共查询到20条相似文献,搜索用时 31 毫秒
1.
Wenxia Pan Wenhong Zhang Wei Ma Chengkang Wu 《Plasma Chemistry and Plasma Processing》2002,22(2):271-283
Argon DC plasma jets in stable laminar flow were generated at atmospheric pressure with a specially designed torch under carefully balanced generating conditions. Compared with turbulent jets of short length with expanded radial appearance and high working noise, the laminar jet could be 550 mm in length with almost unchanged diameter along the whole length and very low noise. At gas feeding rate of 120 cm3/s, the jet length increases with increasing arc current in the range of 70–200 A, and thermal efficiency decreases slightly at first and then leveled off. With increasing gas flow rate, thermal efficiency of the laminar jets increases and could reach about 40%, when the arc current is kept at 200 A. Gauge pressure distributions of the jets impinging on a flat plate were measured. The maximum gauge pressure value of a laminar jet at low gas feeding rate is much lower than that of a turbulent jet. The low pressure acting on the material surface is favorable for surface cladding of metals, whereas the high pressure associated with turbulent jets will break down the melt pool. 相似文献
2.
Wenxia Pan Xian Meng Chengkang Wu Xi Chen 《Plasma Chemistry and Plasma Processing》2006,26(4):335-345
The generation, jet length and flow-regime change characteristics of argon plasma issuing into ambient air have been experimentally examined. Different torch structures have been used in the tests. Laminar plasma jets can be generated within a rather wide range of working-gas flow rates, and an unsteady transitional flow state exists between the laminar and turbulent flow regimes. The high-temperature region length of the laminar plasma jet can be over an order longer than that of the turbulent plasma jet and increases with increasing argon flow rate or arc current, while the jet length of the turbulent plasma is less influenced by the generating parameters. The flow field of the plasma jet has very high radial gradients of plasma parameters, and a Reynolds number alone calculated in the ordinary manner may not adequately serve as a criterion for transition. The laminar plasma jet can have a higher velocity than that of an unsteady or turbulent jet. The long laminar plasma jet has good stiffness to withstand the impact of laterally injected cold gas and particulate matter. It could be used as a rather ideal object for fundamental studies and be applied to novel materials processing due to its attractive stable and adjustable properties. 相似文献
3.
Modeling results are presented to compare the characteristics of laminar and turbulent argon thermal plasma jets issuing into ambient air. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of ambient air into the laminar and turbulent argon plasma jects, respectively. It is shown that since only the molecular diffusion mechanism is involved in the laminar plasma jet, the mass flow rate of ambient air entrained into the laminar plasma jet is comparatively small and less dependent on the jet inlet velocity. On the other hand, since turbulent transport mechanism is dominant in the turbulent plasma jet, the entrainment rate of ambient air into the turbulent plasma jet is about one order of magnitude larger and almost directly proportional to the jet inlet velocity. As a result, the characteristics of laminar plasma jets are quite different from those of turbulent plasma jets. The length of the high-temperature region of the laminar plasma jet is much longer and increases notably with increasing jet inlet velocity or inlet temperature, while the length of the high-temperature region of the turbulent plasma jet is short and less influenced by the jet inlet velocity or inlet temperature. The predicted results are reasonably consistent with available experimental observation by using a DC arc plasma torch at arc currents 80–250 A and argon flow rates (1.8–7.0)×10−4 kg/s. 相似文献
4.
A special bi-anode plasma torch that can change the anode arc root position without changing working gas flow rate has been developed to investigate the effect of anode arc root position on the behavior of the plasma jet. It has two nozzle-shaped anodes at different axial distances from the cathode tip. The arc root can be formed at anodes either close to the cathode tip (anode I) or far away from it (anode II) to obtain different attachment positions and arc voltages. The characteristics of pure argon plasma jets operated in different anode modes were measured in the field free region by using an emalpy probe, and the thermal efficiency of the torch was determined by measuring the temperature differences between cooling water flowing in and out of the torch. The results show that compared with the normal arc operated in anode I mode, the elongated arc operated in anode II mode significantly reduced the plasma energy loss inside the torch, resulting in a higher temperature and a higher velocity of the plasma jet in the field free region. 相似文献
5.
Hai-Xing Wang Fu-Zhi Wei Xian Meng Xi Chen Dong-Sheng Han Wen Xia Pan 《Plasma Chemistry and Plasma Processing》2011,31(1):127-138
An experimental study is conducted to investigate the entrainment characteristics of a turbulent thermal plasma jet issuing
from a DC arc plasma torch operating at atmospheric pressure. The mass flow rate of the ambient gas entrained into the turbulent
plasma jet is directly measured by use of the so-called “porous-wall chamber” technique. It is shown that a large amount of
ambient gas is entrained into the turbulent plasma jet. With the increase of the gas mass flow rate at the plasma jet inlet
or the plasma torch exit, the mass flow rate of entrained ambient gas almost linearly increases but its ratio to the jet-inlet
mass flow rate decreases. The mass flow rate of the entrained gas increases with the increase of the arc current or jet length.
It is also found that using different ways to inject the plasma-forming gas into the plasma torch affects the entrainment
rate of the turbulent plasma jet. The entrainment rate expression established previously by Ricou and Spalding (J. Fluid Mech.
11: 21, 1961) for the turbulent isothermal jets has been used to correlate the experimental data of the entrainment rates
of the turbulent thermal plasma jet, and the entrainment coefficient in the entrainment rate expression is found to be in
range from 0.40 to 0.47 for the turbulent thermal plasma jet under study. 相似文献
6.
Xiuquan Cao Deping Yu Meng Xiao Jianguo Miao Yong Xiang Jin Yao 《Plasma Chemistry and Plasma Processing》2016,36(2):693-710
Thermal plasma jets have been widely used in various materials processing techniques. However, the conventional thermal plasma torches usually generate turbulent plasma jets with the disadvantages of high axial temperature gradient, a short jet length and difficulties in the process control relatively, limiting its applications to materials processing. Therefore, this paper proposes a new laminar plasma torch (LPT) working with pure nitrogen to generate laminar plasma jet (LPJ). Its design and structural characteristics, e.g. segmented anode, axial gas injection, parallel water cooling structure, etc., are detailed to ensure the stability, the favorable temperature and velocity distribution of the generated LPJ. Experiments on the characteristics of the LPT showed that the generated LPJ possessed high specific enthalpy (ranging between 10 and 90 kJ/g), long jet length (maximum length: 480 mm) and low axial temperature gradient, and its output power a current and the gas flow rate. In addition, the thermal efficiency of the LPT was experimentally determined to be ranging between 25 and 45 %. Furthermore, experiment and simulation on the application of the LPJ for surface quenching verified the even radial temperature distribution of the plasma jet and high heat flux density brought to the surface. 相似文献
7.
Modeling study is performed to reveal the special features of the entrainment of ambient air into subsonic laminar and turbulent
argon plasma jets. Two different types of jet flows are considered, i.e., the argon plasma jet is impinging normally upon
a flat substrate located in atmospheric air surroundings or is freely issuing into the ambient air. It is found that the existence
of the substrate not only changes the plasma temperature, velocity and species concentration distributions in the near-substrate
region, but also significantly enhances the mass flow rate of the ambient air entrained into the jet due to the additional
contribution to the gas entrainment of the wall jet formed along the substrate surface. The fraction of the additional entrainment
of the wall jet in the total entrained-air flow rate is especially high for the laminar impinging plasma jet and for the case
with shorter substrate standoff distances. Similarly to the case of cold-gas free jets, the maximum mass flow-rate of ambient
gas entrained into the turbulent impinging or free plasma jet is approximately directly proportional to the mass flow rate
at the jet inlet. The maximum mass flow-rate of ambient gas entrained into the laminar impinging plasma jet slightly increases
with increasing jet-inlet velocity but decreases with increasing jet-inlet temperature. 相似文献
8.
Modeling study is performed to reveal the momentum and heat/mass transfer characteristics of a turbulent or laminar plasma
reactor consisting of an argon plasma jet issuing into ambient air and interacting with a co-axially counter-injected argon
jet. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed
to treat the diffusion of argon in the argon–air mixture for the laminar and the turbulent regimes, respectively. Modeling
results presented include the streamline, isotherm and argon mass fraction distributions for the cases with different jet-inlet
parameters and different distances between the counter-injected jet exit and the plasma torch exit. It is shown that there
exists a quench layer with steep temperature gradients inside the reactor; a great amount of ambient air is always entrained
into the plasma reactor; and the flow direction of the entrained air, the location and shape of the quench layer are dependent
on the momentum flux ratio of the plasma jet to the counter-injected cold gas. Two quite different flow patterns are obtained
at higher and lower momentum flux ratios, and thus there exists a critical momentum flux ratio to separate the different flow
patterns and to obtain the widest quench layer. There exists a high argon concentration or even ‘air-free’ channel along the
reactor axis. No appreciable difference is found between the turbulent and laminar plasma reactors in their overall plasma
parameter distributions and the quench layer locations, but the values of the critical momentum flux ratio are somewhat different. 相似文献
9.
Measurements of composition, temperature, and velocity in atmospheric argon plasma jets are reported, using enthalpy probes. The plasma jets are generated by a commercial type plasma gun and the measurements are expected to be of particular interest for industrial applications such as plasma spraying. Emphasis has been on the central and downstream regions of the plasma flame. The entrainment of air into the jet was found to be very high, even close to the axis of the jet. Gas samples analyzed with a gas chromatograph showed demixing of the air, i.e., nitrogen is more abundant in the jet than at room temperature. The high air entrainment has a strong cooling effect on the plasma, resulting in a rapid temperature drop along the axis. The influence of the argon flow rate and of the arc current on the jet's conditions was parametrically studied. Matching of the quantities measured in the jet with the torch input confirmed the validity of the results, and the relevance of enthalpy probe diagnostics in thermal plasma jets. 相似文献
10.
Influence of the Gas Injection Angle on the Jet Characteristics of a Non-transferred DC Plasma Torch
The non-transferred direct current (DC) plasma torch has been widely used in various industrial applications due to its special jet characteristics. The jet characteristics are determined by different factors, including the working parameters, the torch construction, the gas injection angle (GIA) etc. As there is little study on the influence of the GIA on the jet characteristics, experimental study on the GIA’s effects on the jet characteristics has been carried out on a specially designed non-transferred DC plasma torch, whose GIA can be changed by replacing a gas injection component. The arc voltages and thermal efficiencies of the plasma torch, the specific enthalpies and jet lengths of the plasma jets at different working conditions were obtained and analyzed. It has been found that the GIA greatly affects the arc voltage, the thermal efficiency, the specific enthalpy and the jet length. Based on these findings, plasma torch with appropriate GIA could be used to help generating the plasma jet with desired characteristics. 相似文献
11.
When materials processing is conducted in air surroundings by use of an impinging plasma jet, the ambient air will be entrained
into the materials processing region, resulting in unfavorable oxidation of the feedstock metal particles injected into the
plasma jet and of metallic substrate material. Using a cylindrical solid shield may avoid the air entrainment if the shield
length is suitably selected and this approach has the merit that expensive vacuum chamber and its pumping system are not needed.
Modeling study is thus conducted to reveal how the length of the cylindrical solid shield affects the ambient air entrainment
when materials processing (spraying, remelting, hardening, etc.) is conducted by use of a turbulent or laminar argon plasma
jet impinging normally upon a flat substrate in atmospheric air. It is shown that the mass flow rate of the ambient air entrained
into the impinging plasma jet cannot be appreciably reduced unless the cylindrical shield is long enough. In order to completely
avoid the air entrainment, the gap between the downstream-end section of the cylindrical solid shield and the substrate surface
must be carefully selected, and the suitable size of the gap for the turbulent plasma jet is appreciably larger than that
for the laminar one. The overheating of the solid shield or the substrate could become a problem for the turbulent case, and
thus additional cooling measure may be needed when the entrainment of ambient air into the turbulent impinging plasma jet
is to be completely avoided. 相似文献
12.
G. M. W. Kroesen D. C. Schram J. C. M. de Haas 《Plasma Chemistry and Plasma Processing》1990,10(4):531-551
The plasma in a cascaded arc in argon with flow is studied both experimentally and theoretically. The plasma pressure has been measured as a function of axial position in the are channel with a Baratron pressure transducer. The electron density and the electron temperature have been determined as a function of axial position using Hβ-Stark broadening and line-continuum emissivity ratio, respectively. Comparison of the gas pressure measurements with an equilibrium model suggests that the /low is laminar. A one-dimensional nonequilibrium model based on the electron- and heavy-particle number balances and the heavy-particle energy balance is presented. The measured axial profiles of the electron density agree well with the model predictions, especially in the most upstream part of the arc channel. The plasma is strongly ionizing. Temperature equilibration takes about 20 mm of arc length, depending on the argon flow. 相似文献
13.
Direct current (dc) plasma torch with inter-electrode inserts has the merits of fixed arc length, relative high enthalpy and
may show advantages in future plasma processes where stability and controllability are must-have. Energy fluctuations in the
plasma may result from power supply ripple, arc length variation, and/or acoustic oscillation. Using an improved power supply
with a flat waveform, the characteristics of an argon plasma energy instabilities under reduced pressure were studied by means
of simultaneously monitoring the arc voltage and arc current spectrum. Dependence of the arc fluctuation behavior on the plasma
generating parameters, such as the current intensity, the plasma gas flow rates and the vacuum chamber pressure were investigated
and discussed. Results show that the plasma torch has a typical U-shaped voltage-ampere characteristic (VAC). The correlation
between the VAC and the probability of energy distributions was studied. Through pressure measurements at the cathode cavity
and the vacuum chamber, the existence of sonic flow in the inter-electrode insert channel was confirmed. 相似文献
14.
A two-fluid model of turbulence for a thermal plasma jet 总被引:2,自引:0,他引:2
15.
Effects of Nozzle Length and Process Parameters on Highly Constricted Oxygen Plasma Cutting Arc 总被引:1,自引:0,他引:1
Qianhong Zhou Hui Li Feng Liu Shaofeng Guo Wenkang Guo Ping Xu 《Plasma Chemistry and Plasma Processing》2008,28(6):729-747
The influence of nozzle length and two process parameters (arc current, mass flow rate) on the plasma cutting arc is investigated.
Modeling results show that nozzle length and these two process parameters have essential effects on plasma arc characteristics.
Long nozzle torch can provide high velocity plasma jet with high heat flux. Both arc voltage and chamber pressure increase
with the nozzle length. High arc current increases plasma velocity and temperature, enhances heat flux and augments chamber
pressure and thus, the shock wave. Strong mass flow has pinch effect on plasma arc inside the torch, enhances the arc voltage
and power, therefore increases plasma velocity, temperature and heat flux. 相似文献
16.
Yugesh Vadikkeettil Ravi Ganesh Ramachandran Kandasamy Vidhi Goyal Kailsha Chandra Meher 《Plasma Chemistry and Plasma Processing》2018,38(4):759-770
The characteristics of the plasma jet emanating from a dc non-transferred plasma torch is affected by many factors including arc current, type of gas, gas flow rate, gas injection configuration and torch geometry. The present work focuses on experimental investigation of the influence of shroud gas injection configuration on the I–V characteristics and electro-thermal efficiency of a dc non-transferred plasma torch operated in nitrogen at atmospheric pressure. The plasma gas is injected into the torch axially and shroud gas is injected through three different nozzles such as normal, sheath and twisted nozzles. The effects of flow rates of plasma/axial gas and arc current on I–V characteristics and electro-thermal efficiency of the torch holding different nozzles are investigated. The I–V characteristics and electro-thermal efficiency of the torch are found to be strongly influenced by the shroud gas injection configuration. The effect of arc current on arc voltage decreases with increasing the axial gas flow rate. At higher axial gas flow rate (>?45 lpm), the I–V characteristics of the plasma torch are similar irrespective of the nozzle used. The variation of electro-thermal efficiency with arc current is almost similar to that of arc voltage with arc current. As expected, the electro-thermal efficiency is increased when the axial gas flow rate is increased and at higher axial gas flow rate, it is not influenced by the arc current and shroud gas configuration. The plasma torch with normal nozzle may be better in the range of operating conditions used in this study. 相似文献
17.
Tianyang Li Christophe Rehmet Yan Cheng Yong Jin Yi Cheng 《Plasma Chemistry and Plasma Processing》2017,37(4):1033-1049
Methane pyrolysis via thermal plasma was investigated experimentally on a 2 kW DC arc plasma setup in argon atmosphere. Two widely applied methane pyrolysis profiles, i.e., pre-mixing methane and argon before fed into plasma torch, and injecting methane into pure argon plasma jet at torch outlet, were compared. Performances of methane pyrolysis regarding to methane conversion, acetylene selectivity, acetylene specific energy requirement (SER), and plasma stability were concluded. Results showed that pre-mixing methane and argon before fed into plasma torch would be efficient in converting methane and acetylene production, with higher conversion of methane and lower SER to acetylene at a given specific energy. Also, methane in arc zone would cause periodic fluctuations of plasma voltage and power, which could be reduced by controlling methane fraction in feed. On the other hand, when methane was injected into argon plasma jet at torch outlet, the energy efficiency in converting methane and producing acetylene would be lower. And the plasma would barely participate in the reaction other than providing heat, but the erosion of electrode was much slower and slighter. It was also validated that the SER of acetylene was limited by the thermal loss of the setup due to size-effect of reactor. 相似文献
18.
This paper attempts to predict the noise emission characteristics of a turbulent argon thermal-plasma jet issuing into ambient
air. The flow, temperature and concentration fields and turbulence characteristics of the turbulent plasma jet are computed
at first, and then the noise emission from the plasma jet to a sideline far-field observer is calculated using the approach
proposed by Fortuné and Gervais (AIAA J. 37(1999)1055) for predicting the noise emission from a turbulent, hot but non-ionized,
air jet after some modification. The diffusion of ambient air into the turbulent argon plasma jet is handled using the turbulence-enhanced
combined–diffusion-coefficient method. Velocity fluctuation correlations (aerodynamic noise source) in the plasma jet are
calculated still using the K-ɛ two-equation turbulence model, but the temperature-velocity fluctuation correlations (entropic
noise source) within the jet are calculated by solving a second-order turbulent Reynolds heat-flux transport equation in order
to better deal with the contribution of temperature fluctuation to the noise emission. It is shown that among the contributions
of aerodynamic noise source, entropic noise source and their mixed effect, the entropic noise source (i.e. the temperature-velocity
fluctuation correlations) is dominant for the noise emission from the turbulent plasma jet to the sideline observer. The noise
intensity increases with increasing plasma jet temperature or velocity. The predicted noise frequency spectrum characteristics
and noise intensity levels are shown to be reasonably consistent with available experimental data. 相似文献
19.
Heat Generation and Particle Injection in a Thermal Plasma Torch 总被引:1,自引:0,他引:1
Vardelle A. Fauchais P. Dussoubs B. Themelis N. J. 《Plasma Chemistry and Plasma Processing》1998,18(4):551-574
The operation of plasma guns used for plasma spraying involves a continuous movement of the anode arc root. The resulting fluctuations of voltage and thermal energy input introduce an undesirable element in the spray process. This paper deals with the effects of these arc instabilities on the plasma jet, and the behavior of particles injected in the flow. The first part refers to the formation of the plasma jet. Measurements show that the static behavior of the arc depends strongly upon the plasma-forming gas mixture, especially the mass flow rate, of the heavy gas, injection mode, nozzle diameter, and arc current. These parameters control the electric field in the arc column, the arc length, its stability, and the gas velocity and temperature. The dynamic behavior of the arc is examined to determine how the tempeature and velocity of the plasma gas vary with voltage variations. Relationships between the gas velocity at the nozzle exit and the lifetime of the arc roots, and the independent operating parameters of the gun can be established from a dimensional analysis. The second part discusses the interaction between the plasma jet and the particles injected into the flow. The parameters controlling particle injection and trajectory are examined to determine how injection velocity must vary with particle size and density to achieve a given trajectory. The effect of the transverse injection of the powder carrier gas is investigated using a 3-D computational fluid dynamics code. Finally, the effect of the jet fluctuations on particle trajectory is studied under the assumption that the jet velocity follows the voltage variation. The result is a continuous variation of the particle spray jet position in the flow. Experimental observations confirm the model predictions. 相似文献
20.
J. F. Brilhac B. Pateyron J. F. Coudert P. Fauchais A. Bouvier 《Plasma Chemistry and Plasma Processing》1995,15(2):257-277
This work was devoted to the study of the dynamic and static behavior of de vortex plasma torch with a well-type cathode (power level below 100 kW). The dynamic behavior of the torch was characterized by the fulctuations of arc voltage and current, plasma jet radiation, and acoustic pressure. Characteristic frequencies of the arc root movement inside the torch were observed. By numerical simulation (with the numerical codeMelodie, it was shown that the position of the erosion diameter) of the axial velocity along the cathode channel near the wall. The static behavior of the torch was inverstigated for different cathode designs. The variations of voltage U with arc current I, gas flow rate G nature of the gas and cathode design were represented by semiempirical relationships established between dimensionless numbers. By dimensional analysis, the behavior of this torch was compared with that of two powerful torches: the Aerospatiale and the Plasma Energy Corporation torches. 相似文献