Heat Transfer Characteristics of Square Micro Pin Fins under Natural Convection

In order to comply with the recent demand for downsizing of the electric equipment, the minia- turization and the improvement in heat transfer performance of a heat sink under natural air-cooling are increasingly required. This paper describes the experimental and numerical investigations of heat sinks with miniature/micro pins and the effect of the pin size, pin height and the number of pins on heat transfer characteristics of heat sinks. Five types of basic heat sink models are investigated in this study. The whole heat transfer area of heat sinks having the different pin size, pin height and the number of pins respectively is kept constant. From a series of experiments and numerical analyses, it has been clarified that the heat sink temperature rises with increase in the number of pins. That is, the heat sink with miniaturized fine pins showed almost no effect on the heat transfer enhancement. This is because of the choking phenomenon occurred in the air space among the pin fins. Reflecting these results, it is confirmed that the heat transfer coefficient reduces with miniaturization of pins. Concerning the effects of the heat transfer area on the heat sink performance, almost the same tendency has been observed in other three series of large surface area, that is, higher pin height. Furthermore as a result of studying non-dimensional convection heat transfer performance, it was found that the relation between the Nusselt number (Nu) and the Rayleight number (Ra) is given by Nu = 0.16 Ra0.52.

Enhancement of Heat Transfer Using Pins Swimming in Non-isothermal Fluidic Systems: Exact Solutions

Heat transfer to pins swimming in non-isothermal fluidic systems is theoretically analyzed. Four different cases are considered: [A] pins aligned longitudinally in flowing fluid having constant temperature gradient, [B] pins aligned transversely in flowing fluid flow with constant temperature gradient, [C] pins moving longitudinally towards a heated surface, and [D] pins moving transversely towards the heated surface. The Appropriate unsteady energy transport equations are solved and closed form solutions for the fin temperatures are obtained. Accordingly, different performance indicators are calculated. It is found that heat transfer to the swimming fin increases as the fin thermal length, Peclet number and fluid temperature difference along the fin length increase. It decreases as fluid temperature index along the motion direction increases. Moreover, the swimming pins of case C are found to produce the maximum system effective thermal conductivity. In addition, pins of case B with thermal lengths above 11 produce system thermal conductivity independent on the thermal length. Meanwhile, pins of case A having thermal lengths above 10 produce system thermal conductivities less responsive to the thermal length. The system thermal conductivity is noticed to increase as the thermal length and Peclet number increase. Eventually, pins of case D produce system thermal conductivities that are independent on the transverse temperature. Finally, the results of this work provide a basis for modeling super convective fluidic systems that can be used in cooling of electronic components.

基于拉氏变换的变温度PIN二极管动态建模

为了实现车载设备的传导噪声预估,二极管模型需在车载环境的宽温度变化范围内对动态特性进行准确描述。以PIN型功率二极管为研究对象,分析了定温度的拉式变换解析模型,引入温度参数,将原模型参数与温度的关系以函数的形式表达,实现任意结温下的模型参数温度修正,从而建立变温度的PIN二极管动态特性模型。在此基础上,通过温箱实验对感性负载条件下的功率二极管动态性能进行测试,实现了模型参数抽取和模型验证。结果显示,该模型在25~120℃的宽温度范围内,可实现反向恢复特性的精确模拟,各项特性参数的误差均较小,不会出现PSPICE通用模型的异常振荡问题,能满足宽频率及宽温度范围的汽车级EMI预估需求。

应用不同深度学习代理模型的灯笼型扰流柱通道换热性能分布预测方法比较

为提高对灯笼型扰流柱通道端壁面的换热分布预测能力,构建并比较了几种深度学习代理模型的预测性能。应用数据驱动的思想,建立了多项式响应面模型(RSM)、径向基函数模型(RBF)、径向基神经网络模型(RBFNN)、Kriging模型和总体平均近似模型(Ensemble)共5种传统代理模型以及5种基于生成对抗网络的深度学习代理模型,包括有残差网络的pix2pix模型、pix2pixHD模型、CycleGAN模型、StarGAN模型以及单一无残差网络的pix2pix模型,以扰流柱截面参数为设计变量,通过拉丁超立方抽样分别得到了样本数为50、25、12、6和3的训练集,并根据扰流柱尾迹高换热区分布特点,将训练样本数为50的数据集分为宽样本数据集和窄样本数据集,比较了不同代理模型对端壁面换热性能的预测精度、计算成本和泛化能力。结果表明:有残差网络的pix2pix模型相比于无残差网络的pix2pix模型,预测精度得到有效提高,在样本数为50的情况下,面平均值预测误差从0.68%降低到0.32%,平均相对误差从6.89%降低到6.41%,而且当训练样本数减少时,有残差网络的模型预测能力更加突出;传统代理模型的时间成本可忽略不计,但深度学习模型的单卡训练时间较长,且增加残差网络后的模型计算成本更高;当训练样本数为50时,传统代理模型和深度学习模型之间预测精度差异不大;当训练样本数逐渐减少时,深度学习代理模型展示出更高的预测精度和泛化能力;Kriging模型虽然泛化能力强,但是预测结果趋同;RSM模型、RBF模型和Ensemble模型泛化能力最差,训练样本数较少时,预测结果严重失真。可见在换热性能预测方面,深度学习模型在预测精度与泛化能力上均有显著优势,尤其适合于小样本问题,对提高灯笼型扰流柱截面设计效率具有参考价值。

带扰流柱阵的双层壁间隙冲击传热特性

数值模拟研究了带扰流柱阵的双层壁结构间隙靶面、冲击面以及不同位置扰流柱面努塞尔数详细分布,分析了各个表面努塞尔数平均值随冲击雷诺数的变化规律以及不同位置扰流柱面换热特性差别。结果表明,靶面努赛尔数的第二峰值大于滞止点处的努赛尔数。冲击面展向平均努赛尔数最大值位于靠近冲击孔的第2排和第4排扰流柱处。在5排扰流柱中,与冲击孔处于同一排(第3排)的扰流柱面平均努塞尔数最大,表现为翻卷汇流传热特性;第2排和第4排扰流柱面平均努塞尔数次之,表现为翻卷绕流传热特性。远离冲击孔的第1排和第5排扰流柱面平均努塞尔数最小,表现为绕流传热特性。冲击雷诺数从1×104增加到6×104,第3排扰流柱面和第2、4排扰流柱面以及第1、5排扰流柱面努塞尔数增幅分别为306%、325%和360%。

Experimental Study on Heat Transfer Enhancement of Natural Circulation Liquid Cooling System for Electronic Component

The present research is an experimental study on heat transfer characteristics of a natural circulation cooling system for electronic components. A smooth chip and two micro-pin-finned chips were tested. The chip is mounted on the base of a rectangular horizontal duct located at the bottom of 250 mm high natural circulation loop.FC-72 is used as a coolant. The test conditions are set that the operation pressure of experimental system is 1. 013× 105 Pa, the flow rate of FC-72 is 150 g/min and the subcoolings are 10 K, 25 K and 35 k, respectively. Effect of the subcooling on nucleate boiling and critical heat flux(CHF) were investigated. The results show that subcoolingis found to significantly affect CHF for all chips and micro-pin-finned chips sharply enhanced the boiling heat transfer, CHF of micro-pin-finned chips are 2.5～3 times as large as that of smooth chip at the same subcooling.

Numerical investigation on thermal properties of micro-pin-fin cooler

Micro-pin-fin cooler mounted on the power chip enables the heat removal to meet modern microsystem requirement.Carbon nanotubes (CNTs) have been proven as a potential material for micro-coolers due to the superior thermal conductivity,good mechanical property and so forth,and there appear various applications of CNTs in the micro-cooler technology.In the present paper,an analysis of the thermal and hydraulic characteristics of the micro-pin-fin heat sink was conducted,where air was used as the cooling medium and an impinging jet was introduced to enhance the heat transfer.Three-dimension computational fluid dynamics (CFD) simulations were carried out for micro-pin-fin coolers with various parameters,including the pin-fin size and pattern as well as the jet velocity and nozzle diameter.The flow field and thermal properties of the micro-pin-fin heat sink were obtained,and the heat removal efficiency was evaluated.

A NEW METHOD TO JUDGE THE ENHANCEMENT OF HEAT TRANSFER IN THE TRAILING EDGE OF TURBINE BLADES

An experimental investigation of heat transfer and friction loss with taper pin fin configurations were dealed with. Three kinds of configurations-full cross pin fins (FCF), shorter round pin fins (SRF) and taper pin fins (TPF), with different height md spacing of the fins and friction loss are compared with each other. Besides, a new dimensionless number, the ratio of f to Nu, cslled here the specific friction loss, is put forward to judge the validity of configurations. The comparison obviously shows that:

Evaluation on Heat Transferring Performance of Fabric Heat Sink by Finite Element Modeling

Considering the limitation in current manufacturing technology of commercial pin fin heat sinks,a new fabric heat sink has been designed. However,it is lack of an understanding of the heat transferring performance of this new kind of heat sink. In this study,the finite element method (FEM) was used to predict the heat transferring performance of fabric heat sink under the condition of natural convection and forced convection, and its heat transferring performance was compared with that of pin fin heat sink. The results showed that in the condition of natural convection the heat transferring performance of pin fin heat sink was better than that of fabric heat sink, and vice versa under the forced convection condition.

微针肋结构大过冷度强化沸腾传热实验研究

高热流密度电子设备发热功率日益增加,需要新的高效冷却技术保障其安全运行,采用大过冷度流体与微针肋结构结合的方式是提升冷却能力的一种新思路。本文实验观测了微针肋结构过冷沸腾现象,测试了不同过冷度(50、60、75、95℃)对表面传热系数与临界热流密度的影响,确定了大过冷度下微针肋结构相比光滑结构表面传热系数的强化比例,研究了微针肋结构过冷沸腾强化传热机理。结果表明:当热流密度超过200 W/cm^(2)时,微针肋结构过冷沸腾换热表面传热系数随过冷度增大而增大;微针肋结构临界热流密度随过冷度上升而上升,当过冷度由50℃升至95℃,临界热流密度由228 W/cm^(2)升至400 W/cm^(2)以上;微针肋结构表面传热系数均比同工况下的光滑结构表面传热系数大,微针肋结构的强化传热效果在大热流密度、大过冷度下更为显著,当热流密度为250 W/cm^(2)、过冷度为95℃时,强化比例为1.28。

R134a在菱形离散肋微小通道内的流动沸腾换热实验研究

研究了制冷剂R134a在角度分别为30°、60°和90°的菱形离散肋微小通道内的流动沸腾换热特性。微小通道内菱形离散肋分布区域长300 mm、宽20 mm,进口处饱和压力为(700±5)kPa,其他工况范围为:干度0~1,质量流率200~500 kg(/m2·s),热通量10~30 kW/m2。实验结果表明:离散肋中的流动沸腾换热受到核态沸腾和对流沸腾的共同作用,传热系数随质量流率和热通量的增加而增加,但随着干度的升高,热通量的作用减弱并趋于消失。此外,离散肋结构对流动沸腾换热有显著影响,相同工况下,90°菱形离散肋的传热系数高于30°和60°,且在高干度更显著。最后,基于实验数据和分析结论,提出了一个适用于预测不同结构离散肋微小通道中流动沸腾传热系数的计算关联式。

环形流道内扰流柱参数优化的模拟分析

套管结构可以通过多种被动式方法提高其传热效率。通过正交试验和数值模拟的方法,以努塞尔数Nu、阻力系数f和综合评价指标PEC作为评价指标,制定了6因素3水平的正交试验,对设置在套管环形流道内不同参数的扰流柱进行了数值模拟。对正交试验结果进行极差分析,得到各因素对Nu、f和PEC的影响趋势,并最终以PEC确定了扰流柱的最优参数组合(PEC=1.973)。基于场协同理论进行分析,具有扭转角的椭圆形扰流柱大幅提高了环形流道内流体的径向速度,约可达到轴向速度的4%,显著提升了换热壁面处的温度场和速度场的协同效果,起到了强化传热的作用。

刮板转载机凸槽寿命优化设计研究

基于刮板转载机凸槽中板位于爬坡与悬空段的过渡地带,极易出现磨损破坏的问题,通过对刮板转载机凸槽中板出现磨损的原因机理进行分析,从改变凸槽升角、设计可更换中底板凸槽以及改变凸槽材质提高强度三个方面对凸槽进行优化设计,以提高刮板转载机凸槽使用寿命。

短扰流柱排端壁的平均换热实验

为了深入了解涡轮叶片尾缘扰流柱区域的端壁换热情况,针对梯形通道高稠度短扰流柱排,沿着弦向和径向都有出流或只沿径向出流的情况,通过实验测量了端壁换热分布,研究了弦向出流比(0.25～1)和弦向出流雷诺数(3×104～9×104)对端壁换热的影响。实验结果表明:(1)端壁平均努塞尔数随着雷诺数增加而增加,随着出流比的增加先下降,出流比从0.75到1平均努塞尔数又略增加。(2)不同出流比情况下,沿着弦向和径向努塞尔数的变化具有不同的规律。

带射流孔板矩形扰流柱通道内的壁面换热

为了研究射流冲击对扰流柱通道内壁面的换热,以及射流孔板与扰流柱前缘的距离对通道换热的影响规律,采用薄膜加热片作为加热器提供等热流边界条件,对带射流孔板的矩形扰流柱通道进行了换热实验。实验结果表明在通道中布置扰流柱可以明显增强通道换热,且在扰流柱排前放置射流孔板可进一步对换热进行强化。实验还发现,射流孔板与扰流柱的间距不同,对通道换热的影响不同;在间距相同的情况下,射流孔板对不同位置的扰流柱换热的影响程度也有所差异。

流体横掠方形微针肋阵列热沉的传热特性

本文以去离子水为冷却工质,对流体横向冲刷方形微针肋阵列热沉的传热特性进行了实验研究.结果表明流体横掠微针肋热沉具有优越的换热特性。实验得出,冷却液的雷诺数对热沉的换热性能影响显著;总热阻随着泵功的增大而降低;对流换热热阻在总热阻中所占比例较小,流体吸热焓变热阻成为影响热沉性能的主导因素.

不同直径及形状的短扰流柱群的流阻及换热

采用放大的模型对装有五排短扰流柱的涡轮叶片尾缘的冷却通道的流阻特性及通道端壁表面上的局部换热系数进行了测量 ,重点研究了扰流柱直径及形状的影响。结果表明 :(1 )扰流柱直径越大 ,压力损失系数越大。在相同条件下 ,圆柱形扰流柱排的压力损失系数要大于圆锥形扰流柱排的压力损失系数 ;(2 )圆柱形扰流柱排内的换热强化系数的增长速度比圆锥形扰流柱排要快 ,而且达到的最大值也较大。扰流柱直径越大 ,在相同的局部位置处换热强化系数越大 ;(3 )当扰流柱直径增加时 ,其阻力的上升要比换热的上升快的多。与圆柱形扰流柱相比 。

斜针翅纵向流管束研究及混合管束设计应用

分析斜针翅管束纵向流流动特性 ,建立了斜针翅管束流动的流体力学及传热强化理论模型 ,并对不同规格斜针翅管进行试验研究和比较 ,可节省材料 1 /3,压降仅为光管折流板换热器的 1 /2 0 ,强化传热为光管 2～ 3倍 ,并设计应用于炼油换热器中。

三维整体针翅强化传热管的传热和压降性能研究与比较

在概述国外三维针翅管的基础上介绍了作者的实验研究结果 ,并与Q .Liao等人的 14种管结果比较 。