Heat transfer augmentation in a circular tube with delta winglet vortex generator pairs

Winglet pairs are promising longitudinal vortex generators which can be used to produce streamwise vortices that do not decay until further downstream and consequently increase heat transfer rate with comparatively lower pressure penalty. This paper deals with the effect of delta winglet vortex generator (DWVG) pairs on thermal and flow behaviors in a circular tube for Reynolds numbers (Re) range of 5000–25000. The DWVG pairs involved are the pitch ratio (PR = 9.6), four attack angles (α = 10° 20° 30° and 40°), three winglet height (h = 5 mm, 7.5 mm and 10 mm) and three spacing between leading edges (s = 10 mm, 15 mm and 20 mm). The experimental results indicate that the Nusselt number (Nu) increases with Re while friction factor (f) decreases with Re. Nusselt number and friction factors both are increasing with attack angle and winglet height, while the middle spacing yields the highest Nu and f. Maximum Nusselt number increment (Nu/Nu ₀ ) with the DWVG pairs was observed as being 73% larger than that of smooth tube, while the maximum friction factor increment (f/f ₀ ) was 2.5 times larger. Thermal enhancement factor (TEF) decreases with Re. The largest TEF obtained, 1.44, is with the combination of α30°s15h7.5 at Re = 5000. Compared with other types of VGs in published experimental research papers, the current DWVG pairs show better thermal performance than many of them. Vortices downstream of the DWVG are visualized with smoke flow for better understanding of the flow behavior.