Velocity Measurement of Induced Flow by a Laser Focusing Shock Wave

The objective of this study is to apply the shock wave for control in a micro channel. The shock wave was generated by a laser focusing of pulsed laser beam in the channel. Using a pulse laser to generate a shock wave, a non-stationary flow was induced in the small space between the parallel plates. The spherical and cylindrical shock propagations were observed with schlieren method. The shock Mach number decreases with time and approaches to unity. As reported in the previous investigations, the shock speed was attenuated in a short distance and time. In the present experiment, It was not found a remarkable difference in the shock speed between the spherical and cylindrical shock experiments. Subsequently, the flow induced by the cylindrical shock wave was studied using PIV technique. A smoke tracer was used in the experiment and its velocity was measured within 100 μs. A numerical simulation was carried out to investigate the momentum relaxation between the gas and smoke particle. A suitable shock initiation model was introduced in the simulation. The experimental results show that a wide acceleration and deceleration zone exist behind the shock wave. Also, the relaxation distance in the experimental data is much longer than that in numerical simulation.     

光纤激光多功能加工设备设计方案

该设备的方案为选配光纤传导的大功率光纤激光器,将聚焦系统连接在机器人手腕上,集成为柔性激光加工设备,运动的执行机构为激光束扫描运动的机器人＋工件旋转的转台系统,可以实现各种形状零件的全自动激光加工。     

Why self‐cleaning is important for solar thermal receivers?

Surface cleaning remains essential for the sustainable operation of high performance solar thermal receivers. Cleaning of optical surfaces, such as solar troughs and absorbers, requires energy intensive efforts because of the large surface area involvement such as those observed in solar farms. In addition, self‐cleaning of such surfaces becomes demanding because of lowering the cleaning costs, reducing the waste of resources, such as clean water, and minimizing the complication of the mechanical systems incorporated. Self‐cleaning of surfaces is associated with the low adhesion between the surface and the foreign particles; in which case, these particles can be removed easily from the surfaces in a cost‐effective way. The surface energy and contact area of the surface are two main important parameters influencing the particle adhesion on the surfaces. In this case, reducing the surface energy and forming micro/nano size pillars on the surface through texturing lower the particle adhesion on the surfaces significantly. In solar thermal energy harvesting applications, metallic or composite materials are used and texturing the surface remains challenging in terms of cost and precision of operation when conventional texturing methods are used. One of the methods to create surface texture consisting of micro/nano pillars is to use the laser beam ablation. This results in hierarchical distribution of surface texture with desired pillar heights1. In addition, laser surface texturing offers significant advantages over the conventional techniques. Some of these advantages include fast processing, precision of operation, and low cost. Although the laser processing involves with high temperature processing and thermally induced stresses remain important, the defects sites can be minimized via controlling the process parameters during the texturing. Introducing the assisting gas on the texturing surface enables to generate compounds such as oxide or nitride species, which lower the surface energy considerably. Consequently, investigation of laser texturing of solar energy materials while incorporating the assisting gas becomes essential. In the present perspective, the laser surface texturing of solar energy materials for thermal power applications is presented together with challenges and future perspectives. Specifically, the followings will be presented: (1) the texture characteristics of laser treated metallic and ceramic surfaces; (2) wetting state of the textured surface, and optical properties of textured surface in terms of absorption of the solar irradiation.     

Characteristics of plasma induced by interaction of a free-oscillated laser pulse with a coal target in air and combustible gas

Plasma in the air is successfully induced by a free-oscillated Nd:YAG laser pulse with a peak power of 10^2–3 W. The initial free electrons for the cascade breakdown process are from the ablated particles from the surface of a heated coal target, likewise induced by the focused laser beam. The laser field compensates the energy loss of the plasma when the corresponding temperature and the images are investigated by fitting the experimental spectra of B²∑⁺ → X²∑⁺ band of CN radicals in the plasma with the simulated spectra and a 4-frame CCD camera. The electron density is estimated using a simplified Kramer formula. As this interaction occurs in a gas mixture of hydrogen and oxygen, the formation and development of the plasma are weakened or restrained due to the chaining branch reaction in which the OH radicals are accumulated and the laser energy is consumed. Moreover, this laser ignition will initiate the combustion or explosion process of combustible gas and the minimum ignition energy is measured at different initial pressures. The differences in the experimental results compared to those induced by a nanosecond Q-switched laser pulse with a peak power of 10^6–8 W are also discussed.     

Fast LBIC in-line characterization for process quality control in the photovoltaic industry

The photovoltaic industry asks for fast, non-destructive techniques for in-line characterization tools in solar cells production. We shall show in this paper that the use of the light beam induced current technique (LBIC) is capable to get in a few seconds time photocurrent maps of large area solar cells and to correlate these data with the cell efficiency. The samples analysed in this study are industrial 10×10 cm² multicrystalline silicon solar cells. The LBIC setup works with three laser sources at 633, 780 and 830 nm, taking the laser beam power below 1 mW. The laser beams are moved on the wafer surfaces using a galvanometer x–y scanner system and the beam size on the focus has a diameter of about 65 μm. We demonstrated the possibility to obtain quantitative information about the cell quality in <3 s from the photocurrent maps with a pretty good correlation with the efficiency data.     

Combustion of nanofluid fuels with the addition of boron and iron particles at dilute and dense concentrations

The combustion characteristics of nanofluid fuels containing additions of boron and iron particles were investigated. The effects of particle materials, loading rate, and type of base fuel on suspension quality and combustion behavior were determined. The burning behaviors of dilute and dense suspensions were compared, and the results for dense nanosuspensions showed that most particles were burned as a large agglomerate at a later stage when all the liquid fuel had been consumed. Sometimes this agglomerate may not burn if the energy provided by the droplet flame is insufficient. For dilute suspensions, the burning characteristics were characterized by a simultaneous burning of both the droplet and the particles, which integrated into one stage. The fundamental mechanism responsible for bringing the particles out of the droplet, which is a prerequisite condition for them to burn, is different for n-decane- and ethanol-based fuels. For the former, the particles are brought out of the droplet by a disruptive behavior of the primary droplet, which was characterized by multiple-time disruptions and with strong intensity. This was caused by the different boiling points between n-decane and the surfactant. For ethanol-based fuels having no added surfactant, the particles are also brought out of the droplet by disruptive behavior, but characterized by continuous disruptions of mild intensity. This was very likely caused by a continuous water absorption by the ethanol droplet during its burning process.     

一种测量柴油喷雾SMD分布的新方法：荧光／散射光图象处理法

本文介绍了一种测量柴油喷雾２维索特平均直径（ＳＭＤ）空间分布的新方法。这种方法基于喷雾的荧光及米氏散射原理。本文的主要发现是：当普通商用柴油被波长为５３２ｎｍ的绿色Ｎｄ：ＹＡＧ激光诱导时，能发出波长为５７０ｎｍ的荧光，因此试验时所用的柴油不必添加任何物质，柴油的理化特性不会改变。喷雾的荧光和散射光图象透过不同的滤光片被ＣＣＤ摄像头接收，通过计算图象中各点两种光的强度对比，就能求得油滴直径的分布。本文介绍了试验结果并进行了分析。     

铝传感层蒸镀速率对飞秒激光抽运探测热反射方法测量热导率影响的研究

对飞秒激光抽运探测热反射实验中的一个关键因素传感层进行了研究,发现铝传感层的蒸镀速率对飞秒激光抽运实验有着很大的影响。分别在3种不同类型的硅片和玻璃片基底上用不同的蒸镀速率蒸镀了100nm的铝膜蒸镀速率控制在2×10-10到15×10-10 m/s。通过扫描电子显微镜(SEM)和X射线反射(XRR)研究了蒸镀铝膜表面的形貌及铝膜的厚度。基于飞秒激光抽运探测热反射方法对基底的热导率进行了测量,发现随着蒸镀速率的增大,不同基底测量得到的热导率呈现一致的规律。结果表明,蒸镀速率越大,铝传感层的晶粒越大,传感层的体积热容越小,当蒸镀速率大到一定程度时,由于晶粒的不规则度越来越大,反过来又影响体积热容的大小,从而影响了飞秒激光抽运探测热反射。     

Experimental investigations of spray generated by a pressure swirl atomizer

This paper has focused on the droplets behavior of kerosene RP-3 spray produced by a pressure swirl atomizers in terms of spray pattern, droplet size spatial distribution, mean droplet size, and distribution index with variations of pressure differential. The analyses have been carried out experimentally with the aid of optical diagnostic methods. The spray pattern, such as spray cone angle and fuel spatial distribution, has been measured by the technique of planar laser induced fluorescence of kerosene. A method for correction of fuel distribution measurement error induced by laser attenuation in spray is proposed and validated. The droplet size spatial distribution in central axis plane of the spray has been measured by a planar droplet sizing method which combining laser induced fluorescence and Mie scattering. The spray pattern in axial center plane and cross-sectional plane perpendicular to axis of the atomizer indicate that the droplets in spray concentrate around the outer periphery and in a narrow annular zone at the near-field of fuel injector exit, and then disperse to produce a solid spray at downstream of the spray. The analyses of droplet size spatial distribution, Sauter mean diameter, and distribution index with pressure differential clearly show the presence of droplets collision and its adverse effects on droplet size uniformity. The spray outline, droplet mass spatial distribution, and droplet size spatial distribution, droplets dispersion and collision in the process of atomization provide a great insight into the processes of atomization and spray development, which are key information for fuel injector design and quality control. The visualizations of spray pattern and droplet size spatial distribution with variations of pressure differential for pressure swirl atomizer are key issues in swirl cup or internally staged airblast fuel injectors because pressure swirl atomizer provides primary atomization or pilot spray which affects the quality of air/fuel mixing in lean-burn combustion. Moreover, a well-defined and complete database regarding the isothermal hollow cone spray is provided for validation of spray model.     

Thermoreactions between molten salts and water at high cooling rates

New experimental results of the investigations of “steam explosion” in ionic melts are described. The existence of a long lived local area of increased optical density is established. Caused by initial perturbation (like water droplet, laser beam, etc.), the area moves through the melt and disappears simultaneously with the explosion. Also, characteristic electromagnetic radiation is for the first time registered in the ultraviolet and infrared spectra during the time before the explosion. The results described show that although the water can trigger the process (in accordance with the “steam explosion” concept), the phenomena related to the explosion are intrinsic to the phase transformations in the melt. These phenomena can hardly be understood in the framework of the standard theory of the first-order phase transitions.     

Performance enhancement of polymer electrolyte membrane fuel cell by PtCo3 nanoporous film as high mass-specific power density catalyst using laser deposition and processing

This study demonstrates the performance of polymer electrolyte membrane fuel cell (PEMFC) can be significantly enhanced with laser deposited and processed PtCo₃ nanoporous film as the catalyst layer. The PtCo₃ nanoporous film is fabricated by first depositing a thin film of stacked PtCo₃ nanoparticles on gas diffusion layer using pulsed laser deposition and then subjecting the film to scanning continuous-wave laser processing. The heating by the scanning laser beam increases electrochemical surface area substantially, which could be ascribed to formation of Pt skin on each nanoparticle via Pt surface segregation. Furthermore, the durability of the catalyst under accelerated degradation test is greatly enhanced, which may be attributed to inhibition of Co leaching by the Pt skin and strengthening of the contacts between nanoparticles caused by concomitant sintering effect. Application of such catalyst to the cathode of a PEMFC exhibits a 2.4-fold increase in current density with respect to that without laser processing, raising the cathode mass-specific power density to 8.79 kW g⁻¹ with 1 atm oxygen and 12.0 kW g⁻¹ with 1.5 atm oxygen.     

Water sorption–desorption in Nafion membranes at low temperature,probed by micro X-ray diffraction

Using a micro X-ray beam, the structure of a water swollen Nafion^® membrane, alone or in a membrane electrode assembly (MEA) designed for fuel cells, was studied upon cooling down to −70 °C. By scanning the membranes along their thicknesses, the water sorption–desorption process was investigated as a function of cooling/heating stages. From the scattering curves, it was deduced that the state of the water at a sub-zero temperature is glassy inside the membrane and ice crystals are observed only outside it. In the case of the MEA, this growth can be destructive since this formation is localised inside the active layers.     

Measurement of droplet size distribution in core region of high-speed spray by micro-probe L2F

In order to investigate the distribution of droplet sizes in the core region of diesel fuel spray, instantaneous measurement of droplet sizes was conducted by an advanced laser 2-focus velocimeter （L2F）. The micro-scale probe of the L2F is made up of two foci and the distance between them is 36 lain. The tested nozzle had a 0.2 mm diameter single-hole. The measurements of injection pressure, needle lift, and crank angle were synchronized with the measurement by the L2F at the position 10 mm downstream from the nozzle exit. It is clearly shown that the droplet near the spray axis is larger than that in the off-axis region under the needle full lift condition and that the spatial distribution of droplet sizes varies temporally. It is found that the probability density distribution of droplet sizes in the spray core region can be fitted m the Nukiyama-Tanasawa distribution in most injection periods.     

熔池传热与流动的非定常数值模拟

对受移动激光速间隙加热的工件中的非定常传热与熔体流动进行了数值模拟，对不同热源移动速度下熔池行为的模拟结果进行了比较，当激光束移动速度比较高时形成的熔池浅而短，而移动速度比较较时形成的熔池深而长，所得工件代表点上材料的温度随时间变化以及加热或冷却速率，可以进一步用于研究被焊工件中的热应力或组织结构变化。     

Photothermal effects induced by laser excitation in a scanning tunneling microscope

Numerous near-field experimental methods use laser excitation inducing non-negligible thermal effects that are not taken into account. We particularly investigate the case of a scanning tunneling microscope. The analysis of the experimental results gives the thermal contribution of the probe in the measured signal when the probe is irradiated by the laser beam. The phase of the signal allows the distinction between the effects induced by the probe and the sample. We show that the experimental set-up can measure the periodic thermal expansion of the sample with the picometer resolution.     

Solar energy harvesting and self-cleaning of surfaces by an impacting water droplet

The self-cleaning of surfaces via impacting water droplets is examined pertinent to solar energy applications. The mechanism of spreading and retraction of the impacting droplet onto the dust and cleaned hydrophobic surfaces is considered in the analysis. The spreading factor of the impacting droplet is formulated incorporating the energy balance on the hydrophobic surface. High-speed photography is used to monitor the impacting droplet behavior. The functionalized silica particles coating is introduced towards generating hydrophobic wetting state on the glass surfaces. Environmental dust particles are characterized prior to self-cleaning impacting tests. It is found that spreading rate predicted for the impacting droplet agrees well with that obtained from the experiments. The droplet Weber number incorporated in the experiments does not result in droplet breaking on the surface upon impacting. The dusts are dissimilar in shapes and consist of several elements. Impacting droplet gives rise to cloaking of the dust on the surface during spreading and retraction. Almost all the dusts are removed from the surface through the impacting droplets; however, few dust residues are left on the impacting surface, which cover only 0.03% of the total surface area. Hence, we have demonstrated that self-cleaning of a surface can be achieved by an impacting droplet for renewable energy applications.     

乳化油微爆的统一模型

关于乳化油的微爆在以往的文献中提出过各种计算模型,对油包水与水包油两种乳化油的微爆有不同看法。提出一种油包水与水包油微爆的统一模型并给出了微爆强度的定义,试图对微爆的强弱做定量的描述。通过该模型讨论了一些主要因素,如液滴初始直径,分散相直径及水含量等对微爆强度的影响规律。计算结果表明,该模型的理论预测与实验事实基本一致。另外还讨论了其他因素如添加剂的性质、气体溶解度等的作用。     

Micro‐CT scanning analysis for inner structure of porous media

A micro‐CT scanner was employed to investigate inner characteristics of porous media, and particular cakes were taken as samples. By obtaining the inner pore structure and inner structure reconstruction, porosity and its variation in the samples, water distribution in pores, and other inner characteristics were determined and explored. When the sample was dried after immersion in water, the solid frame shrank, some pores became larger and the porosity increased, while the sample not immersed in water did not change much after being dried. The experiments indicate that micro‐CT scanning is an effective technology to study the inner structure of porous materials with pores larger than tens of microns and also can be used to explore some important transport performance. ©2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(4): 208– 214, 2007; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/htj.20155 Copyright © 2004 Wiley Periodicals, Inc.     

Thermoelastic Material Response Due to Laser Pulse Heating in Context of Four Theorems of Thermoelasticity

This article describes the study of induced temperature and stress fields in an elastic half-space in the context of classical coupled thermoelasticity (Biot) and generalized thermoelasticity (Lord–Shulman, Green–Lindsay and Green–Naghdi) in a unified system of equations. The medium is considered to be made of an isotropic homogeneous thermoelastic half-space. The bounding plane of the surface is heated by a non-Gaussian laser beam with pulse duration of 2 ps. An exact solution of the problem is first obtained in Laplace transform space. Because the response is of more interest in the transient state, the inversion of Laplace transforms were carried out numerically. The derived expressions were computed numerically for copper, and the results are presented in graphical form.