Numerical study of winglets vortex generator effects on thermal performance in a circular pipe Y. Xu, M.D. Islam * , N. Kharoua Department of Mechanical Engineering, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates article info Article history: Received 28 April 2016 Received in revised form 24 October 2016 Accepted 25 October 2016 Keywords: Heat transfer enhancement Heat exchanger Circular pipe Winglets vortex generator abstract Various technologies have been developed to enhance heat transfer and, ultimately, to develop more efficient compact heat exchanging devices. In this research, heat transfer and wall friction in a pipe, with vortex generators (VGs) insert, are numerically investigated. The effects of different attack angles and blockage ratios of VGs fitted inside a smooth pipe are investigated. CFD simulations, with and without VGs insert, were conducted for an air flow with Reynolds numbers in the range 6000e33000 and for a constant heat flux on the pipe model surface. Four VGs are fitted in a circular pattern on the inner surface of the pipe. The different sets of delta winglets were characterized by four attack angles b (0  , 15  , 30  and 45  ) and three blockage ratios B (0.1, 0.2 and 0.3). The Nusselt number and friction coefficient results show the influence of the VGs insert on thermal performance. The results indicate that the best set of parameters for thermal performance enhancement (TPE) is b ¼ 30  and B ¼ 0.1. The internal flow behavior and the distribution of the Nusselt number, friction coefficient and turbulent kinetic energy, were explored to explain the effects of the VGs, with different configurations, on the overall Nusselt number, friction coefficient and thermal performance enhancement. The extent of the turbulent wake, in the axial direction, was estimated to explain the different performance of the configurations considered. © 2016 Elsevier Masson SAS. All rights reserved. 1. Introduction Vortex generator refers to the components that can generate vortices to enhance the heat transfer performance of heat ex- changers, especially those of compact type. Thus, the size of the heat exchanger can be reduced and more energy can be saved. The effect of turbulence promoters/VGs on thermal performance has been studied extensively. There are two types of mechanisms for the enhancement of thermal performance by vortex generators. One, is active heat transfer enhancement using mechanical force to generate vortices. Another type, is passive heat transfer enhance- ment which does not rely on mechanical force but on the fluid flow itself with some roughness structure to generate vortices [1]. There are mainly two kinds of vortices widely studied including longi- tudinal vortex with the axis that is parallel to the main flow stream and transverse vortex which axis is perpendicular to the main flow. As longitudinal vortex can last for a long distance far down the VGs, it leads to a more thermal performance enhancement in conjunction with the same pressure drop. Consequently, longitu- dinal VGs are more efficient. In research, considering the conve- nience of operation and lowering the cost, only one circular pipe or rectangular duct is considered instead of a whole heat exchanger. Pipe and rectangular ducts are two usual configurations considered when investigating vortex generators either experimentally or numerically. Twisted or helical insert tapes, wire coils, turbulence rings and conical vortex generators are usually employed to in- crease the heat transfer performance of heat exchangers with cir- cular pipes. While, ribs, wings, winglets and baffles are mainly employed to increase the heat transfer performance of heat ex- changers with ducts, channels or fins. As vortex generators are widely used in compact heat ex- changers, laser curtain seals, mixing biofuel [2], cooling of electrical devices, aero-engines and gas turbines [3,4], solar collectors, the flow structures and thermal performance characteristics in duct flow have been studied extensively for many years. However, in these practical applications, the air was used instead of liquids as the heat transfer flow medium [5]. The thermal performance of VGs with different shapes, heights (blockage ratios), distances along flow direction (pitch ratios), attack angles, inclination angles, and arrangements were studied * Corresponding author. E-mail address: dislam@pi.ac.ae (M.D. Islam). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts http://dx.doi.org/10.1016/j.ijthermalsci.2016.10.015 1290-0729/© 2016 Elsevier Masson SAS. All rights reserved. International Journal of Thermal Sciences 112 (2017) 304e317