液态金属软表面池沸腾传热的实验研究

池沸腾是一种高效的传热方式,目前主要通过刚性固体表面改性强化沸腾传热.本文以乙醇为工质,实验研究了光滑铜表面和液态金属软表面池沸腾传热.发现液态金属软表面可有效降低沸腾起始点(ONB)壁面过热度,饱和沸腾时, ONB壁面过热度从光滑铜表面的约18℃降低到软表面的约6℃,沸腾传热系数最大提高了149%.与光滑铜表面相比,液态金属软表面增加了汽泡核化穴数量,减小了汽泡尺寸,提高了汽泡脱离频率.观察到软表面弹性毛细波和汽泡射流现象.弹性毛细波增强了壁面热边界层热质传递.发现汽泡脱离过程中,汽泡尾部在液态金属薄层内形成残余核化穴,残余核化穴快速长大,与上升的大汽泡聚合,形成汽泡射流现象.弹性毛细波及汽泡射流解释了液态金属软表面强化池沸腾传热的机理.     

基于激光毛化技术的5052铝合金粘接试验研究

为了提高5052铝合金的粘接性能,利用脉冲光纤激光的短脉冲和高峰值功率的特性,对铝合金试件进行了激光毛化试验研究。通过正交实验法,研究了平均功率、扫描速度、脉冲频率和脉冲宽度等工艺参数对激光毛化质量的影响,以及各工艺参数的影响权重,并求得最佳工艺参数,最佳工艺参数为平均功率90 W、扫描速度10 mm/s、脉冲频率1000 kHz、脉冲宽度200 ns。根据优化后的工艺参数,加工获得了粗糙度2.35 μm,然后对激光毛化后的铝合金试件进行单搭接拉伸试验,研究发现粘接强度随着粗糙度的增大而增大,当粗糙度到达一定程度时,粘接强度反而会随着粗糙度的增大而减小。另外,粘接强度还跟铝合金表面的微织构的类型及疏密程度都有很大关系。     

系统变量对低温环境下超急速爆发沸腾的影响

本文以脉冲激光为热源,以高响应、高精度铂薄膜电阻为测温和加热元件,进行了低温工质(液氮)超急速爆发沸腾的实验研究。利用显微放大摄影系统对不同工况的沸腾过程进行了拍照,发现不同系统变量:如试件表面状况、激光参数对低温工质(液氮)超急速爆发沸腾行为有着重要的影响。     

微重力池沸腾现象中的汽泡行为特性

本文对准稳态加热条件下的微重力核态池沸腾中的汽泡行为特征进行了实验研究,分析了初始核化过程、汽泡数密度、汽泡尺寸及其运动速度等的变化趋势,探讨了过冷度及加热历史等因素对相关特性的影响.实验发现:微重力条件下汽泡生成后沿加热面不停地移动;原生汽泡会聚合形成大汽泡,大汽泡不断捕掠小汽泡而长大,直到大汽泡覆盖整个加热面;汽泡生长速度随过冷度增加而变慢.     

短脉冲红外激光诱导等离子体微刻蚀单晶高温高压金刚石

采用短脉冲激光诱导等离子体辅助加工技术加工金刚石微结构,研究短脉冲红外激光的光强、脉宽、重复频率、靶材与金刚石基片之间的距离等加工参数对金刚石的加工线宽、槽深以及加工效果的影响。当用脉冲宽度大于4 ns的激光作用在方向良好的单晶金刚石上时,光热作用明显,诱导产生金属等离子团的能量密度达到一定阈值且复合短脉冲激光能量作用下,单晶金刚石表层温度迅速上升至600°C以上,此时金刚石表层产生了刻蚀微结构;当用脉冲宽度小于4 ns的激光轰击靶材表面时,短脉冲激光轰击靶材诱导金属等离子团,可实现背面溅射相关金属靶材,当等离子体密度达到微刻蚀阈值时也可实现金刚石背部刻蚀以及石墨化。短脉冲红外激光的脉宽、重复频率决定了沉积/刻蚀加工效果。本文研究表明短脉冲激光诱导等离子体辅助加工技术是一种新型可靠的金刚石微结构加工工艺。     

微通道内种子汽泡抑制沸腾不稳定性研究

两相流动不稳定性是在各种水力学直径通道内所遇到的复杂现象,这些不稳定性对热能系统可能造成有害影响。本文以丙酮为工质,在微通道入口处设置微汽泡发生器,采用脉冲电压激励产生可控微汽泡,以微汽泡为种子尝试对通道内沸腾不稳定性的控制。实验研究表明,低频种子汽泡热控能够减轻各参数的脉动幅度;高频种子汽泡热控能彻底抑制沸腾不稳定性。本研究结果为解决微通道内沸腾不稳定性提供了一种崭新有效的手段。     

钨在瞬态热流下熔融重凝对脉冲参数的依赖性

为了研究钨在瞬态热流下达到熔融状态后，不同脉冲参数对其熔融重凝行为的影响，实验观察了钨在脉宽5 ms与0.1 ms的脉冲辐照下熔融重凝行为的特征，并考虑熔融层流动驱动力、冷却速率、温度梯度等多项因素，分析了分层结构与柱状晶对热源参数的依赖性。通过计算两种热源参数下的热作用特性分析了钨在脉宽0.1 ms的脉冲辐照下出现柱状晶而在脉宽5 ms的脉冲辐照下未出现的原因。研究发现，高流强和短脉宽的脉冲束流易于促进形成分层结构，其原因是较高流强能引起材料表层熔化层流动，同时较短脉宽能使熔化层流痕来不及恢复平整，而被快速冷却固化；当样品在瞬态热流下发生熔化时，较短的脉宽有利于形成柱状晶，较长的脉宽有利于形成等轴晶粒和出现晶粒长大。     

超急速温升沸腾热流密度研究

本文以不干涉测温系统的激光为热源,以高响应、高测量精度的铂薄膜电阻为测温元件和加热元件,从而保证了研究丙酮液体在超急速温升条件下的沸腾热流密度时瞬态测量的精确性。利用显微镜和数码相机对沸腾过程进行照相,计算了丙酮液池的温度场、热流密度及其蒸汽量,得出了沸腾曲线,发现了过冷沸腾中汽泡并不淹没的现象。     

异质原子对液体气泡成核影响的分子动力学模拟

采用分子动力学方法模拟了池沸腾中液体层加入异质原子对气泡成核的影响.分析了异质原子能量参数对液体起始气泡成核时间和温度的影响及其机理.结果表明,当异质原子能量参数小于液氩能量参数时,液体起始气泡成核时间缩短,起始温度降低.当异质原子剂能量参数大于液氩能量参数时,液体起始气泡成核时间增加,起始温度升高.异质原子在壁面上的吸附及在液体中的扩散行为影响固液界面性质,较大能量参数的异质原子扩散系数较小,更多能量参数较大的原子吸附在固体表面上使得壁面势能壁垒增加,导致沸腾时间延迟,液体需要吸收更多的热量克服势能壁垒,进而提高沸腾起始温度.能量参数较小的异质原子扩散系数较大,异质原子更容易分散到液体中,使得壁面附近液体层势能减小,液体层更容易气泡成核行为.     

等离子屏蔽效应下飞秒激光照射金箔的分子动力学模拟

采用耦合了双温度模型的分子动力学方法对飞秒激光烧蚀金箔的传热过程进行了模拟研究,考虑了非傅里叶效应,探究了不同激光能流密度下等离子体羽流的屏蔽作用.根据密度分布将激光烧蚀过程中的金箔划分为过热液体层、熔融液体层和固体层,并比较了不同激光能量密度下过热液体层表面发生的相爆炸沸腾现象以及表面温度的变化情况.结果表明,随着激光能量密度的增大,等离子体的屏蔽比例几乎呈线性增大.在激光的烧蚀过程中,金箔的上表面最先经历液体层以及过热液体层,并且随着时间的推移,液体层和过热液体层逐渐向金箔底部移动.过热液体层发生体积移除的相爆炸沸腾是金箔烧蚀的主要方式,随着激光能量的增大,爆炸沸腾发生的时间提前,并且结束的时间相应延后,持续时间变长.     

考虑金属蒸汽的钨极惰性气体保护焊电弧与熔池交互作用三维数值分析

基于局域热平衡状态假设并考虑金属蒸汽的作用, 建立了钨极惰性气体保护焊电弧与熔池交互作用的三维数学模型. 电弧等离子体的热力学参数和输运系数由温度和金属蒸汽浓度共同决定, 并使用第二黏度近似简化处理金属蒸汽在氩等离子中的输运过程. 在考虑熔池流动时, 主要考虑了浮力、电磁力、表面张力和等离子流拉力的作用. 通过对麦克斯韦方程组、连续性方程、动量守恒方程、能量守恒方程和组分输运方程的耦合求解, 得到了金属蒸汽在电弧中的空间分布、电弧和熔池的温度场、速度场和电流密度分布等重要结果. 通过与未考虑金属蒸汽的结果对比, 研究了熔池上表面产生的金属蒸汽对电弧等离子体行为的影响, 以及电弧等离子对熔池行为的影响. 结果表明, 金属蒸汽主要富集在熔池上表面附近; 金属蒸汽对电弧等离子体有明显的收缩作用, 而对等离子速度和电势影响较小; 金属蒸汽的出现对熔池上表面速度分布和剪切力分布以及熔池形貌并无明显影响. 求解结果与已有的实验结果和计算结果符合良好.     

过热液体中蒸汽泡上升过程的格子Boltzmann三维数值模拟

应用格子Boltzmann相变模型,在三维空间研究蒸汽泡在过热液体中生长、上升和变形等动力学行为.为研究传热传质对蒸汽泡运动的影响,对比模拟相同条件下气泡在等温环境中上升的物理过程.结果表明：蒸汽泡在过热液体中上升发生的变形程度较小,意味着相变对蒸汽泡的影响和表面张力一样使汽泡保持初始的形状.蒸汽泡在过热液体中的上升速度较小,说明随着汽泡生长拖拽力的影响比浮力大.蒸汽泡生长率在初始阶段达到最大值,随后会趋于一个恒定的值.随着汽泡体积增大和上升速度的增加,其对流场的扰动也越来越剧烈.蒸汽泡生长和上升引起的对流运动对温度场的演化造成很大的影响.     

浓度边界层中成长汽泡的界面特性

对于双组分池内核态沸腾，热边界层中生成的汽泡，同时也处于浓度边界层中．本文建立了边界展中汽泡表面张力模型，对温度和浓度引起的表面张力变化进行分析．结果表明，表面张力从汽泡顶部到底部是递增的，从而形成液体沿界面自上而下的Marangoni流．通过汽泡底部微层相界面方程的求解，探讨了微层变形特征，由此分析汽泡脱离机理．微层变形与文献[1]中指出的接触线的变化从本质上是一致的．     

Explosive vaporization of a water layer on a flat microheater

Explosion vaporization of a water layer of different thickness, induced by pulse heating in an inhomogeneous temperature field on the surface of a flat microheater coated with a submicron silicon-carbide layer, is experimentally studied. An optical method is used for recording the vaporization time history and dynamics of the steam blanket. Pulsed laser irradiation is applied for high-time-resolution photography of the vaporization process. The dynamics of filling the heater surface with the vapor phase and the lifetime of the main vapor bubble and the satellite bubble are estimated. Dependences of the vaporization temperature on the heater temperature growth are obtained. The initial temperature is 30°C; the temperature growth rate on the heater surfaces is about 180 MK/s.     

一个新的核态沸腾汽泡动力学模型

本文在经典汽泡动力学理论基础上，提出了描述汽泡生长过程的综合界面模型．本模型的核心在于汽泡内部的热力学过程的详细分析及汽液界面的传热、传质过程的详细描述．并对汽泡生长过程进行了模拟计算，给出了动力学控制阶段的时间范围．本模型对汽泡生长、汽膜发展的理论分析及数值模拟提供了良好的基础．     

Investigation of contact line dynamics under a vapor bubble at boiling on the transparent heater

The paper presents the results of an experimental study of dynamics of vapor bubble growth and departure at pool boiling, obtained with the use of high-speed video recording and IR thermography. The study was carried out at saturated water boiling under the atmospheric pressure in the range of heat fluxes of 30?150 kW/m². To visualize the process and determine the growth rates of the outer bubble diameter, microlayer region and dry spot area, transpa-rent thin film heater with the thickness of 1 μm deposited on sapphire substrate was used in the experiments, and video recording was performed from the bottom side of the heating surface. To study integral heat transfer as well as local non-stationary thermal characteristics, high-speed infrared thermography with a frequency of up to 1000 FPS was used. High-speed video recording showed that after formation of vapor bubble and microlayer region, dry spot appears in a short time (up to 1 ms) under the vapor bubble. Various stages of contact line boundary propagation were ob-served. It was shown that at the initial stage before the development of small-scale perturbations, the dry spot propaga-tion rate is constant. It was also showed that the bubble departure stage begins after complete evaporation of liquid in the microlayer region.     

考虑液面凹陷时金属熔化与蒸发的数值研究

将关于液面凹陷的Young-Laplace方程与关于金属溶池的流体力学方程组及关于金属蒸发的ＢＧＫ方程联立求解，在给定的电子枪加热条件下，获得了熔池流场和温度场图像及金属蒸气密度、速度和温度分布.数值计算结果表明，随电子枪功率增加，金属的蒸发速率增加，蒸气的密度增大、温度降低而速度升高.与假设液面为平面的情况相比，考虑液面凹陷后求得的液面温度较低，金属的蒸发速率较小，并且这种差别随电子枪功率的增加而扩大.因此对于高功率电子枪加热金属蒸发，必须考虑液面凹陷的影响才能得到符合实际的结果. 关键词： 液面凹陷 Young-Laplace方程 熔池 金属蒸发     

Bubble boiling in droplets of water and lithium bromide water solution

The dynamics of growth and interaction of vapor bubbles in droplets of pure water and LiBr water solution on a horizontal wall were investigated in a wide superheating range. The growth rates of bubbles were determined both in a distillate droplet and in a salt solution droplet. The bubble growth rate in a pure water droplet at the final stage is somewhat lower than in pool boiling. The bubble growth rate in a salt solution is substantially lower than for pure water. Due to the bubble density maldistribution, the vapor flow density is appreciably higher at the droplet edges than on the droplet axis. Collective behavior of the bubbles possesses both stochastic character and elements of self-organization. The thermal measurements were carried out by means of high-speed video and blowup thermal imager.     

Laser-induced microjet: wavelength and pulse duration effects on bubble and jet generation for drug injection

The expansion of the laser-induced bubble is the main mechanism in the developed microjet injector. In this study, Nd:YAG and Er:YAG lasers are used as triggers of the bubble formation. The impact of the laser parameters on the bubble dynamics is studied and the performance of the injector is evaluated. We found that the main cause of the differences in the bubble behavior comes from the pulse duration and wavelength. For Nd:YAG laser, the pulse duration is very short relative to the bubble lifetime making the behavior of the bubble close to that of the cavitation bubble, while in Er:YAG case, the high absorption in the water and long pulse duration change the initial behavior of the bubble making it close to a vapor bubble. The contraction and subsequent rebound are typical for cavitation bubbles in both cases. The results show that the laser-induced microjet injector generates velocity which is sufficient for the drug delivery for both laser beams of different pulse duration. We estimate the typical velocity within 30–80 m/s range and the breakup length to be larger than 1 mm suitable for trans-dermal drug injection.     

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.