R600a喷雾冷却系统换热过程

以R600a压力式封闭系统喷雾冷却过程为研究对象,对其换热过程进行分析。对液滴撞击热面后的状态进行建模,分析了其运动状态。通过忽略液膜的对流换热,引入韦伯数来简化并修正雾滴与热源表面的对流换热系数关联式;借鉴二次成核理论,通过单位时间内,单位面积上覆盖的雾滴数量对核态沸腾换热系数关联式修正。通过上述分析,以对流换热和核态沸腾换热两种机理为中心,建立了新的换热系数关联式。通过与其他文献的关联式、实验测量值进行比较、不同工质进行比较、不同实验系统比较,发现该式预测值和实验测量值偏差在20%以内,能够很好地预测压力式封闭系统喷雾冷却过程的换热系数。     

开式单喷嘴喷雾冷却均匀性实验研究

针对高热流密度激光介质高效散热与均匀冷却技术需求,设计并搭建了以去离子水为冷却工质的开式单喷嘴喷雾冷却实验平台,实验研究获得了不同热流密度（16～110 W/cm²）、不同冷却工质流量（200～300 mL/min）以及不同喷雾高度（15～25 mm）下单相喷雾冷却换热系数及其冷却均匀性效果。结果表明：该实验工况下,不同热流密度条件下喷雾高度及工质流量对于单相喷雾冷却换热效率及温度均匀性影响显著;喷雾高度15 mm、工质流量200 mL/min时获得最大对流换热系数为5.93 W/（cm²·K）;喷雾高度15 mm、工质流量250 mL/min时面积20 mm×20 mm的热源表面温度均匀性最佳可优于0.6 ℃。     

高功率激光器喷雾冷却的实验研究

以水为冷却介质,采用Spray公司的TG0.3机械雾化实心圆锥喷嘴,在体积通量为0.044,0.049和0.053 m3/(m2·s)情况下,对刻有不同微结构槽道冷却面的无沸腾区换热性能进行实验研究。结果表明：刻有微结构的表面可明显增强换热效果;壁面刻有高为0.2 mm的微结构槽道且壁面温度为52 ℃时,体积通量为0.044 m3/(m2·s则热流可达260 W/cm2,通量为0.053 m3/(m2·s则散热功率高达376 W/cm2,完全可以满足当前高功率激光器的散热需求。对于光滑面以及槽肋高为0.1和0.2 mm的换热面,其换热能力随着体积通量的增加而增强;换热面高度为0.4 mm时,通量对换热的影响变得较微弱。微结构槽道不仅增加了换热面积,还有利于液膜扩散,减小液膜厚度,增强换热。在三种不同的流量通量下,高度为0.2 mm的微结构槽道换热性能最佳。     

封闭式喷雾冷却系统启动时间

以蒸馏水为冷却工质,对冷凝器内置的封闭式光滑表面喷雾冷却系统进行了启动时间变化规律的实验研究,结果表明喷雾系统不同模块最优启动顺序为冷凝器、泵和加热器。采取6种不同的启动顺序,启动后加热表面的温度不均匀度均小于0.25 ℃。在最优启动顺序下,研究了冷却水流量、泵高度、加热功率和喷雾流量对启动时间的影响。随冷却水流量和喷雾流量增加启动时间缩短,随泵高度增加启动时间略微增大,随加热功率增大启动时间先增大后减小。启动时间影响程度由大到小的因素依次为喷雾流量、冷却水流量、加热功率及泵相对于加热表面的高度。喷雾工质流量由10 mL/min增大到35 mL/min时,封闭式喷雾系统的启动时间缩短了5 000 s。同样操作条件下,封闭式系统的启动时间比开放式系统延长15%~20%。     

封闭式喷雾冷却系统启动时间

 以蒸馏水为冷却工质,对冷凝器内置的封闭式光滑表面喷雾冷却系统进行了启动时间变化规律的实验研究,结果表明喷雾系统不同模块最优启动顺序为冷凝器、泵和加热器。采取6种不同的启动顺序,启动后加热表面的温度不均匀度均小于0.25 ℃。在最优启动顺序下,研究了冷却水流量、泵高度、加热功率和喷雾流量对启动时间的影响。随冷却水流量和喷雾流量增加启动时间缩短,随泵高度增加启动时间略微增大,随加热功率增大启动时间先增大后减小。启动时间影响程度由大到小的因素依次为喷雾流量、冷却水流量、加热功率及泵相对于加热表面的高度。喷雾工质流量由10 mL/min增大到35 mL/min时,封闭式喷雾系统的启动时间缩短了5 000 s。同样操作条件下,封闭式系统的启动时间比开放式系统延长15%~20%。     

喷雾腔压力及喷嘴孔径对相变喷雾冷却性能的影响

为了满足大功率激光器件高热流密度及低表面温度的冷却需求,以R22为冷却工质,实验研究了在闭式系统中改变喷雾腔压力及喷嘴孔径对相变喷雾冷却中临界热流密度、冷却温度等冷却性能的影响,实验结果表明：在喷雾入口压力为0.8 MPa,喷雾高度为22 mm,入口温度为-3 ℃的实验条件下,当喷雾腔压力在0.2~0.4 MPa范围内变化时,随着喷雾腔压力的升高,临界热流密度值（CHF）先增大后减小,存在最优的临界热流密度,冷却壁面温度随着喷雾腔压力的升高而上升;当改变喷嘴孔径时,CHF存在最优值,过小及过大的孔径均会影响喷雾冷却性能;当喷嘴孔径为 0.4 mm,喷雾腔压力为0.34 MPa时, CHF值最高,为276.1 Wcm-2,其对应的被冷却表面温度为26.8 ℃,表面换热系数为 66 640 Wm-2K-1。     

润滑油对基于制冷循环的喷雾冷却系统性能的影响

针对高功率固体激光器的散热需求,设计了一种基于制冷循环的喷雾冷却系统,并对其换热性能进行研究。由于制冷系统的压缩机需要润滑,冷却液中不可避免地会混有润滑油。润滑油会对冷却液的粘度、表面张力产生影响,并可能产生油膜,从而对冷却液的换热过程产生强化或抑制。因此,通过实验研究,分析了润滑油对喷雾冷却换热性能的影响。实验结果表明,冷却液中存在润滑油,会增大冷却液流经喷嘴过程中的阻力,减小流量,但根据本系统的应用情况,可以忽略;在低热流密度时,含有一定量的润滑油更有利于热源表面温度的均匀分布,高热流密度时,2%(质量分数)的含油量对温度分布不均的影响可以忽略;润滑油的存在可以提高临界热流密度,使得系统的散热能力得到提高,更有利于其在高功率固体激光器散热领域的应用。     

Effects of flash boiling injection on in-cylinder spray,mixing and combustion of a spark-ignition direct-injection engine

Higher engine efficiency and ever stringent pollutant emission regulations are considered as the most important challenges for today's automotive industry. Fast evaporation and combustion technique has caused unprecedented attention due to its potential to solve both of the above challenges. Flash boiling, which features a two-phase flow that constantly generates vapor bubbles inside the liquid spray is ideal to achieve fast evaporation and combustion inside direct-injection (DI) gasoline engines. In this study, three spray conditions, including liquid, transitional flash boiling and flare flash boiling spray were studied for comparison under cold start condition in a spark-ignition direct-injection (SIDI) optical gasoline engine. Optical access into the combustion chamber includes a quartz linear and a quartz insert on the piston. In separate experiments, we recorded the crank angle resolved spray morphology using laser scattering technique, and distribution of fuel before ignition employing laser induced fluorescence technology, as well as time-resolved color images of flame with high-speed camera. The spray morphology during the intake stroke shows stronger plume-plume and plume-air interaction under flash boiling condition, as well as smaller penetration. Then around the end of compression (before ignition), the fuel distribution is also shown to be more homogeneous with less cyclic variation under flash boiling. Finally, from the color images of the flame, it was found that with the increase of superheat degree, the diffusion rate of blue flame (generated by excited molecules) is higher, which is considered to be related with the larger fractal dimension of the flame front. Also, the combustion is more complete with less yellow flame under flash boiling.     

微结构表面封闭式喷雾冷却传热特性

以蒸馏水为工质,在闭式循环喷雾冷却系统上,变化喷雾流量,研究了表面几何结构对喷雾传热性能的影响。从对流换热和相变换热比例关系的角度,对喷雾换热机理进行了实验研究。结果表明:与光滑表面相比,微结构表面可明显增强喷雾换热强度,这主要归因于相变换热的增强。表面温度较低时,直肋面换热效果最好 ;增大流量,光面换热增强,而直肋面变化不明显。表面温度较高时,方肋面换热效果最好;随着流量增大,所有面换热均增强。对于微结构表面,相变换热份额均大于50%,故而以相变换热为主;而光滑表面,即使在温度较低时,相变换热份额也大于20%。临界热流密度与三相接触线长度正相关,流量为15.9 mL/min时,方肋面、直肋面和光面的临界热流密度依次为159.1,120.2,109.8 W/cm2,蒸发效率分别为96.0%,72.5%,67.1%。     

微结构表面封闭式喷雾冷却传热特性

以蒸馏水为工质,在闭式循环喷雾冷却系统上,变化喷雾流量,研究了表面几何结构对喷雾传热性能的影响。从对流换热和相变换热比例关系的角度,对喷雾换热机理进行了实验研究。结果表明：与光滑表面相比,微结构表面可明显增强喷雾换热强度,这主要归因于相变换热的增强。表面温度较低时,直肋面换热效果最好 ;增大流量,光面换热增强,而直肋面变化不明显。表面温度较高时,方肋面换热效果最好;随着流量增大,所有面换热均增强。对于微结构表面,相变换热份额均大于50%,故而以相变换热为主;而光滑表面,即使在温度较低时,相变换热份额也大于20%。临界热流密度与三相接触线长度正相关,流量为15.9 mL/min时,方肋面、直肋面和光面的临界热流密度依次为159.1,120.2,109.8 W/cm2,蒸发效率分别为96.0%,72.5%,67.1%。     

Thermal modeling and the optimized design of metal plate cooling systems for single concentrator solar cells

A metal plate cooling model for 400× single concentrator solar cells was established.The effects of the thickness and the radius of the metal plate,and the air environment on the temperature of the solar cells were analyzed in detail.It is shown that the temperature of the solar cells decreased sharply at the beginning,with the increase in the thickness of the metal plate,and then changed more smoothly.When the radius of the metal plate was 4 cm and the thickness increased to 2 mm or thicker,the temperature of the solar cell basically stabilized at about 53 C.Increasing the radius of the metal plate and the convective transfer coefficient made the temperature of the solar cell decrease remarkably.The effects of Al and Cu as the metal plate material on cooling were analyzed contrastively,and demonstrated the superiority of Al material for the cooling system.Furthermore,considering cost reduction,space holding and the stress of the system,we optimized the structural design of the metal plate.The simulated results can be referred to the design of the structure for the metal plate.Finally,a method to devise the structure of the metal plate for single concentrator solar cells was given.     

Flame spray synthesis under a non-oxidizing atmosphere: Preparation of metallic bismuth nanoparticles and nanocrystalline bulk bismuth metal

Metallic bismuth nanoparticles of over 98% purity were prepared by a modified flame spray synthesis method in an inert atmosphere by oxygen-deficient combustion of a bismuth-carboxylate based precursor. The samples were characterized by X-ray diffraction, thermal analysis and scanning electron microscopy confirming the formation of pure, crystalline metallic bismuth nanoparticles. Compression of the as-prepared powder resulted in highly dense, nanocrystalline pills with strong electrical conductivity and bright metallic gloss.     

Precise measurement of the line width of the photoassociation spectra of ultracold molecules by a frequency shifter

We propose a technique to precisely measure the line width of the photoassociation spectra of the excited cesium molecule by using a frequency shifter to generate two laser beams with a precise frequency difference. A series of photoassociation spectra are recorded with two laser beams induced molecular lines, whose peak separation serves as an accurate frequency ruler to measure the line width of the photoassociation (PA) spectra. The full width half maximum line width was studied as a function of PA laser intensity. The extrapolated value at zero laser intensity is (34.84 ± 0.22) MHz. By analyzing other broadening mechanisms, a value of (32.02 ± 0.70) MHz was deduced. It is shown that this scheme is inexpensive, simple, robust, and is promising for applications in a variety of other atomic species.     

Precise measurement of the line width of the photoassociation spectra of ultracold molecules by using a frequency shifter

We propose a technique to precisely measure the line width of the photoassociation spectra of the excited cesium molecule by using a frequency shifter to generate two laser beams with a precise frequency difference. A series of photoassociation (PA) spectra are recorded with two laser beam induced molecular lines, whose peak separation serves as an accurate frequency ruler to measure the line width of the PA spectra. The full width half maximum line width was studied as a function of PA laser intensity. The extrapolated value at zero laser intensity is (34.84 ± 0.22) MHz. By analyzing other broadening mechanisms, a value of (32.02 ± 0.70) MHz was deduced. It is shown that this scheme is inexpensive, simple, robust, and is promising for applications in a variety of other atomic species.     

Pool boiling heat transfer and quench front velocity during quenching of a rodlet in subcooled water: Effects of the degree of subcooling

Pool boiling heat transfer and quench front propagation were investigated during quenching of cylindrical stainless steel rodlets in subcooled water. The degree of subcooling was varied from 0°C (saturated) to 40°C at an increment of 10°C at atmospheric pressure. The results showed that the increase of degree of subcooling accelerates quenching, with the total quenching time being shortened from 90 second (saturated) to 12 second (subcooled by 40°C). As revealed by the boiling curves that were obtained via solving an inverse heat conduction problem in cylindrical coordinates, boiling heat transfer is enhanced significantly for all boiling modes with raising the degree of subcooling. At the highest degree of subcooling of 40°C, the critical heat flux is improved by nearly 300% as compared to that in saturated water. In addition, the rewetting temperature (i.e., Leidenfrost point) was found to increase as a nearly linear function of the degree of subcooling. The quench front was observed to propagate upward from the bottom of the rodlet, which is accelerated noticeably with increasing the degree of subcooling. The average quench front velocity was shown to agree well with the predicted value of an existing theoretical model that was modified to take the influence of subcooling into consideration.     

Characterization of a nano line width reference material based on metrological scanning electron microscope

The line width (often synonymously used for critical dimension, CD) is a crucial parameter in integrated circuits. To accurately control CD values in manufacturing, a reasonable CD reference material is required to calibrate the corresponding instruments. We develop a new reference material with nominal CDs of 160 nm, 80 nm, and 40 nm. The line features are investigated based on the metrological scanning electron microscope which is developed by the National Institute of Metrology (NIM) in China. Also, we propose a new characterization method for the precise measurement of CD values. After filtering and leveling the intensity profiles, the line features are characterized by the combination model of the Gaussian and Lorentz functions. The left and right edges of CD are automatically extracted with the profile decomposition and k-means algorithm. Then the width of the two edges at the half intensity position is regarded as the standard CD value. Finally, the measurement results are evaluated in terms of the sample, instrument, algorithm, and repeatability. The experiments indicate efficiency of the proposed method which can be easily applied in practice to accurately characterize CDs.     

HIRFL新B1聚束器冷却系统设计

在利用 MAFIA程序计算得到的新 B1聚束器腔体表面电流分布的基础上 ,提出了新 B1聚束器的冷却方案 ,并计算了水流及水管表面的电流分布 ,得到了冷却所需要的流量。最后估计了由于工作温度的升高所引起的并联阻抗的减小及频率的漂移。     

二次谐波转换对激光脉宽影响的实验研究

 实验研究了KDP晶体二次谐波转换非线性效应对脉宽测量精度的影响。结果表明：在小信号近似下,长度为1～3cm的KDPⅡ类晶体,对于近高斯脉冲,其基频脉宽与倍频脉宽存在21/2的关系;对于非近高斯脉冲,基频脉冲半宽与倍频脉冲波形的1/4高宽相等;在输入功率密度和能量转换效率较高时,基频与倍频脉宽无固定关系。     

单光子探测器APD无源抑制特性研究

为了选择高性能单光子探测器件，采用无源抑制方法对工作在盖革模式下的雪崩光电二极管（APD: avalanche photodiode）特性进行了测量。利用APD两端的电压在雪崩后趋于稳定的特性，获得了一种确定暗击穿电压的方法。特性测量实验结果表明：降低温度能加宽APD的最佳工作区域范围，并提高最佳增益值，从而使APD具有更高的灵敏度。通过对EG&;G系列APD和外延APD暗电流和信噪比特性进行比较，发现外延 APD具有良好的噪声性能和信噪比性能，适用于单光子探测。