液滴撞击冷铝表面的冻结沉积特性

对单液滴撞击冷表面的动态特性进行试验研究,通过快速可视化观测,分析了直径2.6 mm液滴撞击冷铝表面的冻结行为.结果表明,液滴撞击冷表面动态行为可分为铺展、回缩以及冻结沉积阶段.冻结阶段包括铺展过程冻结以及回缩过程冻结.液滴撞击冷壁面前期(0～3.5 ms),We起主导作用,壁面温度对液滴铺展行为几乎无影响。但壁面温度对液滴回缩及冻结过程影响较大.相同We下,壁面温度T-20℃,液滴最大铺展因子基本相同,达到冻结的平衡态铺展因子均小于最大铺展因子,且随壁面温度降低而逐渐增加,为回缩过程冻结.壁面温度T≤-20℃时,液滴最大铺展因子与平衡态铺展因子相同,为铺展过程冻结。本文试验条件下壁面温度-20℃可作为区分液滴撞击冷壁面铺展与回缩冻结的临界温度.     

基于响应面的铝板表面液滴冻结过程的实验研究

本文以铝板表面上液滴冻结过程为研究对象，研究多因素交互作用在液滴冻结过程中的影响规律。基于响应面实验方法，对不同的冷壁表面温度、空气温度、液滴初始温度及液滴体积的液滴冻结过程展开实验研究。结果表明：冷壁表面温度对液滴的冻结时间影响最大，其次是液滴体积和空气温度，而液滴初始温度对液滴冻结时间影响最小。由方差分析可知，冷壁表面温度和液滴体积之间的交互作用对液滴冻结时间表现显著，其他交互项作用不显著。     

壁面温度对疏水表面上水滴冻结的影响

本文研究了冷壁面温度对疏水表面(θ=108.8°)上单个水滴冻结过程的影响,比较了水滴的冻结时间及冻结前后的形态变化。结果表明,水滴冻结所需要的时间随冷表面温度的降低而减小,冷表面温度越低,水滴冻结后其顶端越容易出现树枝状霜晶生长。最后,根据能量守恒原理建立了冷表面上水滴冻结的数学模型,给出了相应的表达式,进而分析了壁面温度对水滴冻结时长的影响,计算结果与实验结果一致。     

疏水表面结霜融霜过程中液滴行为研究

研究了自然对流条件下具有微纳结构铝基疏水表面上结霜、融霜过程中液滴的动态行为,分析了表面方位及接触角对冻结前冷凝和融霜后排液的影响。结果表明:水平疏水表面上冷凝液滴的直径和基底直径随时间而增加,液滴的接触角随时间的变化有一定波动,但呈减小趋势;放置一液滴在疏水表面上冷凝时,周围形成的小液滴以该液滴为中心,离它越远,小液滴尺寸越大;融霜后疏水表面上呈现规则的球缺状液滴,而裸铝表面上是不规则的液滴。     

液滴撞击加热壁面传热实验研究

本文采用高速摄像仪对水滴和乙醇液滴撞击加热壁面后的蒸发过程进行了实验观测, 分析了液滴撞击加热壁面后的蒸发特性参数. 实验中, 两种液体初始温度均为20 ℃, 不锈钢壁面初始温度范围为68-126℃. 水滴初始直径为2.07 mm, 撞击壁面时Weber 数为2-44; 乙醇液滴初始直径为1.64 mm, Weber数为3-88. 结果表明, 液滴受到重力、表面张力及流动性的影响, 在蒸发过程的大部分时间内, 水滴高度持续降低而接触直径几乎不变; 蒸发后期, 液滴发生回缩, 水滴的接触直径、高度和接触角出现振荡现象. 乙醇液滴的接触角随时间的增加呈现先减小随后保持不变的趋势, 而接触直径和高度则持续减小, 直到液滴完全蒸发. 液滴蒸发总时长与液体物性和壁面温度有关, 随壁面温度的升高而减小, 与液滴撞击壁面时的Weber 数无关. 同时, 随着壁面温度的升高, 液滴显热部分占总换热量的比重增大, 显热部分能量不可忽略, 本文实验条件下得到水滴的平均热流密度为0.014-0.110 W·mm^-2.     

石墨表面熔融硅的润湿行为研究

熔融硅在石墨表面的润湿规律对于超薄硅片的横向拉模制造尤为重要.本文利用COMSOL软件模拟了理想条件下熔融硅在光滑石墨表面的润湿过程,并借助高温高真空接触角测量仪对高温条件下石墨表面熔融硅的润湿性能开展了实验研究.考察了石墨表面粗糙度(R_a=0.721μm与R_a=0.134μm)、环境温度(1737—1744 K)、恒温持续时间(10—30 s)等因素对润湿角的影响.结合固-液、气-液界面的压力、速度分布图,分析了恒定温度、毛细效应下表面张力变化对润湿过程的影响机制.研究结果表明,相同温度下,石墨表面硅液滴的润湿角随石墨表面粗糙度增大而减小.对同一粗糙度表面,润湿角在相同温度下随保温时间的增加略微减小,且变化的幅度随温度升高而减小.当液滴半径远小于5 mm时,表面张力在润湿过程中起主导作用;当液滴半径大于5 mm时,液滴自身重力的影响不可忽略.     

钛表面液滴冻结的实验研究

基于可视化实验系统,研究了钛表面静止液滴的冻结行为,分析了底板倾斜角度对液滴的相变时间和接触直径的影响,以及液滴体积大小对相变时间的影响.结果 表明:底板倾斜角度对液滴相变时间的影响并不是单调的,随着底板倾斜角度增加相变时间呈现先减小再增加最后又减小的趋势,底板倾斜角度为0°以及35°时,相变时间最长;液滴的接触直径随...     

分形理论结合相变动力学的冷表面结霜过程模拟

运用分形理论并结合相变动力学模拟冷表面上结霜过程.在相变动力学基础上成功模拟了结霜初始阶段水蒸气在冷表面上凝结、液滴生长及冻结过程,随后运用分形理论的有限制的扩散凝聚(diffusion limited aggregation,DLA) 模型模拟了霜晶在冻结液滴表面上的形成生长过程.模拟结果与实验结果取得良好的一致,模拟过程中凝结液滴出现及冻结的时间与实验结果几乎完全符合;液滴冻结之前其表面接触半径随时间变化的模拟结果与实验结果基本一致,同时模拟霜层厚度与实验测得霜层厚度也非常接近.研究结果对于探讨分形理 关键词： 分形 相变动力学 结霜 模拟     

冷表面温度对动态结霜过程霜层特性影响的实验研究

本文利用直流风洞型冷表面结霜实验台,对冷表面温度变化条件下,动态结霜过程的霜层特性进行了实验研究。分析了结霜条件为:空气温度10.5℃、相对湿度80%、风速3m/s、冷表面温度-3.5～-19.7℃范围内,结霜过程的结霜速率、除霜频率、霜层高度、结霜量、霜层密度及霜层导热系数等霜层物性参数的变化规律。     

加热基底上附壁液滴蒸发动力学实验研究

为了了解液滴表面热流型演变规律及其对蒸发速率的影响,采用稳态蒸发实验研究了加热基底上乙醇和水滴的蒸发过程。液滴半径固定在R=2.5 mm,高度变化范围为0.4至1.4 mm。结果表明,随着液滴高度降低,乙醇液滴表面出现了中心不均匀的温度波动、热流体波及Bénard-Marangoni流胞,而水滴表面则没有明显温度波动。随着基底温度升高,液滴内部热对流增强;随着液滴高度降低,乙醇液滴蒸发速率先减小、后增大,而水滴蒸发速率则无明显变化。实验结果证实热对流对液滴蒸发速率具有明显促进作用。     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.