AMS02 WAKE侧散热板热系统的优化模拟

为了确保电子系统在AMS02运行的全过程于规定的温度范围内工作,采用热容-热阻结点网络和集总参数法,对WAKE侧散热板的暂态特性进行了热分析计算,并结合导热仿生优化原理,对高导热材料的内嵌热管进行了优化布置。模拟结果表明,沿减小温度梯度的高导热材料进化方向布置热管较增加其密度,传热更有效。当基板与高导热材料的导热系数比为1∶(1.8×105),合理布置20%面积比的热管完全可以满足散热板的温度梯度要求,并使连接在散热板上的所有电子箱和电源箱都能在规定的温度范围内工作。     

周期性边界条件下埋地热油管道传热影响因素的分析

建立土壤多孔介质模型,采用有限容积法对地表温度周期性波动条件下埋地热油管道非稳态传热过程进行数值计算。考虑了土壤中水相、气相迁移对管道传热的影响,对比分析了有、无保温层及保温层厚度、保温层导热系数、土壤导热系数、土壤含水率、管径、埋深等因素对埋地管道非稳态传热规律的影响。研究表明：保温层厚度、导热系数、土壤导热系数对埋地热油管道非稳态传热的影响相对较大。管径、埋深对管道传热的影响相对次之,且埋深对管道的影响冬季远要大于夏季,而土壤含水率对管道传热的影响相对较小。     

基于分形理论的翅片管气化器霜层热导率

基于分形理论的DLA模型,数值模拟了翅片管气化器表面霜层生长过程,同时对霜层生长形态进行了实验观测,得到了不同时刻的霜层生长图像。计算了气化器表面霜层剖面孔隙面积分布分形维数与分形孔隙率,模拟图像与实验图像的对比表明两者取得良好的一致,验证了数值模拟的合理性。在此基础上建立了霜层导热的分形模型,采用热阻法给出了霜层热导率表达式。计算结果表明,用该方法确定的霜层热导率与实测得到的霜层有效热导率值域范围是相符的。并通过与其他导热模型的比较,验证了将剖面面积分布分形维数引入导热模型以确定霜层热导率的可行性,从而为霜层热导率的理论研究开辟了一条新路。     

Modeling of the temperature field in a porous thermal barrier coating

Thermal barrier coatings (TBCs) are advanced materials systems with low thermal conductivity. One of the reasons for the low thermal conductivity in TBCs is that they contain porous structures created by a network of micro-voids. In the present investigation, experimental and analytical studies of heat transfer in TBCs having different levels of porosity were performed. The ceramic coatings were analyzed using scanning electron microscopy to calculate the level of porosity and micro-pore size distribution. A two-dimensional FE model was then developed, where a stochastic method was used to define randomly distributed porous structures equivalent to porosities of 1%, 3%, and 5%. The results showed that the heat flux and temperature gradient were affected by the interactions between neighboring micro-pores and micro-pores/ceramic coatings, and that the effect of the micro-pores was limited to a small area (2–2.5 times the micro-pore radius). Based on the obtained results, a set of effective thermal conductivity equations are proposed which more clearly describe the heat transfer process in a porous TBC structure. Two different equivalent thermal resistance models were used to study the heat transfer process under low porosity (<3%) and high porosity (>3%) conditions.     

高热流密度器件水冷散热器结构性能的实验研究

与平行流水冷散热相比,现阶段对喷射流水冷散热的研究报道较少。为了研究喷射流结构的散热效果,本文设计了两种喷射流结构的水冷散热器,搭建了以去离子水为冷却介质的液冷散热器实验台,调节实验的热流密度及冷却水流量在不同条件下观察芯片温度及散热器底板温度的变化,得到了不同热流密度下芯片温度、散热器底板温度及热阻随冷却水流量的变化规律。散热器内部针翅结构和冷却水流动方式的改变可使芯片温度降低5~8℃,散热器底板平均温度也相应降低4℃左右,且底板温度梯度较小。同时,热阻的变化随流量增大逐渐变缓,散热器结构改进使热阻减小了7%~8%。实验结果表明,改进结构的散热器能有效加强边角区域流体的扰动,提升散热器整体的换热效率,表明喷射流水冷散热是一种高效的散热方式。     

多孔介质中受限层流冲击射流的流动与换热特性

基于两区(two-domain)模型采用基于预处理的时间推进法对铺设有多孔介质层的恒温平板在受限层流冲击射流作用下的流动与换热特性进行了研究,其中多孔区域动量方程采用Brinkman-Forchheimer拓展Darcy模型,能量方程则采用局部热平衡(LTE)模型,并对porous/fluid交界面切应力跳跃条件对多孔介质冲击射流的影响进行了分析。流体的控制方程采用基于密度的有限体积法来求解,并针对于多孔区域低速流动特点采用相对应的预处理矩阵来消除控制方程的刚性。还对Reynolds数、孔隙率、Darcy数、热导率比、多孔介质层厚度等参数的变化对流动结构及换热特性的影响进行了分析。研究结果表明,在目前的计算条件下,在其他参数一定时,Reynolds数、孔隙率对通道内流动结构的影响有限;Darcy数、多孔介质层厚度则对流动结构的影响很大;上述参数对受冲击平板的总体换热性能均有明显的影响。在受冲击平板上铺设适当厚度的高渗透率、高热导率的多孔材料能有效地增强换热性能。     

In situ spectroscopic ellipsometric study of porous alumina film dissolution

Porous alumina films have been synthesised by anodisation of high purity aluminium in 0.4 M phosphoric acid at constant current density. The dissolution process of those coatings in 2 M phosphoric acid has been investigated by in situ spectroscopic ellipsometry (SE) combined with electrochemical measurements. A sharp drop of the open current potential is observed due to film removal. The dissolution time depends on total barrier layer thickness. Those results are confirmed by SE measurements. A three layer optical model has been used to interpret SE spectra. During the dissolution process, the porous top layer has a quasi-constant thickness, whereas porosity increases significantly. The dissolution proceeds within the pores. Only the thickness of pure alumina barrier layer (upper part) decreases during the dissolution process whereas the interface barrier layer (lower part) does not change.     

Fluorination effects of carbon black additives for electrical properties and EMI shielding efficiency by improved dispersion and adhesion

Electrospinning and heat treatment were carried out to get nano sized carbon fibers (CFs) as a matrix for shielding the electromagnetic interference (EMI). In order to improve the electrical conductivity and EMI shielding efficiency of electrospun CFs, carbon black (CB) was fluorinated and embedded into the electrospun CFs. Electrospun fiber sheets embedded fluorinated CB were heat-treated at different temperatures to determine the effects on electrical properties. It is demonstrated that fluorination treatment of CB and heat treatment of electrospun sheets at higher temperature lead to higher electrical conductivities and EMI shielding efficiencies, because fluorination significantly improved its dispersion in electrospun CF webs and created good adhesion between the CB and the CFs. The electrical conductivity of carbon composite sheets (webs) reached ∼38 S/cm, and a high EMI shielding efficiency was obtained (∼50 dB).     

肋片和多孔介质强化梯级相变储热系统性能的对比研究

针对于相变材料（PCM）导热性能差引起的梯级相变储热系统传热速率低的问题,利用三维数值仿真研究肋片和多孔介质对梯级相变储能系统放热性能的强化作用,在此基础上提出了梯度孔隙率进一步提升系统的放热性能,从PCM的放热速率和放热效率两个方面对梯级相变储能系统的不同强化方法进行了分析对比。结果表明肋片在显热放热阶段强化传热作用更显著,而多孔介质在潜热放热阶段强化传热更显著。整个放热过程只加入多孔介质比只加入肋片表现出更好的放热性能。同时添加肋片和多孔介质时,梯级相变系统放热性能最优,PCM完全凝固时间减少了40%。三种孔隙率梯度工况下,系统的放热效率无明显差异,但在负梯度孔隙率情况下,放热速率更高且更均匀。相比于正梯度孔隙率的情况,负梯度孔隙率具有更优的热性能。     

肋片和多孔介质强化梯级相变储热系统性能的对比研究

针对于相变材料（PCM）导热性能差引起的梯级相变储热系统传热速率低的问题,利用三维数值仿真研究肋片和多孔介质对梯级相变储能系统放热性能的强化作用,在此基础上提出了梯度孔隙率进一步提升系统的放热性能,从PCM的放热速率和放热效率两个方面对梯级相变储能系统的不同强化方法进行了分析对比。结果表明肋片在显热放热阶段强化传热作用更显著,而多孔介质在潜热放热阶段强化传热更显著。整个放热过程只加入多孔介质比只加入肋片表现出更好的放热性能。同时添加肋片和多孔介质时,梯级相变系统放热性能最优,PCM完全凝固时间减少了40%。三种孔隙率梯度工况下,系统的放热效率无明显差异,但在负梯度孔隙率情况下,放热速率更高且更均匀。相比于正梯度孔隙率的情况,负梯度孔隙率具有更优的热性能。     

烧结床层的热质分析

基于烧结生产的复杂物理化学过程,建立了烧结床层传热、传质和流动的二维非稳态数学模型,考虑了孔隙率、物料颗粒当量直径等床层结构影响参数的变化,并对气固传热系数进行了修正。通过数值计算,获得了烧结床层的温度场、结构变化和烟气的流场、温度场、浓度场等。烟气出口温度、床层总压降与生产实测值吻合较好,验证了数学模型的正确性。进一步分析了燃料配比、风量和给料温度等操作参数对烧结过程的影响。研究结果表明：燃烧带的厚度、最高温度随着烧结过程的进行而逐渐增加。床层孔隙率、颗粒当量直径的变化主要发生在燃烧带的熔融、冷凝阶段。料层压损最大的是燃烧熔融层,其次是混合料带,最小的是烧结矿层。增加焦粉含量、提高烧结混合料的初温,有利于提高成矿质量;风量过大时,会造成成矿质量下降、生产成本提高。     

Low thermal conductivity of atomic layer deposition yttria-stabilized zirconia (YSZ) thin films for thermal insulation applications

Thermal insulation applications have long required materials with low thermal conductivity, and one example is yttria (Y₂O₃)-stabilized zirconia (ZrO₂) (YSZ) as thermal barrier coatings used in gas turbine engines. Although porosity has been a route to the low thermal conductivity of YSZ coatings, nonporous and conformal coating of YSZ thin films with low thermal conductivity may find a great impact on various thermal insulation applications in nanostructured materials and nanoscale devices. Here, we report on measurements of the thermal conductivity of atomic layer deposition-grown, nonporous YSZ thin films of thickness down to 35 nm using time-domain thermoreflectance. We find that the measured thermal conductivities are 1.35–1.5 W m⁻¹ K⁻¹ and do not strongly vary with film thickness. Without any reduction in thermal conductivity associated with porosity, the conductivities we report approach the minimum, amorphous limit, 1.25 W m⁻¹ K⁻¹, predicted by the minimum thermal conductivity model.     

金属基有机硅树脂涂层复合材料的导热性能

以锌粉为导热填充剂对环氧有机硅树脂进行改性,考察了改性环氧有机硅树脂涂层干膜中锌粉含量对涂层导热系数的影响,分析了涂层厚度对碳钢基材导热性能的影响. 结果表明,环氧有机硅树脂涂层的导热系数约为0.19 W/(m?K),其耐温能力在200℃以上,可保证涂层在中低温烟气余热回收换热器表层长期工作而不发生任何热反应;添加锌粉可改善环氧改性有机硅涂层的导热性能,涂层干膜锌粉25wt%时,涂层材料导热系数达0.35 W/(m?K),较未添加锌粉时增大了84%. 复合材料的导热系数随涂层厚度增加而下降,无涂层的碳钢导热系数为47.59 W/(m?K),涂层厚度为200 ?m时,导热系数降至34.33 W/(m?K).     

影响换热器用多孔表面管沸腾换热的主要因素

综述了换热器用多孔表面管沸腾传热的各种影响因素,既介绍了如孔穴直径、多孔层厚度及孔隙率等多孔层的特征参数的特性;也介绍了材料物性、加工工艺、工质物性、压力与温度等对沸腾传热的影响.同时对各种因素对沸腾换热的具体影响进行了分析归纳,对今后多孔表面强化沸腾传热的研究提出了一些自己的建议.     

异形整体式热管散热器传热实验与分析

针对电子电气设备散热和均温的需求,提出了一种新型结构形式的异形整体热管散热器：平板热管形式的蒸发段与具有高肋化比翅片的冷凝铜管集成。对该热管进行了传热实验与分析,研究热管在不同工况下温度数值及分布,探究影响热管性能的因素和规律,验证其传热能力。结果表明：在各种工况下热管温差始终在1.75℃之内,均温性能良好。加热功率和对流散热状况对热管启动性能、总体热阻、当量热导率、传热系数都有影响。随着加热功率和对流速度增加,热管启动时间和热阻均降低,当量热导率和传热系数则逐渐上升。热阻最低为0.189℃·W^-1,最佳当量热导率为20964 W·m^-1·K^-1。相比于同等尺寸的传统热管,热阻降低了37%,传热效率提升15%。     

分层土壤中竖直埋管换热器传热特性

基于移动有限长线热源理论,考虑土壤分层及存在渗流现象,建立了埋管传热解析模型。利用热响应试验验证了该模型的正确性。对比分层模型与均质模型所给出的埋管散热过程的土壤温度响应表明:埋管周围土壤沿轴向分层使其温度分布也呈现分层而非均匀的特点,各层土壤中钻孔壁处温度趋于稳定所需的时间及数值因各层物性的差异而不同。对整个埋管区土壤均质或分层但存在渗流作用以及部分土壤层存在渗流作用等3种情况进行了计算分析,发现:若整个分层土壤中存在渗流,而视土壤为均质并忽略渗流的影响,则对换热能效系数低估可达6.3%;埋管散热在各土壤层中的热作用距离因存在渗流与否偏差可达43%。可见,为了准确评估钻孔周围渗流作用下分层土壤中的温度分布特性,应利用分层模型计算各土壤层中的热作用距离,以最大值确定管间距,否则会导致管群布置时管间距选取偏小。     

涂层厚度对传热系数的影响研究

通过建立模拟试验装置,不断地增加钢管表面的涂层厚度,记录下不同涂层厚度条件下管内热水的进出口温度以及冷却水的进出口温度。然后对试验数据进行分析,得出涂层厚度对传热效率的影响规律。     

热弥散对地埋管换热器全尺度传热的影响

开展地埋管换热器的传热研究是增强其传热性能以及提高热泵系统能效比的关键,现有的地埋管换热器传热模型多忽略了由地下含水层空间非均质性而产生的热弥散传热。因此,基于渗流影响下的全尺度模型和求解热弥散系数的速度一次方模型,提出了考虑热弥散影响的地埋管换热器全尺度传热模型。研究表明此模型所适用的地下水渗流速度范围为1×10^-8~1×10^-6 m/s;热弥散效应主要在中长时间尺度下体现,渗流速度、热弥散度以及孔隙率是影响传热过程的主要因素。地下水渗流速度和热弥散度越大,孔隙率越小,热弥散效应越强。综合考虑三类影响因素,渗流速度对钻孔传热的影响最大,热弥散度次之,孔隙率的影响最弱;对钻孔群而言,热弥散对上游钻孔传热过程的影响最大,中游次之,下游最小;在所研究的参数范围内,热弥散可使钻孔稳态传热能力提升5.52%~49.93%。     

Effects of improved porosity and electrical conductivity on pitch-based carbon nanofibers for high-performance gas sensors

Pitch-based carbon fibers with multi-walled carbon nanotubes (MWCNTs) were fabricated via an electrospinning method and used as gas sensor electrodes. The pitch-based carbon fibers were treated at various temperatures to investigate the effect of the reaction temperature. The electrospun fibers were thermally treated to produce carbon fibers, and the resulting material was chemically activated to increase the number of active sites for efficient gas adsorption. The activation process improved the porous structure by increasing the specific surface area by approximately 86-fold. Due to the improved porosity and electrical conductivity, gas adsorption sites were enlarged and electron transfer was improved, resulting in a high-performance NO gas sensor with improved sensitivity and rapid response time. The improved porosity was attributed to the chemical activation process, and the enhanced electrical conductivity was attributed to the heat treatment and the addition of MWCNTs.     

树脂基热防护材料烧蚀传热的关键参数影响性分析

针对树脂基热防护材料烧蚀传热的关键参数分析问题,采用碳化层-热解层-原始材料层一维模型,分析了热防护材料烧蚀热解响应过程的传热特性,得出了不确定参数影响传热过程的敏感性大小,结果表明：材料的热导率对树脂基热防护材料烧蚀传热特性影响最大,密度、比热容和材料厚度影响程度较大,其他参数的影响性可以忽略。同时建立了温度和不确定参数之间的量纲1准则关系式,为热防护材料的选取设计以及模型参数修正提供了参考依据。