低雷诺数下二自由度圆柱涡激振动时的流动和传热

采用数值方法研究了低雷诺数下流体扰流圆柱发生涡激振动时流体的流动、圆柱的振动及流体与圆柱之间的传热特性。采用有限体积法求解Navier-Stokes方程获得流场,同时采用四阶龙格-库塔算法求解圆柱的运动特性,分析了不同雷诺数和折合速度U_r对二自由度圆柱涡激振动及其传热的影响。结果表明：在不同折合速度下圆柱振动会呈现“8”形运动轨迹,涡脱落模式为“2S”;圆柱涡激振动时传热强弱受自身涡脱落频率和振动幅度的影响;对于二自由度涡激振动圆柱,当U_r=4时圆柱涡脱落频率最接近圆柱的固有频率,圆柱振动剧烈,振动幅度大,同时传热最强,且二自由度涡激振动圆柱的时间平均努塞尔数明显大于一自由度圆柱。     

错列锯齿翅片流动及传热特性的参数化数值与试验研究

采用CD40柴油机油为工作介质,在发动机冷却系统实际工况下,对21种几何尺寸的错列锯齿型不锈钢翅片进行了研究。考察了齿高、齿距、齿开窗宽度、齿厚、齿型角、错齿距对错列锯齿型翅片流动及传热特性的影响,结果表明:在研究参数范围内,齿高为3.2 mm、齿距为14 mm、开窗宽度为1.5 mm、齿型角为70°、齿厚为0.5 mm、错齿距为齿距的1/4时,翅片的综合性能最优。通过仿真数据拟合,得到了基于雷诺数和无量纲参数s/ht、/s、t/b的传热因子、摩擦因子的关联式。70%的传热因子试验值与关联式值的相对偏差在±10%以内;摩擦因子试验值与关联式值的相对偏差在5%~15%,可用于该型翅片的选型和前期设计。     

复合翅片传热与流动特性的数值模拟

结合纵向涡发生器和开缝的优点,提出一种复合强化传热翅片,并对该种翅片的传热与流动特性进行数值计算.由于翅片结构比较复杂,在三角形小翼及X形开缝区域应用阶梯逼近进行网格划分.复合翅片的强化作用在于两方面:一是在三角形小翼和X形开缝后的流场均产生了纵向涡,对流体产生较强的扰动;二是带状开缝的间断表面抑制了边界层的增长,平均...     

翅片螺距和翅片高度对折齿型螺旋翅片管束传热及阻力特性的影响

折齿型螺旋翅片管是在平齿型螺旋翅片管的基础上改进而成的一种新型翅片管.为获取其在错列布置下的传热和阻力特性的变化规律,对5组试件进行了模化试验.结果表明:当pf/do在0.096～0.134时,空气侧Nu数随翅片螺距的增大而增大,当pf/do在0.134～0.233时,翅片螺距对于Nu数影响很小;Eu数随着翅片螺距的增大而减小;当Hf/do在0.365～0.471时,存在最优翅片高度,使得空气侧Nu数最大;Eu数随着翅片高度的增大而增大.     

基于低雷诺数κ-ε模型冲击射流传热的数值模拟

冲机射流在机翼除霜、电子元器件散热等工程领域应用广泛,为进一步研究其传热机理,利用两种低雷诺数κ-ε模型对单股冲击射流的传热进行数值模拟,并与标准κ-ε模型及雷诺应力模型的计算结果进行对比。结果表明,低雷诺数κ-ε模型在两种工况下的计算结果均优于其他模型。湍流热扩散系数作为求解温度场最重要的参数,对传热的计算结果影响极大,通常用湍流普朗特数作为常数求解。在低雷诺数κ-ε模型的基础上,分别通过湍流普朗特数函数求解和t2-εt方程直接求解进行改进,并将计算结果进行比较。结果表明,使用湍流普朗特数函数形式求解湍流热扩散系数能给出更好的预测结果,直接求解仍需进一步改进。     

H型翅片椭圆管束传热与阻力特性的实验研究

对H型翅片椭圆管束的管外侧传热与阻力特性进行了模化实验研究,得到了H型翅片椭圆管束的管外侧传热与阻力特性变化规律,分析了横向管间距s_1与纵向管间距s_2对H型翅片椭圆管束传热与阻力特性的影响。研究表明:在研究范围内,随着s_1的增大,管外侧传热性能变差、阻力减小;随着s_2的增大,管外侧传热性能明显变差,阻力明显减小;且纵向管间距s_2对传热与阻力性能的影响比横向管间距s_1更明显。     

球突翅片的传热流动特性及等效热阻数值分析

为了提高翅片管换热器的传热系数和减小压降,提出了一种球突型翅片,通过数值模拟研究其传热与流动性能,同时应用(火积)耗散理论对其传热的不可逆性进行分析。计算结果表明:与平片相比,其传热能提高26.21%～39.53%,而阻力系数仅提高16.62%～27.04%,同时综合性能增加16.54%～32.56%;这说明该翅片具有高传热系数低压降的特点,是一种性能优良的翅片。通过(火积)耗散分析可以看出:球突翅片的等效热阻减小,其传热的不可逆性减弱。     

脉动燃烧器不同形式风冷翅片传热与流动特性的数值模拟

结合脉动燃烧器高温燃烧室和尾管部分需要风冷的实际情况,采用数值模拟方法针对几种典型翅片的传热与流动特性进行了数值计算,比较分析了不同形式翅片的表面传热系数、№数和表面摩擦系数,最终为脉动燃烧器风冷翅片选择了较为合理的翅片形式。     

翅片参数与PCM材料对散热器传热影响实验研究

为了研究翅片数量、倾斜角度及相变材料对带有纵向翅片的电源散热器传热性能的影响,在恒定热负荷为16 W的条件对含相变材料和不含相变材料两类工况下的5种不同结构散热器进行了实验研究.通过搭建的由实验段、直流电源、数据采集设备等组成的实验系统开展实验,分析了翅片数量(1～5片)、散热器倾角(0°～90°)和相变材料(正二十烷)对散热器传热性能的具体影响,并通过含相变材料散热器的温升变化特征以及相变传热特征对散热器热性能进行了评估.结果表明,倾斜角度和翅片数对液相相变材料对流单元的形成以及传热和运行时间起着关键作用.对于0°倾角的含相变材料散热器,相比1个翅片,5个翅片达到最大允许温度所需的时间增加了80％,而当倾角增加到90°时翅片数的增加对时间的影响不显著.对于所有的倾斜角度,各个测点的相变材料(PCM)温度值都随着翅片数的增加而减小.当倾角增加时,由于浮力诱导流的增强促使对流液相相变区域变大;在单翅片工况下,60°倾角相比0°倾角状态下的工作时间延长78.6％.     

水平放置离散矩形直翅片强制对流的传热研究

实验了不同空气流速时水平放置离散矩形直翅片组的稳态传热着重探讨了翅片列间距对翅片组散热性能和阻力损失的影响,用离散翅片的长度作为定性尺寸,得到了计算直线排列的离散翅片组平均对流传热系数的无量纲传热准数关系式     

Heat Transfer and Friction Characteristics of Perforated Fin with a Longitudinal Slot under Forced Convection

Extended surfaces are used in a variety of heat transfer applications owing to their ability in reducing the convection resistance by exposing a large surface area to the surrounding fluid. Surface modification in the form of perforations is a passive method of increasing the heat transfer rates with the additional benefit of weight reduction. This work deals with numerical investigation of heat transfer and friction from a perforated fin (with and without slot) subjected to forced convection. The perforated fin with slot has been found to have a maximum enhancement in heat transfer with the simultaneous increase in frictional losses versus that of a solid fin. Further, the perforated fin without slot has been able to transfer heat at a relatively higher rate with a considerable reduction in energy loss due to friction in comparison to a solid fin.     

Influence of geometrical factors and pressing mould wear on thermal-hydraulic characteristics for steel offset strip fins at low Reynolds number

Aiming at achieving a more comprehensive understanding of influence factors on thermal-hydraulic characteristics for high-pressure-direction type steel offset strip fins, this paper constructed a single fin core assembly test rig with lubricant oil as work media at low Reynolds number. Six fin schemes were experimentally investigated only varying in fin height and fin wavelength due to mould restrictions. Twenty fin schemes changing in fin width, fin angle, fin thickness and staggered fin wavelength were simulated for supplement, and the geometrical influences in fin performance were put forward. Both Taguchi method and uniform design were introduced to study the contribution of each geometrical factor to fin global thermal-hydraulic performance, verifying that fin wavelength and fin height have the most significant contributions. Based on signal to noise ratio distributions, fin geometries were optimized for 16% elevation of global performance at Reynolds number of 430, then the ‘cost-based fin performance’ was put forward and compared quantitatively, proving that above optimum fin scheme is also economical. Finally, mould wear influence in fin performance was tested in different wear periods of a vacuum heat-treated Cr12MoV mould pair, then three stages of initial wear, stable wear, and overdrive wear were defined based on pressing times. The influences were analyzed and regulations were deduced relating to mould pressing times and oil Reynolds number. The suggested lifetime for this type of mould pair is 1050 thousand pressing times. The paper intends for fin performance evaluation, optimization, and pressing mould improvement.     

百叶窗翅片结构对空气侧流动换热影响的研究进展

百叶窗翅片作为换热器主要翅片形式之一,其结构对空气侧流动换热特性有着重要影响。本文总结了近年来国内外在百叶窗翅片结构对空气侧流动换热影响方面的研究,包括翅片间距、翅片高度、翅片厚度、翅片深度、百叶窗间距及开窗角度对空气侧换热系数、压降、流动效率、传热因子和摩擦因子的影响。研究表明:传热因子随开窗角度和翅片深度的增加而增大,随翅片间距的增加而减小;摩擦因子随开窗角度的增大而增大,随百叶窗间距的增大而减小;其中,开窗角度与翅片深度分别是影响空气侧流动和换热的最主要因素。最后,在百叶窗结构的基础上,提出了对翅片表面进行处理以及使用新型翅片结构等途径来进一步强化空气侧流动换热的建议。     

两种针形管单排管／单管传热及阻力性能试验研究

在一低速风洞中，针对两种不同开头的高肋针形管进行了单排管／单管传热及阻力性能的试验研究，获得了相应的阻力及传热准则关系，并将其与光管及低肋针形管进行了性能比较，为工业设计应用提供了有价值的数据。     

低气压下翅片管换热器空气侧换热特性的实验研究

为研究低气压环境下翅片管换热器空气侧的换热特性,对不同气压环境下空气侧流速和翅片间距对平翅片管换热器空气侧换热特性的影响进行了实验分析。实验环境气压范围为40～100 kPa,换热器迎面风速为1.0～3.5 m/s,翅片间距2～3 mm。研究表明:实验工况下环境气压40 kPa时空气侧传热因子仅为常压下的30.42%～46.41%;低气压环境空气侧流速和翅片间距对空气侧换热的影响趋势与常压数据基本保持一致;不改变换热器结构,环境气压的变化仅影响空气物性,而对空气的流动状态的影响不大;翅片间距影响随Re的减小和环境气压的降低而减弱,两种翅片间距模型空气侧传热因子平均差异在环境气压为100 kPa时为12.07%,40 kPa时缩小为3.00%。     

等雷诺数法在板式换热器传热试验中的应用

在换热器的传热性能试验中,对流换热系数的测定是重要的一个组成部分,对流换热系数的测定方法有很多种,但都有各自的应用范围和条件。对于板式换热器而言,用等雷诺数法来获得其对流换热系数是比较合适的一种方法,文中介绍了等雷诺数法的原理及其在板式换热器中的应用。通过实例,介绍了具体的试验及计算方法。     

The Influence of Strip Location on the Pressure Drop and Heat Transfer Performance of a Slotted Fin

In this article, a numerical study is conducted to predict the air-side heat transfer and pressure drop characteristics of slit fin-and-tube heat transfer surfaces. A three-dimensional steady laminar model is applied, and the heat conduction in the fins is also considered. Five types of slit fins, named slit 1, slit 2, slit 3, slit 4, and slit 5, are investigated, which have the same global geometry dimensions and the same numbers of strips on the fin surfaces. The only difference among the five slit fins lies in the strip arrangement. Slit 1 has all the strips located in the front part of the fin surface, then, following the order from slit 1 to slit 5, the strip number in the front part decreases and, correspondingly, the strip number in the rear part increases, so that all the strips of slit 5 are located in the rear part. Furthermore, slit 1 and slit 5, slit 2 and slit 4, have a symmetrical strip arrangement along the flow direction. The numerical results show that, following the order from slit 1 and slit 5, the heat transfer rate increases at first, reaching a maximum value at slit 3, which has the strip arrangement of “front coarse and rear dense”; after that, it begins to decrease, as does the fin efficiency. Although they have the symmetrical strip arrangement along the flow direction, slit 5 has 7% more Nusselt number than slit 1, and slit 4 also has 7% more Nusselt number than slit 2, which shows that strip arrangement in the rear part is more effective than that in the front part. Then the difference of heat transfer performance among five slit fins is analyzed from the viewpoint of thermal resistance, which shows that when the thermal resistances in the front and rear parts are nearly identical, the optimum enhanced heat transfer fin can be obtained. This quantitative rule, in conjunction with the previously published qualitative principle of “front sparse and rear dense,” can give both quantitative and qualitative guides to the design of efficient slotted fin surfaces. Finally, the influence of fin material on the performance of enhanced-heat-transfer fins is discussed.     

Wall‐Confined Flow and Heat Transfer Around a Square Cylinder at Low Reynolds and Hartmann Numbers

This article discusses the results obtained through a two‐dimensional numerical simulation following a finite volume approach on the forced convection heat transfer for the hydromagnetic flow around a square cylinder at low Reynolds and Hartmann numbers. The magnetohydrodynamic (MHD) flow of a viscous incompressible and electrically conducting fluid is assumed to take place in a rectangular channel subjected to externally imposed magnetic fields and the cylinder is fixed within the channel. The magnetic fields may be applied either along the streamwise or transverse directions. Simulations are performed for the range of kinetic Reynolds number 10 ≤ Re ≤ 60 with Hartmann number 0 ≤ Ha ≤ 15 and for different thermal Prandtl numbers, Pr = 0.02 (liquid metal), 0.71 (air), and 7 (water) for a blockage ratio β = 0.25. A steady flow can be expected for the above range of conditions. Besides the channel wall, the magnetic field imparts additional stability to the flow as a consequence of which the recirculation region behind the obstacle reduces with increasing magnetic field strength for a particular Re. The critical Hartmann numbers for the complete suppression of flow separation in the case of a transversely applied magnetic field are computed. The rate of heat transfer is found almost invariant at low Re whereas it increases moderately for higher Re with the applied magnetic field. The heat transfer increases in general with the Reynolds number for all Hartmann numbers. Finally, the influence of obstacle shape on the thermohydrodynamic quantities is noted. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(5): 459–475, 2014; Published online 3 October 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21091     

翅柱复合型冷却器表面传热性能的数值研究

提出一种用于油液冷却的新型翅柱复合型冷却器,并应用SIMPLE算法对于其二维简化模型在不同工况下的表面传热及流动阻力进行数值模拟。将模拟结果与试验和经验关联式相比较,吻合情况良好,表明此算法和所简化的模型是合理的。根据数值模拟结果研究这种传热表面的传热机理,分析翅柱位置及几何参数对流动与传热的影响,对该新型翅片的推广及优化设计有较大帮助。