A numerical investigation in buoyancy effects on micro jet diffusion flame

The buoyancy effect on micro hydrogen jet flames in still air was numerially studied.The results show that when the jet velocity is relatively large(V≥0.2 m/s),the flame height,width and temperature decrease,whereas the peak OH mass fraction increases significantly under normal gravity(g=9.8 m/s^2).For a very low jet velocity(e.g.,V=0.1 m/s),both the peak OH mass fraction and flame temperature under g=9.8 m/s^2 are lower than the counterparts under g=0 m/s^2.Analysis reveals that when V≥0.2 m/s,fuel/air mixing will be promoted and combustion will be intensified due to radial flow caused by the buoyancy effect.However,the flame temperature will be slightly decreased owing to the large amount of entrainment of cold air into the reaction zone.For V=0.1 m/s,since the heat release rate is very low,the entrainment of cold air and fuel leakage from the rim of tube exit lead to a significant drop of flame temperature.Meanwhile,the heat loss rate from fuel to inner tube wall is larger under g=9.8 m/s^2 compared to that under g=0 m/s^2.Therefore,the buoyancy effect is overall negative at very low jet velocities.

Partially-Premixed Combustion Characteristics and Thermal Performance of Micro Jet Array Burners with Different Nozzle Spacings

In order to develop a burner with uniform temperature field,the combustion characteristics and thermal performance of partially premixed methane/air jet flames were experimentally studied by using micro jet array burners.The circular tubes of 1.0-mm inner diameter and 1.5-mm outer diameter were used as nozzles.The effects of nozzle spacing and equivalence ratio on flame phenomenology,temperature distribution and pollutant emissions were respectively investigated by camera photography,thermocouple measurement and sampling analysis.Results show that there are two clean flame patterns:clean merged-flame and clean non-merging flames.The flame patterns depend on the strength of flame interaction,the equivalence ratio of the mixture and the quantity of air entrainment through the gap between nozzles.The burners with small nozzle spacing such as 2 mm and 2.5 mm tend to produce fully merged flame with low equivalence ratio limit and the corresponding temperature fields are very uniform with fluctuations less than 0.3%,but a small increase in equivalence ratio will lead to rapid deterioration of combustion property.The burner with a medium spacing of 3 mm can produce partially merged flame in a wide equivalence ratio range with low emissions,and the temperature fluctuation can be less than 0.5%(<7 K)in the optimal region.The burner with a large spacing of 4 mm will basically form independent array flames with the largest temperature fluctuation over 1%,while it can achieve clean combustion under high equivalence ratio due to large air entrainment.Comprehensive analysis shows that the micro jet array burner with medium nozzle spacing of 3 mm has the best combustion characteristics and thermal performance.

Experimental and numerical study on a micro jet cooling solution for high power LEDs

An active cooling solution based on close-looped micro impinging jet is proposed for high power light emitting diodes (LEDs). In this system, a micro pump is utilized to enable the fluid circulation, impinging jet is used for heat exchange between LED chips and the present system. To check the feasibility of the present cooling system, the preliminary experiments are conducted without the intention of parameter optimization on micro jet device and other system components. The experiment results demonstrate that the present cooling system can achieve good cooling effect. For a 16.4 W input power, the surface temperature of 2 by 2 LED array is just 44.2°C after 10 min operation, much lower than 112.2°C, which is measured without any active cooling techniques at the same input power. Experimental results also show that increase in the flow rate of micro pump will greatly enhance the heat transfer efficiency, however, it will increase power consumption. Therefore, it should have a trade-off between the flow rate and the power consumption. To find a suitable numerical model for next step parameter optimization, numerical simulation on the above experiment system is also conducted in this paper. The comparison between numerical and experiment results is presented. For two by two chip array, when the input power is 4 W, the surface average temperature achieved by a steady numerical simulation is 34°C, which is close to the value of 32.8°C obtained by surface experiment test. The simulation results also demonstrate that the micro jet device in the present cooling system needs parameter optimization.

A review on the erosion mechanisms in abrasive waterjet micromachining of brittle materials

The fabrication of miniature structures on components with high-integrity surface quality represents one of the cutting edge technologies in the 21st century.The materials used to construct such small structures are often difficult-to-machine.Many other readily available technologies either cannot realise necessary precision or are costly.Abrasive waterjet(AWJ)is a favourable technology for the machining of difficult-to-machine materials.However,this technology is generally aimed at large stock removal.A reduction in the scale of this technology is an attractive avenue for meeting the pressing need of industry in the production of damage-free micro features.This paper reviews some of the work that has been undertaken at UNSW Sydney about the development of such an AWJ technology,focusing on the system design currently employed to generate a micro abrasive jet,the erosion mechanisms associated with processing some typical brittle materials of both single-and two-phased.Processing models based on the findings are also presented.The review concludes on the viability of the technology and the prevailing trend in its development.

Efect of Micro Abrasive Slurry Jet Polishing on Properties of Coated Cemented Carbide Tools

Owing to the popularization of coating technology, physical Vapor Deposition (PVD) coated tools have become indispensable in the cutting process. Additionally, the post-treatment of coated tools applied to industrial production can efectively enhance the surface quality of coating. To improve the processing performance of coated tools, micro abrasive slurry jet (MASJ) polishing technology is frst applied to the post-treatment of coated tools. Subsequently, the efects of process parameters on the surface quality and cutting thickness of coating are investigated via single-factor experiments. In the experiment, the best surface roughness is obtained by setting the working pressure to 0.4 MPa, particle size to 3 μm, incidence angle to 30°, and abrasive mass concentration to 100 g/L. Based on the results of the single-factor experiments, combination experiments are designed, and three types of coated tools with diferent surface qualities and coating thicknesses are obtained. The MASJ process for the post-treatment of coated tools is investigated based on a tool wear experiment and the efects of cutting parameters on the cutting force and workpiece surface quality of three types of cutting tools. The result indicates that MASJ machining can efectively improve the machining performance of coated tools.

A Voice Coil Powered Controllable Micro-Jet Injection System

A medical device of micro-jet injection for drug delivery is described in this paper. The device is powered by a Lorentz force driver (or voice coil motor, VCM) and is able to perform pulsed injection through controlling the direction of the current passing through the device. The driving force and the resulting injection pressure are also controllable through control of the current intensity of the VCM. A physical model was established by combining the existing jet injection model with the relationship of the driving force obtained from a finite-element-method (FEM) analysis, and was verified by experimental measurements. The numerical calculation of the physical model reveals the relationship between the injection pressure and the current intensity of VCM under system conditions. In normal cases, the injection dose can be varied. Thus the relationship between the current intensity of VCM and the dose value was numerically obtained under the condition for the maximum injection pressure to be above a threshold value. These results can be used for optimization of the device.

The Enhanced Measurements of Laser Micro-jet Processing

In the paper, the enhanced measurements of the laser micro-jet processing are discussed. In fact, within pure water breakdown threshold of laser, the less focal which focused with the appropriate focusing lens and the small nozzle of water chamber enhance the laser power density ,at the same time, the laser beam transport in the wave guided water with the proper total reflection angle. The laser power which depended on the properties of the diameter, the coupling water chamber and the coupling efficiency of the micro-jet and laser beam expect of the properties of laser.

Rapid manufacturing of SiC molds with micro-sized holes using abrasive water jet

Silicon carbide (SiC) is highly wear resistant with good mechanical properties, including high temperature strength, excellent chemical resistance, and high thermal conductivity and thermal shock resistance. SiC molds, which can be produced with diverse microstructural features, are now widely used in glass molding owing to their excellent characteristics, and also have potential applicability in IT industries. SiC molds are traditionally fabricated by silicon micromachining or dicing. The fabrication cost of silicon micromachining is very high, however, because several expensive masks are needed. Furthermore, the fabrication time is very long. Meanwhile, it is difficult to make micro-patterned molds with arbitrary shapes using dicing saws. Abrasive water jet (AWJ) is widely applied to cut and drill very brittle, soft and fibrous materials. It offers high energy density, the absence of a heat affected zone(HAZ), high performance, and an environment friendly process. In spite of these advantages, micro-hole drilling via conventional AWJ processing suffers from notable shortcomings. We proposed a new abrasive supplying method of AWJ. The proposed method reduces frosting phenomena, and provides micro-machining of AWJ. The characteristics of a hole machined was investigated by the proposed AWJ process according to the ratio of water and abrasives. With the optimal experimental conditions, 3×3 array SiC molds with the diameter of 700 μm and depth of 900 μm were successfully manufactured.

3种根管封闭剂根尖封闭性能的Micro-CT研究

目的使用微计算机断层扫描技术(micro-computed tomography,Micro-CT)分析比较3种根管封闭剂(C-Root SP、iRoot SP和AH-Plus Jet)在单尖法根管充填中对根管充填封闭性能。方法收集90颗符合纳入标准的下颌单根管离体前磨牙,使用ProTaper Next预备至X3,然后随机分为3个实验组C-Root SP组(A组)、iRoot SP组(B组)、AH-Plus Jet组(C组),均采用单尖法进行根管充填,并进行1周的口腔环境模拟,最后用Micro-CT测量根管内气泡体积及其所占比例来评价比较3种根管封闭剂的封闭性能。结果A、B、C组在根管的不同部位(下1/3段、中1/3段、上1/3段)均存在可识别的气泡。在下1/3段中,A组的气泡体积(0.01±0.01)mm3与C组(0.12±0.08)mm^(3)相比存在统计学差异(P<0.05),与B组(0.02±0.01)mm^(3)相比则没有统计学差异;在中1/3段与上1/3段,组间无统计学差异。在根管全长范围内,A组的气泡总体积最小(0.15±0.03)mm^(3),B、C两组分别为(0.22±0.11)mm^(3)、(0.34±0.28)mm^(3),A组与B组相似,均优于C组。结论C-Root SP的根尖封闭性能与iRoot SP接近,且优于AH-Plus Jet,C-Root SP可以作为良好的根管封闭剂选择。

采用DC Arc Plasma JetCVD方法沉积微/纳米复合自支撑金刚石膜

在30kW级直流电弧等离子体喷射化学气相沉积(DC Arc P lasm a Jet CVD)设备上,采用Ar-H2-CH4混合气体,通过调节甲烷浓度以及控制其他沉积参数,在Mo衬底上沉积出微/纳米复合自支撑金刚石膜。实验表明,当微米金刚石膜层沉积结束后,在随后的沉积中,随着甲烷浓度的增加,金刚石膜表面的晶粒大小是逐渐减小的。当甲烷浓度达到20%以上时,金刚石膜生长面晶粒呈现菜花状的小晶团,膜体侧面已经没有了粗大的柱状晶,而是呈现出光滑的断口,对该层进行拉曼谱分析显示,位于1145 cm-1附近有一定强度的散射峰出现。这说明所沉积的晶粒全部变为纳米级尺寸。

微氧射流技术在慢性伤口患者中的应用

目的:探讨微氧射流技术在慢性伤口患者中的应用效果。方法:选取2022年11月1日~2023年10月31日收治的60例慢性伤口患者作为研究对象,采用随机数字表法分为研究组和对照组各30例,对照组接受常规换药处理,研究组在对照组基础上接受微氧射流技术;比较两组愈合疗效、住院情况、生活质量[采用健康调查简表(SF-36)]、细菌阳性率。结果:研究组创面愈合总有效率高于对照组(P<0.05);研究组换药消耗时间、伤口愈合时间、换药次数、创面深度均低于对照组(P<0.01);干预后,两组SF-36评分均高于干预前(P<0.05),且研究组高于对照组(P<0.01);研究组铜绿假单菌、大肠杆菌、金黄色葡萄球菌及其他菌总阳性率低于对照组(P<0.05)。结论:微氧射流技术能改善慢性伤口患者愈合情况,抑制细菌繁殖,降低细菌阳性率,提高创面愈合疗效和患者生活质量。

自激振荡射流微通道换热特性及热力学分析

该文对自激振荡射流微通道内流体换热性能进行模拟研究,结果表明在不同入口质量流量(1.0~6.0 g/s)条件下,自激振荡器内高压区域的移动造成了微通道散热器内射流的偏转,形成射流周期性循环摆动的同时强化了流体区域的扰动。此外,经对比分析后发现相较射流微通道内流体区域其冲击范围更广,因此其均温性能也优于射流微通道。与此同时,该文运用热力学原理分析评价自激振荡技术对射流微通道内流体流动换热性能的影响,发现自激振荡器所产生的振荡射流可以减小射流微通道内流体对流换热过程中的不可逆损失,其热能传输效率高达到98.48%。

射流清洗对微灌过滤器滤网清洗效果及损伤研究

为探究射流清洗技术在农业微灌过滤器清洗过程中的应用,开展射流清洗试验研究,分析入射压力、喷嘴口径、喷嘴扩散角度等3个参数对微灌过滤器滤网清洗效果及表面损伤程度的影响规律.试验结果表明:清洗效果随入射压力、喷嘴口径、喷嘴扩散角度的增大而提升,但提升幅度不断减小;滤网表面损伤程度随入射压力、喷嘴口径的增大而增大,随喷嘴扩散角度的增大而减小,其幅度逐渐减小;通过方差分析可知,射流参数对清洗效果影响程度从大到小依次为喷嘴口径、入射压力、喷嘴扩散角度,对滤网表面损伤影响程度从大到小依次为喷嘴口径、喷嘴扩散角度、入射压力;通过TOPSIS综合评价得到最佳射流参数组合为入射压力0.3 MPa,喷嘴口径3 mm,喷嘴扩散角度80°.

微磨料气射流加工结构化拓扑鱼鳞表面的实验研究

为了在脆性难加工材料上制备结构化鱼鳞表面,首先提取了鱼鳞表面的拓扑特征,建立了鱼鳞单元的表面模型,并基于鱼鳞模型进行搭接,得到结构化鱼鳞表面;然后根据微磨料气射流加工原理,分析了气射流加工拓扑鱼鳞实验的可行性与影响结构化鱼鳞表面形貌的重要工艺参数;最后采用单因素实验法对重要加工参数进行实验分析,得到了较好的拓扑鱼鳞表面单元,并以此进行排布得到结构化表面。研究结果表明,微磨料气射流加工结构化拓扑鱼鳞表面较为合理的工艺参数组合是加工时间为10 s、气射流加工压力为0.5 MPa、靶距离为10 mm以及射流角度为30°,在此工艺条件下,鱼鳞表面的形态和尺寸可能会受到一定的影响,但鱼鳞表面的拓扑属性保持不变。

强激光加载下铝、锡微喷特性实验研究

基于皮秒激光驱动的高能X射线源具有微焦点、高亮度和高分辨的优良特性,已成为研究金属材料微层裂和微喷射现象的热门诊断方法之一。锡因其熔点较低而成为微喷研究熔化影响的首选材料,铝由于熔点相对较高,不易发生冲击或卸载熔化形成液滴喷射,因此,铝微喷实验研究结果相对较少。基于“星光-III”激光装置的纳秒加载源和皮秒束诊断源,发展了皮秒激光驱动高能X射线背光照相技术,研究了表面含V型几何缺陷的金属锡、铝微喷特性,通过延时控制,获取了两种典型金属微喷射过程的高分辨物理图像,得到了喷射物质的形态分布、平均速度和面密度分布等重要信息。采用SPH方法对实验结果进行了数值模拟分析,揭示了实验中喷射物的形成机理以及高速喷射物质的来源。

微磨料气射流加工技术研究现状与展望

微磨料射流加工技术是一种基于高能流体的磨粒冲蚀磨损加工技术,已广泛应用于难加工材料、复杂三维型面、光滑表面的加工。为进一步提高磨料射流加工过程中的精准控形控性能力,国内外学者开展了诸多基础加工理论与工艺探索等方面的研究工作。本文在概述磨料射流加工技术发展的基础上,全面总结了国内外学者在微磨料气射流加工(MAJM)技术中的微磨料气射流束发散效应及其抑制策略、材料力学性能对材料冲蚀去除模式的影响、材料冲蚀加工过程磨料嵌入抑制策略、微结构冲蚀加工几何特征等方面的主要研究成果,并对微磨料射流加工技术的难点与发展趋势进行了展望。

磁场辅助场致射流微细放电加工研究

场致射流放电加工方法利用高压电场诱导形成的微细射流与工件间放电来蚀除材料。虽无电极损耗,但微细射流的柔性较大且易受外界干扰,导致放电分散性和随机性较强,因此难以对其进行精准控制。为此,设计了一种四极性磁场施加装置,通过磁场对离子束的约束作用控制射流,并分析了磁场的作用机理,研究了磁场约束对场致射流放电加工的蚀坑宽度、深度及表面粗糙度的影响规律。结果表明,随着磁感应强度的增大,放电蚀坑的最大深度和表面粗糙度值都减小;磁场散焦方向上蚀坑的宽度增加,而聚焦方向上蚀坑的宽度减小,同时放电时的极间距离也减小。经试验验证,所提出的磁场辅助场致射流放电加工方法适用于具有沟槽类型特征的部件加工。

基于离子液射流定域电沉积的微镍柱成形研究

为提高定域电沉积三维金属微结构的加工效率和质量,采用离子液射流定域电沉积方式制备直径50~100μm、高度450~500μm的微镍柱,探究了工艺参数对沉积速率、微镍柱直径和微观形貌的影响。结果表明,在选定的参数范围内,微镍柱直径和沉积速率随着电压的增大同时增大,占空比对微镍柱沉积速率的影响高于对平均直径的影响,而脉冲频率对于微镍柱沉积速率影响不大。通过体式显微镜观察微镍柱微观形貌呈尖端效应,表明在阳极尖端半径对应区域和离子液射流范围内的共有区域具有电沉积定域性和优先性。合理控制阳极运动轨迹,调节参数为极间电压4.4 V、占空比50%、脉冲频率10 kHz时,可获得轮廓平直、表面致密的直径为61μm微镍柱。

静电溶吹微纳无机纤维制备技术及其应用进展

静电纺丝技术存在生产效率低和对高黏纺丝溶液限制大等问题,阻碍了高性能无机纤维的大规模产业化进程,而静电溶吹技术基于静电力与气流力的耦合作用,可助力微纳纤维的高产高质。为推广静电溶吹纺丝技术并为微纳无机纤维生产提供理论和实践依据,综述了该技术的基础研究,包括原理和先进设备;具体分析了该技术生产无机纤维的影响因素,尤其明晰了产业化的可控工艺;总结了目前国内外静电溶吹技术制备无机纤维的应用进展,通过与其它纺丝技术的对比阐明了静电溶吹技术目前存在的优势和挑战,并指出协同提升纤维质量和产量、赋予纤维结构/功能多样化以及研发多孔轻质无机纤维和超细无机纱线是该技术未来的重要发展方向。

GH4169镍基高温合金射流电解加工微坑阵列试验研究

表面微坑阵列在机械密封、摩擦磨损、表面润滑、传热散热等方面有巨大作用。为了能在GH4169镍基高温合金上射流电解加工微坑阵列,通过测试合金在不同溶液中的极化曲线,优选了射流电解加工合金的电解液,并在此基础上设计试验,探究了加工电压、射流速度对射流电解加工微坑的影响规律,并通过优选工艺参数加工出微坑阵列。最后,以质量分数10%的NaNO_(3)溶液为电解液,以加工电压25 V、射流速度10 m/s加工出平均深径比为0.376、深径比标准偏差为0.004372的加工精度高、一致性好的微坑阵列结构。