波纹管内流动与传热规律的数值计算

采用三维层流及低雷诺数湍流模型对波纹管内流动与传热性能进行了数值模拟,模拟结果与试验结果吻合良好.通过数值计算拓宽了波纹管流动与传热关联式的参数范围,发现在较大雷诺数(RP)范围内波纹管阻力系数随Re的变化趋势表现为指数规律.考察了不同波纹高度、波纹间距对流动与传热的影响,并对模型参数进行了综合性能评价,结果表明:波纹高度对波纹管内流动与传热的影响较波纹间距更显著;波纹管结构的强化传热性能只有在高Re条件下才得以体现,Re越大,波纹管综合性能因子也越高.通过数值计算得到了波纹管流动与传热的最优结构参数及最佳传热雷诺数范围.     

低Re下板式换热器性能的实验研究及热力学分析

实验测定了两种不同板式换热器在低Re条件下(200相似文献   

管间距对开缝翅片管换热器传热与阻力特性的影响

为了获得管间距对开缝翅片管换热器传热与阻力特性的影响规律,对5种不同翅片管换热器进行了数值模拟研究,并进行了模化试验验证。结果表明:开缝翅片管束的传热和阻力特性与翅片侧气体的Re数有关,随着Re数增大,翅片侧Nu数增大,摩擦因子f逐渐减小;纵向间距S2对开缝翅片管换热器的综合流动传热性能的影响较大。数值模拟与试验结果偏差较小,采用数值模拟方法能够比较准确地分析开缝翅片管换热器的传热与阻力特性。     

人字形波纹板传热与阻力特性的数值模拟和试验研究

对板式低温多效海水淡化技术中的人字形波纹板式换热器进行了单相流换热试验和数值模拟,试验结果表明其换热系数较高,在低Re数下就能达到3000 W/(m~2·K)以上。通过建立三维模型,运用CFD软件对人字形波纹板内部的传热及阻力特性进行数值模拟,研究了波纹倾角β、波高h和波纹间距λ对人字板传热以及流动阻力的影响,模拟结果与试验值误差都在15%以内。数值模拟结果表明,波纹倾角从30°增大到60°,传热因子j约提高60%,继续增大波纹倾角,传热因子反而降低,摩擦因子f随着波纹倾角的增大而增大;波高从3 mm增大到6 mm,传热因子j约提高5%,但摩擦因子f增大1倍;波纹间距从5 mm增大到20 mm,传热因子j约降低30%,但同时摩擦因子f降低70%。建议板式低温多效海水淡化系统的板型参数:波纹倾角60°,波高3~4 mm,波纹间距15~20 mm。     

波纹翅片管换热器空气侧传热与流动特性的数值研究

应用三维数值模拟的方法对1～5排波纹翅片管换热器空气侧传热与流动特性进行了研究,并通过实验验证了模型的可靠性。得出了不同Re数下管排数、管径尺寸与换热系数及空气侧压降的关系。利用场协同原理对不同Re数下空气侧换热特性进行了分析,即从流场和温度场相互配合的角度审视对流换热机制。     

水平方管底面加热熔盐混合对流模拟研究

利用数值模拟方法研究熔融盐在底面加热水平方管内的混合对流传热过程,分析熔融盐混合对流的流动和传热特性等。得到了沿流动方向各横截面处的温度、速度分布图及流线图,并对传热特性以及Nu数的变化规律进行探讨。结果表明,在高热流密度情况下非均匀加热壁面时,浮升力效应使主流核心区的形状随流动距离的增加而发生变化;在通道横截面上形成对称的二次涡流;局部Nux数在相同热流密度下随Re数的增加而增大;在Re数相同数时Nux数会随着Ri数的增加而增加;数值模拟点与湍流混合对流传热关联式的吻合度较高,其偏差在±15%以内。     

开缝布置方式对开缝翅片管换热器 传热与阻力特性的影响

为了获得开缝布置方式对开缝翅片管换热器传热与阻力特性的影响规律,对5种不同翅片管换热器进行了数值模拟研究,并进行了模化试验验证。结果表明：增加开缝会提高翅片管换热器的传热性能,但阻力也随之增加;与开缝位置相比,开缝数量对开缝翅片管换热器传热与阻力特性的影响更大;在Re=4800~7500日时,开缝翅片管换热器综合流动传热性能 随着Re数的增大而增大;在5种翅片中,开缝翅片的综合流动传热性能高于普通平直翅片;数值模拟与试验结果偏差较小,采用数值模拟方法能够比较准确地分析开缝翅片管换热器的传热与阻力特性。     

等温热源微通道单相液体层流换热特性

对Dh=0.82mm的矩形微通道阵列内等温热源作用下层流传热特性进行了实验和数值模拟。实验中使用常温自来水提供等温热源。微通道流体流动雷诺数Re=100～900,传热温差50K,将所得数据与常规尺度均匀壁温加热下N-S方程的数值解法结果进行对比。结果表明,在Re300时,Nu数随着Re数的增加而增加;而在Re350时,实验所得Nu数近似为常数。将发展入口条件的数值模拟结果与实验结果比较,前者比后者高7.2%。     

水平外波纹管降膜管间流型转变

相比于光管,外波纹管的壁面凹凸结构可增加换热面积,从而提高水平管降膜蒸发器的传热特性。准确预测溶液管间流型是提高传热传质性能的关键步骤。建立了水平管降膜实验装置,研究管间距、溶液浓度、管型对降膜流动过程管间流型转变的影响。结果表明:随着雷诺数(Re)的增加,管间流动形态依次出现滴状流、滴柱状流、柱状流和柱片状流和片状流;对于每一种流型转变所对应的Re,外波纹管明显低于光管,并随管间距的增大而增大,随NaCl溶液质量分数的增大而减小;相较于光管,两种外波纹管更易在较低Re下获得各稳定流型。流型转变边界可以通过流体Re与伽利略数(Ga)之间的函数关系式描述。通过拟合实验数据,得到了3种管的流型转变关系式。     

Cu-H_2O纳米流体流动与强化换热的数值模拟研究

模拟纳米流体在三维管道中的流动和强化传热过程,运用数值计算方法研究纳米流体的流动特性和传热机理,探究不同纳米颗粒体积分数和不同纳米颗粒大小在不同雷诺数(Re)下对纳米流体的流动和传热特性的影响。基于DPM模型对纳米流体在圆管中的对流换热进行了数值模拟研究,研究结果表明,在一定范围内,每增加0.5%的体积分数,纳米流体的传热性能平均增强7.82%。随着纳米颗粒的减小,纳米流体的传热系数不断增加。     

两种波纹通道内周期性充分发展对流换热的数值研究

应用数值方法研究了流体在正弦型和三角型两种波纹通道内周期性充分发展的层流流动与换热特性,分析了恒壁温条件下,雷诺数Re对流动与换热性能的影响,并对两种通道的摩擦阻力系数厂与努谢尔特数Nu分别进行了比较。结果表明：两种通道流动阻力的相对大小因雷诺数Re的大小而异;正弦型波纹通道的换热速率比三角型通道的换热速率大,因此换热器通道表面的选择应综合考虑。     

The effects of geometrical parameters of a corrugated channel with in out-of-phase arrangement

Numerical investigations of forced turbulent convective flow and heat transfer in a corrugated channel of plate heat exchanger are carried out. The continuity, momentum and energy equations were solved by means of a finite volume method (FVM). The top and bottom walls of the corrugated channel are heated at constant heat flux boundary conditions. The effects of geometrical parameters of corrugated tilt angles, channel heights and wavy heights using water as a working fluid on the thermal and flow fields as well as on the performance of evaluation criterion are studied. The corrugated channel with three different corrugated tilt angles of 20°, 40° and 60° with different channel heights of 12.5, 15 and 17.5 mm and different wavy heights of 2.5, 3.5 and 4.5 mm are tested. This investigation covers Reynolds number and heat flux in the range of 8000–20,000 and 0.4–6 kW/m², respectively. The numerical results indicate that the wavy angle of 60° and wavy height of 2.5 mm with channel height of 17.5 mm are the optimum parameters and they have a significant effect on the heat transfer enhancement. It is found that using wavy channel is a suitable method to increase the thermal performance and getting higher compactness of the heat exchanger.     

Experimental study on heat transfer enhancement of wavy finned flat tube with longitudinal vortex generators

The performance of direct air-cooled condensers in power plant is affected significantly by air-side flow and heat transfer characteristics of the wavy finned flat tube. Experimental investigations were conducted for air-side flow and heat transfer with and without delta winglet pairs punched on the surface of the wavy fin. The different temperature fields of the heated wavy fin surface with and without delta winglet pairs were obtained by the infrared thermography technology. Both experiments and numerical simulations showed that a substantial increase in the heat transfer with six delta winglet pair generators on the wavy fin was obtained with the Reynolds number varying from 1500 to 4500, which was the range of the air flow in practical direct air-cooled condensers. The average Nusselt number increased by 21–60% with the Reynolds number varying from 1500 to 4500 and the average friction factor increased by 13–83% with the Reynolds number varying from 500 to 4500 in experiments. The average performance evaluation criteria, PEC, can be up to 1.31 with six delta winglet pairs punched on the wavy fin surface, indicating the high potential of heat transfer enhancement to direct air-cooled condensers by longitudinal vortex generators.     

Experimental Study on Heat Transfer and Pressure Drop Characteristics of Four Types of Plate Fin－and－TUbe Heat Exchanger Surfaces

ＥｘｐｅｒｉｍｅｎｔａｌＳｔｕｄｙｏｎＨｅａｔＴｒａｎｓｆｅｒａｎｄＰｒｅｓｓｕｒｅＤｒｏｐＣｈａｒａｃｔｅｒｉｓｔｉｃｓｏｆＦｏｕｒＴｙｐｅｓｏｆＰｌａｔｅＦｉｎ－ａｎｄ－ＴＵｂｅＨｅａｔＥｘｃｈａｎｇｅｒＳｕｒｆａｃｅｓ￥Ｈ．Ｊ．Ｋａｎｇ；Ｗ．Ｌ...     

Heat and mass transfer analysis of a wavy fin-and-tube heat exchanger under fully and partially wet surface conditions

In this study a mathematical model of heat and mass transfer performance of a wavy fin-and-tube heat exchanger under wet surface condition is presented. The heat exchanger is a counterflow heat exchanger in which humid air and liquid are flowing in opposite direction. A water film that causes evaporative cooling of the humid air is circulated on the humid air side. The heat and mass transfer equations are first derived for fully wet heat exchanger and then by defining a wettability parameter, these equations are obtained for partially wet heat exchanger. In modeling, values of Lewis number and wettability parameter are not necessarily specified as unity. The temperature distributions of humid air, liquid and water film, and relative humidity distribution of humid air are obtained numerically. The theoretical results are found to be in good agreement with the available experimental measurements.     

Numerical simulation of heat transfer enhancement in wavy microchannel heat sink

In this paper, heat transfer and water flow characteristics in wavy microchannel heat sink (WMCHS) with rectangular cross-section with various wavy amplitudes ranged from 125 to 500 μm is numerically investigated. This investigation covers Reynolds number in the range of 100 to 1000. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using the finite-volume method (FVM). The water flow field and heat transfer phenomena inside the heated wavy microchannels is simulated and the results are compared with the straight microchannels. The effect of using a wavy flow channel on the MCHS thermal performance, the pressure drop, the friction factor, and wall shear stress is reported in this article. It is found that the heat transfer performance of the wavy microchannels is much better than the straight microchannels with the same cross-section. The pressure drop penalty of the wavy microchannels is much smaller than the heat transfer enhancement achievement. Both friction factor and wall shear stress are increased proportionally as the amplitude of wavy microchannels increased.     

Heat transfer enhancement and pressure drop for fin-and-tube compact heat exchangers with wavy rectangular winglet-type vortex generators

In the current work, heat transfer enhancement and pressure loss penalty for fin-and-tube compact heat exchangers with the wavy-up and wavy-down rectangular winglets as special forms of winglet are numerically investigated in a relatively low Reynolds number flow. The rectangular winglets were used with a particular wavy form for the purpose of enhancement of air side heat transfer performance of fin-and-tube compact heat exchangers. The effect of Reynolds numbers from 400 to 800 and angle of attack of 30° of wavy rectangular winglets are also examined. The effects of using the wavy rectangular winglet, conventional rectangular winglet configuration and without winglet as baseline configuration, on the heat transfer characteristics and flow structure are studied and analyzed in detail for the inline tube arrangements. The results showed that the wavy rectangular winglet can significantly improve the heat transfer performance of the fin-and-tube compact heat exchangers with a moderate pressure loss penalty. In addition, the numerical results have shown that the wavy winglet cases have significant effect on the heat transfer performance and also, this augmentation is more important for the case of the wavy-up rectangular winglet configuration.     

An investigation of the performance of compact heat exchanger for latent heat recovery from exhaust flue gases

An experimental and theoretical investigation of the utilization of finned tube compact heat exchanger for a heat recovery steam generator (HRSG) to recover both sensible and latent heat is presented in this paper. The heat transfer and pressure drop characteristics of the fin-and-tube heat exchanger are theoretically studied. A correlation of the combined convection–condensation heat transfer is derived by using the heat and mass transfer analogy models. The experimental results have shown that the Colburn factor (j) and the friction factor (f) for humid air, simulating the exhaust of HRSG, are larger than those for dry air. It has been also found that the f factor difference between humid air and dry air decreases as the air side Reynolds number increases and both the f factor and the j factor for humid air increase with the increase of water vapour concentration. A scheme for the design of compact heat exchanger for HRSGs is presented.     

Laminar Forced Flow and Heat Transfer in Cross-Corrugated Triangular Ducts

The fluid flow and heat transfer characteristics in a cross-corrugated triangular channel are studied under laminar forced flow and uniform wall temperature conditions. Both the local and the periodic mean values of friction factor and wall Nusselt numbers in the hydro and thermally developing entrance region are investigated. It is found that at higher Reynolds numbers, recirculations in the lower wall valleys are a dominant factor for flow and heat transfer, while at lower Reynolds numbers, parallel flows in the upper wall corrugation are the predominant factor. Compared with a parallel flat plates duct, the Nusselt numbers in a cross-corrugated triangular duct can be enhanced, and can be even higher at higher Reynolds numbers. The growth of steady recirculations and the concomitant periodic disruption and thinning of the boundary layer promote enhanced transport of heat as well as momentum. Effects of heat transfer enhancement are more obvious under higher Reynolds numbers. Two correlations are proposed to predict the periodic mean values of Nusselt numbers and friction factors for Reynolds numbers from 10 to 2000.     

Fully-Developed Flow in a Furrowed Wavy Channel: Characterization of Unsteady Flow Regimes and Its Effect on Thermal-Hydraulic Performance

Numerical study is done at various Reynolds numbers (100–2000), for periodically fully developed flow and heat transfer in five geometrically different wavy channels. Time signal analysis is done to distinguish various unsteady flow regimes and a flow regime map is proposed; demarcating steady, two types of periodic as well as quasi-periodic, and chaotic flow regimes. Temporal variation of flow structure and Nusselt number as well as friction factor are presented – in the various unsteady regimes – and discussed for a unified cause-and-effect study. Effect of different flow regimes on the thermal-hydraulic performance of wavy as compared to plane channel is presented. Effect of Prandtl numbers (0.01–100) is also studied.