毛细微槽内的相变传热的实验研究

本文对矩形毛细微槽竖直板的相变传热特性进行了实验研究。结果表明毛细微槽对相变换热具有很大的促进作用。当壁面过热度较小时,相变换热形式主要是三相接触线附近的蒸发换热机制。而当过热度较大时,微槽内发生剧烈的沸腾。微槽内相变换热的临界热负荷有两种产生机理:其一是当微槽长度较大时微槽内由于流动阻力而产生的液体输运临界;另一机理是当微槽长度较小时的池内沸腾临界现象,亦即由动态微液层模型决定的临界机理。实验还得到了微槽强化传热的最佳优化尺寸。     

多孔介质流动沸腾微观模型实验研究

多孔通道内流动沸腾广泛应用于热管、高效换热器、航天热防护等领域,认识孔隙尺度相变典型行为及特点有助于理解多孔介质内沸腾传热机理,进而改进多孔介质内部流体相变模型。基于此,本文搭建了二维多孔介质内流动沸腾实验台,对微米级多孔通道内部流动沸腾现象及气泡行为进行研究。实验发现在毛细力、黏性力和惯性力的共同作用下,多孔介质内的流动沸腾形态显著区别于直通道内的沸腾,存在局部气泡堵塞、合并和液膜蒸干、再润湿等行为。该研究有助于增强对相变过程的理解,并为多孔结构优化设计提供指导。     

微管内流动沸腾流型的可视化研究

本文设计了一套针对微圆管内流动沸腾现象进行实验研究的实验台,并对微圆管内流动沸腾的流型变化及分布进行了可视化实验研究。在一定质量流率、进口温度情况下,利用高速摄影仪对均匀受热的内径为530μm的水平布置的石英玻璃毛细管内水的流动沸腾流型进行拍摄,得到了相应条件下微管内的相变流型分布图。     

芯片微通道沸腾相变过程中流动交变现象探析

亚微米芯片通道中的沸腾相变极不稳定,在某些条件下,在芯片微通道的同一位置处会出现不同相态流体的交变现象,并伴随有剧烈的热力参数波动。本文通过显微观测和高速摄影技术,对芯片微通道沸腾相变过程中不同相态流体交变现象的成因及特点进行了揭示和分析,这对认识高热流密度条件下芯片微热沉中的流动非稳定性问题具有重要意义。     

CO2/丙烷混合工质管内流动沸腾换热的数值模拟

通过在FLURENT软件中植入相变UDF模块,实现了对混合工质流动沸腾换热的数值模拟.模拟对象为质量比50/50的CO2/丙烷混合工质在内径4 mm长1.4 m的光管和微肋管内的流动沸腾换热,工况如下,入口流速为0.487 m/s、入口温度为-5°C、操作压力为2.0928 MPa、壁面热流密度为100 kW/m2.对...     

沸腾核化的异相扰动与叠加模型

本文提出沸腾核化的异相扰动与叠加模型,讨论各种扰动特性及对核化的作用。应用新的模型,可定性分析解释不同非均匀核化的物理过程与机制,进一步充实对微小尺度液体提出“汽化空间”与“拟沸腾”两个新概念的内涵并给出更明确的物理含意。     

R32/R134a在微通道内流动沸腾特性

研究非共沸混合工质R32/R134a(质量比,25%/75%)在水平微尺度通道内流动沸腾换热规律。在各种工况下进行了非共沸混合工质R32/R134a在水平微尺度管道内流动沸腾换热的实验,考察了质量流量G、热流密度q、质量干度x对微尺度通道内流动沸腾换热系数的影响。研究表明:在热流密度、质量流量都较低的区域,对细管道,换热系数与热流密度的关联度较大;而对微管道,换热系数受影响的因素比较多,并在干度为0.6时出现"干涸"现象,使得换热系数急剧下降。在质量流量高的区域,对细管道,热流密度对换热系数的影响很小;而对微尺度管道,当干度为0.06时换热系数发生转变,随质量干度的增加先减小后增大,热流密度增大到一定的阶段后,换热系数不再随热流密度变化。     

微型超高压力标定系统在近代物理教学中的应用

搭建了便携式的超高压力标定系统,结合CCD成像技术,在微区原位测量样品的压力及观察相变状态.利用红宝石荧光高压特殊性质,标记荧光峰位,精确计算样品压力.     

过冷沸腾时器内液体振动的频域分析

一、引言 在核反应堆、重力辅助热管等装置中,在过冷沸腾工况下,器内液体经常会产生激烈的压力脉动,在有关文献中,这种现象常被称为间歇效应、间歇沸腾或冲击沸腾。在我们开发的真空相变热水器中也经常出现这种不稳定现象。显然这种不稳定对设备的安全性和传热特性等会产生不良影响,所以有必要对振动的产生条件和机理进行研究。近年来,我们针对真空相变热水器中的这类现象进行了理论研究和可视化实验,并得到了     

单柱微通道流动沸腾换热数值模拟研究

高功率电子芯片的安全运行需要高效的散热技术。流动沸腾换热由于高换热系数受到广泛关注。为精确模拟微通道内流动沸腾复杂两相流过程,本文提出了耦合VOF方法的在相界面处迭代求解能量源项的相变模型。针对单微柱微通道内流动沸腾换热过程进行了数值模拟,分析了瞬态两相流过程及温度场演变规律,查明了热流密度及进口过冷度的影响机制。结果表明,由于局部蒸汽的覆盖,不同工况下微通道内流动沸腾存在热阻的转折点,高热流密度对应更高的气泡生长速度和成核面积,高过冷度会延缓转折点,但整体热阻将升高。     

微槽结构和工质对槽内流动沸腾的影响

本文报道实验测试甲醇等低沸点工质在微槽内流动沸腾特性的结果，并对所呈现的起始沸腾区有明显壁温回落等现象作出分析，从而在一定程度上揭示出微型槽道和工质本身特性对流动沸腾特性的影响。     

Novel dynamical effects and glassy response in a strongly correlated electronic system

We find an unconventional nucleation of a low-temperature paramagnetic metal phase with a monoclinic structure from the matrix of a high-temperature antiferromagnetic insulator phase with a tetragonal structure in a strongly correlated electronic system BaCo0.9Ni0.1S1.97. Such unconventional nucleation leads to a decrease in resistivity by several orders with relaxation at a fixed temperature. The novel dynamical process could arise from the competition of strain fields, Coulomb interactions, magnetic correlations, and disorders. Such competition may frustrate the nucleation, giving rise to a slow, nonexponential relaxation and "physical aging" behavior.     

Analytical calculation of the nucleation rate for first order phase transitions beyond the thin wall approximation

First order phase transitions in general proceed via nucleation of bubbles. A theoretical basis for the calculation of the nucleation rate is given by the homogeneous nucleation theory of Langer and its field theoretical version of Callan and Coleman. We have calculated the nucleation rate beyond the thin wall approximation by expanding the bubble solution and the fluctuation determinant in powers of the asymmetry parameter. The result is expressed in terms of physical model parameters. Received: 18 August 1999 / Published online: 14 October 1999     

非晶一次晶化过程微观组织演化和生长动力学的相场法研究

在晶化物理模型中添加扩散系数对晶化过程的影响, 采用相场方法研究初始形核率和初始形核半径对一次晶化过程中微观组织和生长动力学的影响。结果表明: 随着初始形核率的增加, 相同时间内非晶一次晶化的晶粒数量逐渐增加, 晶粒尺寸逐渐减小。晶化分数随着演化时间和初始形核率的增加逐渐增大, 初始形核率越大, 相同演化时间内的晶化分数越高。不同初始形核半径情况下, 非晶一次晶化过程中的晶粒数量和尺寸随着演化时间的增加基本保持不变。晶化分数随着演化时间的增加而增大。不同初始形核率和初始形核半径情况下所对应的生长指数均小于1, 表明初始形核率和初始形核半径对晶化方式无影响, 均为一次晶化。改变初始形核率和初始形核半径可调控一次晶化微观组织结构, 而晶粒尺寸及晶化分数直接关系到合金性能。     

Localization of plastic deformation in calcium fluoride crystals at elevated temperatures

The strain distribution was experimentally studied in CaF₂ crystals subjected to compression tests along [110] and [112] at a constant strain rate at temperatures T = 373–1253 K. At T > 845 K, the plastic deformation in deformed samples is found to be strongly localized in narrow bands, where the shear strain reaches several hundred percent. The physical deformation conditions are determined under which the plastic flow loses its stability and, as a result, the deformation is localized. The temperature dependence of the critical stress of the transition to a localized flow is found. A scenario is proposed for the nucleation and development of large localized shears during high-temperature deformation of single crystals.     

Color-tunable organic microcavity laser array using distributed feedback

Distributed feedback microstructures play a fundamental role in confining and manipulating light to obtain lasing in media with gain. Here, we present an innovative array of organic, color-tunable microlasers which are intrinsically phase locked. Dye-doped helixed liquid crystals were embedded within periodic, polymeric microchannels sculptured by light through a single-step process. The helical superstructure was oriented along the microchannels; the lasing was observed along the same direction at the red edge of the stop band. Several physical and technological advantages arise from this engineered heterostructure: a high quality factor of the cavity, ultralow lasing threshold, and thermal and electric control of the lasing wavelength and emission intensity. This level of integration of guest-host systems, embedded in artificially patterned small sized structures, might lead to new photonic chip architectures.     

Micromachining of complex channel systems in 3D quartz substrates using Q-switched Nd:YAG laser

A novel microchannel fabrication technology for quartz using a Q-switched Nd:YAG laser is presented. Complex 3D channel systems inside quartz substrates can be constructed directly using a laser beam by controlled fracturing, and high-quality microchannels can be fabricated by melting quartz using a laser-induced plasma. The behavior of laser-induced plasmas in drilling microchannels is discussed. The diameter of the microchannels can be controlled from 25 to 200 μm. The average roughness of the interior channel wall is less than 0.2 μm. Currently, microchannels longer than 4 mm in fused-quartz cubes can be achieved using laser-induced plasmas. Received: 15 January 2001 / Accepted: 5 June 2001 / Published online: 30 October 2001     

Convenient preparation of ITO nanoparticles inks for transparent conductive thin films

We synthesized Pt and Ag nanowires using a mesoporous silica, SBA-15, as templates. The obtained nanowires are a few micrometers (~4???m) long and 7?nm in diameter. The nanowires are free from bundling and, thus, can be separated as single nanowires. The successful synthesis of such nanowires requires a few considerations. In general, SBA-15 has microchannels on the walls through which the mesopores are interconnected when synthesized at 100?°C or higher. We, therefore, synthesized SBA-15 at a low temperature (80?°C) to eliminate the microchannels. Impregnation of the metal precursors and reduction of them into metals forms metal particles outside the pores in addition to the desired metal nanowires inside the pores. Surface alkylation of SBA-15 prohibits the nucleation of metal on the external surface and exclusively forms the nanowires. Finally, the introduction of surface passivating agent, an alkylthiol, during the removal of the template keeps the nanowires from interacting with one another. The Pt and Ag nanowires so-synthesized were characterized by electron microscopy.