fernndez A.F. Doval F. Lusquios M. Boutinguiza R. Soto M. Prez-Amor 《Optics and Lasers in Engineering》2006,44(11):1158-1171
The aerodynamic features of the gas flow during laser fusion cutting are an essential factor influencing the cut performance. For this reason it has been a subject of some studies to explain the interactions of the gas jet with the workpiece and to design different gas injection systems with the aim of preventing the drawbacks of the conventional cutting heads.An off-axis cutting head with a de Laval nozzle to inject a supersonic gas jet has been previously demonstrated to be an effective design to achieve a complete removal of the molten material from the cutting front and to avoid the formation of the recast layer. In the present work, the fundamentals and procedures to adjust the main factors determining the efficiency of this gas injection system are described. Specifically, the gas flow inside the cut kerf is analysed by means of flow visualization using the Schlieren technique. 相似文献
11.
小功率等离子体射流的流特性 总被引:3,自引:0,他引:3
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。 相似文献
12.
Numerical modelling of a direct current non-transferred thermal plasma torch for optimal performance
A laminar steady-state 2D axisymmetric model of a direct current (DC) thermal plasma torch using a magneto-hydrodynamic approach has been developed. The model takes into account the entire torch system comprising the plasma gas injection, the inner region of the torch, and the jet exiting into the ambient environment. Numerical results are obtained for two different power inputs chosen from published experimental data. The temperature predictions at the torch exit are found in good agreement with experimental results. Velocity analysis of the plasma jet has been presented and the impact of electromagnetic force on jet velocity is analysed. The Lorentz force arising due to the coupling of fluid motion and electromagnetic forces shoots up the jet velocity to significantly high values near the cathode tip. Temperature and velocity profiles are in good agreement with the characteristics of a long laminar plasma jet. An operating value of heat transfer coefficient (h) has been suggested for optimal torch operation, thus ensuring a low anode erosion rate and acceptable thermal efficiency. The argon torch has the maximum temperature and longest jet length among the plasma gases considered. 相似文献
13.
Collares M.P. Pfender E. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1997,25(5):864-871
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 相似文献
14.
Wide ranges of technological applications involve arc plasma devices as the primary plasma source for processing work. Recent findings exhibit the existence of appreciable thermal non-equilibrium in these so-called thermal plasma devices. Commercially available magnetohydrodynamic codes are not capable of handling such systems due to unavailability of non-equilibrium thermodynamic and transport property data and self-consistent models. A recipe for obtaining mechanical design of arc plasma devices from numerical simulation incorporating two-temperature thermal non-equilibrium model is presented in this article with reference to the plasma of the mixture of molecular gases like nitrogen and oxygen. Such systems are technologically important as they correspond to the plasma devices operating with air, oxygen plasma torches in cutting industries and plasma devices using nitrogen as shielding gas. Temperature field, associated fluid dynamics and electrical characteristics of a plasma torch are computed in a systematic manner to evaluate the performance of a conceived design using a two-fluid CFD model coupled with a two-temperature thermodynamic and transport property code. Important effects of different nozzle designs and plasma gases obtained from the formalism are discussed. Non-equilibrium thermodynamic properties are computed using modified two-temperature Saha equations and transport properties are computed using standard Chapman–Enskog approach. 相似文献
15.
Although plasma torches have been commercially available for about 50 years, areas such as plasma gun design, process efficiency,
reproducibility, plasma stability, torch lives etc. have remained mostly unattended. Recent torch developments have been focusing
on the basic understanding of the plasma column and its dynamics inside the plasma torch, the interaction of plasma jet and
the powders, the interaction of the plasma jet with surroundings and the impingement of the jet on the substrate. Two of the
major causes of erratic and poor performance of a variety of thermal plasma processes are currently identified as the fluctuations
arising out of the arc root movement on the electrodes inside the plasma torch and the fluid dynamic instabilities arising
out of entrainment of the air into the plasma jet. This paper reviews the current state of understanding of these fluctuations
as well as the dynamics of arc root movement in plasma torches. The work done at the author’s laboratory on studying the fluctuations
in arc voltage, arc current, acoustic emissions and optical emissions are also presented. These fluctuations are observed
to be chaotic and interrelated. Real time monitoring and controlling the arc instabilities through chaos characterization
parameters can greatly contribute to the understanding of electrode erosion as well as improvement of plasma torch lifetime. 相似文献
16.
Hrabovsky M. Konrad M. Kopecky V. Sember V. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1997,25(5):833-839
Experimental investigation of an electric arc stabilized by a water vortex was carried out in a DC arc plasma torch for the power range 90-200 kW. Volt-ampere characteristics of the arc as well as the power balance were determined separately for the part of the arc column stabilized by water and for the remaining part between the nozzle exit and the external anode. The temperature of arc plasma close to the nozzle exit was determined by emission spectroscopy. Negatively biased electric probes in the ion collecting regime were used for determination of the plasma flow velocity. The measured temperatures up to 27000 K, and velocities up to 7 km/s are higher than the values commonly reported for plasma torches with DC arcs stabilized by a gas flow. Mass and energy balances within the arc chamber were determined from the experimental results. The radial transport of the energy by radiation was identified as a decisive process controlling the arc and plasma properties. The balance of radial energy transport was studied. The ratio of energy spent for evaporation of the water to the energy absorbed in the evaporated mass is very low in the water stabilized arc. This is the principal cause of high plasma temperatures and velocities found by the measurements 相似文献
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18.
基于同轴传输线结构设计了两种不同喷嘴结构的大气压微波等离子体射流(MW-APPJ)装置,其工作频率2.45 GHz,工作气体为氩气,分别研究了两种不同喷嘴结构对等离子体放电特性产生的影响。仿真结果表明,MW-APPJ在气体喷嘴处会产生高强度的电场,经过优化结构,实现在频率2.45 GHz下,喷嘴处的场强满足氩气电离的击穿场强阈值要求。同时,利用多物理场耦合仿真软件对装置的气流分布进行了稳态模拟,并通过实验对比分析了两种喷嘴结构下大气压氩等离子体射流的基本特性。实验结果表明,不同的喷嘴结构会影响等离子体装置的反射系数随输入功率的变化规律,但并不影响等离子体射流长度随输入功率的变化规律和反射功率随进气流量的变化规律;同时,在大气压下,稳态微波等离子体射流呈现出类金属性,等离子体中的电子只能在很薄的区域中吸收微波能量,因而造成微波的反射功率较大。 相似文献
19.
Erich Kranz 《等离子体物理论文集》1971,11(1):13-21
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. 相似文献