Recent research has shown that flat plate double-pass solar air heaters (DPSAH) exhibit higher thermal performance compared to conventional flat plate single-pass solar air heaters (SPSAH). However, scientific literature on design and performance evaluation of a curved DPSAH is scarce. In this paper, we systematically investigated various designs of DPSAH and reported its performance characteristics using a validated numerical model. Higher outlet air temperature by about 5 degrees C was observed when the DPSAH absorber plate is located at the mid of the insulating wall and transparent glass cover. Furthermore, putting asymmetric semi-circular roughened surfaces shows better performance than symmetric circular shapes as the reattachment of vortices with the absorber plate is more frequent in the former case. Two new correlations were developed for Nusselt number (Nu) and friction factor (f) as a function of Reynolds number (Re) and relative roughness height (d/H). The values of Nu and f obtained from the developed correlations agree well with data from the model. (C) 2020 Elsevier Ltd. All rights reserved.