In the present study, turbulent flow and heat transfer inside a three-dimensional tube with elliptic and circular cross sections are simulated using finite volume method. The main aim of this study is to investigate the effects of ellipse aspect ratio and twisting of the tube wall on the flow characteristics and heat transfer. According to the obtained results, by converting the cross section from circular to elliptical and reducing the aspect ratio of elliptic tube with smooth wall, the friction factor and heat transfer increase. Investigating on the effect of wall twisting shows that, due to generation of secondary flow, the heat transfer and friction factor increase. By reducing the twist pitch, the generated vortices get merged into a large vortex and affect the flow across the whole cross section, causing the mixing to improve and heat transfer to increase. According to the obtained results, in the elliptical tube with an aspect ratio of 1.65, at twist pitches of 0.4 and 0.2, the heat transfer increases compared to a smooth tube with a similar cross section by 5% and 20%, respectively; and by a sudden reduction of twist pitch from 0.2 to 0.1 along the path, the heat transfer improves up to 30%. The effect of addition of multi-wall carbon nanotube nanoparticles to the base fluid inside the twisted elliptical tube with an aspect ratio of 1.65 on the heat transfer and flow characteristics is also analyzed, 28–32% in heat transfer for Reynolds number of 4000 and 16,000. According to the obtained results, the performance evaluation criterion (PEC) of nanofluid with low volume fraction and at lower Reynolds numbers is less than 1; however, for the flow of nanofluid with a volume fraction of 0.03, the heat transfer enhancement prevails over the increase in the pressure drop and the PEC is more than 1.

中文翻译：

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椭圆形扭曲管内纳米流体流动特性和传热的数值研究

在本研究中，使用有限体积方法模拟了具有椭圆形和圆形横截面的三维三维管内的湍流和热传递。这项研究的主要目的是研究椭圆纵横比和管壁扭曲对流动特性和传热的影响。根据获得的结果，通过将横截面从圆形转换为椭圆形并减小具有光滑壁的椭圆形管的长宽比，可以增加摩擦系数和传热。对壁扭曲的影响进行的研究表明，由于产生了二次流，传热和摩擦系数增加了。通过减小扭曲螺距，生成的涡流合并成一个大涡流，并影响整个横截面的流动，导致混合改善，传热增加。根据获得的结果，在长径比为1.65的椭圆管中，在扭曲节距为0.4和0.2的情况下，与具有类似横截面的光滑管相比，传热分别增加了5％和20％；并且通过沿路径将扭曲间距从0.2突然减小到0.1，传热率提高了30％。还分析了将多壁碳纳米管纳米颗粒以1.65的纵横比添加到扭曲椭圆管内部的基础流体中对传热和流动特性的影响，对于雷诺数为4000时，传热率为28-32％。 16,000 根据得到的结果，低雷诺数下低体积分数的纳米流体的性能评价标准（PEC）小于1。然而，