Numerical investigation of nanofluid flow characteristics and heat transfer inside a twisted tube with elliptic cross section

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.