Experimental study of thermal performance and flow behaviour with winglet vortex generators in a circular tube

Abstract

Vortex generators (VGs) are among the various technologies that have been developed to enhance heat transfer and ultimately increase the efficiency of compact heat exchanging devices. VGs create longitudinal vortices which do not decay until further downstream and consequently increase heat transfer rate with comparatively lower pressure drop. In this research, thermal performance and flow behavior with VGs insert in a circular tube is investigated experimentally. The effects of attack angles, blockage ratios, pitch ratios and arrangements of VGs on the thermal performance were studied. Experimental measurements with and without VGs insert in a tube were conducted for airflows within Reynolds numbers ranging from 6000 to 33,000, with for a constant heat flux on the tube surface. Four Delta winglets VGs were inserted in a circular pattern on the inner surface of a tube with the help of a narrow rod. The different sets of delta winglets were characterized by four attack angles β (0°, 15°, 30° and 45°), three blockage ratios B (0.1, 0.2, 0.3), three different row values, N (4, 8 and 12) and three relative pitch ratios PR (4.8, 2.4 and 1.6).The experimental results indicate that the Nusselt number decreases with pitch ratio, but increases with Reynolds number, attack angle and blockage ratio. Nusselt number increment (Nu/Nu0) decreases with Reynolds number and pitch ratio, but increases with blockage ratio and attack angle. Maximum Nusselt number increment (Nu/Nu0) with the VGs was observed as being almost 2 times larger than that of smooth tube, while the maximum friction factor increment (f/f0) was 4.8 times larger. Thermal performance enhancement (TPE) decreases with blockage ratio, attack angle, Reynolds number and pitch ratio. The largest TPE obtained was 1.45. In order to understand the mechanism of heat transfer enhancement, details of the flow behavior were also studied with a flow visualization experiment employing a high speed camera and smoke generator.