International Journal of Thermal Sciences 160 (2021) 106655 Available online 15 October 2020 1290-0729/© 2020 Elsevier Masson SAS. All rights reserved. Constructal design of different ribs for thermo-fuid performance enhancement of a solar air heater (SAH) Ashok K. Barik a, * , Aurovinda Mohanty b , Jnana R. Senapati c , Mohamed M. Awad d a Mechanical Engineering Department, College of Engineering and Technology (CET) Bhubaneswar, Odisha, 751029, India b Mechanical Engineering Department, Veer Surendra Sai University of Technology (VSSUT) Burla, Sambalpur, 768018, Odisha, India c Mechanical Engineering Department, National Institute of Technology (NIT) Rourkela, Odisha, India d Mechanical Power Engineering Department, Faculty of Engineering, Mansoura University, Mansoura, 35516, Egypt A R T I C L E INFO Keywords: Constructal theory T-rib Solar air heater SST k  ωmodel ABSTRACT In this paper, the constructal theory has been applied to evolve six new types of ribs (T-rib, left arm inclined T- rib, right arm inclined T-rib, Y-rib, concave and convex T-ribs) for a solar air heater (SAH). The fuid fow and heat transfer characteristics of the SAH with these ribs have been numerically investigated by solving the mass, momentum, energy, and turbulence equations employing the fnite volume technique of ANSYS Fluent R16. The SST k  ωmodel has been used to solve the turbulence quantities, especially, the turbulent kinetic energy (k), and the specifc dissipation rate (ω). The performance of the SAH has been obtained by varying different prominienet parameters of importance, such as the pitch (20 mm–90 mm), stem height (3 mm–11 mm), the inclination angle of the rib (0 o -60 ◦ ) and Reynolds number (5000–18,000). The optimal lengths for the pitch (i.e., 80 mm) and for the stem height (i.e., 8 mm) have been found out for T-ribs. It has been observed that the Nusselt number for T- rib increases by 191% in a range of Reynolds number, 5000–18,000, and at the optimum pitch and stem height. It is also observed that the Nusselt number marginally increases with the inclination angle of the rib. The concave T-rib and convex T-ribs have been evolved from the basic T-rib. Among all the considered ribs, the T-rib is found to be the best heat transfer enhancer keeping the friction factor at a low value. We found a higher Nusselt number and a lower friction factor for the present SAH with the T-ribs, as compared to that of the SAHs considered by previous researchers. The thermo-hydraulic performance (THPP) of a SAH with present rib (i.e., T-rib or the concave T-rib) has been observed to be more than SAH with other shapes of ribs used by previous researchers. 1. Introduction The energy from fossil fuels pollutes the environment due to harmful emissions. These emissions lead to the global warming, the depletion of ozone layer. These effects may put the entire lives on this earth in danger in the future unless no serious effects are made to curb the environ- mental pollution to protect the ozone layer. Thus, alternate energy sources known as renewable energy will completely replace conven- tional energy in the coming years. These energy sources are non- polluting, and therefore, regarded as clean/green energy. The energy obtained from the sun is the major source of green solar energy that has been harnessed and stored for household and industrial applications. Solar energy is used for various practical applications, like heating of the space, drying of food products, curing of industrial products, room heating, and many more. One such device is called the solar air heater (SAH), in which the solar energy is used to heat the air. An abundant research [1–8] has been carried out by various re- searches across the world to improve the effciency of the solar air heater. One of the easiest ways to improve the effciency of SAHs is to install ribs of different geometrical shapes. These surface ribs typically alter the thermo-fuid dynamics due to the generation of secondary vortices as a result of fow separation and reattachment. Different ribs, such as the herringbone rib [9], twisted rib [10], transverse rib [11], broken transverse ribs [12], V-rib and multiple V-rib [13–20], hybrid rib [21], square rib [22], and inclined rib [23] have been used to augment the heat transfer rate in a SAH. Jin et al. [24] numerically studied thermo-fuid characteristics of a SAH with multiple V-ribs to obtain the optimum performance. They argued that the heat transfer enhancement was due to the generation of both primary and secondary vortices in the stream-wise direction as well as the inter-rib region. The effects of * Corresponding author. E-mail addresses: ashokbarik.mech@gmail.com (A.K. Barik), m_m_awad@mans.edu.eg (M.M. Awad). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: http://www.elsevier.com/locate/ijts https://doi.org/10.1016/j.ijthermalsci.2020.106655 Received 1 June 2020; Received in revised form 10 September 2020; Accepted 30 September 2020