Thermal performance and flow behavior of winglets vortex generators in a circular tube

Abstract

Various technologies have been developed to enhance heat transfer and ultimately to develop more efficient compact heat exchanging devices. Vortex generators/turbulence promoters generate vortices which reduce boundary layer thickness and yield a better mixing of the fluid to enhance heat transfer. Vortex generators (VGs) create longitudinal vortices which do not decay until further downstream and consequently increase heat transfer coefficient with comparatively lower pressure drop in this region. In this research, thermal performance of winglets longitudinal VGs is investigated experimentally and numerically. Delta winglet VGs of different heights, attack angles and arrangements are inserted inside a pipe of inner diameter D = 52 mm. Research with and without VGs is conducted for air flow within a range of Reynolds numbers 6000 to 33000 and a constant heat flux of 694 W/m 2 on the tube surface. The influence of the winglet VGs on thermal performance and pressure dissipation is investigated through Nusselt number and friction factor. The objectives of the current research are to investigate the thermal performance enhancement compared with smooth tube and identify the parameter which induces the highest thermal performance augmentation in the circular tube. In order to understand the mechanism of heat transfer enhancements, details of the flow behavior are also investigated with the help of flow visualization experiment by employing high speed camera and smoke generator. The experimental results indicate that Nu decreases with pitch ratio but increases with Re, attack angle, and blockage ratio. Nusselt number increment (Nu/Nu0) decreases with Re and pitch ratio but increases with blockage ratio and attack angle. The maximum Nusselt number achieved is 133.40 but the maximum Nusselt number increment (Nu/Nu0) is 1.98. The maximum friction factor is 0.13 while the maximum friction factor increment ( f/f0) is 4.88. Thermal performance enhancement (TPE) decreases with blockage ratio, attack angle, Re and pitch ratio, the largest TPE is 1.45. Hence, blockage ratio = 0.1 with pitch ratio = 2.4, attack angle = 0° and Re = 6000 is the best parameter for TPE. The simulation results indicate that the maximum Nusselt number is 118.59 but the maximum Nusselt number increment (Nu/Nu0) is 1.75. The maximum friction factor is 0.11 while the maximum friction factor increment (f/f0) is 3.2. The maximum TPE is 1.31. The best set of parameters for TPE is β = 30°, B = 0.1, and Re = 6000. Keywords: Heat transfer enhancement, heat exchanger, circular tube, winglets vortex generator, flow visualization, horseshoe vortex, longitudinal vortex.

Date of Award	2016
Original language	American English