金属熔体近壁面流动剪切模型及其对金属凝固影响的理论研究

本文建立了金属熔体近壁面流动剪切模型, 并分析了流动剪切对金属凝固的影响. 针对A356合金计算结果表明:层流流动的熔体内部剪应力随垂直斜板表面距离的增大而减小, 随着流动长度的增加先急剧下降之后趋于稳定; 紊流流动的熔体所受的剪应力随着垂直倾斜板表面距离的增大先急剧下降之后趋于稳定, 随着流动长度的增加而不断增大; 斜板倾角越大, 斜板上相同位置的熔体层受到的剪应力越大; 熔体垂直斜板表面距离越小, 柱状晶所承受的弯曲应力越大; 斜角越大, 斜板上相同位置的柱状晶的弯曲应力越大; 随着熔体在倾斜板表面流动长度的增加, 在层流阶段, 倾斜板表面柱状晶根部所受的弯曲应力先急剧下降之后趋于平稳, 而在紊流阶段, 弯曲应力是缓慢增加的; 理论分析表明柱状晶在熔体近壁面流动过程受到的最大弯曲应力低于αup -Al晶粒的屈服强度, 所以斜板上熔体流动产生的弯曲力不能将柱状晶折断, 只能将晶粒冲刷游离到熔体中使晶粒增殖, 与实验结果相符合. 所以本模型可以很好地解释熔体近壁面流动过程中的剪切本构关系以及剪应力对凝固组织的影响.     

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应用SolidWorks和ANSYS软件设计了自由液态锡表面流体结构模型,计算了初始流速和热通量不同时液态锡的速度变化和温度变化,得到了流动液态锡的速度分布和温度分布。结果表明,垂直流动方向液态锡流速较为均匀,沿流动方向液态锡流速逐渐增大、液态锡液面厚度逐渐变薄。初始温度为600K的条件下,热通量为1MW·m?2时,液态锡出口温度为623.38K;热通量为5MW·m?2时,液态锡出口温度为720.18K。在相同条件下使用液态锂作为计算流体,结果表明出口处液态锂的温度低于液态锡的温度。     

自由液态锡表面结构的流体模型

应用SolidWorks 和ANSYS 软件设计了自由液态锡表面流体结构模型,计算了初始流速和热通量不同时液态锡的速度变化和温度变化,得到了流动液态锡的速度分布和温度分布。结果表明,垂直流动方向液态锡流速较为均匀,沿流动方向液态锡流速逐渐增大、液态锡液面厚度逐渐变薄。初始温度为600K 的条件下,热通量为1MW·m⁻² 时,液态锡出口温度为623.38K;热通量为5MW·m⁻² 时,液态锡出口温度为720.18K。在相同条件下使用液态锂作为计算流体,结果表明出口处液态锂的温度低于液态锡的温度。     

运动状态下聚能战斗部侵彻披挂反应装甲靶板的数值模拟

运用LS-DYNA有限元程序模拟了不同横向飞行速度(150、200、300、400、500m/s)和侵彻角度(30°、45°、60°)情况下聚能战斗部对披挂反应装甲后效靶板的侵彻过程,讨论了射流所受干扰情况及其对后效靶板的侵彻结果。研究结果表明:当侵彻角度一定时,射流对靶板表面的切割长度随速度的增大而增大,且在侵彻角度为30°时增大速率最快;但射流侵彻深度随速度的增大而减小,且在侵彻角度为60°时减小速率最慢。当飞行速度一定时,射流对靶板表面的切割长度和侵彻深度均随侵彻角度的增大而减小,且表面切割长度降幅随速度的增大呈先增大后减小的趋势,在速度为300m/s时,降幅最大,为59.6%;而侵彻深度降幅随速度的增大呈先减小后增大的趋势,在速度为350m/s时,降幅最小,为39.3%。最后通过理论方法分析了数值模拟结果,论证了数值模拟方法的正确性。     

冰腔池火边沿高度对冰壁侧腔形成的影响

本文利用透明的石英玻璃油池,研究了冰区池火中初始油池边沿高度对冰壁侧腔长度的影响。实验发现随着初始油池边沿高度的增加,侧腔长度呈现出先增加后减小的趋势。分析结果表明,这主要由于油池边沿高度会影响氧气和燃料蒸汽的分布,进而改变边界层厚度,而在边界层厚度的变化改变了燃烧速率,进一步导致燃烧时长发生变化。借助瞬态能量方程和能量守恒方程,发现侧腔长度和燃烧时长存在一定的线性关系。     

2,5-二甲基呋喃-空气混合气层流燃烧速率的测定

在定容燃烧弹中利用高速纹影摄像法和球形扩展火焰研究了常压下不同燃空当量比和初始温度时2,5-二甲基呋喃-空气混合气的层流火焰特性,获得了拉伸和无拉伸火焰传播速率,无拉伸层流燃烧速率和马克斯坦长度。研究结果表明:2,5-二甲基呋喃-空气混合气的无拉伸火焰传播速率和无拉伸层流燃烧速率在燃空当量比1.2附近达到最大值,它们都随着初始温度的增加而增加。随着初始温度的增加,无拉伸火焰传播速率的峰值位置向浓混合气(燃空当量比增大)的方向移动。随着燃空当量比的减小和初始温度的增加,马克斯坦长度增加,表明火焰前峰面的稳定性增强。基于实验结果数据,拟合得到了2,5-二甲基呋喃-空气混合气的无拉伸层流燃烧速率的关系式。     

湍流边界层流动结构研究用低Re数水洞试验台的设计及测试

自行设计制造了一台低Re数循环水洞试验台,收缩段采用维辛斯基(Witozinsky)曲线设计。变频电机调速,试验段流速范围0 2～1.0 m/s。用一维LDV系统测量了水洞试验段入口中心处的流速和湍流度随电机频率的变化,和来流速度为U_∞=0.26 m/s的平板湍流边界层速度和湍流度分布;并用PIV系统检测边界层内层低速条带结构。试验结果表明,试验段入口湍流度随着流速增大而下降,放置于试验段的平板其边界层已达到充分发展状态,并且检测到其底层低速条带结构,试验台可用于湍流边界层流动结构测试研究。     

污垢对圆管内对流换热场协同的影响

针对不同结垢厚度的圆管进行数值模拟,分析在层流和紊流两种流动状态下,污垢对速度场和温度场以及它们之间的协同程度-协同角及协同数的影响.数值模拟结果表明,对于层流和紊流,其平均场协同数均随着污垢厚度的增加而增加,随着雷诺数的增加而减小;平均场协同角则相反;紊流时场协同数大于层流时的场协同数,协同角小于层流时的协同角;紊流...     

复合冲击和复合旋流冷却特性的对比研究

针对应用旋流冷却和气膜冷却对涡轮叶片前缘区进行冷却系统设计的科学命题,首次建立了包含冷气腔、冲击/旋流腔、气膜孔和燃气主流通道的复合冲击和复合旋流冷却模型,利用数值方法在相同几何和气动条件下对比了复合冲击和复合旋流冷却的流动换热特性.研究结果表明:冲击孔/旋流喷嘴流量沿X方向逐渐增大,吹风比沿X方向先增大后减小.复合冲击冷却的气膜孔流量和吹风比沿X方向略有增大,复合旋流冷却气膜孔流量和吹风比沿X方向减小.复合旋流冷却平均Nu比复合冲击冷却提升13.0%.气膜孔会扰动冷气流动,使附近小范围区域Nu提升.与复合冲击冷却相比,复合旋流冷却气膜孔的冷气流量和速度大,肾型对涡强度高,绝热气膜冷却效率减小.     

歧管式微通道流动特性的研究

岐管式微通道(MMC)热沉具有热阻小、结构紧凑、冷却液流量小、流速低、沿流动方向温度分布均匀等优点.本文针对以去离子水为介质的岐管式微通道(宽W=100 μm,深H=300 μm)的流动特性进行了实验研究,实验的雷诺数范围为5O～3500.结果表明工质在微通道内流态由层流向紊流转变的临界雷诺数提前,此外数值模拟结果与实验值也吻合较好.最后在实验基础上,拟合出工质在层流和紊流下的流动阻力经验关联式.     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Hybrid Wavelength and Code Division Multiple Access in Optical Networks

A review of schemes for multiple access in fiber optic networks shows that a hybrid of wavelength and code division multiple access (WCDMA) combines the best features of both. In particular, the hybrid scheme retains the large information carrying capacity of wavelength division multiple access (WDMA) and flexibility of code division multiple access (CDMA). In this paper WDMA, optical CDMA (OCDMA), and WCDMA networks are discussed. In OCDMA networks, concept of incoherent and coherent coding including inverse decoding and matched filter is introduced. The delay performance of both WDMA and WCDMA networks, under the simple suboptimum access protocols based on cyclic search, is computed. It has been shown quantitatively that tuning delay significantly affects the delay performance of both WDMA and WCDMA networks. Futhermore, delay performance of WCDMA networks is always better than the WDMA networks for the same tuning delay, load, and number of users.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

Sorting of bending magnets for the SSRF booster

The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper.     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Resonance scattering of canonical elastic shells in absorbing fluid medium

Resonance scattering of elastic spherical shell and cylindrical shell while the surrounding fluid medium has absorption is studied. The normal mode solution derived using exact elastic theory and the separation of variables is still applicable. However, the scattering form function has to be modified for the absorbing medium, otherwise the unreasonable result would be obtained. The backscattering form function in the absorbing medium is redefined, and the form function of elastic spherical and cylindrical shell with vacuum or solid matter filled is calculated in various absorption conditions. The results show that the absorption of surrounding fluid leads to notable attenuation of the coincidence resonances in the mid-frequency, but it has a little influence on the low-frequency resonance scattering induced by the filler inside the shell.     

Single-beam self-referenced phase-sensitive surface plasmon resonance sensor with high detection resolution

A versatile and low-cost single-beam self-referenced phase-sensitive surface plasmon resonance(SPR)sensing system with ultra-high resolution performance is presented.The system exhibits a root-mean-square phase fluctuation of ±0.0028.over a period of 45 min.i.e.a resolution of±5.2×10-9 refractive index units.The enhanced performance has been achieved through the incorporation of three design elements:a true single-beam configuration enabling complete self-referencing so that only the phase change associated with SPR gets detected,a differential measurement scheme to eliminate spurious signals not related to the sensor response,and the elimination of retardation drifts by incorporating temperature stabilization in the liquid crystal phase modulato .Our design should bring the detection sensitivity of non-labeling SPR biosensing closer to that achievable by conventional fluorescence-based techniques.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.