Numerical analysis of heat transfer enhancement and flow structure of alternating oval tubes by considering different alternate angles under turbulent flow

Abstract

<span>In this research, the convective heat transfers of turbulent water fluid flow in alternating oval tubes is studied using computational fluid dynamics. The purpose of the study is to analyze the heat transfer enhancement and secondary internal flows under different alternate angles. Also, comparing the effect of two schemes</span><span>for the domain discretization to be used in the solution variables' gradients on simulation results is investigated. The secondary flow causes an increase in the numbers of multi-longitudinal vortices (MLV) by changing the angle of pitches. These phenomena permit the cold fluid flow to stream in more paths from center to tube wall and better condition for mixing of fluids. Consequently, the heat transfer enhances by using the alternating oval tubes. However, forming the multi-longitudinal vortices causes an increase in pressure drop. Also, by raising the angle of pitches, the friction factor and the average of Nusselt number are amplified. It is also observed that the average heat transfer coefficient in the transition range is more than other areas. The mean Nussult numbers of this kind of tubes in the angles of 40</span><span>, 60</span><span>, 80</span><span>,</span><span> and 90</span><span> improved 7.77%, 14.6%, 16.93%, and 24.42%, respectively in comparison with the round tube. The performance evaluation criteria (<em>PEC</em>) for all alternating oval tubes under the constant inlet velocity boundary condition indicated that the highest value (<em>PEC</em>=1.09) had been obtained at the lowest Reynolds number (<em>Re</em>=10,000) in the alternating oval tube 90°.</span>