Numerical and experimental studies on DC plasma spray torch

One of the challenging problems in the plasma spray technique is reproducibility of the coating quality. This problem is mainly associated with arc fluctuations, which affect the plasma jet temperature and velocity, inside the plasma torch. In this study, 3D numerical models are developed to study the arc behavior inside the torch and effect of arc fluctuations on plasma jet temperature and velocity. Plasma arc is simulated for different operating parameters. Different arc sizes are predicted by using thermo-dynamical principle of minimum entropy production for given torch power. The influence of arc current and gas flow rate on the Ar-N₂ plasma arc characteristics, plasma jet and torch efficiency is presented. Predicted torch efficiencies and arc voltages are comparable with measurements. At the nozzle exit, velocity shows stronger three-dimensional effect than temperature. Plasma jets are simulated using different nozzle exit profiles obtained from the plasma arc model and their temperature and velocity distributions are clarified.     

Optical investigations of multicomponent plasma of capillary discharge. Supersonic outflow regime

We present the results of spectroscopic investigations of the plasma of an impulse discharge in a capillary with an ablation wall made of carbon-containing polymer, within the discharge pulse parameters providing the supersonic flow regime of a plasma jet. Based on a 2D-matrix high resolution spectra containing H_α, Cu I, Cu II, C I, C II, and CN and the Swan molecular bands, we obtain spatiotemporal distributions of the electron number density and the plasma temperature in the capillary and the supersonic plasma jet. We reveal the peculiarities of the spatial distribution of the electron number density and of the spectral component intensity within both above stated zones, conditioned, in particular, by achievement, in the hot central zone, of an electron temperature above the “normal” temperature, as well as by essential nonisobaricity of the initial section of the plasma jet. The emission properties of the high-temperature jet core–the intensities and the profiles of the H_α and H_β Balmer lines, relative intensities of the C II and O II ion lines–registered with high temporal (10 μs) and spatial (20–30 μm) resolution make it possible to discover the main regularities in the spatiotemporal distributions of pressure, temperature, and ionization degree in the capillary and in the supersonic heterogeneous jet of the erosion discharge. Due to the presence in the flow of the molecular components displaying their emission properties at the jet periphery, we manage to obtain information on the plasma parameters within the zone of formation of the “intercepting” shocks in the supersonic jet.     

The application of hybrid RANS/ILES approach for the investigation of the effect of nozzle geometry and mode of efflux on the characteristics of turbulence of exhaust jets

The efficient hybrid RANS/ILES approach is used for the investigation of the effect of nozzle geometry and parameters of flow at the nozzle exit section on the characteristics of turbulence in the exhaust jet. The calculations are performed for nozzles of different types such as conical and chevron nozzles and the nozzle of double-flow turbojet engine (TJE) with chevrons on the core nozzle. The effect of the foregoing parameters on the level of fluctuations of velocity and pressure in the investigated jets is demonstrated. The effect of off-design mode of efflux on the parameters of turbulence in the jet is investigated under conditions of supersonic efflux of the jet. The effect of misalignment of the core and fan nozzles on the flow in the jet is considered for the nozzle of double-flow TJE. Grids containing about 10⁶ cells are used for the calculations. The accuracy of the results is confirmed by comparison with the known experimental data.     

量热探针在热等离子射流测量中的应用

为了测量等离子射流的温度和速度,设计了量热探针测量系统,采用补偿式的量热法来测量热等离子体 的比焓得到热等离子体的温度;也可作为水冷皮托管来测量射流的动压头确定射流流速。系统采用Φ0.8 mm探 针实际测量最高温度达8515K。根据等离子喷枪的输入功率和热效率估算出喷枪出口处射流的温度和速度,与探 针的测量结果比较吻合。     

On the gas flow rate through a Mach disk in an underexpanded jet

An air jet with an excess static pressure at a regime of escape into quiescent air is considered. The range of Mach number variation is 1–3, and that of the inefficiency ratio 1–1000. An analysis has been made on the basis of the well-known experimental-calculated approximation dependences for stationary expanding flows. We solve the inverse problem where the parameters at the nozzle exit and the geometric characteristics of the initial section of the jet are known and it is required to find the remaining quantities corresponding to them. In the present analysis, this is the rate of flow through a central discontinuity and the integral characteristic of the magnitude of the total pressure loss. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 82, No. 2, pp. 320–325, March–April, 2009.     

Nanokristalline Werkstoffe hergestellt mit dem Thermischen HF‐Plasma

Nano‐crystalline materials manufactured with the Thermal RF‐Plasma The Inductively Coupled Plasma (ICP) at atmospheric pressure is particularly suited for melting and evaporation of materials. The electrodeless ICP can be generated without limitation of the kind of plasma forming gases. Therefore, using an argon‐oxygen gas mixture as sheath gas of the ICP nanophase coatings can be processed by synthesis with metal‐organic liquid precursors injected in the hot plasma core. For depositions, the plasma jet has to be supersonic. For particles which impinge onto the substrate placed near the nozzle exit of the plasma torch thin and dense coatings are obtained with crystallite sizes of 30‐ 40 nm. The composition and the grain size of as‐deposited coatings are analyzed by XRD.     

球头体逆向喷流减阻的数值模拟研究

为研究逆向喷流对超声速球头体减阻的影响,该文结合标准k-ε湍流模型,通过求解轴对称和三维Navier-Stokes方程,数值模拟了超声速球头体逆向冷喷流流场,着重分析了喷口总压、喷口尺寸及攻角对流场模态和减阻效果的影响。计算结果显示:喷流能使球头体受到的阻力明显减小;随着喷流总压的增大,在不同喷口尺寸和攻角下,流场均先后经历长射流和短射流穿透模态;存在最大减阻临界喷流总压值,该值与喷口尺寸比呈近似的线性关系,在所研究参数范围内最大减阻可达54.7%;随着攻角的增大,流场的不对称性加强,减阻效果下降。该文的研究对超声速钝体减阻技术在工程上的应用具有一定的参考价值。     

Effect of the Angular Distribution of Cathode Ions on the Current Flow in a High-Current Vacuum Arc

The effect of the angular distribution of cathode ions on the parameters of a vacuum arc is investigated. It is shown that, at current densities characteristic of a high-current arc, collisions of flows of fast ions from different cathode spots may result in a high value of the ion temperature at the cathode plasma boundary. The boundary in current density of the region of a stable current flow as a function of the width of angular distribution of cathode ions is determined. It is found that, in the region of supersonic flow of ions, the permissible value of the current density drops rapidly as the angular distribution width increases, and in the subsonic region, it rises rapidly. An expression is obtained for the maximum possible ion sound velocity and for the equivalent critical ion velocity. Two- and three-fluid hydrodynamic models are used to perform calculations that illustrate characteristic distributions of parameters in the discharge gap. Analysis of the calculation results makes it possible to discuss the characteristic features of current flow in a vacuum discharge.     

Heat and mass transfer during in-flight nitridation of molybdenum disilicide powder in an induction plasma reactor

The present study reveals some of the important parameters which control the in-flight nitridation of molybdenum disilicide (MoSi₂) powders when carried out in an induction thermal plasma reactor. Initially, gradients of temperature, velocity and concentration were evaluated, using an enthalpy probe system, for the plasma flow without injection of MoSi₂ powders. Radial profiles were then measured at the torch exit to examine the mass and energy transfer mechanisms occurring under different nitridation conditions. These measurements were performed using an induction plasma torch connected to a 50 kW radio-frequency (r.f.) power supply, the torch being attached to a water cooled cylindrical reactor. The process operating conditions studied were plasma plate power, chamber pressure, sheath gas composition, composition and flow rate of quench gas. The effect of last named parameter on the nitridation of the powders was found to be the most important parameter in the nitridation process. The results show that there is an optimum flow rate value for each type of quench gas and the temperature and concentration mapping demonstrates that the combination of high temperatures and high concentrations of N₂ are necessary to reach maximum nitridation levels in MoSi₂.     

Calculation of the velocity of a plasma flow in the nozzle exit section of a coaxial-electrode Hall accelerator

The velocity of a plasma jet in the nozzle exit section and the pressure in the discharge zone of a coaxial-electrode Hall accelerator have been calculated on the basis of the experimentally measured enthalpy, temperature, and electron concentration near the indicated section within the framework of a model of the magnetic hydrodynamics of a plasma flow. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 3, pp. 102–108, May–June, 2006.     

低压等离子喷涂氧化铝涂层的特性

以细小的氧化铝为热喷涂粉末,采用低压等离子喷涂制备了沉积率高于50%,孔隙率低于2%的氧化铝涂层.研究了不同工艺下低压等离子喷涂氧化铝涂层的沉积率、相组成和显微结构,并对低压等离子功率和真空室压力工艺参数对涂层的影响进行了分析.研究结果表明,所制备的涂层以α-Al₂O₃和γ-Al₂O₃相并存;随着功率和压力提高,涂层的孔隙率有明显的降低,但压力达到23.7kPa时功率影响较小.此外,还对等离子焰流中的粒子温度和速度进行了计算.结果表明,在23.7kPa压力下保证粒子充分熔融的前提下使粒子具有较高的运动速度.     

Plasma-dynamic discharges in transverse supersonic air flows

An experimental investigation is performed of modes of outflow into stationary air and of supersonic flow of a plasma jet developed by an impulse plasma generator using a plasma-dynamic discharge of magnetoplasma compressor at low values of energy deposition. A theoretical model is suggested which enables one to scale the experimental results to the injection of plasma jet into a flow of large cross section. The possibility is demonstrated of using such jets for bulk ignition of supersonic flows of combustible mixture.     

Braking of a “Magnetized” Sphere in a Hypersonic Rarefied Plasma Flow

The dependence of the drag coefficient of a “magnetized” (with a self-generated magnetic field) sphere on the ratio of the magnetic pressure to the gas dynamic pressure in a hypersonic rarefied plasma flow is determined experimentally. The dependences are obtained for a wide range of angles between the vectors of the incident flow velocity and the magnetic field, as well as between the velocity vectors of the incident hypersonic plasma flow and a subsonic plasma jet injected from the surface of the sphere. It is shown that the injection of a subsonic plasma jet from the surface of a “magnetized” sphere in a hypersonic rarefied plasma flow increases its drag coefficient by several times in comparison with an “unmagnetized” sphere.     

Supersonic flow regimes with a pulsating laser radiation energy supply

A comparative analysis of the gas flow regimes behind the region of laser energy supply to a supersonic jet is performed depending on the laser radiation focusing conditions. A relationship established between the energy and gasdynamic parameters reveals a significant difference in the flow parameters (velocity, stagnation pressure) behind the extended and localized (point) thermal plasma sources for the identical initial conditions in a supersonic jet with the Mach numbers M=1.5–10.     

In-flight behavior of lanthanum zirconate (La2Zr2O7) particles in gas tunnel type plasma jet and its coating properties

Numerical simulation of in-flight particles in thermal plasma jet is one of the most important research fields. It has been used to analyze the influence of spray parameters on jet characteristics and to improve the quality of coating during plasma spraying. In this study, in-flight behavior of a group of lanthanum zirconate (La₂Zr₂O₇) particles with different diameters (10–60 μm) in gas tunnel type plasma jet is investigated by numerical modeling. The influence of torch power on the plasma jet is investigated and its interaction with different sizes of La₂Zr₂O₇ particles is studied under optimized spraying conditions. The resultant coating properties are also investigated and correlated with simulation results. The simulation results showed that the plasma jet temperature and velocity increased while increasing the torch power. Consequently, the in-flight particle temperature and velocity profile also increased with respect to the torch power.     

Transverse electric discharges in supersonic air flows: Simulation of the effects influencing the heating of gas in discharge channel

The Navier-Stokes equations are used to numerically investigate the gasdynamic aspects of interaction between a transverse discharge and supersonic flow in a model of heat source. The effect made by the jet and by the gasdynamic interaction of discharge channels on temperature is analyzed for plane flows with a pair of identical heat sources. Axisymmetric flows are calculated for a uniform incident flow. A marked variation of the flow parameters in the discharge zone and in the wake is observed if the power of energy input corresponds to, or is higher than, the gas enthalpy flow incident via the characteristic cross section of heat source. The calculation results are compared with experimental data. Explanations are provided of the results of experiments in heating a supersonic flow by a transverse electric discharge.     

真空电弧等离子射流蒸发法沉积超微粉

在真空电弧等离子喷涂设备上设计安装了超微粉的制备装置。制备出 Fe,Si,Ni,Mo,TiAl,Ti_3Al 和 Si_3N_4等超微粉。通过控制真空室工作压力和等离子弧电流,超微粉的粒度可分别为5,10,20和50nm。观察了粉的形状、分布,测试了沉积速率。     

Mass addition of a supersonic jet

Presented are results of an experimental determination of the mass addition for a supersonic jet in flow regimes with n = P_ch/P_a 1. An empirical formula is given relating the mass addition in the initial section to the Mach number M _a at the nozzle exit, the ratio n, and the distance from the nozzle exit section to the section examined.     

Extent of the subsonic domain in a supersonic underexpanded jet

A method is proposed to determine the extent of the subsonic domain in a supersonic underexpanded jet. The method is based on representing the flow parameters in the form of series.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 38, No. 1, pp. 44–48, January, 1980.