Influence of the processing conditions on the characteristics of the clad layers produced with laminar plasma technology

Laminar plasma technology was used to produce ceramic hardened layers of Al₂O₃-40% mass Ni composite powders on stainless steel substrates. In order to investigate the influences of processing conditions on the morphologies of the surface modified layers, two different powder-feeding methods were tested, one with carrier gas called the powder injection method, and the other without carrier gas called powder transfers method. The microscopic investigations demonstrate that the cross-section of the clad layers consists of two distinct microstructural regions, in which the Al₂O₃ phases exhibit different growth mechanisms. When the powder transfers method is adopted, the number density and volume fraction of the Al₂O₃ particles increase considerably and their distributions exhibit zonal periodical characteristics. When the powder-feeding rate increases, the microstructure of the Al₂O₃ phases changes from a small globular to a long needle shape. Finite element simulations show that the transient thermo-physical features of the pool substances, such as solidification rate and cooling rate, influence strongly the mechanisms of the nucleation and the directional growth of the Al₂O₃ phases in the thermal processing.     

Plunging method for Nd : YAG laser cladding with wire feeding

The effects of wire feeding direction and position, cladding time, and cladding speed on the quality of cladding layer for laser cladding with wire feeding are studied. Experiment results indicate that the wire feeding direction and position are important for wire laser cladding. By adopting the correct wire feeding direction and position, the wire can be plunged into the melt pool and can be melted by the heat of the molten metal. With the increase of cladding speed, the dilution of clad layer can be reduced and the growth of the grain size of heat-affected zone of the base metal can be limited. If proper cladding parameters are used, the clad layers will have a good surface shape, sound metallurgical bonding with base metal, low dilution, and the effect of heating on heat-affected zone metal can be limited. The hardness of the clad layer and heat-affected zone are checked. It indicates that with the increase of cladding speed, the hardness of the clad layer and heat-affected zone increases.     

Influence of processing variables on the formation of La2Zr2O7 in transferred arc plasma torch processing

The transferred arc plasma (TAP) torch process has various noteworthy features such as extremely high temperatures, low environmental impact and short processing time which makes it the most suitable technique for synthesizing ceramic composite materials. Furthermore, it is a direct two-step technique which by its virtue of high temperature and power density paves way for high production rate. Hence in this study, an effort has been made to utilize the TAP torch processing technique for the bulk production of La₂Zr₂O₇ with time effectiveness from the mixture of La₂O₃ and ZrO₂ powders (1:2 mol ratios) which were ball milled for 4 h. For this purpose, transferred arc plasma torch was specially designed in laboratory scale level and the operating parameters were optimized in order to achieve maximum La₂Zr₂O₇ formation efficiency. In this study, the phase and microstructure formation of the processed samples was analyzed via X-ray diffraction (XRD) and scanning electron microscope (SEM) images respectively. Moreover, EDX analysis was incorporated to highlight the superior influence of the longer processing time on the stoichiometric ratio of ZrO₂/La₂O₃ in the processed sample as against input power and the gas flow rate.     

Temperature field measurement of an array of laminar premixed slot flame Jets using Mach-Zehnder interferometry

An experimental study was performed to investigate the influence of Reynolds number (Re) and non-dimensional jet-to-jet spacing (S/D_h) on flame shape, structure and temperature field of an array of laminar premixed slot flame jets. Mach-Zehnder interferometry technique is used to obtain an insight to the overall temperature field between single, twin and triple slot flame jets. The slot jets with large aspect ratio (L/W), length of L=60 mm and width of W=6 mm were used to eliminate the three-dimensional effect of temperature field. The effect of jet-to-jet spacing was investigated on flame characteristics under the test conditions of 200≤Re≤400 and equivalence ratio (φ) of unity. The present measurement reveals that the variation of maximum flame temperature with increment of Reynolds number is mainly due to heat transfer effects and is negligible while the flame height is increased. For the cases of twin and triple flame jets by increasing Reynolds number and decreasing non-dimensional jet-to-jet spacing (S/D_h), the interferences between the jets are increased and the jets attracted each other. Strong interference was observed at S/D_h=1.15. For the case of triple jets at this S/D_h, the central jet was suppressed while the side jets deflected towards the inner jet. The interference between jets was found to reduce the heat flux in the jet-to-jet interacting zone due to incomplete combustion. Also the optimum jet-to-jet spacing of triple flame jets is obtained at each Reynolds number to enhance the heat transfer performance of the jets.     

Investigation of cladding and coating stripping methods for specialty optical fibers

Fiber optic sensing technology is used extensively in several engineering fields, including smart structures, health and usage monitoring, non-destructive testing, minimum invasive sensing, safety monitoring, and other advanced measurement fields. A general optical fiber consists of a core, cladding, and coating layers. Many sensing principles require that the cladding or coating layer should be removed or modified. In addition, since different sensing systems are needed for different types of optical fibers, it is very important to find and sort out the suitable cladding or coating removal method for a particular fiber. This study focuses on finding the cladding and coating stripping methods for four recent specialty optical fibers, namely: hard polymer-clad fiber, graded-index plastic optical fiber, copper/carbon-coated optical fiber, and aluminum-coated optical fiber. Several methods, including novel laser stripping and conventional chemical and mechanical stripping, were tried to determine the most suitable and efficient technique. Microscopic investigation of the fiber surfaces was used to visually evaluate the mechanical reliability. Optical time domain reflectometric signals of the successful removal cases were investigated to further examine the optical reliability. Based on our results, we describe and summarize the successful and unsuccessful methods.     

Different configurations of transferred atmospheric pressure plasma jet and their application to polymer treatment

The employment of atmospheric pressure plasma jets (APPJs) in a large sort of applications is limited by the adversities related to the size of the treated area and the difficulty to reach the target. The use of devices that employ long tubes in their structure has contributed significantly to overcome these challenges. In this work, two different plasma systems employing the jet transfer technique are compared. The main difference between the two devices is how the long plastic tube was assembled. The first one uses a copper wire placed inside a long plastic tube. The other device has a metallic mesh installed in a concentric arrangement between two coaxial plastic tubes. As a result, the two APPJ systems exhibit different properties, with the wire assembly being more powerful, also presenting higher values for the electrical current and rotational temperature when compared to the mesh mounting. X-ray photoelectron spectroscopy (XPS) demonstrates that both configurations were capable of inserting O-containing functional groups on the polypropylene (PP) surface. However, the transferred plasma jet with wire assembly was able to add more functional groups on the PP surface. The results from XPS analysis were corroborated with water contact angle measurements (WCA), being that lower WCA values were obtained when the PP surface presented higher amounts of O-containing groups. Furthermore, the results suggest that the APPJ with wire configuration is more appropriate for material treatments, while the transferred jet with mesh arrangement tends to present lower electrical current values, being more suitable for biological applications.     

层流氩等离子体射流温度的测量

 采用给水冷管状静电探针施加负偏置电压、并使探针以一定速度垂直于射流轴线扫过层流氩等离子体射流的方法,测量探针所收集到的累积离子饱和电流随侧向位置的变化,利用Abel变换推导出了局部离子饱和电流密度沿射流径向的分布;采用自制的水冷动压探针,以动态扫描法测量了射流动压沿射流径向的分布;根据局部离子饱和电流密度和射流动压的测量数据,由理论关系式推导出了等离子体射流横截面上的温度分布,同时,采用谱线相对强度法测量了等离子体射流的激发温度。结果表明：两种方法得到的等离子体射流中心温度吻合较好,所得到的射流中心温度随弧电流加大而增大的变化趋势也一致。     

Reactive plasma cladding of TiC/Fe cermet coating using asphalt as a carbonaceous precursor

A new process of preparing Ti-Fe-C composites powder for reactive plasma cladding, precursor carbonization-composition process, was developed. TiC/Fe cermet coatings were synthesized by reactive plasma cladding of the composite powder. XRD and SEM were employed to analyze the phase composition and microstructure of the composite powder and coating. The hardness and wear resistance of the coating was tested. Results show that: The compound powder prepared by precursor carbonization-composition process has very tight structure, which can avert the question of raw powder breaking-up in cladding process. The TiC/Fe cermet coating by reactive plasma cladding consists of alternate, laminated layers as following: the layers in which the round nanoscale TiC particles are dispersed within the α-Fe matrix and the layers of TiC accumulation. The TiC/Fe cermet coating by reactive plasma cladding shows superior hardness and wear resistance: The surface hardness of the TiC/Fe cermet coating is 68 ± 6 (HR30 N). In the same fretting conditions, the wear resistance of Ni60 coating is twelve times than that of the TiC/Fe cermet coating.     

Influence of laser remelting on the microstructure and phases constitution of plasma sprayed hydroxyapatite coatings

In this paper, the plasma sprayed coatings were treated by laser remelting. The morphologies, elements analysis and phases of both sprayed and remelted coatings were studied by means of electron probe microanalysis (EPMA), X-ray diffraction (XRD) and so on. The results show that the structure of the sprayed coatings is coarse, the amorphization of HA is tremendous, and the bonding state between the coating and the substrate is mechanical combination. After the sprayed coatings were treated by laser remelting in a proper conditions, the properties of the coatings are improved greatly. The microstructure of remelted coatings is columnar and cellular dendritic crystal which is homogeneous and compact, and the coating consists of HA, -TCP and CaO phases, the Ca/P ratio of transition layer is close to 1.67, but the Ca/P ratio of surface layer is higher than that of HA because of the loss of P.     

层流氩等离子体射流温度的测量

采用给水冷管状静电探针施加负偏置电压、并使探针以一定速度垂直于射流轴线扫过层流氩等离子体射流的方法,测量探针所收集到的累积离子饱和电流随侧向位置的变化,利用Abel变换推导出了局部离子饱和电流密度沿射流径向的分布;采用自制的水冷动压探针,以动态扫描法测量了射流动压沿射流径向的分布;根据局部离子饱和电流密度和射流动压的测量数据,由理论关系式推导出了等离子体射流横截面上的温度分布,同时,采用谱线相对强度法测量了等离子体射流的激发温度。结果表明：两种方法得到的等离子体射流中心温度吻合较好,所得到的射流中心温度随弧电流加大而增大的变化趋势也一致。     

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.     

Wall-impinging laminar premixed n-dodecane flames under autoignitive conditions

Laminar one-dimensional (1D) flames in a stagnation flow stabilised at a wall are used to study flame–wall interaction under diesel engine conditions. The thermochemical conditions correspond to that of the Engine Combustion Network (ECN) Spray A reference case. A range of inflow velocities is considered, where the lowest inflow velocity is chosen such that the flame is detached from the inlet. The presence of a wall is shown to have a significant impact on the flame structure and emission formation. The 1D flame and homogeneous reactor results exhibit two distinct reaction zones due to low- and high-temperature chemistry (LTC and HTC, respectively). The burner-stabilised flames are overall dominated by autoignition for all inflow velocities. For the impinging jet flames, the response of the LTC reaction zone follows closely that of the burner-stabilised flames up to relatively high inflow velocities. The HTC reaction zone, however, deviates strongly from the burner-stabilised flames, already at low inflow velocities and quenches at high inflow velocities. A budget analysis revealed a strong contribution from diffusion in the HTC reaction zone, resulting in an increasing importance of deflagrative combustion as opposed to autoignition. This trend was attributed to enhanced strain rates at higher inlet velocity leading to higher gradients. Wall heat transfer was also investigated. The highest wall heat transfer rates were observed for mixtures between $\Phi =1.0$ and $\Phi =1.5$ and for inlet velocities just below the quenching limit. This was attributed jointly to the higher peak product temperatures for these mixtures and to their enhanced resilience to quenching under strain which leads to higher temperature gradients at the wall just before quenching. NO formation was studied. The highest NO formation was observed near $\Phi =1.0,$ though the response to strain rate was different for stoichiometric and rich mixtures, which was attributed to differing NO formation pathways.     

针板型大气压氦气冷等离子体射流的二维模拟

大气压冷等离子体射流的传播机理一直是人们研究的一个热点. 本文采用自洽的二维等离子体流体模型, 研究了大气压氦气冷等离子体射流在自身环境气体中以及在介质管中的传播问题. 得到了电子密度、电离速率、空间电场以及电子温度等参量的时空分布规律, 分析了介质管大小以及介质管介电常数对射流放电性质的影响, 得到了一种提高电子密度和射流尺寸的新方法. 关键词： 大气压冷等离子体射流 等离子体子弹 数值模拟 流体模型     

层流等离子体制备球形氧化铝粉末的实验研究

为研究非转移弧层流等离子体制备面向新材料领域的μm级球形氧化铝粉末的能力,使用自制的非转移弧分段式阳极层流等离子体球化设备,以载气送粉的方式,对η相的不规则μm级三氧化二铝粉末进行等离子体球化处理,并采用均匀设计法,研究等离子体发生器和送粉器不同的工作参数对氧化铝粉末球化率的影响规律。结果表明,实验所采用的直流非转移弧层流等离子体发生器能有效制备球化率接近100%的高球化率球形氧化铝粉末。实验发现,高球化率、高分散性和粒径大小均匀的球形氧化铝粉末可在不同工艺参数组合下制备,并证明了采用非转移弧分段式阳极层流等离子体发生器可实现较低功率下制备较高球化率球形氧化铝的可行性。实验还通过XRD与PDF卡片索引技术对球化前后的氧化铝粉末进行了物相定性分析,发现η相的氧化铝粉末经射流作用转化成了Corundum型的氧化铝粉末。     

Impact of symmetry breaking on the Flame Transfer Function of a laminar premixed flame

This work presents a numerical study of the acoustic response of a laminar flame with tunable asymmetry. A V-shaped premixed flame is stabilised in the wake of a cylindrical flame holder that can be rotated. The configuration is symmetric when the flame holder is fixed but increasing its rotation rate breaks the symmetry of the flow. This configuration is submitted to acoustic forcing to measure the effect of rotation of the flame holder on the Flame Transfer Functions. It appears that the asymmetry of the two flame branches changes their respective time delays, resulting in interference in the global unsteady heat release rate fluctuations. Consequently, the Flame Transfer Function exhibits dips and bumps, which are studied via laminar Direct Numerical Simulation. Potential applications for the control of combustion instabilities are discussed.     

Production of long,laminar plasma jets at atmospheric pressure with multiple cathodes

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.     

TDLAS-based in situ measurement of absolute acetylene concentrations in laminar 2D diffusion flames

We report the first quantitative and calibration-free in situ C₂H₂ measurement in a flame environment using direct Tunable Diode Laser Absorption Spectroscopy(TDLAS). Utilizing a fiber-coupled Distributed Feedback diode laser near 1535 nm we measured spatially resolved, absolute C₂H₂ concentration profiles in a laminar non-premixed CH₄/air flame supported on a modified Wolfhard-Parker slot burner with N₂ purge slots to minimize end flames. We developed a wavelength tuning scheme combining laser temperature and current modulation to record with a single diode laser a mesh of 37 overlapping spectral windows and generate an ∼7 nm (30 cm⁻¹) wide, high-resolution absorption spectrum centered at 1538 nm. Experimental C₂H₂ spectra in a reference cell showed excellent agreement with simulations using HITRAN2004 data. The enhanced wavelength coverage was needed to establish correct C₂H₂ line identification and selection in the very congested high temperature flame spectra and led to the P17e line as the only candidate for in situ detection of C₂H₂ in the flame. We used highly efficient optical disturbance correction algorithms for treating transmission and background emission fluctuations in combination with a multiple Voigt line Levenberg-Marquardt fitting algorithm and Pt/Rh thermocouple measurements to achieve fractional optical resolutions of up to 4 × 10⁻⁵ OD (1σ) in the flame (T up to 2000 K). Temperature dependent C₂H₂ detection limits for the P17e line were 60 to 480 ppm. By translating the burner through the laser beam with a DC motor we obtained spatially resolved, absolute C₂H₂ concentration profiles along the flame sheet with 0.5 mm spatial resolution as measured with a knife edge technique. The TDLAS-based, transverse C₂H₂ concentration profiles without any scaling are in excellent agreement with published mass spectrometric C₂H₂ data for the same flame supported on a similar burner, thus validating our calibration-free TDLAS measurements.     

阴极透镜层流电子枪调制特性

 通过求解第一类边界条件的拉普拉斯方程,获得了典型的三电极阴极透镜中轴上电位的解析表达式。以此为基础,分析了阴极透镜各几何参数及电位参数对其聚焦性能的影响,得知通过增大调制极电位或其孔径、增加调制极与加速极的间距或缩小调制极与阴极的间距的方法,可以保证在改善初始粒子时间、空间弥散特性的基础上,相对减小轴向电场的非均匀性以减弱其会聚电场强度,从而使带电粒子束在阴极透镜区域中不能形成交叠点,达到构成其中电子运动具有层流性质的阴极透镜层流电子枪的工程应用目的。另外也获得了采用厚度较小的调制极有助于整个电子光学系统性能优化的结论。     

The effect of preferential diffusion on soot formation in a laminar ethylene/air diffusion flame

The influence of preferential diffusion on soot formation in a laminar ethylene/air diffusion flame was investigated by numerical simulation using three different transport property calculation methods. One simulation included preferential diffusion and the other two neglected preferential diffusion. The results show that the neglect of preferential diffusion or the use of unity Lewis number for all species results in a significant underprediction of soot volume fraction. The peak soot volume fraction is reduced from 8.0 to 2.0 ppm for the studied flame when preferential diffusion is neglected in the simulation. Detailed examination of numerical results reveals that the underprediction of soot volume fraction in the simulation neglecting preferential diffusion is due to the slower diffusion of some species from main reaction zone to PAH and soot formation layer. The slower diffusion of these species causes lower PAH formation rate and thus results in lower soot inception rate and smaller particle surface area. The smaller surface area further leads to smaller surface growth rate. In addition, the neglect of preferential diffusion also leads to higher OH concentration in the flame, which causes the higher specific soot oxidation rate. The lower inception rate, smaller surface growth rate and higher specific oxidation rate results in the lower soot volume fraction when preferential diffusion is neglected. The finding of the paper implies the importance of preferential diffusion for the modeling of not only laminar but maybe also some turbulent flames.     

激光重熔对等离子喷涂ZrO2-20%Y2O3热障涂层隔热性能的影响

采用大气等离子喷涂（APS）技术在铝基体表面制备氧化锆（ZrO2-20%Y2O3,质量分数）热障涂层,并用脉冲激光对其进行重熔处理,研究了激光重熔对涂层组织形貌、物相转变和隔热性能的影响。研究结果表明激光的比能量对涂层的成型及性能有重要影响,过高的比能量会使涂层表面粗糙度增加,涂层成型变差。在选用合适的低比能量激光重熔条件下,扫描电镜观察结果表明经激光重熔可消除喷涂态涂层的孔隙和层状结构。对粉末和重熔前后的涂层进行了X射线衍射分析,结果表明喷涂及重熔过程中都没有发生相变;隔热试验结果表明重熔后涂层的隔热温度有所下降。