湿工况下平直翅片对流传热传质数值研究

建立了湿空气流经平直翅片通道并伴有凝结现象发生的三维对流传热传质的数值模型,在空气进口雷诺数Re为190～3770,进口相对湿度φ_(in)为50%～90%的范围内,对干湿两种工况,平直翅片通道内的换热流动进行了对比研究。结果表明:湿工况换热系数为干工况换热系数的2.8～3.1倍,干工况翅片效率比湿工况翅片效率高35.8%～41.9%。当翅片为部分湿工况时,翅片效率随进口相对湿度的增大而增大,换热系数随进口相对湿度的增大而减小;当翅片为全湿工况后,进口相对湿度对翅片效率和换热系数的影响微弱。     

水平内微肋管局部凝结换热性能实验与数值求解

以Ｒ１１为工质，蒸汽凝结压力为１４７－２６５ｋＰａ，质量流率４ｔｙ１５３ｋｇ／ｍ２ｓ，本文对二维内微肋管和三维内微肋管水平管内凝结分层流区局部换热系数进行了系统的实验。与光管比较，二维内微肋管和三维内微肋管局部凝结换热系数分别提高了１４７－７８３％和２６１－９９７％。本文首次从理论分析入手建立了二维内微肋管水平管内凝结分层流区局部换热系数分析模型并进行了数值求解。计算结果与本文实验相当吻合。     

带内螺旋翅片的T形翅片管传热特性研究

以钢质光滑管、T形翅片管及带内螺旋翅片的T形翅片管为元件，以氟利昂113—水为介质的单管池沸腾传热试验的结果表明，在试验范围内，带内螺旋翅片的T形翅片管的总传热系数是光滑管的1．9～2．6倍，是T形翅片管的1．21～1.29倍。用Wilson图解分析法得到的这种新管型的管内单相流体给热系数的方程为：hi＝0．0244λ／DiRe(0.8)Pr(0.3)，管外沸腾给热系数的方程为：h0＝5104．3q(0．04)。对试验结果和这种新管型的传热强化机理进行了讨论。     

椭圆形和圆形翅片管流动与传热的数值研究

对椭圆管椭圆翅片间的流动与传热规律进行了三维数值研究,分析了不同翅片间距、迎面风速对表面换热系数和流动阻力的影响;与具有相同结构参数(相同的基管当量直径和翅片厚度、表面积)的圆管圆翅片进行比较表明,在相同条件下,两者的表面换热系数相差不大,但椭圆管椭圆翅片间流动阻力却有明显的减小.场协同分析表明,翅片迎风侧的换热要优于背风侧;通过适当增加迎风侧翅片面积,减小背风侧翅片面积,可以在强化换热的同时,减小流动阻力.     

斜翅型冷凝强化换热管传热性能的实验研究

对一种斜翅型外翅片带内螺纹的冷凝强化换热管进行传热性能的实验研究。管外冷凝换热的制冷剂为R134a,管内对流换热的介质为水。分别在定热流密度与定水流速的条件下进行一系列工况的实验,得到相应的实验数据。在定热流密度条件下,利用Wilson图解法得到管内的换热系数数据及相应的计算关联式。在定水流速的条件下,利用分离方法得到管外冷凝换热系数数据及相应的计算关联式。将强化管换热系数数据与光管换热系数的理论计算值进行了比较,结果表明:冷凝强化换热管管内对流换热的强化倍率为2.4,管外凝结换热系数随壁面过冷度的增加而增大,管外凝结换热的强化倍率为:1.78～3.92。     

高效传热管内凝结换热性能及阻力性能的实验研究

本文以ＨＦＣ１３４ａ和ＨＣＦＣ２２为工质对光管及两种不同槽型的强化传热管（ＤＡＥ－２管与ＤＡＥＣ管）的水平管内凝结换热进行了对比实验研究、研究发现,ＤＡＥ－２管平均换热系数比光管提高了１４０％～１７０％,而单位长度阻力损失增加了５０％～１００％,ＤＡＥＣ管平均换热系数比光管提高了１６０％～２００％,同时单位长度阻力损失增加了７０％～１３０％。此外,本文给出了ＤＡＥ－２管和ＤＡＥＣ管平均换热系数及阻力损失的计算关联式,可用于冷凝器设计。     

分离式热管换热器传热特性的实验研究

本文在自行设计分离式热管实验装置的基础上,对其传热特性进行了实验研究。其工作温度为170～250℃,热流密度为25～50 kW/m~2。蒸发段和冷凝段构成相同,均是由7根直径30 mm的无缝钢管短管束组成,管长为160 mm,带有紧套的钢帛环形肋片结构尺寸为:外径45 mm、厚1 mm、片间距4 mm。实验结果表明,在本实验条件下,分离式热管的最佳充液率按管束总容量计为18%～38%。根据实验结果拟合了最佳充液率(24%)下蒸发段内部平均沸腾换热系数和冷凝段内部凝结换热努塞尔数综合关系式。     

多孔细径微肋管内CO2流动蒸发换热特性的研究

对亚临界二氧化碳在带有微肋的微细通道内的蒸发换热特性进行了实验研究.实验段为长0.6 m,内径1.7 mm的八孔带0.16 mm高微肋的铝制扁管.实验中参数的变化为:蒸发温度1～15 ℃,质量流速100～300 kg/m2s,热流密度1.67～8.33 kW/m2,干度0.1～0.9.实验结果表明,二氧化碳在带有微肋的微细通道中的蒸发换热系数高于其在光滑微细通道内的换热.二氧化碳的流动蒸发换热系数主要受热流密度和蒸发温度的影响,基本上是换热系数随热流密度及蒸发温度的增加而增加,但同时临界干度前移及滞后,而质量流速对换热系数的影响较弱;压力损失随质量流速和热流密度的增加以及蒸发温度的降低而增加.     

小热管强化传热的研究

对五种内径相近的小热管在不同工怍温度(Tv)、热流密度(qe)及倾角(θ)下的传热性能进行了实验研究。五 种热管带有不同吸液芯结构:微肋、丝网芯、加网芯微肋、金属粉末烧结芯及光滑表面。获得了各热管的蒸发换热系数和 冷凝换热系数随(Tv)、(qe)及(θ)等的变化规律,讨论了吸液芯结构等对热管蒸发及冷凝换热系数的影响。     

弧形通道散热器流动与传热的数值模拟研究

以某航空电子元器件的散热器为研究对象,建立了弧形翅片散热器微流道单元流域的三维模型。根据高空运行温度和压力设置边界条件,利用计算流体力学数值模拟方法对散热器弧形翅片的流场、传热性能等进行分析研究。通过实验数据拟合的关联式对模拟结果进行验证,对其强化传热效果与平板翅片进行比较。结果表明,弧形翅片具有很好的换热效果,雷诺数在780～1 450范围内,弧形翅片的传热效果比平板翅片提高8.43%～18.06%。相同工况下,弧形翅片的散热量较多、散热性能较好、传热效率较高。     

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.