Numerical study of heat transfer and flow behavior in a circular tube fitted with varying arrays of winglet vortex generators

Winglet vortex generators (WVGs) mounted inside a circular tube, generate longitudinal vortices and offer increased turbulence level with a comparatively lower pressure penalty for a higher heat exchange performance. In this study, the thermal enhancement and flow structure arising from radially-arrayed winglets mounted at different attack and inclination angles, as well as different winglet lengths, are analyzed by CFD simulation. The flow and heat transfer behaviors are presented in the turbulent flow region for air flow, with Reynolds number ranging from 6000 to 27,000. Vortex generators are arranged inside the tube as a series of four rings, with each ring having 4 WVGs on the inner surface of a circular tube. The present research investigates the characteristics of WVGs which include four winglet inclination angles (alpha = 0 degrees, 10 degrees, 20 degrees and 30 degrees), five winglet attack angle (beta = 0 degrees, 10 degrees, 20 degrees and 30 degrees and 45 degrees), and three winglet lengths (L = 10 mm, 15 mm, 20 mm). The data shows that winglets in a tube result in a considerable enhancement of Nusselt number and friction factor. It is found that Nusselt number and friction factor augment with the increase of attack angle or length, yet declines with the increase of inclination angle. The maximum of Nu is 86.88, at L = 20 mm, PR = 4.8, alpha = 0 degrees, and beta = 45 degrees. The largest Nusselt number ratio, Nu/Nu(0), defined as a ratio of augmented Nusselt number to the Nusselt number for fully developed smooth flow is 136%, which was obtained at a lower Reynolds number (Re = 6000). The results also illustrated that WVGs could generate longitudinal and transverse vortices to induce both impingement flow and recirculation zone leading to higher heat transfer and comparatively lower pressure drop. Nusselt number contours reveal that the wake zone behind WVGs displays the intensity of heat transfer along the tube.