对流换热边界条件的多孔材料主动散热性能

通过推导2种不同换热边界条件下平板夹层多孔材料的散热指标, 研究了考虑对流换热因素的平板夹层多孔材料主动散热性能, 得到了影响材料散热性能的因素。分析了在确定的相对厚度下, 不同构型多孔材料的相对密度与散热指标的关系, 并得出正六边形构型的散热指数最大。随着相对厚度的增大, 最大散热指标和最优相对密度增大较快, 当相对厚度大于20时, 最大散热指标和最优相对密度变化较小并最终趋于定值。由上述结果可以得到相对应的最小质量, 随着最小质量的增大, 最大散热指标增大并最终趋于定值。在相同的最大散热指标下, 随着表面换热系数比值的增大, 最小质量逐渐减小。最后考虑承载因素对结构进行了优化分析, 正六边形构型的多孔材料具有明显的综合性能优势。      

Kagome 蜂窝夹层平板的多功能优化设计

采用“等效介质模型”, 分析了复合材料夹层蜂窝平板的整体换热系数。在此基础上, 针对轻质蜂窝(相对密度小于0. 3) , 选取蜂窝面内等效剪切刚度表征夹层结构的承载能力, 综合考虑散热能力和结构承载能力, 比较了Kagome 蜂窝与传统的正三角形、正方形和正六边形蜂窝的性能。结果显示Kagome 蜂窝不但散热性能很好, 而且通过对蜂窝相对密度和尺寸的优化, 在结构质量相同的情况下, Kagome 蜂窝的综合性能比传统蜂窝具有明显优势。      

芯材增强对夹层圆柱壳动力学性能影响的有限元分析

将表层、增强材料与芯材分开,应用有限元分析软件ANSYS,采用8节点SOLID45实体单元,对增强型夹层圆柱壳建立物理模型,进行自由振动及瞬态动力学过程分析。考虑树脂材性、尺寸以及分布等参数的变化,分析了点阵增强和齿槽增强对夹层圆柱壳动力学性能的影响,将两种增强方式进行了对比。结果显示,树脂柱及树脂齿槽均可改变圆柱壳的振动特性,对降低瞬态荷载下的动力响应有积极作用。其中树脂材性的影响较小,而点阵和齿槽的尺寸与分布对圆柱壳动力学性能的影响较为明显,分析显示,点阵增强对于提高结构固有频率比齿槽增强更好一些,而齿槽增强对于降低端部受冲击荷载时的动力位移比点阵增强更好一些。     

二维多孔材料散热性能分析与设计

二维多孔材料存在一个易于流动的方向并具有较大的面密度 , 因此在具有良好的比刚度和比强度的同时也具有良好的散热性能 , 研究强迫对流下的散热性能对其多功能化设计具有重要意义。本文中利用数值方法求解考虑二维多孔材料内部流体流动规律、 热传导和对流换热影响的流固耦合热传输问题 , 分析了多孔率和微结构尺寸对散热性能的影响并进行了最优参数设计 ; 通过分析比较 5种具有典型微结构形式的二维多孔材料的散热性能 , 给出了微结构形式对散热性能的影响。提出了以需要满足的散热性能为约束条件 , 以满足需求的设计参数的可调范围(设计参数的允许变化范围)为设计目标的最优散热结构设计理念。以此理念得到的设计结果 , 更有利于根据其他性能的要求对材料进行多功能化设计。分析表明 , 具有正六边形微结构的二维多孔材料的散热性能最优 , 并有利于实现轻质多功能化设计。     

有限潜深状态下圆柱壳固有振动特性分析

基于波传播法和虚源法,建立有限潜深状态下圆柱壳-声场耦合数学模型,并考虑了自由液面和流体静压的影响。采用数值方法求解耦合系统特征方程,得到了有限潜深状态下圆柱壳的模态频率。数值分析表明:较无限域中圆柱壳模态频率特性而言,自由液面的存在致使结构模态频率相应增大,而流体静压则使得结构模态频率相应减小;当潜深较小时,自由液面的影响相对较显著,而当潜深较大时,流体静压的影响相对较显著;流体静压对有限潜深状态下圆柱壳各阶模态频率特性的影响程度不尽相同,从而导致在一定潜深条件下,圆柱壳模态频率阶次出现变化;当潜深较大时,圆柱壳面临失稳。     

微间隙结构的温度场测量与分析

随着高功率、高性能集成电路的快速发展,芯片的散热面临着巨大挑战。微间隙结构具有长度和宽度尺寸较大、高度尺寸微小的特点,容易在芯片散热面构造,有利于实现高热导率的微流体散热。本文提出了微间隙结构温度场的测量方法,采用3×3阵列的9个0603型贴片式温度传感器,测量在两相流通过被热源加热的微间隙结构时的温度场。微间隙的四个角的温度最高,中间区域温度较低。微间隙的温度场分布为两个区域:有效散热区和热点区,呈"十"字型分布。研究了微间隙结构在气液两相流的散热技术下的散热性能及特点,并得出气液两相流在微间隙中的散热效果优于单相流。     

流场中填充吸声材料夹层板结构的声振耦合特性

采用等效流体模拟吸声材料,建立了外部流场作用下填充吸声材料夹层板结构的声振耦合模型,应用波动分析方法研究结构中声的透射特性,分析了入射声波入射角和方位角、流场流速和流向、夹层结构几何尺寸等参数对填充吸声材料夹层板结构声振耦合特性的影响。仿真计算表明吸声材料提高了双层板结构的隔声性能;隔声性能随着面板厚度和夹层厚度的增加而提高,随着入射角和方位角的增大而减小;在计算频段内(0~5000Hz),逆流入射时传声损失随着马赫数的增大而减小,顺流入射时却随着马赫数的增大而增大。     

冲击块与金属圆柱壳相互作用的缓冲吸能分析

应用Johnson Cook本构方程, 结合Singace叠缩模型,考虑冲击作用引起材料的应变强化效应、应变率强化效应和温度效应,研究了冲击物体和金属圆柱壳相互作用的能量转化过程。运用角度增量叠缩法计算了圆柱壳的塑性变形能,根据冲击块和圆柱壳所组成系统能量守恒得到了冲击块速度位移、冲击块和圆柱壳之间瞬时载荷的解析表达式,以及圆柱壳塑性变形引起的温度分布函数。通过对不同材质的金属圆柱壳在冲击作用下塑性变形过程的计算分析,并和有限元计算结果比较,证明了本文计算方法的正确性。     

升、降温火灾下混凝土瞬态热应变的计算

该文首先借助等效热膨胀系数将混凝土瞬态热应变转化为自由热膨胀应变;接着基于已有一维瞬态热应变模型和通用有限元程序的开放接口,自编程序实现了考虑升、降温火灾条件和三维应力状态下混凝土瞬态热应变的计算;最后利用相关试验数据对方法验证。结果表明考虑瞬态热应变,结构在升、降温阶段的变形会增大,且计算结果更接近试验值;进一步表明了采用等效热膨胀系数的方法考虑瞬态热应变的合理性。     

射频离子推力器放电室结构优化研究

射频离子推力器因结构简单、推力精确可调、工作寿命长等因素,已被广泛应用于航天领域,其中放电室的离子密度是决定推力性能的关键性要素。本文基于电感耦合放电过程建立了二维轴对称流体模型,分别研究了放电室在不同长径比和不同结构等条件下的离子密度变化规律。结果表明:增大射频功率和气压可以提升离子密度,放电室长度在3.5-4 cm时离子密度较高。在放电室直径相同条件下,模型1圆柱结构与模型2圆台结构体积保持一致,减小表面积可以提升离子密度;模型3球壳结构、模型4球壳圆柱结构与模型1圆柱结构的总长度一致,最优的复合结构模型4与单一圆柱结构模型1相比,离子密度提高18.2%,若两者密度相等时,最优的复合结构模型4可以节省15%功率输入或降低10%放电室内气压。     

The thermal conductivity and heat capacity of gaseous argon

This paper presents new absolute measurements of the thermal conductivity and of the thermal diffusivity of gaseous argon obtained with a transient hot-wire instrument. We measured seven isotherms in the supercritical dense gas at temperatures between 157 and 324 K with pressures up to 70 MPa and densities up to 32 mol · L^–1 and five isotherms in the vapor at temperatures between 103 and 142 K with pressures up to the saturation vapor pressure. The instrument is capable of measuring the thermal conductivity with an accuracy better than 1% and thermal diffusivity with an accuracy better than 5%. Heat capacity results were determined from the simultaneously measured values of thermal conductivity and thermal diffusivity and from the density calculated from measured values of pressure and temperature from an equation of state. The heat capacities presented in this paper, with a nominal accuracy of 5%, prove that heat capacity data can be obtained successfully with the transient hot wire technique over a wide range of fluid states. The technique will be invaluable when applied to fluids which lack specific heat data or an adequate equation of state.     

The Thermal Conductivity, Thermal Diffusivity, and Heat Capacity of Gaseous Argon

This paper presents new absolute measurements for the thermal conductivity and thermal diffusivity of gaseous argon obtained with a transient hot-wire instrument. Six isotherms were measured in the supercritical dense gas at temperatures between 296 and 423 K and pressures up to 61 MPa. A new analysis for the influence of temperature-dependent properties and residual bridge unbalance is used to obtain the thermal conductivity with an uncertainty of less than 1% and the thermal diffusivity with an uncertainty of less than 4%. Isobaric heat capacity results were derived from measured values of thermal conductivity and thermal diffusivity using a density calculated from an equation of state. The heat capacities presented here have a nominal uncertainty of 4% and demonstrate that this property can be obtained successfully with the transient hot wire technique over a wide range of fluid states. The technique will be useful when applied to fluids which lack specific heat data.     

杆件倾角对复合点阵夹芯结构主动换热性能的影响

对具有内热源的复合材料点阵夹芯结构的流体流动和热传输特性进行了数值研究,采用等效热网络法和以结构槽道高度为度量的无量纲参数分别对结构的热源热场和主动换热性能进行了分析和表征,揭示了流体流动及热源温度对芯体杆件倾角变化的响应,给出了一定热流密度下倾角与热源最高温度之间的函数关系,并对基于倾角的结构热/力性能进行了初步的优化分析。结果表明,该复合材料点阵夹芯结构具有优异的主动换热特性,且随着杆件倾角的增大,结构的换热效率提高,结构最高温度降低。     

模压氮化硼/聚对苯二甲酸乙二醇酯复合材料的导热机制与散热效果

电子电器设备中元器件的高密度集成使得散热问题日益突出,对导热材料的需求不断增长。本文以聚对苯二甲酸乙二醇酯(PET)为基体,六方氮化硼(h-BN)作为导热填料,通过熔融共混法制备h-BN/PET复合材料,考察了h-BN含量和PET基体聚集态结构对复合材料导热性能的影响,分析了复合材料的导热机制,并从材料应用的角度探讨了复合材料导热系数的温度依赖性和散热效果。结果表明,PET基体的结晶度和h-BN含量对复合材料的最终导热系数均有贡献,复合材料的导热系数随结晶度和h-BN含量的增加而提升。h-BN发挥了异相成核作用,显著加快了PET的结晶速度,提高了PET的结晶度。模压成型中h-BN受剪切应力驱使在PET基体中沿流动方向取向,导致复合材料呈现明显的各向异性特征。面内方向h-BN的有序排列为声子提供了更为通畅的传导通道。当h-BN含量为50wt%时,复合材料的面内与面间导热系数分别达到最大值3.00 W·(m·K)^-1和2.19 W·(m·K)^-1。h-BN/PET复合材料具有良好的散热效果,h-BN含量越高,复合材料的冷却速率越快,散热过程中温度...     

Novel thermal interface materials: boron nitride nanofiber and indium composites for electronics heat dissipation applications

With increased power density and continued miniaturization, effective thermal dissipation is of significant importance for operational lifetime and reliability of electronic system. Advanced thermal interface materials (TIMs) with excellent thermal performance need to be designed and developed. Here we report novel TIMs consisted of boron nitride (BN) nanofibers and pure indium (In) solder for heat dissipation applications. The BN nanofibers are fabricated by electrospinning process and nitridation treatment. After surface metallization by sputtering, the porous BN film is infiltrated with liquid indium by squeeze casting to form the final solid composites. The new composites show the in-plane and through-plane thermal conductivity respectively of 60 and 20 W/m K. The direction dependence thermal properties of the TIM are due to the anisotropic thermal performance of BN nanofibers in the composite. A low thermal contact resistance of 0.2 K mm²/W is also achieved at the interface between this new composite and copper substrate. These competent thermal properties demonstrate the great potential of the BN–In TIMs in thermal management for electronic system.     

瞬态热带恒功率法测量导热率

基于传统热桥法基本原理,提出了新的非对称结构模型--瞬态热带恒功率法用于测量材料的导热率。非对称结构模型将热源热带与温度传感热带电路彼此独立,消除了热带电阻自热以及环境温度变化带来的影响。通过短时间内快速测量恒流电路中两热带的电压差随时间的变化关系准确测量材料的导热率。依据此模型设计了热带加热片,建立了基于LabVIEW测试软件的实验装置自动测量平台,实验测量结果与参考材料导热率具有良好的一致性,相对偏差在3％左右,验证了该理论模型的有效性。     

基于流固耦合的弹载电子设备瞬态热仿真分析方法

在导弹飞行过程中由于工况环境剧烈变化,其整流罩蒙皮温度和热载荷急剧变化,这将对导弹的作战性能产生重大影响。为保证弹载电子设备的设计满足精度要求,提出了一种基于流固耦合非定常数值模拟的设计计算方法。针对弹载电子设备在高速运行过程中的瞬态热载荷是一个典型流固耦合问题,通过求解流体与固体表面的热传递方程,计算获得流场内各点的能量分段函数。运用MATLAB软件计算拟合出流场每块结构微元对应的瞬态功率曲线分段函数,采用将功率曲线分段函数加载至Fluent的热仿真分析方法,直观预测整个弹载电子设备的工作温度及其环境温度的瞬态变化。通过对某型号反辐射导引头及其弹载电子设备气动加热问题进行具体计算分析,获得了该型号装备在各个时刻流场内导引头及其弹载电子设备的瞬态温度空间分布,所得结果与实际有较好的吻合度。结果表明该方法对同类电子设备的瞬态热分析和相应散热设计具有一定参考作用。     

Luminaries-level structure improvement of LEDs for heat dissipation enhancement under natural convection

Heat dissipation enhancement of LED luminaries is of great significance to the large-scale application of LED. Luminaries-level structure improvement by the method of boring through-hole is adopted to intensify heat dissipation. Furthermore, the natural convection heat transfer process of LED luminaries is simulated by computational fluid dynamics (CFD) model before and after the structural modification. As shown by computational results, boring through-hole is beneficial to develop bottom-to-top natural convection, eliminate local circumfluence, and finally form better flow pattern. Analysis based on field synergy principle shows that boring through-hole across LED luminaries improves the synergy between flow field and temperature field, and effectively decreases the thermal resistance of luminaries-level heat dissipation structure. Under the same computational conditions, by luminaries-level structure improvement the highest temperature of heat sink is decreased by about 8°C and the average heat transfer coefficient is increased by 45.8%.     

Effect of number of turns and configurations on the heat transfer performance of helium cryogenic pulsating heat pipe

The pulsating heat pipe (PHP) is a potential alternative to highly conductive metals such as copper for long distance heat transfer. Effective thermal conductivity and heat transfer capacity of a PHP are two of the most critical factors for practical applications. In this paper, a helium based PHP, which consists of 48 parallel tubing sections, was developed. The lengths of the evaporator, adiabatic and condenser sections are 50 mm, 100 mm and 50 mm respectively. The condenser section was thermally anchored to a Gifford-McMahon cryocooler (GM cryocooler) with a cooling capacity of 1.5 W at 4.2 K. A maximum effective thermal conductivity of 12330 W/m∙K was obtained when 1.1 W heat was applied to the evaporator section at a fill ratio of 70.5%. With the same geometric parameters and operational parameters, the effect of the number of turns on the heat transfer performance was figured out by comparing the 48-turn PHP with an 8-turn PHP. The results show that the temperature difference between the evaporator and condenser sections of the 48-turn PHP is much smaller than that of the 8-turn PHP. The dry-out temperature response, effective thermal conductivity and heat transfer capacity of them are obtained and analyzed. Furthermore, two configurations of the 48-turn PHP, a parallel configuration and a series configuration, are defined. An optimum configuration is proposed and makes a reference to the design of a cryogenic PHP for applications.