Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts Numerical investigations on heat transfer characteristics of curved rectangular winglet placed in a channel Hemant Naik, S. Harikrishnan, Shaligram Tiwari ∗ Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, 600036, India ARTICLE INFO Keywords: Heat transfer enhancement Curved rectangular winglet vortex generator Nusselt number Secondary flow intensity Entropy generation Thermal performance factor ABSTRACT Flow and heat transfer characteristics of curved rectangular winglet vortex generators (RWVGs) are investigated numerically. Three-dimensional numerical computations have been carried out for flow through a channel with curved RWVG mounted on its bottom wall. Effect of curvature of RWVG having concave and convex shapes with respect to the flow facing surface, has been addressed in terms of arc angle which is varied in the range from 15° to 120° for fixed arc length. Effect of curvature in concave and convex RWVG on flow and heat transfer char- acteristics has been compared with that of plane RWVG. Temperature and flow field characteristics near the plate surface have been presented and discussed with the help of temperature contours and streamline plots. Enhancement in heat transfer and pressure loss are examined by using Nusselt number and friction factor re- spectively. Maximum enhancement in heat transfer is found to be 22% for concave shape RWVG having arc angle equal to 75° as compared to channel in absence of RWVGs. The mechanism of heat transfer enhancement is explored with the help of secondary flow intensity and field synergy principle. Thermodynamic performance of various RWVGs has been analysed by calculating entropy generation rate caused by heat transfer and friction. Finally, overall thermo-hydraulic performance of various curved RWVGs has been reported and compared with that of plane RWVG. 1. Introduction Limitation of energy sources has raised the need of energy saving in many industries that has formed most important area of research in recent times. Many industrial applications require efficient energy transfer by high performance heat exchange, such as refrigeration and air conditioning, electronic cooling, aerospace engineering, chemical engineering and automobile industry. In present thermal industries, energy saving can be achieved by enhancing heat transfer and reducing pumping power. In heat exchanger devices, thermal resistance of gas side is much higher than that of liquid side and contributes heavily to overall performance. Thus, enough scope exists towards improvement of thermal performance of these devices on gas side. Augmentation in heat transfer can be achieved by disruption in the growth of thermal boundary layer and enhanced fluid mixing that can be accomplished by producing secondary flow [1]. Vortex generator (VG) as ‘wing’ or ‘winglet’ mounted on fin surface is one of the effective methods for secondary flow generation. Biswas et al. [2] numerically and Valencia et al. [3] experimentally investigated and reported that use of VGs on the flat fin surfaces are beneficial in terms of augmen- tation of heat transfer with less penalty in pressure drop. Longitudinal vortices are generated by the VGs in the main flow direction that dis- rupt the growth of thermal boundary layer and enhance fluid mixing thereby higher heat transfer is achieved [4–6]. VGs are usually mounted as protrusions on a surface at an angle of attack (β ) with re- spect to the flow direction either as external devices or by punching out from the surface itself. Four basic configurations of VGs are widely known, such as delta wing [5,6], rectangular wing [5–7], delta winglet (DW) [5,6,8–10] and rectangular winglet (RW) [5,6,11,12]. Wings and winglets are distinguished based on attachment of edges of a VG to the plate. If the trailing edge of VG is attached to the plate, it is referred to as a ‘wing’ and if the chord length is attached it is named as a ‘winglet’. Heat transfer and flow field characteristics have been investigated experimentally by Fiebig [6] in a rectangular channel with four dif- ferent types of VGs (delta wing, rectangular wing, DW and RW). It has been reported that for the same heat transfer rate, pressure drop is lower for winglet type VGs as compared to wing-type VGs. Tiggelbeck et al. [5] performed flow and heat transfer experiments with these four different types of VGs and reported that for higher angles of attack ( > β 30 o ) and higher Reynolds number (Re > 3000), winglet type VGs perform better than wing type VGs. The mounting arrangement of VG in the form of winglet pair is identified as common flow up (CFU) or https://doi.org/10.1016/j.ijthermalsci.2018.03.028 Received 17 November 2017; Received in revised form 4 March 2018; Accepted 31 March 2018 ∗ Corresponding author. E-mail address: shaligt@iitm.ac.in (S. Tiwari). International Journal of Thermal Sciences 129 (2018) 489–503 1290-0729/ © 2018 Elsevier Masson SAS. All rights reserved. T