Numerical simulation and laser-based imaging of mixture formation, ignition, and soot formation in a diesel spray

Laser-based imaging of fuel vapor distribution, ignition, and soot formation in diesel sprays was carried out in a high-pressure, high-temperature spray chamber under conditions that correspond to temperature and pressure in a diesel engine. Rayleigh scattering and laser-induced incandescence are used to image fuel density and soot volume fraction. The experimental results provide data for comparison with numerical simulations. An interactive cross-sectionally averaged spray model based on Eulerian transport equations was used for the simulation of the spray, and the turbulence-chemistry interaction was modeled with the representative interactive flamelet (RIF) concept. The flamelet calculation is coupled to the Kiva3V computational fluid dynamics (CFD) code using the scalar dissipation rate and pressure as an input to the RIF-code. The flamelet code computes the instationary flamelet profiles for every time step. These profiles were integrated over mixture fraction space using a prescribed β-PDF to obtain mean values, which are passed back to the CFD-code. Thereby, the temperature and the relevant species in each CFD-cell were obtained. The fuel distribution, the average ignition delay as well as the location of ignition are well predicted by the simulation. Furthermore, simulations show that the experimentally observed injection-to-injection variations in ignition delay are due to temperature inhomogeneities. Experimental and simulated spatial soot and fuel vapor density distributions are compared during and after second stage ignition.     

Well-crystallized zinc oxide quantum dots with narrow size distribution

In this study, pulsed laser ablation, online annealing, and following size classification using a differential mobility analyzer (DMA) were employed to fabricate quantum dots (QDs) of zinc oxide (ZnO). The irregularly shaped ZnO particles were obtained at annealing temperature less than 873 K, which gradually transformed into spherical QDs with increasing the annealing temperature. Finally, ZnO QDs with narrow size distribution having spherical shapes were successfully obtained at temperatures above 1173 K under the DMA classification at a nominal size of 10 nm. TEM observation demonstrated that the ZnO QDs obtained by this process were well-crystallized single crystallites with a wurtzite structure. Further, ZnO QDs with average sizes in the range of 4.8–8.1 nm were successfully fabricated by reducing the specified sizes of DMA. These features of the fabricated ZnO QDs are favorable for investigation of intrinsic quantum size effect in ZnO.     

CCD立靶测量系统目标捕获性能研究

在CCD立靶测量系统工作原理的基础上,从CCD的灵敏度与曝光量、背景照度与目标照度、CCD的分辨能力和目标与背景的对比度等几个方面研究CCD立靶测量系统的目标捕获性能,给出了相关的计算方法.特别对暗、小、快目标的全天时捕获,提出了采用激光选通照明的方法来保证系统的捕获率.通过实验验证了计算方法和照明方法的有效性.     

Laser assisted removal of synthetic painting-conservation materials using UV radiation of ns and fs pulse duration: Morphological studies on model samples

In an effort to establish the optimal parameters for the cleaning of complex layers of polymers (mainly based on acrylics, vinyls, epoxys known as Elvacite, Laropal, Paraloid B72, among others) applied during past conservation treatments on the surface of wall paintings, laser cleaning tests were performed with particular emphasis on the plausible morphological modifications induced in the remaining polymeric material. Pulse duration effects were studied using laser systems of different pulse durations (ns and fs) at 248 nm. Prior to tests on real fragments from the Monumental Cemetery in Pisa (Italy) which were coated with different polymers, attention was focused on the study of model samples consisting of analogous polymer films cast on quartz disks. Ultraviolet irradiation is strongly absorbed by the studied materials both in ns and fs irradiation regimes. However, it is demonstrated that ultrashort laser pulses result in reduced morphological alterations in comparison to ns irradiation. In addition, the dependence of the observed alterations on the chemical composition of the consolidation materials in both regimes was examined. Most importantly, it was shown that in this specific conservation problem, an optimum cleaning process may rely not only on the minimization of laser-induced morphological changes but also on the exploitation of the conditions that favour the disruption of the adhesion between the synthetic material and the painting.     

Laser forming of a bowl shaped surface with a stationary laser beam

Despite a lot of research done in the field of laser forming, generation of a symmetric bowl shaped surface by this process is still a challenge mainly because only a portion of the sheet is momentarily deformed in this process, unlike conventional sheet metal forming like deep drawing where the entire blank undergoes forming simultaneously reducing asymmetry to a minimum. The motion of laser beam also makes the process asymmetric. To counter these limitations this work proposes a new approach for laser forming of a bowl shaped surface by irradiating the centre of a flat circular blank with a stationary laser beam. With high power lasers, power density sufficient for laser forming, can be availed at reasonably large spot sizes. This advantage is exploited in this technique. Effects of duration of laser irradiation and beam spot diameter on the amount of bending and asymmetry in the formed surface were investigated. Laser power was kept constant while varying irradiation time. While varying laser spot diameter laser power was chosen so as to keep the surface temperature nearly constant at just below melting. Experimental conditions promoted almost uniform heating through sheet thickness. The amount of bending increased with irradiation time and spot diameter. It was interesting to observe that blanks bent towards the laser beam for smaller laser beam diameters and the reverse happened for larger spot diameters (~10 times of the sheet thickness). Effect of spot diameter variation has been explained with the help of coupled thermal-structural finite element simulations.     

Subwavelength structures for high power laser antireflection application on fused silica by one-step reactive ion etching

In this paper we report a simple method to fabricate a novel subwavelength structure surface on fused silica substrate using one-step reactive ion etching with two-dimensional polystyrene colloidal crystals as masks. The etching process and the morphologies of the obtained structure are controlled. We show that the period of the obtained fused silica pillar-like arrays were determined by the initial polystyrene nanoparticle size. The height of pillar arrays can be adjusted by controlling the etching duration, which is proved to be of importance in tailoring the antireflection properties of subwavelength structures surface. The novel subwavelength structures surface exhibit excellent broadband antireflection properties, but the size of the pillar affects the antireflective properties in short wavelength region. We anticipate this method would offer a convenient and scalable way for inexpensive and high-efficiency high power laser field designs.     

Laser cutting of Kevlar laminates and thermal stress formed at cutting sections

Laser cutting of Kevlar laminates is carried out and thermal stress field developed in the cutting region is predicted using the finite element code. Temperature predictions are validated through the thermocouple data. The morphological changes in the cutting section are examined by incorporating optical and scanning electron microscopes. It is found that temperature predictions agree well with the thermocouple data. High values of von Mises stress are observed at the cutting edges and at the mid-thickness of the Kevlar laminate due to thermal compression formed in this region. The laser cut edges are free from whiskers; however, striation formation and some small sideways burning is observed at the kerf edges.     

Stilbene 420激光染料的光谱特性分析

配制了Stilbene 420染料溶液。测量了其吸收光谱。以调Q倍频Nd∶YAG激光为抽运源,实现了对Stilbene 420染料的激光光谱和荧光光谱的分析。激光光谱在425 nm处获得最强峰值,半峰高宽(FWHM)为1 nm,光谱范围为420～440 nm。荧光光谱峰值在428.5 nm,与激光最强峰值相差3.5 nm。最高染料转换效率为9.26%。     

激光诱导冲击成形的研究进展

激光冲击成形是一种利用高能短脉冲激光诱导的高幅冲击波的力效应使板料产生塑性变形的新技术。本文介绍了激光冲击成形的原理,系统阐述了成形工艺参数对成形质量的影响。在总结激光冲击成形研究现状的基础上,指出激光动高压加载驱动成形方法是激光冲击成形的发展方向。最后提出动高压加载驱动成形可以在微体积成形方面进行应用拓宽。     

基于景象投射技术的激光引信半实物仿真系统

在激光引信的研制过程中,半实物仿真是能够较为全面地评测其工作性能的重要手段,介绍了基于激光景象投射(LSP)技术的激光引信半实物仿真(HILS)系统作为仿真试验的平台。该系统能够有效地模拟来自目标和背景的激光回波特性,并以激光阵列的形式投射到激光引信接收端,从而获取激光引信在弹目交会过程中的实时反馈信息,为激光引信的研制改进提供了可靠的数据支持。