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

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.