Effects of roughness elements on laminar flow and heat transfer in microchannels

A model of laminar flow and heat transfer in rough microchannels is developed and analyzed numerically to compare the effect of roughness elements on the thermal and hydrodynamic characteristics. In this model, the rough surfaces are configured with triangular, rectangular and semicircular roughness elements, respectively. Here, the effects of the Reynolds number, roughness height, and roughness element spacing on pressure drop and heat transfer in rough microchannels are all investigated and discussed. The results indicate that the global heat transfer performance is improved by the roughness elements at the expense of pressure head when compared to the smooth channel. Differing from the smooth microchannels, both the Poiseuille number and average Nusselt number of rough microchannels are no longer constant with Reynolds numbers and are larger than the classical value. Especially, the difference from the effects of three types of roughness elements is identified. With the increasing roughness height, the flow over surfaces with semicircular and triangular roughness elements induces stronger recirculation and flow separation. This contributes to heat transfer enhancement but also increases the pressure drop. However, the influence of the rectangular roughness element height is weaker than that of semicircular and triangular elements. In addition, the effects of the spacing of roughness elements on the Poiseuille number and average Nusselt number are in decreasing order for semicircular, triangular and rectangular roughness elements, respectively.     

Computational analysis of the flow of pseudoplastic power-law fluids in a microchannel plate

The flow of pseudoplastic power-law fluids with different flow indexes at a microchannel plate was studied using computational fluid dynamic simulation.The velocity distribution along the microchannel plate and especially in the microchannel slits,flow pattern along the outlet arc and the pressure drop through the whole of microchannel plate were investigated at different power-law flow indexes.The results showed that the velocity profile in the microchannel slits for low flow index fluids was similar to the plug flow and had uniform pattern.Also the power-law fluids with lower flow indexes had lower stagnation zones near the outlet of the microchannel plate.The pressure drop through the microchannel plate showed huge differences between the fluids.The most interesting result was that the pressure drops for power-law fluids were very smaller than that of Newtonian fluids.In addition,the heat transfer of the fluids through the microchannel with different channel numbers in a wide range of Reynolds number was investigated.For power-law fluid with flow index (n =0.4),the Nusselt number increases continuously as the number of channels increases.The results highlight the potential use of using pseudoplastic fluids in the microheat exchangers which can lower the pressure drop and increase the heat transfer efflciency.     

Thermal performance and entropy generation for nanofluid jet injection on a ribbed microchannel with oscillating heat flux: Investigation of the first and second laws of thermodynamics

In current numerical study, forced flow and heat transfer of water/NDG (Nitrogen-doped graphene) nanofluid in nanoparticles mass fractions (φ) of 0, 2% and 4% at Reynolds numbers (Re) of 10, 50, 100 and 150 are simulated in steady states. Studied geometry is a two-dimensional microchannel under the influence of nanofluid jet injection. Temperature of inlet fluid equals with T_c=293 K and hot source of microchannel is under the influence of oscillating heat flux. Also, in this research, the effect of the variations of attack angle of triangular rib (15°, 30°, 45° and 60°) on laminar nanofluid flow behavior inside the studied rectangular geometry with the ratio of L/H=28 and nanofluid jet injection is investigated. Obtained results indicate that the increase of Reynolds number, nanoparticles mass fraction and attack angle of rib leads to the increase of pressure drop. By increasing fluid viscosity, momentum depreciation of fluid in collusion with microchannel surfaces enhances. Also, the increase of attack angle of rib at higher Reynolds numbers has a great effect on this coefficient. At low Reynolds numbers, due to slow motion of fluid, variations of attack angle of rib, especially in angles of 30°, 45° and 60° are almost similar. By increasing fluid velocity, the effect of the variations of attack angle on pressure drop becomes significant and pressure drop figures act differently. In general, by using heat transfer enhancement methods in studied geometry, heat transfer increases almost 25%.     

Prediction of Nusselt Number and Friction Factor of Water-Al2O3 Nanofluid Flow in Shell-and-Tube Heat Exchanger with Helical Baffles

Heat transfer and flow field of water-Al₂O₃ nanofluid were simulated three-dimensionally in the shell-side of shell-and-tube heat exchanger with helical baffles. The effects of Reynolds number and volume fraction on heat transfer and pressure drop were evaluated. Increasing the volume fraction and Reynolds number intensified both heat transfer and pressure drop. Reduction of the Reynolds number increased the friction factor, but no considerable change was observed in the friction factor by increasing the volume fraction at constant Reynolds number. Heat transfer of the nanofluid revealed greater dependency on the volume fraction of particles at lower Reynolds numbers. Models of Nusselt number and friction factor were obtained in the heat exchanger in terms of Reynolds number and volume fraction using neural network. The neural network predicted the output variables with great accuracy.     

Turbulent heat transfer and pressure drop in plain tube bundles

The influence of turbulence intensity on heat transfer and pressure drop in tube bundles was measured in an open wind tunnel. The bundles consisted of one to three rows of plain tubes with in-line and staggered tube arrangements. The vertically arranged tubes were heated by saturated steam, condensing inside, and cooled outside with air in cross-flow. The turbulence intensity behind different grids varied in the air stream between 0.8% and 25%, and the air-side Reynolds numbers ranged from 1.5 × 10⁴ to 1.5 × 10⁵. The enhancement of the heat transfer coefficients for the bundles is due to an increase in the level of turbulence and also to a decreasing number of rows. The increase in the Nusselt number is about 42% for a single row and about 14% for a tube bundle with three rows.

From the pressure drop experiments it follows that over a wide range of Reynolds numbers the drag coefficient is almost independent of the inlet turbulence intensity.

In this paper the experimental results for single rows and for bundles with three rows are presented. The influence of the transverse and longitudinal pitch to diameter ratio on heat transfer and pressure drop is discussed.     

扭曲三叶管传热与流阻性能的数值研究

扭曲管换热器是一种新型高效换热器。在扭曲管强化传热机理研究的基础上,提出了一种新的扭曲管管型--三叶管。验证了标准k-ω湍流模型在圆管及扭曲椭圆管计算中的精确度,并采用该湍流模型对扭曲三叶管Re在4000～20000范围内的传热和流阻性能进行了研究。计算结果表明,扭曲三叶管的Nusselt数比扭曲椭圆管大,虽然压差增大较多但综合传热性能比扭曲椭圆管高。这是由于扭曲三叶管特殊的三叶区结构以及过渡区曲率的变化,使得三叶管内的螺旋流动比椭圆管更为复杂,速度场与温度梯度场的协同程度更好。随着Reynolds数的增大扭曲三叶管的压差及Nusselt数都逐渐增大,但综合性能逐渐降低,在低Reynolds数下扭曲三叶管的强化传热效果较为明显。内切圆直径及过渡圆弧直径越小,扭曲三叶管的综合性能越好,其中内切圆直径的影响更为显著。     

纵流式换热器的数值分析

利用有限体积法,计算了孔板开孔度分别为0.2,0.3和0.5的孔板支撑纵流式换热器,在雷诺数为5 000,10 000,15 000和20 000时的传热和压降情况,讨论了流体的雷诺数、支撑板的开孔度及开孔方式对传热系数和压降的影响。结果表明,在低雷诺数工况下选用较大的开孔度,换热器的综合性能更高;综合考虑换热和压降,对于不同的开孔度宜选用不同的开孔方式;研究结果对换热器的设计有参考价值。     

xperimental Study of Heat Transfer Enhancement and Friction Loss Induced by Inserted Rotor-assembled Strand （I） Water

The single-phase pressure drop and heat transfer in a rotor-assembled strand inserted tube were measured using water as the working fluid. Experiment using a smooth tube was carried out to calibrate the experimental system and the data reduction method. In the experiment, fixed mounts were used to eliminate the entrance effect. The experimental results of smooth tube show that employment of fixed mounts leads to a visible bias of friction factor at relative low Reynolds numbers, although it does not significantly affect the Nusselt numbers. The measured data of inserted tube reveal that rotor-assembled strand can significantly improve heat transfer with the Nusselt number increased by 101.6%-106.6% and the overall heat transfer coefficient increased by 58.1%-67.4% within the Reynolds number range of 20000 to 36000. Meanwhile, friction factor increases by 52.2%-84.2% within the same Reynolds number range. The correlations of Nusselt number and friction factor as function of the Reynolds number and Prandtl number were determined through multivariant linear normal regression.     

螺旋隔板三维翅片管传热实验研究与数值模拟

文章对冷却水在换热器管程流动并与壳程的热油逆流换热条件下,对螺旋隔板三维翅片管换热器的传热与压降性能进行了实验研究,并与光滑管进行了对比。在相同壳程Reynolds数下,三维翅片管的壳程Nusselt数是光滑管的2.2—2.9倍,而压降是光滑管的2.3倍左右。采用计算流体力学软件F luent 6.0对螺旋隔板三维翅片管和光滑管换热器进行了数值模拟。结果表明,螺旋流条件下光滑管表面速度矢量均匀、稳定,而三维翅片表面的速度矢量因翅片激发流体而产生湍动和不规则的二次流,从而强化了流体的对流传热。对于螺旋隔板三维翅片管换热器,壳程Nusselt数和压降的数值模拟结果与实验计算值吻合良好,最大偏差分别为6.3%和9.8%。     

叉排微柱群内顶部缝隙对传热效率的影响

以去离子水为工质,实验研究了直径为500 μm、高度为500 μm以叉排排列的微柱群内顶部缝隙对传热效率的影响规律。采用电加热棒进行加热,测量微柱群板Reynolds数在8～400之间在不同顶部缝隙时的进出口温度与流量,获得微柱群内流动阻力系数及Nusselt数,进而掌握微柱群内传热效率与Re关系。研究结果表明,在较低Re时,顶部缝隙对微柱群内流动阻力和Nu影响较小;随着Re增加,其对流动阻力与Nu的影响越来越显著。微柱群传热效率一开始随着Re增加而快速增加,随着Re进一步增加,其传热效率开始缓慢增加甚至出现下降。根据实验结果,尽管微柱群顶部缝隙存在降低了其内Nu,但提高了微柱群的传热效率。     

自旋扭带强化传热及旋转特性

旋转扭带是在固定式扭带基础上发展而来的, 因其特殊结构所以能在管内流体作用下产生自旋效果。本文对自旋式扭带旋转特性及强化传热特性进行研究。通过理论及实验研究管内自旋扭带旋转特性后得出:扭率越小, 扭带克服阻力起始旋转需要的流体速度越小;扭带转速与管内流体流速呈一次线性关系, 且扭带节距不变时线性比例基本保持不变。通过实验研究后得出:自旋扭带能达到很好的强化传热性能, 扭率越小其强化传热性能越明显, 同时阻力特性也越明显, 在雷诺数为4×10³～4×10⁴、扭率为3～8时, 换热管内摩擦因子增至1.7～3.5倍, 努赛尔数增幅为10%～37%。本文使用评估指标η对扭带进行综合评价, 得出扭率为7的自旋扭带具有最佳的综合性能。并分别拟合出摩擦因子及努赛尔数与雷诺数、扭率之间的关联式, 提出一种工程上自旋扭带选型方法。     

仿蜂巢分形微管道网络中的流动与换热

受自然界中蜂巢结构分形特征的启发,设计和加工了仿蜂巢分形微管道网络,并进行了参数优化.在微管道截面参数、对流传热系数、传热温差均相同的条件下,对流动与换热特性的理论分析表明：加热底面积相同时,仿蜂巢分形微管道网络所能带走的热量可达平行阵列微管道网络的5倍以上;不计分流、合流效应,总换热量一定时,仿蜂巢分形微管道网络所需的泵送功率约为传统平行阵列微管道网络的1/10.恒定热流条件下的去离子水层流对流换热实验也证明：仿蜂巢分形微管道网络比传统的平行阵列微管道网络具有更高的Nusselt数和更低的流动压降.这种分形微管道网络除用于电子器件冷却,还可用于微燃料电池极板、微混合器、微生化反应器等微化工系统结构设计.     

内置螺旋弹簧换热管内流动与传热三维数值模拟

为研究内置螺旋弹簧换热管单管强化传热原理,采用Fluent软件对内置螺旋弹簧换热管内流体流动与传热特性进行数值模拟,考察了弹簧的应用对管内流场、压降和换热性能的影响,并分别取螺旋弹簧节距p分别为2 mm、4 mm、5 mm初步研究了弹簧的节距对强化传热效果的影响。模拟结果显示：弹簧管内流体呈螺旋流动状态,管壁附近流体切向速度和径向速度有一定程度的提高,从而加剧了管内流体的混合及边界层的扰动,充分换热,弹簧管进出口温度差较光管有所增加,最高增加了0.9 ℃;相同雷诺数条件下,内置螺旋弹簧换热管Nu数均高于光管,而压降和阻力系数相比光管有明显增加,随着弹簧节距减小换热增强而摩擦阻力系数增加。     

Influence of magnetic field on jet impingement heat transfer with molten salt

Experimental study of jet impingement heat transfer with molten salt under the influence of external constant magnetic field was generated by permanent magnets. Both stagnation correlation and radial distribution of Nusselt number under magnetic field were obtained. The results showed that the Nusselt number with magnetic field became higher than that without magnetic field at stagnation region and jet impingement heat transfer was comparatively enhanced, while in wall jet region, the enhancement of heat transfer was gradually weakened. In addition, when the Reynolds number was constant, the Nusselt number of molten salt increased with increasing of the intensity of magnetic field, and the most enhanced heat transfer existed at the stagnation point. Under the conditions of Reynolds number Re=6400 and the intensity of magnetic field B=2800 Gs, the stagnation Nusselt number of molten salt was about 6 % higher than that without magnetic field. It can be seen that the magnetic field may promote the jet impingement heat transfer of molten salt.     

磁场对熔盐射流冲击传热的影响

以永磁铁构建定磁场,进行外加磁场作用下熔盐射流冲击传热的实验研究,并得到Nusselt数Nu驻点关联式和径向分布。结果表明,在驻点区范围内,Nusselt数较无磁场作用时增大,传热得到比较明显的增强,而在壁面射流区,这种强化传热效果逐渐减弱。此外,当Reynolds数Re一定时,熔盐Nusselt数随着磁场强度的增加而增大,且驻点处强化传热效果最为显著。在Reynolds数Re=6400与磁场强度B=2800 Gs条件下,熔盐驻点Nusselt数Nu₀提高约6％,可见磁场作用对熔盐射流冲击传热具有一定的强化效果。     

扭曲管强化传热性能实验研究

在设计的蒸汽-水换热实验台上测试了5种不同导程的椭圆扭曲管和圆管的传热和流动阻力性能.分别对5种不同导程的扭曲管和圆管管内流体努赛尔数,摩擦系数和管内综合评价因子随雷诺数变化情况进行了对比.实验结果表明:扭曲管的强化传热性能明显,且导程越小,强化传热效果越好.在实验测试的雷诺数范围内,扭曲管的努塞尔数为光滑圆管的1.0...     

空气横掠波纹管束的流动与传热性能

引　言管壳式换热器是石油化工行业应用十分广泛的换热设备 ,它具有易于制造、耐压高等特点 .但是普通管壳式换热器换热效率低 ,因此开发高效传热元件对管壳式换热器的进一步推广应用具有十分重要的意义 .采用翅片管换热器可以大大强化换热 ,但是阻力也增加很多 ,并且容易积垢 ,影响使用 .近年来开发的波纹管换热器由于加工方便、易于制造而越来越受到人们的重视[1～ 4 ] .但对于流体横掠波纹管束时流动传热性能的研究却少见报道 .本文对如图 1所示的波纹管束组成的换热器进行了实验测定 ,研究了 3种排数的阻力与换热规律 ,为设计换热器提供…     

纳米流体在芯片微通道中的流动与换热特性

对去离子水及体积分数分别为0.15%和0.26%的水基γ-Al₂O₃纳米流体在当量直径为194.5 μm的硅基梯形芯片微通道内的层流流动和换热特性进行了实验研究。考察了Reynolds数、Prandtl数以及体积分数对流动换热的影响。结果发现,使用纳米流体后,压降无明显增加,纳米流体的流动阻力特性与去离子水基本相同;对流换热Nusselt数较去离子水有明显提高,且随着体积分数的增加而增加;相同泵功下换热热阻显著下降。实验还发现纳米流体的强化传热效果在较高温度时更加明显。根据实验数据得到了梯形硅微通道内低浓度纳米流体的层流对流换热关联式。研究结果对于集成高效芯片散热系统设计具有重要意义。     

三种内翅片管管内流动与换热特性

在恒热流边界条件下,对以空气为工质的分别带有3种不同纵向内翅片换热管的流动与换热特性进行了实验研究,实验管带有堵塞芯管.3种不同的纵向内翅片为：平直翅片、带有突起的翅片以及波纹形翅片.得出了所测参数范围内的换热与阻力的实验关系式.采用两种不同的标准对所测3根换热管的综合换热性能进行了评价,结果表明,带有波纹形内翅片的换热管在两种评价标准下均表现出了优异的综合换热性能.     

An empirical study on vortex-generator insert fitted in tubular heat exchangers with dilute Cu–water nanofluid flow

The heat transfer enhancement(HTE) in tubular heat exchangers fitted with vortex-generator(VG) inserts is experimentally investigated. The studied four parameters and ranges are: winglets-pitch ratio(1.33, 2.67, and 4),winglets-length ratio(0.33, 0.67, and 1), winglets-width ratio(0.2, 0.4, and 0.6), and Reynolds number(5200to 12200). The testing fluids are the water and Cu–water nanofluid at the volumetric fraction of 0.2%. The results obtained on HTE, pressure drop, and performance evaluation criterion(PEC) are compared with those for water in a smooth tube. It is found that the VG inserts with lower winglets-pitch ratio and higher winglets-length/width ratios present higher values of HTE and pressure drop. Over the range studied, the maximum PEC of 1.83 is detected with the Cu–water nanofluid inside the tube equipped with a VG insert at the winglets-width ratio of0.6 for the maximum Reynolds number, when the heat transfer rate and pressure drop are 1.24 times and 2.03 times of those in the smooth tube. Generalized regression equations of the Nusselt number, friction factor, and PEC are presented for the tubular heat exchangers with the VG inserts for both water and Cu–water nanofluid.It is concluded that the main advantage of the VG inserts is their simple fabrication and considerable performance, particularly at higher Reynolds number.