密封界面微孔结构的三维分形重构技术与泄漏特性

基于分形粗糙表面的三维数值重构技术,对界面微孔结构的分形表征进行详细研究,并应用分形多孔介质输运理论构建界面泄漏机理模型,建立微观形貌与宏观泄漏率之间的理论关系式.用有限元分析法对单粗糙峰的接触变形进行模拟,获得最大孔径的变形位移,实现微观接触力学与微观泄漏模型的有效耦合.应用所提出的预测模型对金属垫片泄漏率进行计算,结果与已有实验结果相吻合.由预测模型可知,粗糙表面分形维数、特征尺度系数以及变形位移量对微孔几何结构影响显著,是影响界面泄漏率的关键因素.     

低温下粗糙表面接触间隙气体热导的研究

对低温非真空环境下粗糙接触界面间隙中介质气体的传热进行了理论分析．依据克努森数的大小,建立了不同传热区域的间隙气体热导理论模型．并对影响接触界面间隙热导的克努森数、普朗特数、热导率、适应系数、压力等参数进行了分析,为实际情况下接触界面的传热提供了理论基础．而且通过实验证明了在界面接触压力较小的情况下,即使对于硬度较小导热性能好的接触固体,间隙气体的导热量仍大于通过实际接触点的导热量．     

粗糙接触界面超声非线性效应的概率模型

提出描述粗糙接触界面超声非线性效应的概率模型,利用分段均匀概率函数描述粗糙接触界面劲度系数变化,结合表面粗糙峰的几何分布特征,得到界面粗糙度和两侧表面相对运动对声波的非线性调制作用。实验观测了铝合金材料的粗糙接触界面的高次谐波现象和阈值现象,进一步分析了归一化非线性参数与界面加载压力、粗糙度之间的关系。实验测量结果与概率模型的理论预测一致,证明了该模型的正确性,为利用超声非线性效应评价粗糙接触界面提供了理论依据。      

低温固体界面间接触导热“hysteresis”现象的研究

文中以强化低温固体界面间的接触导热为出发点 ,实验测定了低温环境下两种金属界面在过度加载前后的接触热导值 ,指出过度加载是一种经济有效的强化导热方法 ,对卸载过程接触热导率大于同次加载过程接触热导的“hysteresis”现象产生机理进行了分析     

粗糙表面形貌对微尺度流动换热的影响

基于分形几何定量描述了多尺度自仿射的粗糙表面形貌,建立了微通道内层流流动换热的理论模型并对表面形貌的影响进行了数值模拟.研究表明,自仿射分形维数直接反映了表面轮廓的不规则度,对于两个具有相同统计粗糙度的轮廓,可能存在不同的分形维数;与常规尺度通道不同,雷诺数、粗糙高度和粗糙表面分形维数都对微通道内层流流动换热有着重要影...     

反复加载情况下低温固体界面间接触导热的研究

文中从固体界面间接触热阻形成机理出发,提出了一种强化接触导热的方法,给出了低温真空环境下,两种金属界面间在反复加载情况下的接触热阻值,对卸载过程接触热导率大于同次加载过程接触热导的实验现象产生机理进行了分析,并指出了下一步的研究方向     

接触热阻理论模型的简化

本文分析了机械加工表面的粗糙度曲线形貌参数的统计特征.基于单点接触热导理论模型和弹性形变理论模型,建立了总的接触热阻与总的压力之间的一般关系,简化了接触热导的计算模型,并与现有的计算方法进行了对比.研究结果表明,简化模型具有很好的精度.     

粗糙表面接触模型的研究现状和展望

工程表面在微观尺度上都是粗糙的.研究粗糙表面的接触行为,有助于更好地理解表面的微观结构影响其功能属性的机理,以便为特定的工程应用设计表面形貌.总结了粗糙表面接触模型的研究现状,并从基体的塑性应变和材料力学行为的尺度效应两个方面对未来的工作进行了展望.     

固-固接触界面热传输研究

提出了固-固接触热传导接触界面层的概念并进行了解释。接触界面层的建立,把微结构的散射和辐射热传导机制统一于界面层内,通过对接触界面层、接触界面层的厚度和界面层热导率等的讨论,初步揭示了接触界面层热传导的机制。在界面层微观传热的机制上,建立了接触界面层热阻预测模型,并把模型预测接触热阻值与扩散失陪模型(DMM)以及实验值进行了比较。结果显示该模型克服了DMM模型预测值偏低的缺陷,这些将对接触界面层的微尺度热分析有一定的帮助。     

高温超导直接冷却的氮化铝与无氧铜接触界面热阻的实验研究

氮化铝(AlN)具有高热导性、高电绝缘性,是超导二元电流引线热截流结构中常用的材料之一。根据稳态导热法建立低温真空实验装置,实验研究了超导冷却系统热截流结构中,界面温度和接触压力对AlN块材与无氧铜(OFHC-Cu)块材间接触界面热阻的影响。在实验温度(90K-210K)和压力(0.273MPa-0.985MPa)条件下,AlN/OFHC-Cu接触界面热阻随接触压力的提高而降低,而当界面温度上升时界面热阻由于热载子热运动的强化而降低,温度较高时,接触界面热阻随压力变化的速率趋缓。低温下AlN/OFHC-Cu间的接触界面热阻是直接冷却超导系统的设计和超导系统的热稳定性方面必需解决的问题。     

二极管端面抽运固体激光器晶体棒与热沉接触热导研究

二极管端面抽运固体激光器中,圆棒晶体采用金属热沉夹持并散热,晶体侧面受到的压力呈非轴对称分布.建立了此状态下晶体棒与热沉间无热界面物质、采用厚度为平均间隙厚度和远大于平均间隙厚度的热界面物质三种情况下接触热导模型.针对前两种模型,采用截断高斯模型和塑性形变模型,讨论了接触热导与装配压力、等效均方根粗糙度的关系.建立了晶体棒与热沉的接触散热模型,对高斯型热耗分布,采用有限元法得到了无热界面物质和采用铟箔作为热界面物质时晶体棒温度的空间分布.结果表明:无热界面物质时,晶体棒与热沉间接触热导随圆心角变化较大,其 关键词： 激光二极管端面抽运固体激光器 热效应 有限元法 接触热导     

Résistance thermique de contact entre isolateurs

The transport of heat between smooth sapphire surfaces has been measured in ultra-high vacuum in the temperature range from 15 to 300°K. The contact consists of a sphere pressed against a plane with a variable load up to 5 kp. In this way the contact shape is well defined and reproducible. The thermal resistance can be separated into two terms: the constriction resistance and the boundary resistance. A model has been developed to describe the last term. Two parameters must be introduced to take into account the phonon scattering at the interface. The experimental results as a function of load and temperature are in very good agreement with such a model. The influence of different surface treatments is also well explained. Additional results for the contact between rough surfaces, which needs a simple extension of the previous theory, are reported.     

Experimental study of different modes of block sliding along interface. Part 2. Field experiments and phenomenological model of the phenomenon

The paper reports the results of field experiments on studying different modes of gravitational sliding of a block on the natural fault surface. Various materials were used as interface filler to model the whole range of deformation events that can be arbitrarily divided into three groups: accelerated creep, slow slip, and dynamic slip. The experiments show that the type of modeled deformation events is defined by both structural parameters of contact between blocks and material composition of the contact filler.Foundations for a new geomechanical model of occurrence of different-type dynamic events were developed. The model is based on the idea that “contact spots” form subnormally to the crack edges during shear deformation; the “spots” are clusters of force mesostructures whose evolution governs the deformation mode. The spatial configuration of “contact spots” remains unchanged during the entire “loading-slip” cycle but changes after the dynamic event occurrence. The destroyed force mesostructures can be replaced by similar structures under intergranular interaction forces when the external influence is fully compensated. Unless “contact spots” are incompletely destroyed, the deformation process dynamics is defined by their rheology. The migration of “contact spots” during deformation of a crack filled with heterogeneous material causes changes in deformation parameters and transformation of the mode itself due to changing rheology of local contact areas between blocks.It is found by fractal analysis that in order for dynamic slip to occur, spatially structured “contact spots” characterized by low fractal dimension must be formed; slow slip events can exist only in a certain parametric domain called the “dome of slow events”. It is found that the probability of slow slip occurrence is higher on fault regions characterized by maximum fractal dimension values: fault tips, fault branching and fault intersection zones.     

Scale-dependent nature of the surface fractal dimension for bi- and multi-disperse porous solids by mercury porosimetry

The surface fractal dimension was calculated by using a mathematical model and mercury intrusion data for a variety of bi- and multi-disperse porous solids including silica gels, alumina pellets, and building stones. The mathematical model was obtained by modifying the well-established scaling relation published previously [Ind. Eng. Chem. Res., 34 (1995) 1383-1386]. It was also verified by comparing with the theoretical surface fractal dimensions for regular fractal structures (Skerpinski tetrahedron and Menger sponge) and the calculated surface fractal dimensions for silica gel and alumina particles via the linear fitting method established previously. The calculation results for various bi- and multi-disperse porous solids have demonstrated that the scale-dependent nature of the surface fractal dimension is ubiquitous. The difference in the surface fractal dimension between pore size intervals usually exists. The estimation of the surface fractal dimension on an average stand may lead to erroneous results.     

A review on the effects of different parameters on contact heat transfer

In this paper, a complete literature review for thermal contact between fixed and periodic contacting surfaces and also thermal contact between exhaust valve and its seat in internal combustion engines is presented. Furthermore, the effects of some parameters such as contact pressure, contact frequency, the contacting surfaces topography and roughness, curvature radius of surfaces, loading–unloading cycles, gas gap conductance and properties, interface interstitial material properties, surfaces coatings and surfaces temperature on thermal contact conductance are investigated according to the papers presented in this field. The reviewed papers and studies included theoretical/ analytical/experimental and numerical studies on thermal contact conductance. In studying the thermal contact between exhaust valve and its seat, most of the experimental studies include two axial rods as the exhaust valve, and seat and the one ends of both rods are considered at constant and different temperatures. In the experimental methods, the temperatures of multi-points on rods are measured in different conditions, and thermal contact conductance is estimated using them.     

Comparison of fractal and profilometric methods for surface topography characterization

In this study microstructural and roughness characterization of surface of aluminium foils used in lithographic printing process was performed by contact and non-contact profilometric methods and fractal analysis. Significant differences in roughness parameters values inferred from stylus method in respect to those inferred from the non-contact measurements were observed. The investigation of correlation between various fractal dimensions obtained from gray-scale SEM micrographs and binary images resulting from median filtering of the original SEM micrographs as well as selected relevant roughness parameters shows that there is a strong correlation between certain roughness parameters and particular fractal dimensions. This correlations permit better physical understanding of fractal characteristics and interpretation of the dynamics of surface roughness change through processing. Generally these correlations are more suitable for parameters obtained by stylus method than those inferred from the laser-based measurements.     

Interfacial energy of the superfluid 3He a-B phase interface in the zero-temperature limit

We have measured the surface energy of the interface between the A and B phases of superfluid 3He in the low temperature limit at zero pressure. Using a shaped magnetic field, we control the passage of the phase boundary through a small aperture. We obtain the interphase surface energy from the over- or undermagnetization required to force the interface through the aperture in both directions, yielding values of the surface tension and the interfacial contact angle. This is the first measurement of the interfacial energy in high magnetic fields and in the zero-temperature limit.