无限弹性基底上热电薄膜的屈曲行为研究

本文研究了热电薄膜粘合到弹性基底结构的屈曲行为．将界面剪切应力和薄膜的轴向应力结合起来,建立了热电薄膜的计算模型,利用边界条件将所求问题转化为一个奇异积分方程．通过使用切比雪夫多项式展开求解奇异积分方程,得到归一化应力强度因子．确定了膜厚度和基材与膜刚度比对薄膜应力和界面应力强度因子的影响．讨论了薄膜长度和厚度比对薄膜应力和界面应力强度因子的影响．结果显示薄膜和基底之间的刚度比对薄膜的应力水平有着较明显的影响．     

基于弹塑性滚动接触疲劳法评价硬质薄膜结合强度

滚动接触疲劳法表征结合强度的剪切应力幅是一个对界面因素敏感,而对非界面因素不敏感的力学参量,能真实反映膜基结合状态,是较为合理的动态结合强度评价方法。但由于应力计算的模型采用弹性力学方法,对膜基结合强度较高而厚度很小的薄膜,往往加大载荷也较难使膜基界面发生分离。本文采用小直径、高弹性模量Al_2O_3球的滚动接触疲劳方式,对高结合强度的硬质薄膜进行评价,并采用有限元方法计算弹塑性变形条件下的应力分布,切应力幅的计算结果具有合理性。改进后的滚动接触疲劳法允许基体发生弹塑性变形,界面切应力大幅度提高,能够在相对较少周次下造成薄膜剥落,失效部位发生于界面,因而更适合于评价结合强度较高的硬质薄膜。     

联合载荷下无裂纹DLC薄膜的内应力有限元分析

本文用有限元分析的方法,对受法向和切向联合载荷作用下的无裂纹类金刚石(DLC)薄膜系统内部应力进行了模拟计算,分析了不同摩擦系数和膜厚比对于内部应力的影响,数值模拟结果表明:较大的摩擦系数可以使得界面处剪切应力和Mises等效应力以及y轴Mises等效应力显著增大,而对于界面处的y向压应力影响不是很大。在一定的范围内(0相似文献   

残余应力对压头诱导的硬质薄膜/韧性基底材料体系界面分层机制的影响

硬质薄膜在工程应用中经常承受高载荷作用。在接触载荷下,薄膜/基底体系通常产生剪切分层破坏和法向分层破坏,并直接影响材料的可靠性。硬质薄膜中较大的残余应力对界面分层破坏影响不容忽视。该文基于内聚力模型,采用有限元方法模拟残余应力对压头诱导的硬质薄膜/韧性基底界面分层破坏的影响规律;给出在不同残余应力下薄膜/基底界面分层破坏时的临界压入深度以及临界载荷;获得考虑残余应力时硬质薄膜/韧性基底界面分层破坏失效图,进而对薄膜材料的工程应用和采用压痕法测量界面结合性能提供指导。     

制冷机和导热带耦合作用的低温容器内BOG蒸发过程模拟分析

采用Ansys Workbench软件,对制冷机和导热带耦合作用的低温容器内BOG蒸发过程进行模拟,研究容器内流体温度场和速度场变化规律。结果表明:容器内液氮形成从侧壁到中心,温度不断降低的热分层;靠近容器壁处液氮温度场和速度场按照从上到下的顺序,从气液界面开始依次发生变化;当底部液氮发生明显的温度分层后,开始由容器底部向气液界面发展,最后液氮主体温度趋于均匀;导热带传递的热量对容器内流体温度场和速度场分布有影响,不同时刻由于气液界面附近导热带温度的不同,导致该处流体温度场形状有差异;容器底部热流体上升过程中在导热带下端的容器底部处形成涡旋,涡旋破碎后的热流体沿导热带两侧向气液界面运动。     

纳米尺度圆孔表面薄膜界面失配位错形核机理

研究了无限大基体内纳米尺度圆孔表面薄膜中界面螺型位错形核的临界条件,薄膜考虑了表/界面效应。运用弹性复势方法,获得了两个区域应力场的解析解答,并导出位错形核能公式,由此讨论了表/界面效应对薄膜界面位错形核的影响规律。算例结果表明,表/界面效应在纳米尺度下对位错形核的影响显著,不同表/界面效应下位错形核的临界薄膜厚度有很大差异,当基体与薄膜的相对剪切模量超过某一值后,只有考虑负的表/界面应力时位错才有可能形核;薄膜厚度在小于某一临界尺寸时负的表/界面应力更容易位错形核,薄膜厚度大于某一临界尺寸时正的表/界面应力更容易位错形核。     

奇异薄层单元及其在双材料界面断裂分析中的应用

构造了一种平面应力奇异薄层单元并证明了其具有-1/2阶奇异性。用此单元研究了双材料界面层的刚度对界面裂纹尖端场的影响。研究发现:对于I 型界面断裂, 减小界面法向刚度对K_Ⅰ、K_Ⅱ的影响远大于减小界面切向刚度, 且法向和切向刚度的减小对K_Ⅱ的影响均大于对K_Ⅰ的影响, 降低法向刚度会显著改变裂尖正应力和剪应力的分布, 而降低切向刚度只明显改变剪应力的分布, 对正应力的分布影响不大;对于界面II 型断裂, 则减小切向刚度对K_Ⅰ、K_Ⅱ的影响远大于减小法向刚度, 且切向、法向刚度的减小对K_Ⅰ影响均大于对K_Ⅱ影响, 降低切向刚度会显著改变裂尖正应力和剪应力的分布, 而降低法向刚度只明显改变裂尖正应力的分布, 对剪应力的分布影响不大。随着界面刚度增大, 应力强度因子和裂尖应力分布均趋近无厚度理想界面情况。     

含双尺度界面复相陶瓷的细观界面滑移应力

基于复相陶瓷显微特征和双尺度界面特性,分析含双尺度界面复相陶瓷内的细观界面滑移应力。首先,基于复相陶瓷宏观、细观和纳观弹性性能,计算双尺度界面复相陶瓷产生弹性变形时的细观平均应力场。然后,在纳观界面位移和应力连续基础上,提出了界面应变模型,确定了纳观界面附近纤维和基体内的位移函数,考虑界面应变的突变值与界面模量间的比例关系,根据纳观界面特性和纤维分布形式,确定出弹性变形条件下外载传递到细观界面上的切应力。最后,基于压痕实验测得复相陶瓷细观界面滑移的屈服切应力,得到细观界面滑移应力的理论计算公式并进行了定量分析。结果表明,复相陶瓷内纳观界面弹性模量越小或泊松比越小时,细观界面越易滑移,复相陶瓷越易产生塑性变形。     

第十七讲薄膜与表面技术基础

6界面与薄膜附着6.1界面层在镀膜工艺中人们经常关心的问题是:能不能在某种材料上沉积所需要的薄膜,沉积薄膜的耐用性如何.如金刚石膜可以在Si硅片上沉积,但为什么不能直接在非常有用的红外材料Ge和ZnS基体上沉积,为什么类金刚石薄膜直接沉积在不锈钢基体上的附着不好.为此,首先需要了解薄膜与基体之间的不同结合的情况和应力产生的原因.薄膜与基体之间总是存在一个界面层,界面层可分为6种不同的形式:(1)力学界面层.     

基于偶应力理论薄膜-基体界面剪应力尺寸效应的研究

将C1自然单元法应用于偶应力理论,建立了偶应力理论无网格法。考虑到薄膜厚度在微米量级,与其材料的特征长度相当,因此基于构建的偶应力理论无网格法,研究了薄膜-基体复合结构界面剪应力的尺寸效应现象。计算结果表明:随着薄膜厚度的减小,无量纲界面剪应力(定义为偶应力理论下的剪应力与经典理论下的剪应力之比值)逐渐变小,尺寸效应逐渐增强。对于给定的粘合强度,偶应力理论下的剪应力要小于经典理论下的剪应力,这意味着偶应力理论下薄膜更不容易发生脱胶失效现象。     

微圆管内R1234ze(E)流动沸腾的环状流特性模拟研究

本文建立了制冷剂R1234ze(E)在微圆管内流动沸腾过程中的环状流模型,对传热和气液两相流动压降进行了模拟研究。综合考虑重力、表面张力及气液界面剪切力的影响,模拟分析了周向液膜不均匀分布特性及该特性对流动与换热的影响,经验证,计算结果与已有实验结果吻合较好,此外还研究了不同因素对环状流区域表面传热系数与压降的影响。模拟结果表明：在流动起始区域,截面液膜厚度的分布受重力作用影响,随着流动沸腾过程的进行,该影响作用开始减弱,且有重力作用时的环状流平均表面传热系数高于无重力作用时的环状流平均表面传热系数,随着重力加速度的增加,环状流的平均表面传热系数不断增大;随着质量流速的增大,表面传热系数与压降均随之增大;随着管径增大,表面传热系数与压降均随之减小。     

空冷/水冷混合冷却竖管内LiBr溶液降膜吸收过程数值解

针对风冷和水冷联合冷却的竖管降膜吸收器,考虑汽液界面的阻力、变膜厚、横向对流和冷却水的冷却作用的影响,建立了降膜吸收过程中热质耦合数学模型和同心管环空内冷却水换热数学模型.计算了沿竖管内表面的液膜厚度、温度、浓度以及冷却水在混合冷却条件下的温度分布等参数.分析了冷却水进口温度、LiBr溶液Re数和PE数等参数对传热系数和吸收速率的影响.数学模型的计算结果与实验数据吻合较好.得出的结论对联合冷却吸收器的设计和优化具有指导意义.     

Evaporation heat and mass transfer in natural convection pipe flow

Abstract

A numerical analysis has been performed to examine film evaporation on natural convection heat and mass transfer in a vertical pipe. Coupled governing equations for liquid film and induced gas flow were simultaneously solved by the implicit finite difference method. Results for interfacial heat and mass transfer coefficients are specifically presented for ethanol film and water film vaporization. The predicted results indicate that the heat transfer from gas‐liquid interface to the gas flow is predominated by the transport of latent heat in association with film evaporation. The results are also contrasted with those of zero film thickness and show that the assumption of extremely thin film thickness made by Chang et al. [5] and Yan and Lin [19] is only valid for a system with a low liquid Reynolds number Re _l1. But as the liquid Reynolds number is high, the assumption becomes inappropriate.     

滴形管外含不凝气体自然对流凝结换热性能研究

本文在自然对流情况下,基于双膜理论和边界层理论,考虑气液界面热阻,建立了滴形管外气液膜厚度及传热系数的数学模型,得到不同初始参数下气液膜厚度、气液膜热阻、气液界面热阻、凝液量和传热系数沿管壁的分布规律。结果表明:其他条件不变,随着混合气压力的增大(由81 325 Pa增至121 325 Pa),液膜厚度增大约7%,传热系数减小约30%。随着不凝气体质量分数的增加(由0.1%增至10%),气膜厚度减小约52%,凝液量减少约85%,传热系数减少约82%。虽然气膜厚度减小,但气膜内不凝气体质量分数增加约58%,气膜热阻增加约61%。对于当量直径相同的滴形管,其他条件不变,滴形管下半部分曲率越大,越易发生液膜分离,传热系数越大。     

Measurement of the surface tensions and the interfacial tensions of n-pentane-water and R 113-water systems

The surface and the interfacial tensions of mutually immiscible liquid systems were experimentally studied. The measured systems are n-pentane-water and R 113-water, which are proposed as heat transfer fluids for a direct-contact heat exchanger to be used for geothermal and waste heat recovery plants. The experimental apparatus was constructed based on the principle of the pendant drop method. Measurements were performed in the temperature range from 20 to 150°C. Based on the correlation of the surface and the interfacial tensions, the temperature dependences of the spreading coefficient, the film pressure, and the work of adhesion in each system were calculated.     

低雷诺数下溴化锂溶液降膜吸收热质传递的数值研究

溴化锂溶液降膜吸收是吸收式空调系统中常见的热质传递形式之一。本文对溶液降膜吸收过程的热质耦合传递分析,建立了溴化锂溶液垂直降膜吸收热质传递的二维数学物理模型,采用CFD-Fluent对模型进行求解。计算得到不同Re下的液膜界面温度、液膜内浓度分布、传热传质通量及传热传质系数等。分析了Re对降膜吸收过程中热质传递的影响。结果表明:当液膜Re150时,液膜界面平均温度与平均传质系数随着Re的增大而增大,而平均传热系数随着Re的增大而减少;平均传热传质通量均是随着Re的增大而先增大后减小,存在一个最佳液膜Re使降膜吸收过程的传热传质通量达到最大,即Re=50时,平均传热和传质通量分别达到最大值7.2 k W/m~2与2.9×10~(-3)kg/(m~2·s)。     

Semianalytic method for heat transfer calculation in the liquid film under conditions of a constant heat flux on the wall

For calculating the heat transfer in the free-falling liquid film, a semianalytic method is offered in which the temperature field in the liquid is presented as a series of basis functions which satisfies boundary conditions. The proposed method is demonstrated by the example of the problem regarding the film heating under conditions of the constant heat flux on the wall taking into account the heat transfer on the interfacial surface. The analytical solution is derived for the thermal initial section, on which the liquid heating occurs in a thin layer near the wall. Calculations using the proposed method are well agreed with the numerical solution obtained by the finite-difference method and with experimental data.     

The effect of increased pressure on interfacial heat transfer in the aluminium gravity die casting process

Contraction and distortion of a casting during cooling within a mould can force their respective surfaces together, with the associated increased interfacial pressure resulting in increased interfacial heat transfer. This problem has been examined for the case of gravity and low pressure die casting of an Al alloy, where an insulating coating is applied to the die cavity to assist filling of the mould. The degree of interfacial pressure was estimated to be, for a typical small die casting, at most about 21 MPa. Repeated applications of a compressive load showed that a freshly applied die coating became thinner and smoother, until a stable situation was reached after about ten applications. The interfacial heat transfer coefficient was estimated to be increased by about 20%, with an increase in the applied pressure by a factor of two, from 7 MPa to 14 MPa, and increased by about 40%, with an increase in the applied pressure by a factor of three, from 7 MPa to 21 MPa. The heat transfer mechanisms between the casting and the die surfaces were evaluated to produce a simple model of interfacial heat transfer which included conduction through the points of actual contact, in parallel with conduction through the interfacial gas between the points of actual contact, both mechanisms being in series with the heat transfer by conduction through the die coating. Evaluation of the model produced agreement with experimentally determined values of the interfacial heat transfer coefficient to within about 15%.     

汽气混合流凝结液膜形成机理研究

汽气混合流体凝结放热过程中凝结所形成的液膜厚度对换热效果的影响较大,因此通过探讨液膜厚度沿管壁的变化,有利于分析强化换热过程。以圆管为例,对汽气混合流体凝结所形成的液膜机理、沿管壁的分布规律及换热性能进行分析,并建立相应的物理及数学模型。通过给定初始条件进行计算,结果表明：液膜沿管壁向下流动过程中其厚度逐渐增加、换热性能逐渐降低;同时分析了管壁温度、汽气混合流体速度及管径等主要参数对液膜的形成及换热性能的影响,为强化凝结换热过程提供了理论基础。     

Simulation study of a hybrid absorber–heat exchanger using hollow fiber membrane module for the ammonia–water absorption cycle

An innovative hybrid hollow fiber membrane absorber and heat exchanger (HFMAE) made of both porous and nonporous fibers is proposed and studied via mathematical simulation. The porous fibers allow both heat and mass transfers between absorption solution phase and vapor phase, while the nonporous fibers allow heat transfer between absorption solution phase and cooling fluid phase only. The application of HFMAE on an ammonia–water absorption heat pump system as a solution-cooled absorber is analyzed and compared to a plate heat exchanger falling film type absorber (PHEFFA). The substantially higher amount of absorption obtained by the HFMAE is made possible by the vast mass transfer interfacial area per unit device volume provided. The most dominant factor affecting the absorption performance of the HFMAE is the absorption solution phase mass transfer coefficient. The application of HFMAE as the solution-cooled absorber and the water-cooled absorber in a typical ammonia–water absorption chiller allows the increase of COP by 14.8% and the reduction of the overall system exergy loss by 26.7%.