Heat transfer and flow behaviors of a wavy corrugated tube

A new design of wavy corrugated tube (WCT) aiming to improve heat transfer by employing multi-longitudinal vortices was put forward. Effects of corrugation number N (N = 1, 2, 3 and 4), arrangement (parallel arrangement, counter arrangement, two-parallel and counter arrangement and five-parallel and counter arrangement), corrugation amplitude A (A = 1 mm, 3 mm, 5 mm and 7 mm), corrugation width W (W = 3 mm, 5 mm, 7 mm and 9 mm) and corrugation height H (H = 0.5 mm, 0.8 mm, 1.1 mm and 1.4 mm) on turbulent fluid flow and heat transfer behaviors were investigated numerically as well as overall thermal performance evaluation criterion (PEC) also being discussed. The operating Reynolds number (Re) ranged from 7500 to 20000 and the constant temperature condition was imposed on the tube wall. Local streamlines, velocity contours, turbulent kinetic energy contours, turbulent intensity contours, temperature contours, heat flux contours and variations of absolute vorticity versus axis distance were plotted to illustrate the heat transfer enhancement mechanisms. Equivalent temperature difference (ETD) stemming from the entransy dissipation extremum principle theory was used to evaluate heat transfer enhancement. It is found that the varying geometric parameters of the WCT play an important role in thermal-hydraulic characteristics. The main findings are that the WCTs exhibits superior heat transfer rate and PEC compared with the plain tube, due to improved flow mixing and enhanced heat flux by multi-longitudinal vortices induced by corrugations. The Nusselt number, friction factor and PEC in the WCTs with both parallel arrangement and counter arrangement increase generally with the rise in corrugation number until N = 3 and then decrease with the further increase of corrugation number from N = 3 to N = 4. The obtained results in ETD are matched with the heat transfer performance for WCTs with parallel arrangement which indicates that the heat transfer ability in the WCTs can be well described. Among the investigated different arrangements, it is found that the WCT with parallel arrangement has the maximum PEC. Furthermore, in results of corrugation amplitude variations versus thermal performance, the PEC in the WCT with A = 5 mm has the largest value. The PEC can be further enhanced by increasing corrugation width with the largest PEC obtained by the WCT with W = 9 mm. For varying corrugation height, the WCT with H = 1.1 mm has the best PEC at most of Re. Under operating conditions and geometric parameters considered, the WCT with A = 5 mm, N = 3, W = 9 mm, H = 1.4 mm and parallel arrangement at Re = 7500 offers the highest PEC value of about 1.56.