Stochastic modeling of atomizing spray in a complex swirl injector using large eddy simulation

Large-eddy simulation of an atomizing spray issuing from a gas-turbine injector is performed. The filtered Navier–Stokes equations with dynamic subgrid scale model are solved on unstructured grids to compute the swirling turbulent flow through complex passages of the injector. The collocated grid, incompressible flow algorithm on arbitrary shaped unstructured grids developed by Mahesh et al. (J. Comp. Phys. 197 (2004) 215–240) is used in this work. A Lagrangian point-particle formulation with a stochastic model for droplet breakup is used for the liquid phase. Following Kolmogorov’s concept of viewing solid particle-breakup as a discrete random process, the droplet breakup is considered in the framework of uncorrelated breakup events, independent of the initial droplet size. The size and number density of the newly produced droplets is governed by the Fokker–Planck equation for the evolution of the pdf of droplet radii. The parameters of the model are obtained dynamically by relating them to the local Weber number and resolved scale turbulence properties. A hybrid particle-parcel is used to represent the large number of spray droplets. The predictive capability of the LES together with Lagrangian droplet dynamics models to capture the droplet dispersion characteristics, size distributions, and the spray evolution is examined in detail by comparing it with the spray patternation study for the gas-turbine injector. The present approach is computationally efficient and captures the global features of the fragmentary process of liquid atomization in complex configurations.     

一种校正XRF光谱基体吸收效应的新方法

根据校正XRF光谱基体吸收效应的基本理论,提出了一种用基本参数法校正基体吸收效应的新方法,该法既不需要标准,又对样品和衬底靶无特别要求,仅用一些基本的物理参数(ω,J,τ)和荧光强度的测量值便可算得样品中待分析元素的含量,方便简单。用岩石标准SY-2,SY-3和MRG-1检验该法,其结果与文献报道的值相符。     

微型化光纤法布里-珀罗传感器腔长一致性的控制研究

研究了光纤法布里-珀罗腔传感器的腔长一致性的控制技术.通过三维调整架组成微调装置来控制全光纤法布里-珀罗腔的腔长;结合法布里-珀罗腔的工作原理,利用光谱分析仪实时检测加工过程中法布里-珀罗腔的腔长并用装置予以修正;以热熔接的方法将毛细玻璃管与光纤加工成法布里-珀罗腔;利用超景深光学显微系统检查加工的法布里-珀罗腔的关键部位的结构,并对熔接点的牢固性进行了检测.实验结果和数据分析显示:法布里-珀罗腔的腔长得到了较好的控制及一致性,熔接部位的变形和对法布里-珀罗腔的性能的影响也很小.该工艺可用于制备全光纤法布里-珀罗腔传感器.     

Optical diagnostics of diesel spray injections and combustion in a high-pressure high-temperature cell

We report on spatially and temporally resolved optical diagnostic measurements of propagation and combustion of diesel sprays introduced through a single-hole fuel injector into a constant volume, high-temperature, high-pressure cell. From shadowgraphy images in non-reacting environments of pure nitrogen, penetration lengths and dispersion angles were determined for non-vaporizing and vaporizing conditions, and found to be in reasonable agreement with standard models for liquid jet propagation and break-up.Quasi-simultaneous two-dimensional images were obtained of laser elastic light scattering, shadowgraphs and spectrally integrated flame emission in a reacting environment (cell temperature 850 K). In addition laser-induced incandescence was employed for the identification of soot-loaded regions. The simultaneously recorded spray images exhibit remarkable structural similarity and provide complementary information about the spray propagation and combustion process. The measurements also reveal the fuel vapor cloud extending well beyond the liquid core and close to the nozzle tip. Ignition takes place close to the tip of the spray within the mixing layer of fuel vapor and surrounding air. Soot is formed in the vapor core region at the tip of the liquid fuel jet. Our results support recently developed phenomenological model on diesel spray combustion.     

太赫兹波段火焰碳黑的光学特性研究

火焰碳黑是碳氢燃料不完全燃烧的重要固体产物,对于一些污染物的生成具有重要影响,其光学特性是光学燃烧诊断的基础。利用太赫兹时域光谱技术研究了0.2～1.6 THz火焰碳黑的光学特性,通过傅里叶变换得到了碳黑的频域光谱,利用定点迭代法获得了太赫兹波段火焰碳黑的复折射率,把太赫兹波段的复折射率与热辐射波段的复折射率进行了比较,此外还对比了两种光学参数提取方法所得到的结果,结果表明,碳黑在太赫兹波段的吸收性较强,其折射率在太赫兹波段与在热辐射波段的差别不是很大,而吸收率在热辐射波段变化更大一些,两种参数提取方法得到的复折射率差别不是很大,研究结果可为太赫兹波技术应用于光学燃烧诊断提供基础性数据,扩展了光学燃烧诊断应用的范围。     

Effects of fuel viscosity on the primary breakup dynamics of a high-speed liquid jet with comparison to X-ray radiography

One of the major concerns in combustion engines is the sensitivity of engine performance to fuel properties. Recent works have shown that even slight differences in fuel properties can cause significant changes in performance and emission of an engine. In order to design the combustion engines with multi-fuel flexibilities, the precise assessment of fuel sensitivity on liquid jet atomization process is a prerequisite since the resulting fuel/air mixture is critical to the subsequent combustion process. The present study is focusing on the effect of physical fuel properties, mostly viscosity difference, on the breakup process of the liquid jet injected into still air. Two different jet fuels, CAT-A2 and CAT-C3, are considered here as surrogates for a fossil-based fuel and a bio-derived high-viscosity alternative fuel. The simulations are performed using the volume-of-fluid (VoF) interface tracking method coupled to Lagrangian particle method in order to capture the breakup instabilities of jets and the resulting droplets. The investigations take the actual geometry of the injector into account to resolve the unsteady flow phenomena inside the nozzle that impact the turbulence transition and atomization. The simulation results are compared to the experimental measurement using X-ray radiography. Both simulation and X-ray measurements consistently describe the effects of different fuels on the fundamental properties of atomization including the breakup length, transverse liquid volume fraction and the droplet sauter-mean-diameter. The application of a Detailed Numerical Simulation approach complemented by unique X-ray diagnostics is novel and providing new understanding and research directions in engine spray dynamics.     

Optical and optoacoustic measurements of the absorption properties of spherical gold nanoparticles within a highly scattering medium

In the present paper the requirements for optical parameter characterization of absorbing materials located within a highly scattering medium has been addressed. The measurement scheme incorporates the optoacoustic technique where a single acoustic transducer is used to detect ultrasonic transients generated from laser irradiation. The absorbing medium is based on different concentrations of spherical gold nanoparticles (SGNP’s), these are currently being considered as non-toxic targeted optical contrast agents for both medical imaging and cancer therapeutics. In this paper we present results which demonstrate the two main advantages the optoacoustic technique has over other measurement schemes. These are the possibility to obtain information on the position and dimensions of absorbing bodies using a time of flight analysis (TOF) and secondly, the higher sensitivity of the optoacoustics compared to optical transmission techniques. The former advantage is of particular interest for imaging applications and the latter for detection and characterization of absorbing materials surrounded by high levels of high scattering mediums. We present for the first time the characterization of SGNP within a highly scattering medium. To further demonstrate the feasibility of the optoacoustic technique, the scattering coefficient of the surrounding medium has also been characterized.     

Optical measurements of the superconducting gap in MgB 2

Far-infrared reflectivity studies on the polycrystalline intermetallic compound MgB₂ with a superconducting transition temperature T _c = 39 K were performed at temperatures 20 K to 300 K. We observe a significant raise of the superconducting-to-normal state reflectivity ratio below 70 cm ^-1 , with a maximum at about 25-30 cm ^-1 , which gives a lower estimate of the superconducting gap of 2Δ(0) ≈ 3-4 meV. Received 7 March 2001 and Received in final form 18 April 2001     

The parametric method of reconstructing the field of a radiator from pressure measurements in its near zone

A method of determining the field of a radiator from pressure measurements in its near zone is proposed. The method is based on parametrization of the field by a set of complex spherical sources. The field of the radiator is calculated using the spherical harmonic amplitudes, which are determined by solving the inverse problem. Results of numerical simulation are presented.     

Scaling of the flow field in a combustion chamber with agas——gas injector

The scaling of the flowfield in a gas--gas combustion chamber is investigated theoretically, numerically and experimentally. To obtain the scaling criterion of the gas--gas combustion flowfield, formulation analysis of the three-dimensional (3D) Navier--Stokes equations for a gaseous multi-component mixing reaction flow is conducted and dimensional analysis on the gas--gas combustion phenomena is also carried out. The criterion implies that the size and the pressure of the gas--gas combustion chamber can be changed. Based on the criterion, multi-element injector chambers with different geometric sizes and at different chamber pressures ranging from 3~MPa to 20~MPa are numerically simulated. A multi-element injector chamber is designed and hot-fire tested at five chamber pressures from 1.64~MPa to 3.68~MPa. Wall temperature measurements are used to understand the similarity of combustion flowfields in the tests. The results have verified the similarities between combustion flowfields under different chamber pressures and geometries, with the criterion applied.     

Spatial scales of extinction and dissipation in the near field of non-premixed turbulent jet flames

Simultaneous high-resolution Rayleigh scattering imaging and planar laser-induced fluorescence (PLIF) of OH are combined to measure the dissipative scales associated with thermal mixing and the structure and scales of extinguished regions of the reaction zone. Measurements are performed throughout the near field (x/d = 5, 10, 15, 20) of two turbulent, non-premixed methane/hydrogen/nitrogen jet flames with Re = 15,200 and 22,800 (flames DLR-A and DLR-B of the TNF workshop). Locally extinguished regions are identified by discontinuities in the OH layers, and the extinction hole sizes are measured. For each flame, the probability density function of the hole sizes is very similar throughout the entire near field, with the most likely hole size being 1.9 mm in DLR-A and 1.1 mm in DLR-B. Extinction events are equally probable at all measurement locations in DLR-A. In the DLR-B flame, there is a progression from frequent extinction close to the nozzle to more continuous reaction zones further downstream. The approximate instantaneous location of the stoichiometric contour is determined using the OH-PLIF images, enabling statistical analysis of dissipative scales conditioned on rich and lean conditions. The widths of the thin, elongated structures that dominate the thermal dissipation field are measured. Statistics of this microscale are qualitatively similar in both flames, with the higher Reynolds number producing smaller scales throughout the flow field. For dissipation layers in rich regions, the layer widths increase significantly with increasing temperature, while on the lean side the layer widths decrease with increasing temperature.     

光学玻璃电流传感头线性双折射的一种测量方法

报告了一种测量光学玻璃电流传感头线性双折射的方法,以琼斯矩阵为数学工具给出了对该方法的理论分析、测量不确定度计算公式,并用实验方法给出了应用实例.此方法的主要优点是光路所用元件容易获得且测量结构简单实用.     

Apertureless near field optical microscopy: a contribution to the understanding of the signal detected in the presence of a background field

Apertureless scanning near field optical microscopy techniques have become a common way of studying surface samples. By using a nano-probe that scatters the electromagnetic non-propagative waves emerging from a given sample, this microscopy provides optical images with a resolution beyond the diffraction limit. Despite a great diversity of works covering a wide variety of physical domains, the formation of the images obtained is not yet fully understood. The purpose of this letter is to assess the influence of the tip position and imposed oscillation of the tip in apertureless SNOM when a background field is added to the scattered near field. We propose a simple analytical model which enables us to account for the experimental results and explains how, depending on the experimental conditions, the near field signal can totally disappear or, on the contrary, be greatly enhanced.     

拉盖尔-高斯光束的近场矢量结构特征

直接以麦克斯韦方程组的解表征拉盖尔-高斯光束。基于麦克斯韦方程组解的矢量角谱表述和电磁光束的矢量结构理论,利用一些数学技巧导出了拉盖尔-高斯光束的TE项和TM项在近场的解析表达式。利用所导出的公式,在近场描绘了拉盖尔-高斯光束的TE项、TM项及整个光束的光强分布。并对角度依赖分别为余弦和正弦关系的拉盖尔-高斯光束的矢量结构进行了比较,结果显示两者整个光束的光斑完全相似,唯一的区别是子瓣的空间方位不同且两者的内部矢量结构完全不相同。     

Optical anisotropy of aligned pentacene molecules on a rubbed polymer corresponding to the electrical anisotropy

We report on the optical anisotropy of a pentacene film on a rubbed (poly)vinylalcohol (PVA) layer related to the electrical performances of the pentacene organic field effect transistors (OFETs) depending on the direction of a current flow. The optical anisotropies of the PVA films are negligible with respect to whether or not rubbing process. In the pentacene OFET on the rubbed PVA layer, however, the optical anisotropy is observed and the anisotropy of the electrical performances directly corresponds to the optical anisotropy of the pentacene thin-film on the rubbed PVA layer.     

Optical bistability and multistability in a four-level quantum system in the presence of plasmonic nanostructure

In this paper, the effect of plasmonic nanostructure on behavior of optical bistability and multistability in a four-level quantum system embedded in a unidirectional ring cavity has been studied. It is found that the distance between plasmonic nanostructures and a four-level quantum system is varied from 4.4 nm to 14.4 nm and has an essential role for controlling the threshold of optical bistability and multistability. Moreover, obtained results show that the group velocity of pulse propagation through the medium is strongly depended on the distance between the plasmonic nanostructure and the four-level quantum system.     

The extended IEM mixing model in the framework of the composition PDF approach: applications to diesel spray combustion

A steady, two-dimensional corner flame is established when fuel and oxidizer enter the reaction zone in mutually perpendicular directions. A model problem in which the velocity fields are linear functions of spatial position is utilized to study the resulting flame. The flame structure is comprised of a diffusion flame surrounded on either side by fuel-rich and fuel-lean partially premixed laminar flames, similar to, but distinct from, triple flames. Using suitable coordinate transformations and change of variables, the governing equations in the thermodiffusive approximation are recast into a form akin to classical triple flames, with the strain rate appearing as the eigenvalue. A new exact integral representation of the solution to the mixture fraction equation is then utilized and high activation energy asymptotics are applied to solve approximately for the resulting flame shape, the imposed strain rate and, most significantly, the position of flame stabilization. This theoretically predicted flame is computed numerically, and comparisons are made between theory and computation.     

Optical transfer function of an optical system with a vortex phase mask in the presence of primary aberrations

The impact of primary aberrations on the optical transfer function of an optical system with a vortex phase mask has been investigated in detail. The optical transfer function of such a system becomes negative in a certain frequency range even in the absence of aberrations, a feature different from the case of an optical system in which center of the point spread function is bright.     

相干光OFDM系统中基于导频辅助的采样时钟频率偏差估计和补偿算法

基于导频辅助法,对相干光正交频分复用系统中采样时钟频率偏差进行估计和补偿,并研究了导频的插入位置对该算法补偿效果的影响,通过对五种不同的导频插入位置进行分析和比较,得到最优导频插入位置.仿真结果表明:本文算法即使在较大的频率偏移情况下也有较好的补偿效果,并且采用该方法得到的光信噪比损耗不到1dB,可以有效地降低系统成本;不同导频插入位置对算法的补偿效果会产生影响,在较小采样频率偏移范围内时,导频平均插入方式为最优.如果采样频率偏移量较大,在导频平均插入不能很好地补偿的情况下,导频应尽量插在低频位置.