液滴束流轴向速度及其弥散的测量

轴向速度弥散是液滴束流在许多应用中的重要指标,本文对单狭缝法测量液滴束流轴向速度弥散作了分析,并发展了三狭缝法测量轴向速度及其弥散,后者适用于大多数液滴束流.     

液态金属在非均匀多孔介质表面的润湿行为研究

基于Wenzel模型和Cassie模型,研究了金属液滴在区域非均匀多孔介质表面的润湿行为。结果表明,大孔、小Young氏接触角区域对液滴更易表现高粘附性;而在小孔、大Young氏接触角区域,由于低粘附性,液滴会向大孔区域表现出爬移行为。提出借助非均匀区域周期性组合的方法来防止液滴飞溅的“自束液”防溢设计思路,为在微重力环境下,液态金属热管、离子电推进器及原子钟等航天星载产品在振动条件下能正常工作提供潜在应用的可能性。     

离子推力器束流均匀性改善方法研究

束流均匀性是离子推力器的一个重要指标,其代表离子推力器栅极整个引出平面上引出束流密度的均匀程度,一般用束流平直度来表示。从产生均匀束流的必要条件出发,讨论了离子束流均匀性影响因素,并对其影响机理进行了分析。根据讨论结果对典型离子推力器的束流均匀性进行了对比分析。最后总结给出了离子推力器束流均匀化设计应遵循的一般原则。     

电流体动力学技术制备纳米超微粉的液滴破碎机理

分析了电流体动力学技术制备纳米粉的液滴破碎机理结果表明：外加电压、电场强度、液体金属的性质直接影响金属液滴的破碎程度     

MECHANISM OF BREAK─UP OF LIQUID DROPLETS GENERATED BY ELECTROHYDRODYNAMIC TECHNIQUE

分析了电流体动力学技术制备纳米粉的液滴破碎机理，结果表明：外加电压、电场强度、液体金属性质直接影响金属液滴的破碎程度。     

电磁阴极磁场分布对磁控溅射系统伏安特性的影响

本文设计了一种新型圆形平面阴极磁控溅射源.该源具有独特的三极线圈结构,改变各线圈励磁电流可调节靶面磁场强度的大小和分布.通过对系统气体放电伏安特性随各线圈励磁电流大小变化规律的分析,以及对距靶面60mm基片台处等离子体束流密度大小和分布的测试,探讨了阴极磁场分布对磁控溅射系统伏安特性的影响.实验结果表明阴极磁场分布模式对气体放电稳定性和等离子体分布影响显著,当阴极磁场呈现收敛型分布时,二次电子被紧密束缚在靶面附近,降低了基片台附近等离子体束流密度,却增大等离子体束流径向分布均匀性.调节非平衡线圈励磁电流,在附加磁场的作用下,阴极磁场呈现发散型分布,二次电子被引向基片台附近,使得基片台附近等离子体束流密度显著增加但径向均匀性变差.     

医院中子照射器I型堆热中子束流孔道等效平面源的模拟计算

采用蒙特卡罗程序MCNP模拟计算了医院中子照射器Ⅰ型堆(IHNI-1)热中子束流孔道出口处的等效平面源.对B堆芯进行了临界搜索计算,模拟计算了热中子束流孔道及出口处中子、γ的束流参数,应用等效平面源模型建立了BNCT等效中子、γ平面源.为人体头颅等效模型剂量分布的快速计算提供了较为可靠的平面源.     

LIPS-200离子推力器束流模型及其应用

建立了基于测量数据基础的LIPS-200离子推力器束流模型,给出了模型中积分发散问题的解决方法。应用模型计算了LIPS-200推力器中远场束流分布,给出了束流发散角、推力修正因子、南北位保推力器安装角等应用实例。     

离子推力器束流密度分布测量

介绍了法拉第筒的测量原理,分析了法拉第筒的测量误差,设计出用于离子推力器束流特性测量的法拉第筒仪器,成功地应用于20 cm离子推力器束流特性的测量.     

微乳液制备纳米颗粒的三维模拟

应用Monte Carlo方法在10×10×100三维网络中模拟了纳米颗粒在W/O微乳液中的形成过程.研究了反应物浓度、反应物配比、液滴尺寸、界面膜韧性对颗粒形成的影响及与颗粒尺寸和颗粒分布之间的关系,并与前人的实验和研究结果进行了分析比较.模拟结果表明颗粒的形成过程可明显分为成核和成长两个阶段;增加反应物的浓度可以使纳米颗粒尺寸增大;反应物过量使颗粒尺寸减小;膜的韧性越大,形成的颗粒越大,而且分布越均匀;颗粒尺寸随液滴尺寸的增加而增加.     

Performance of droplet generator and droplet collector in liquid droplet radiator under microgravity

The Liquid Droplet Radiator (LDR) has an advantage over comparable conventional radiators in terms of the rejected heat power-weight ratio. Therefore, the LDR has attracted attention as an advanced radiator for high-power space systems that will be prerequisite for large space structures. The performance of the LDR under microgravity condition has been studied from the viewpoint of operational space use of the LDR in the future. In this study, the performances of a droplet generator and a droplet collector in the LDR are investigated using drop shafts in Japan: MGLAB and JAMIC. As a result, it is considered that (1) the droplet generator can produce uniform droplet streams in the droplet diameter range from 200 to 280 [μm] and the spacing range from 400 to 950 [μm] under microgravity condition, (2) the droplet collector with the incidence angle of 35 degrees can prevent a uniform droplet stream, in which droplet diameter is 250 [μm] and the velocity is 16 [m/s], from splashing under microgravity condition, whereas splashes may occur at the surface of the droplet collector in the event that a nonuniform droplet stream collides against it.     

Performance of droplet generator and droplet collector in liquid droplet radiator under microgravity

The Liquid Droplet Radiator (LDR) has an advantage over comparable conventional radiators in terms of the rejected heat power-weight ratio. Therefore, the LDR has attracted attention as an advanced radiator for high-power space systems that will be prerequisite for large space structures. The performance of the LDR under microgravity condition has been studied from the viewpoint of operational space use of the LDR in the future. In this study, the performances of a droplet generator and a droplet collector in the LDR are investigated using drop shafts in Japan: MGLAB and JAMIC. As a result, it is considered that (1) the droplet generator can produce uniform droplet streams in the droplet diameter range from 200 to 280 [μm] and the spacing range from 400 to 950 [μm] under microgravity condition, (2) the droplet collector with the incidence angle of 35 degrees can prevent a uniform droplet stream, in which droplet diameter is 250 [μm] and the velocity is 16 [m/s], from splashing under microgravity condition, whereas splashes may occur at the surface of the droplet collector in the event that a nonuniform droplet stream collides against it.     

Printing with Satellite Droplets

Despite their specific methodologies, all current noncontact printing techniques such as inkjet printing (IJP), involve the break‐up of a liquid meniscus during the separation of the ink droplet from the bulk ink reservoir. Often, the break‐up of a liquid meniscus results in the formation of one or more satellite droplet whose volumes are several orders of magnitude smaller than the primary droplet. Many attempts are directed to suppress or control the formation of satellite droplets because they blur the printing result. For the first time, a simple mechanism by which a single satellite droplet is exclusively formed and directed to the substrate by a gas stream while the primary droplet remains attached to a metal rod used for controlling the formation and break‐up of the meniscus is reported. High printing resolution is demonstrated by satellite droplets printing (SDP) without the need for small orifices which are prone to clogging. Furthermore, the droplet generation from a large orifice enables SDP to handle viscous inks which has remained challenging for traditional IJP.     

Review on modelling of corrosion under droplet electrolyte for predicting atmospheric corrosion rate

Atmospheric corrosion of metals is the most common type of corrosion which has a significant impact on the environment and operational safety in various situations of everyday life.Some of the common examples can be observed in land,water and air transportation systems,electronic circuit boards,urban and offshore infrastructures.The dew drops formed on metal surface due to condensation of atmospheric moisture facilitates corrosion as an electrolyte.The corrosion mechanisms under these droplets are different from classically known bulk electrolyte corrosion.Due to thin and non-uniform geometric thickness of the droplet electrolyte,the atmospheric oxygen requires a shorter diffusion path to reach the metal surface.The corrosion under a droplet is driven by the depletion of oxygen in the center of the droplet compared to the edge,known as differential aeration.In case of a larger droplet,differential aeration leads to preferential cathodic activity at the edge and is controlled by the droplet geometry.Whereas,for a smaller droplet,the oxygen concentration remains uniform and hence cathodic activity is not controlled by droplet geometry.The geometry of condensed droplets varies dynamically with changing environmental parameters,influencing corrosion mechanisms as the droplets evolve in size.In this review,various modelling approaches used to simulate the corrosion under droplet electrolytes are presented.In the efforts of developing a comprehensive model to estimate corrosion rates,it has been noted from this review that the influence of geometric evolution of the droplet due to condensation/evaporation processes on corrosion mechanisms are yet to be modelled.Dynamically varying external factors like environmental temperature,relative humidity,presence of hygroscopic salts and pollutants influence the evolution of droplet electrolyte,making it a complex phenomenon to investigate.Therefore,an overview of available dropwise condensation and evaporation models which describes the formation and the evolution of droplet geometry are also presented from an atmo s pheric corrosion viewpoint.     

Vibrating Breakup of Jet for Uniform Metal Droplets

Uniform droplet formation from capillary stream breakup provides promising opportunities for many applications such as solder balls manufacturing, circuit board printing and rapid prototype manufacturing. In this study an apparatus capable of making monosize metal spheres by vibrating breakup has been developed. The droplets were electrically charged to avoid collision and merging with one another during flight. As a result, uniformly sized tin powders （180 μm in diameter） were obtained after cooling and solidification.     

Thermocapillary migration of a planar droplet at moderate and large Marangoni numbers

Thermocapillary migration of a planar non-deformable droplet in flow fields with two uniform temperature gradients at moderate and large Marangoni numbers is studied numerically by using the front-tracking method. It is observed that the thermocapillary motion of planar droplets in the uniform temperature gradients is steady at moderate Marangoni numbers, but unsteady at large Marangoni numbers. The instantaneous migration velocity at a fixed migration distance decreases with increasing Marangoni numbers. The simulation results of the thermocapillary droplet migration at large Marangoni numbers are found in qualitative agreement with those of experimental investigations. Moreover, the results concerned with steady and unsteady migration processes are further confirmed by comparing the variations of temperature fields inside and outside the droplet. It is evident that at large Marangoni numbers the weak transport of thermal energy from outside of the droplet into inside cannot satisfy the condition of a steady migration process, which implies that the advection around the droplet is a more significant mechanism for heat transfer across/around the droplet at large Ma numbers. Furthermore, from the condition of overall steady-state energy balance in the flow domain, the thermal flux across its surface is studied for a steady thermocapillary droplet migration in a flow field with uniform temperature gradient. By using the asymptotic expansion method, a non-conservative integral thermal flux across the surface is identified in the steady thermocapillary droplet migration at large Marangoni numbers. This non-conservative flux may well result from the invalid assumption of a quasi-steady state, which indicates that the thermocapillary droplet migration at large Marangoni numbers cannot reach a steady state and is thus an unsteady process.     

Thermomechanical modeling of molten metal droplet solidification applied to layered manufacturing

Transient and steady-state distributions of temperature and stress along the centerline of a single, initially molten metal droplet deposited onto a comparatively large substrate are examined. After investigating droplet deposition onto a room temperature substrate, the effect of substrate preheating on residual thermal stresses is quantified. Also, deposition of a second droplet is modeled and the effect on residual stresses of localized preheating by the first deposited droplet is assessed. Temperature-dependent conductivity, specific heat and density are used in coupled thermal models of droplet and substrate domains. Mechanical models include temperature-dependent Young's modulus, linear expansion coefficient and creep. Two-dimensional (2D) axisymmetric thermal and mechanical results are compared to one-dimensional (1D) results which approximate conditions along the droplet/substrate centerline. It is found that the more computationally efficient 1D models aid in interpreting the 2D results and provide reliable estimates of maximum stress magnitudes. Methods and results from this investigation are relevant to processes in which molten superheated metal contacts solid metal, such as welding processes. The specific application of interest in this work is droplet-level thermal and mechanical modeling of the microcasting stage of shape deposition manufacturing, which is a layered manufacturing process for the automated manufacture of complex three-dimensional metal parts.     

陶瓷微球新型直接滴定成型工艺研究

以平均粒径为0.1~0.5μm的SnO₂粉为原料,通过凝胶注模成型制备SnO₂浆料,并使用滴定针筒将浆料直接滴定于旋转的金属基超疏水表面,使SnO₂液滴在不同条件下的旋转金属基超疏水表面固化成球。研究了液滴尺寸、成型时间、固含量等因素对微球成型的影响规律,结果表明:直接滴定成型的SnO₂陶瓷微球的球形度十分理想,球形度大于常规滴定成型法,并且可成型出直径0.1~0.5 mm的陶瓷微球;微球的球形度随固含量的增加而增大,随液滴尺寸的增加而减小,随固化时间的增加而减小;当液滴尺寸调整为0.05 mm时,使用凝胶注模工艺优化后的固含量为80%的SnO₂粉浆,在60℃时干燥固化30 min,该条件下获得的SnO₂陶瓷微球具有最高球形度,可达到99.5%;微球球坯烧结前后的表面微观组织结构较为均匀,烧结后可保持完整的球体形状。      

Transient State Machine Enabled from the Colliding and Coalescence of a Swarm of Autonomously Running Liquid Metal Motors

Internally triggered motion of an object owns important potential in diverse application areas ranging from micromachines, actuator or sensor, to self‐assembly of superstructures. A new conceptual liquid metal machine style has been presented here: the transient state machine that can work as either a large size robot, partial running elements, or just divide spontaneously running swarm of tiny motors. According to need, the discrete droplet machines as quickly generated through injecting the stream of a large liquid metal machine can combine back again to the original one. Over the process, each tiny machine just keeps its running, colliding, bouncing, or adhesion states until finally assembling into a single machine. Unlike the commonly encountered rigid machines, such transient state system can be reversible in working shapes. Depending on their surface tension, the autonomously traveling droplet motors can experience bouncing and colliding before undergoing total coalescence, arrested coalescence, or total bounce. This finding would help mold unconventional robot in the sense of transient state machine that could automatically transform among different geometries such as a single or swarm, small or large size, assembling and interaction, etc. It refreshes people's basic understandings on machines, liquid metal materials, fluid mechanics, and micromotors.     

High-speed imaging and CFD simulations of a deforming liquid metal droplet in an electromagnetic levitation experiment

Electromagnetic levitation of a liquid metal droplet is of great interest to study gas–liquid metal reactions. An important prerequisite for the evaluation of the overall mass transfer between the gas and metal is to characterize the geometry of the deforming molten droplet, which determines the interfacial reaction area. In this article, the free surface shape and dynamics of a molten 80%Ni–20%Cr droplet is investigated both experimentally and numerically. The frequencies associated to the oscillatory translational motions of the drop and to the vibrations of its free surface are measured using high-speed video image analysis. A 2D transient model is then presented, in which three interacting phenomena are considered: electromagnetic phenomena, the turbulent flow of liquid metal in the drop and the change in the drop shape. The numerical results presented demonstrate the capabilities of the model.