水在激光作用下的强制流动沸腾模型

通过分析影响沸腾的各种因素,结合水的流动特性以及激光在水中按照指数衰减的特点,利用单元差分方法着重分析了整个光斑所在区域过余温度的分布,借助水沸腾状态与过余温度之间的关系,建立了水在激光作用下的强制流动沸腾模型.环行光束被作为主要研究对象,对环行光束进行了数据模拟和实验验证.在实验中采用照相法记录下气团的形态,与模拟沸腾曲线具有较好的一致性.     

粗糙海面对高斯分布激光光束的反射模型推导

研究激光光束海面反射光强的方向分布特性对海上光电对抗等领域的工程实践具有重要意义.本文采用分形方法模拟粗糙海面,并在海面基准坐标系中建立起描述粗糙海面几何特征的数学方程,然后基于蒙特卡罗方法模拟高斯光束,依据几何光学原理在基准坐标系下推导了高斯光束的海面反射模型,采用该模型可以编程计算激光光束海面反射光强的方向分布.将模拟计算结果与实验结果进行了对比分析,结果表明该模型可以较好地反映激光光束海面反射光强的分布趋势,验证了模型的有效性.     

高光束质量高斯非稳腔固体激光器研究

为了获得高光束质量的脉冲固体激光输出,研究了高斯非稳腔固体激光器的模式分布。运用边界有限元法将谐振腔内光场衍射积分方程转化成矩阵方程组,模拟分析了平凸高斯非稳腔内光阑位置、孔径大小以及高斯镜参数对输出光束模式的影响。基于理论模拟结果对激光器结构参数进行了优化,分别测量了腔内不同光阑位置和孔径下的激光器输出光束振幅及模式分布情况。在光阑半径为1 mm、光阑距高斯镜为150 mm、泵浦电压为900 V的实验条件下,光束质量M_x~2=1. 9、M_y~2=2. 3,激光最大输出能量为280 mJ的高光束质量激光输出。实验结果表明,在腔内加入选模光阑以及优化高斯镜参数可以进一步改善腔内模式分布,获得高光束质量激光输出,这与理论模拟结果基本相符。     

温度扰动对棱镜式激光陀螺特性的影响

考虑温度扰动的情况下,对光在棱镜表面反射和折射的传输矩阵进行了修正.根据激光环形腔自洽理论,建立了温度扰动下的棱镜式激光陀螺腔内光束传输物理模型,分析了温度扰动对陀螺频率偏移、标度因数等参数的影响.理论分析表明,当温度在-40~70℃范围波动时,激光陀螺光学腔长变化量、频率偏移量和标度因数变化量依次为10.04μm、0.011 MHz和1.96×10~(-10).建立了棱镜式激光陀螺变温实验系统,实验结果与理论分析相吻合.     

自聚焦激光束光束质量评价的功率谱密度方法

把功率谱密度分析的方法引入到激光束光束质量的评价中，分析了自聚焦激光束的近场分布，研究了强激光非线性自聚焦的一般规律，给出了在一定强度调制下的光束经过钕玻璃介质发生自聚焦成丝效应的B积分阈值条件，实验和理论模拟结果基本一致.     

卧式螺旋管内过冷沸腾起始点的实验研究

本文对卧式螺旋管内R134a过冷流动沸腾的起始点进行实验研究,根据壁面温度的变化规律判定过冷沸腾起始点。在热流密度逐渐增大的情况下,对过冷沸腾发生时相关参数的不同变化进行了研究,得到了过冷沸腾起始点上壁面过热度和热流密度随位置的变化规律。分析了入口参数对过冷沸腾起始点的热流密度的影响趋势。对实验数据进行回归分析,发展了适用于卧式螺旋管内过冷沸腾起始点热流密度的经验关联式。     

扫描环形光斑激光束优化研究

激光束的形状和能量分布限定了激光的应用范围,为满足不同的激光加工需要,必须对激光束进行变换。针对实验室用CO2激光器在热处理方面的应用,提出了激光扫描环形光斑光束优化法。基于温度场叠加原理建立了扫描环形光斑温度场的数学模型,模拟其温度场,可得到其温度分布特性;通过实验研究,分析了聚焦光斑与扫描环形光斑在激光淬火中对材料的热作用效果。结果表明,扫描环形光斑能改善激光热处理硬化层分布。理论和实验研究表明,激光扫描环形光斑技术可以实现激光热处理光束的优化,有一定的应用前景。     

高能激光环形光束近场和远场传输特性

为满足高能激光环形光束在近场区和远场区的实际应用需求,从电磁波衍射积分方程出发,推导了环形光束光场分布和远场光强分布表达式,并对光场分布和光强分布进行了分析,得到光强分布与高斯光束的有限孔径大小、中心遮拦比和传输距离的关系.引入大气湍流场景,采用相位屏法对环形光束在不同湍流强度下的大气传输进行了数值模拟和分析,研究了受大气湍流影响远场光斑畸变、光斑破碎、光束扩展和漂移等的增强现象.最后开展了环形光束近场区大气传输数值模拟和实验,结果表明:随着传输距离的增加,光斑中心光强越来越强,光斑逐渐趋于均匀,平均光强呈类高斯分布,近场区环形光束扩散和光斑畸变现象受大气湍流影响而增强.     

VOSET方法计算膜态沸腾的不同流态

本文在VOSET界面捕捉方法的基础上对控制方程进行修正,使其能够计算带相变的两相流问题,然后用这种方法计算水平壁面上的膜态沸腾。模拟得到的平均Nu数与Klimenko的关联式的计算结果基本一致。计算结果表明,在较低的壁面过热度下,膜态沸腾呈现气泡状流动;在较高的壁面过热度下,膜态沸腾呈现气带状流动。     

利用红外温度测量方式预测微管内流动沸腾流型

本文利用红外热成像仪对不锈钢微管内水的流动沸腾的壁面瞬态温度场进行了测量。结果表明,相比传统的热电偶测量方式,在微尺度实验中采用红外温度测量瞬态温度场具有更高的可靠性。利用红外温度测量的显示特点,预测不锈钢微管内流动沸腾的流型变化情况,在本文的实验条件下,不锈钢微管内水的流动沸腾流型变化基本上是以单相流、汽塞流和环状流的交替产生为主。     

热激励下碳纳米管与水混合体系传质传热的分子动力学模拟

爆发沸腾换热和纳米流体传热具有很重要的理论和实用意义.但由于爆发沸腾传热过程中,液体内部空间温度梯度大,相变速度快,表现出一定的特殊性,纳米流体传热对其传热过程机理的研究,往往需要在实验条件难以实现的空间和时间极限下,充分研究液相和气相内部的温度、压力和运动状态及其空间分布.采用分子动力学的方法,通过对比研究纯水和碳纳米管/水混合体系爆发沸腾过程,对两种体系密度分布、温度场和应力场研究和对比分析,揭示碳纳米管/水混合体系的强化换热机理,探究碳纳米管对混合体系爆发沸腾换热和纳米流体传热的促进作用.     

Microscopic Boiling of Liquid Nitrogen Induced by a Pulsed Laser Irradiation

An experimental investigation was conducted to explore the characteristics of microscopic boiling induced by firing a microsecond pulsed laser beam on a thin platinum (Pt) film that immerged in the liquid nitrogen (LN₂) cryostat. High-speed photography aided by a high-voltage lighting system was employed to visually observe the bubble formation and the dynamical boiling process of LN₂. A rapid transient temperature-measuring system was designed to record the temperature evolution of the heating surface. Explosive boiling, characterized by bubble cluster, was observed within LN₂ at the early stage of laser heating, and conventional boiling followed after a certain time. The transition time, therefore, was introduced for separating these two different boiling modes. The temperature of Pt film rose sharply to its maximum during laser pulse, with a very high rising rate of about 10⁷ K/s, and then dropped rapidly after laser irradiation. A model of bubble cluster was proposed to describe the explosive boiling heat transfer, and the latent heat released by bubble collapse in explosive boiling was explored as an important mechanism considerably influencing the boiling heat transfer.     

短脉冲激光作用下丙酮液池沸腾现象的瞬态观测

本文对短脉冲激光作用下试件表面的温度变化和丙酮液体汽泡行为进行了瞬态观测,研 究了激光参数和细小金属颗粒等因素对丙酮液体沸腾行为的影响。结果表明：激光参数中,脉宽是 影响温度曲线和汽泡行为的主要因素,随着激光脉宽增大,大汽泡的长大时间和脱离直径减小;汽 泡长大速度与常规沸腾时的完全不同,没有出现等温生长阶段,在激光脉冲后很长时间内汽泡长大 速度快速增大;试件表面覆盖细小金属颗粒后,沸腾过程变得非常剧烈。     

Dependence of the surface roughness of MAPLE-deposited films on the solvent parameters

Matrix-assisted pulsed laser evaporation (MAPLE) was used to deposit layers of poly(9,9-dioctylfluorene) (PFO) to study the relation between the solvent properties (laser light absorption, boiling temperature and solubility parameters) and the morphology of the deposited films. To this end, the polymer was diluted (0.5 wt%) in tetrahydrofuran—THF, toluene and toluene/hexane mixtures. The thickness of the films was equal to 70±20 nm. The morphology and uniformity of the films was investigated by Atomic Force Microscopy and by the photoluminescence emission properties of the polymer films, respectively. It is shown that, although the solubility parameters of the solvents are important in controlling the film roughness and morphology, the optical absorption properties and boiling temperature play a very important role, too. In fact, for matrices characterized by the same total solubility parameter, lower roughness values are obtained for films prepared using solvents with lower penetration depth of the laser radiation and higher boiling temperatures.     

Explosive boiling of water induced by the pulsed HF-laser radiation

The surface evaporation and explosive boiling of water induced by the radiation of a nonchain pulsed HF laser are studied using piezoelectric acoustic pressure transducers. The evolution of pressure signals is studied and the relative contributions of thermal (photoacoustic) and evaporation mechanisms to these signals are determined for a wide range of the laser energy densities. A threshold of bulk explosive boiling with respect to laser pulse energy density (W ₀ = 0.23 J/cm²) is determined.     

自然对流情形下激光辐照液体贮箱的机理

利用激光辐照靶目标时,被辐照部位可能是液体贮箱。通过实验测量与数值模拟的紧密结合,揭示了液体处于自然对流状态时激光辐照下贮箱侧壁温升及液体速度场的演化规律。结果表明:激光辐照初期,铝板中心温升率较高;随着壁面附近液体温度的升高,光斑附近速度边界层内的最大流速增大,传热强度亦增大,导致铝板温升率降低;当铝板吸收的激光能量能够基本被水的对流带走时,铝板中心的温升率趋于零。     

固体热容激光器与热稳态固体激光器温度场和应力场的比较研究

 建立了3维温度场及应力场模型，并采用有限元分析方法对Nd:YAG作为激活介质的固体热容激光器与热稳态固体激光器进行了对比研究。激光二极管阵列距离晶体3.5 mm，阵列功率2×4.2 kW，bar间隔0.4 mm，每bar发光面积为1 μm×5 mm，频率100 Hz，脉宽100 μs，Nd:YAG晶体初始温度20 ℃，冷却水温20 ℃。计算结果表明：热容模式激光介质的表面温度高于中心温度，稳态模式刚好相反, 稳态模式温差极值（19.8 ℃）是热容模式(6.4 ℃)的3.1倍,温度梯度极值(11.9 ℃/mm)是热容模式(2.46 ℃/mm)的4.8倍，热流密度极值(0.136 W/mm²)是热容模式(0.021 W/mm²))的6.4倍；稳态模式激光介质的表面出现张应力，中心出现压应力，热容模式则刚好相反，稳态模式压应力极值(18.27 MPa)是热容模式(12.1 MPa)的1.5倍，张应力极值(38.39 MPa)是热容模式(10.3 MPa)的3.9倍。由于晶体可以承受的压应力的破坏阈值远高于张应力，所以热容模式的固体激光器比稳态模式的固体激光器可以工作在更高的泵浦功率水平上。     

Generation of an electrical signal upon the interaction of laser radiation with water surface

A new physical effect lying in the generation of an electrical signal (ES) upon the interaction of IR laser radiation with the water surface at a laser fluences lower than the plasma formation threshold is studied. An ES amplitude exceeding 15 V is detected. A one-to-one relationship between the observed effect and the bulk explosive boiling of water is established, and a qualitative interpretation is proposed. In the case of irradiation of an open surface, the ES is generated due to bulk explosive boiling accompanied by the evacuation and splashing of the surface layer, the destruction of the double electric layer on the surface, and the spread of an electrified vapor-drop mixture (balloelectric effect). When the surface is covered with a transparent plate, ES generation can be caused by the charge separation upon the detachment of the water surface from the plate by a vapor bubble resulting from boiling and the displacement of the charged water surface upon the expansion and contraction of the bubble.     

Atomic dynamics of explosive boiling of liquid-argon films

Using molecular-dynamics simulation, we study the explosive boiling of thin liquid-argon films adsorbed on a metal surface. This process might be induced by heating the metal substrate by an ultra-fast laser. Upon sudden heating of the metal to temperatures well beyond the critical temperature of Ar, the film starts boiling. While thin films, with thickness below seven monolayers, fragment completely, in larger films only the near-surface Ar layers vaporize. The resulting vapor pressure drives the expansion of the remaining liquid overlayers. By monitoring the space and time dependence of the hydrodynamic variables density, pressure, and temperature, as well as the local thermodynamic state in the Ar sample, we obtain a detailed microscopic picture of the explosive boiling process. Finally, as a result of the fragmentation process, the abundance distribution of the clusters formed in the expansion follows a power-law distribution for cluster sizes m ≲ 10.