Experimental Verification of Model for Liquid-Cooled Staggered Pin Fin Heat Sinks with Top Bypass Flow

Pressure drops and heat transfer over staggered pin fin heat sinks with top bypass flow were experimentally evaluated. The authors considered liquid-cooling applications because there were few data available comparing to air-cooling applications. Empirical equations to predict heat transfer on the endwall were developed by obtaining experimental data on the copper base plate with acrylic pins. A new model for predicting pressure drops and heat transfer over staggered pin fin heat sinks with top bypass flow based on mass, momentum, and energy conservation within the two control volumes is proposed. The first control volume in the model is located within the finned area, and the second is located in the gap between the tip of the pins and the flow channel. This model combines two conditions according to the boundary-layer thickness. A comparison between experimental and calculated results revealed that dimensionless pressure drops and the Nusselt number could be predicted within 30% error for the former and 50% error for the latter.

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.

Effect of Shape and Placement of Twisted Pin Fins in a Rectangular Channel on Thermo-Hydraulic Performance

To enhance the thermo-hydraulic performance of cooling channels,this investigation examines the influence of distinct cross-sectional shapes(i.e.,triangular,rectangular,and hexagonal)of twisted pin fins and their arrangements in straight and cross rows.An ambient air cooling test platform was established to numerically and experimentally investigate the flow and heat transfer characteristics of 360°twisted pin fins at Re=15200-22800.The findings reveal that straight rows exhibit higher Nu values than cross rows for triangular and rectangular twisted pin fins,and Nu increases with Re.In contrast,for hexagonal twisted pin fins,only straight rows at Re=19000 exhibit superior overall thermal performance compared to cross rows.Notably,the heat transfer performance of the cooling channel with hexagonal twisted fins surpasses both triangular and rectangular configurations,especially at high Reynolds numbers(Re=22800).Although the heat transfer coefficient of the cooling channel with hexagonal twisted fins is significantly enhanced by 132.71%compared to the flat channel,it also exhibits the highest thermal resistance and relative friction among the three types of twisted fins,the maximum of which are 2.14 and 16.55.Furthermore,the hydrothermal performance factor(HTPF)of the cooling channels with different types of twisted pin fins depends on the Reynolds number and arrangement modes.At Re=15200,the highest HTPF achieved for the cross-row hexagonal twisted pin fins is 0.99.

基于拓扑优化的车用功率模块Pin-Fin设计方法

集成Pin-Fin散热器可以降低功率模块的结-流热阻,是车用电机控制器热管理的发展趋势。然而,传统参数设计方法无法定量表征Pin-Fin的形貌结构,设计自由度低、设计效果欠佳,难以实现Pin-Fin的热-流协同设计。基于变密度拓扑优化方法,该文提出一种Pin-Fin形貌结构的设计方法,建立Pin-Fin设计的变密度拓扑优化模型,分析模型参数对优化结果和收敛性的影响规律,形成以结-流热阻和冷却液压降为目标的协同优化方法,探索Pin-Fin结构的热-流耦合规律和强化换热方法,得到最优的Pin-Fin散热器形貌结构,搭建变流器实验平台和车用电机控制器样机,对标商业化圆形Pin-Fin结构,以及三角形、菱形和水滴形等其他形貌结构,计及母线电压、负荷电流和冷却液流量的影响,实验结果验证了所提模型方法的可行性和有效性。实验结果表明:基于拓扑优化方法设计的Pin-Fin结构,与传统结构相比可以降低12%的结-流热阻,消除了多芯片结温差异的80%,为车用功率模块和电机控制器研究提供新的思路。

Investigation on flow and heat transfer characteristics in rectangular channel with drop-shaped pin fins

The flow and heat transfer characteristics inside a rectangular channel embedded with pin fins were numerically and experimentally investigated.Several differently shaped pin fins(i.e.,circular,elliptical,and drop-shaped)with the same cross-sectional areas were compared in a staggered arrangement.The Reynolds number based on the obstructed section hydraulic diameter(defined as the ratio of the total wetted surface area to the open duct volume available for flow)was varied from 4800 to 8200.The more streamlined drop-shaped pin fins were better at delaying or suppressing separation of the flow passing through them,which decreased the aerodynamic penalty compared to circular pin fins.The heat transfer enhancement of the drop-shaped pin fins was less than that of the circular pin fins.In terms of specific performance parameters,drop-shaped pin fins are a promising alternative configuration to circular pin fins.

Time-accurate CFD conjugate analysis of transient measurements of the heat-transfer coefficient in a channel with pin fins

Heat-transfer coefficients(HTC)on surfaces exposed to convection environments are often measured by transient techniques such as thermochromic liquid crystal(TLC)or infrared thermography.In these techniques,the surface temperature is measured as a function of time,and that measurement is used with the exact solution for unsteady,zero-dimensional(0-D)or one-dimensional(1-D)heat conduction into a solid to calculate the local HTC.When using the 0-D or 1-D exact solutions,the transient techniques assume the HTC and the free-stream or bulk temperature characterizing the convection environment to be constants in addition to assuming the conduction into the solid to be 0-D or 1-D.In this study,computational fluid dynamics(CFD)conjugate analyses were performed to examine the errors that might be invoked by these assumptions for a problem,where the free-stream/bulk temperature and the heat-transfer coefficient vary appreciably along the surface and where conduction into the solid may not be 0-D or 1-D.The problem selected to assess these errors is flow and heat transfer in a channel lined with a staggered array of pin fins.This conjugate study uses three-dimensional(3-D)unsteady Reynolds-averaged Navier-Stokes(RANS)closed by the shear-stress transport(SST)turbulence model for the gas phase(wall functions not used)and the Fourier law for the solid phase.The errors in the transient techniques are assessed by comparing the HTC predicted by the time-accurate conjugate CFD with those predicted by the 0-D and 1-D exact solutions,where the surface temperatures needed by the exact solutions are taken from the time-accurate conjugate CFD solution.Results obtained show that the use of the 1-D exact solution for the semi-infinite wall to give reasonably accurate“transient”HTC(less than 5%〇relative error).Transient techniques that use the 0-D exact solution for the pin fins were found to produce large errors(up to 160%relative error)because the HTC varies appreciably about each pin fin.This study also showed that HTC measured by transient techniques could differ considerably from the HTC obtained under steady-state conditions with isothermal walls.

Fabrication of micro pin fins on inclined V-shaped microchannel walls via laser micromilling

A laser-micromilling process was developed for fabricating micro pin fins on inclined V-shaped microchannel walls for enhanced microchannel heat sinks.A pulsed nanosecond fiber laser was utilized.The feasibility and mechanism of the formation of micro pin fins on inclined microchannel walls were investigated for a wide range of processing parameters.The effects of the laser output power,scanning speed,and line spacing on the surface morphologies and geometric sizes of the micro-pin fins were comprehensively examined,together with the material removal mechanisms.Micro pin fins with acute cone tips were readily formed on the V-shaped microchannel walls via the piling of recast layers and the downflow of re-solidified materials in the laser-ablation process.The pin-fin height exhibited an increasing trend when the scanning speed increased from 100mm/s to 300 mm/s,and it decreased continuously when the line spacing increased from 5μm to 20μm.The optimal processing parameters for preparing micro pin fins on V-shaped microchannels were found to be a laser output power of 21 W,scanning speed of 100-300 mm/s,and line spacing of 2-5μm.Moreover,the V-shaped microchannels with micro pin fins induced a 7%-538%boiling heat-transfer enhancement over their counterpart without micro pin fins.

基于响应面的车用功率模块Pin-Fin优化设计

集成Pin-Fin散热器可以降低功率模块的结-流热阻,提升车用电机控制器的功率密度和可靠性,是车用功率模块先进热管理技术的发展趋势。然而,Pin-Fin散热器的设计面临电-热-流多物理场耦合的挑战,难于解析表征与定量优化,设计的周期长、成本高。基于响应面优化方法,该文提出一种集成Pin-Fin散热器的模型化设计方法,建立集成Pin-Fin形貌结构与阵列排布的数学描述,给出Pin-Fin散热器热-流性能的理论模型,建立Pin-Fin的多目标优化模型,获得Pin-Fin散热器结构的最优设计。为了验证理论模型和优化设计的可行性和有效性,搭建面对面变流器对拖实验平台,对标商业化车用功率模块的Pin-Fin设计,采用大量固定工况,以及实际车用工况实验结果,对比验证优化后的Pin-Fin能降低车用功率模块7.62%的结-流热阻,减小65%的功率模块损伤,提升车用电机控制器寿命1.8倍,为车用功率模块低热阻集成Pin-Fin散热器提供新的研究思路。

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.

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%。

基于神经网络和人工鱼群算法的惯性延时微通道优化

为了保证中、小口径弹引信的炮口安全性控制要求,设计了具有肋板阻尼结构的惯性延时微通道,同时为了保证其具有稳定的延时性能,研究了该微通道在高离心转速下的延时响应性能。采用了神经网络模型和人工鱼群算法对蛇形微通道内肋板的结构位置进行优化设计。用两相流水平集模型以微通道的延迟时间为研究对象进行了模拟仿真,得到180组样本数据,分析发现肋板结构在微通道内的不同位置与延迟时间呈现出高度的非线性关系。根据样本数据建立神经网络模型用以拟合设计变量与优化目标之间的映射函数,并采用人工鱼群算法对神经网络模型拟合的映射函数的参数进行优化。结果表明,经过结构优化之后,在1 000 g离心环境下微通道中流体的延迟时间从最短的11.446 ms提升到了25.054 ms,延时效果得到了显著提升。最后研究了优化后的结构在中、小口径弹引信使用环境下的延时特性,验证了其满足大部分中、小口径弹引信的延时控制要求。

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。

针翅式散热器模块流动传热特性数值优化

为提升中高温电子器件的散热性能,以针翅式散热器模块为研究对象,研究了热管长度、散热器宽度、针翅直径、针翅间距和针翅高度5个结构参数对翅片散热性能的影响,对正交实验设计的16个组合方案下翅片的流动换热性能进行模拟。以努塞尔数Nu、阻力系数f、传热性能综合评价指标(performance evaluation criteria,PEC)和全因子评价Y作为评价指标,在每个评价指标下利用极差分析主要影响因素和挑选出优化组合。结果表明:影响Nu、f、PEC和Y的最主要因素是针翅间距;最佳优化组合为:针翅间距为2.5 mm,散热器宽度为80 mm,针翅直径为1.5 mm,针翅高度为20 mm,热管长度为25 mm,模块温降为16.28℃,热阻为0.265℃/W。

低温粮食储存用蓄冷冰球传热性能强化研究

基于蓄冷冰球的冰蓄冷是一种封装式的高效蓄冷技术,可以起到“削峰填谷”的作用,在减少能耗的情况下实现粮食的高效储存。本文基于enthalpy-porosity方法对内置了分叉针肋的蓄冷冰球装置进行数值模拟研究,分析了典型的蓄冰过程,明确了肋片对蓄冷冰球装置的必要性。进一步分析了肋片长度比a对蓄冰过程中的温度响应、蓄冰完成时间和蓄冷量的影响,从而为内置分叉针肋的蓄冰装置的设计明确了方向。

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.

Second Law Analysis of the Optimal Fin by Minimum Entropy Generation

Based on the entropy generation concept of thermodynamics, this paper estabfished a general theoretical model for the analysis of entropy generation to optimize fins, in which the minimum entropy generation was selected as the object to be studied. The irreversibility due to heat transfer and friction was taken into account so that the minimum entropy generation number has been analyzed with respect to second law of thermodynamics in the forced cross-flow. The optimum dimensions of cylinder pins were discussed. It＇s found that the minimum entropy generation number depends on parameters related to the fluid and fin physical parameters. Varlatioms of the minimum entropy generation number with different parameters were analyzed.

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

Considering current technology limitation in manufacturing present pin fin heat sinks, a new fabric heat sink has been previously designed. However, there is a lack of an understanding of the heat transferring performance of this new kind of heat sink. Nowadays, finite element analysis has been generally developed for determining heat transfer from in-line and staggered pin fin heat sinks used in electronic packaging applications. In this study, this method is used to predict the heat transfer performance of the new heat sink with woven fabric structure, called fabric pin fin heat sink. Effect of the fin length and the material types made of heat sink on the thermal-structure response of the pin fin was investigated under forced convection. The results show that the minimum temperature of heat sink decreases with an increase of pin fin length,but the decreasing amplitude has decreased. Moreover, the heat transfer performance of fabric heat sink made of continuous carbon Fibers/Polymer (PPS) is worse than that of copper and of aluminum.

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。

截断扰流柱对涡轮叶片前缘冲击冷却的影响研究

通过数值模拟研究了错列排布的截断扰流柱高度及布置方式对涡轮叶片前缘双层壁冲击冷却通道的流动和换热特性的影响。通过对通道内不同截面二维流线以及涡核图分析增强了对弯曲双层壁冷却通道内的流动特征的理解,并进一步结合近靶面极限流线和流动特征图印证该流动特征以及壁面流动,基于此,分析了靶面局部换热特性,并讨论了不同方案下的靶面平均努塞尔数、流阻损失和综合换热性能。数值模拟结果表明,截断扰流柱方案较常规全尺寸扰流柱在换热性能上有明显提升且其流阻有一定降低。当截断扰流柱布置在于冲击孔壁面,截断扰流柱相对高度为0.75时,整个靶面的平均换热和综合换热性能最高。基于本文研究结果,在燃气轮机涡轮叶片前缘双层壁通道中,推荐在冲击孔壁上布置一定高度的截断扰流柱,有助于综合换热性能的提升。