Efficient harvesting of renewable energy such as solar, wind and wave is crucial at present times to meet the increasing demand for the energy in India. Solar air heaters (SAH) which convert solar energy into useful thermal energy for the industrial and agricultural purposes show lower efficiency, inherently due to the low thermal conductivity of the air which results in lesser heat transfer between absorber plate and air. In this paper, we investigate the effect of inclined rectangular fins on the fluid flow and heat transfer inside a rectangular duct SAH. The fins are arranged in such a way that they form a series of converging and diverging passages. The fin height is fixed at half of channel height. Two types of designs are considered: Type 1 (with a 2:1 convergence ratio) and Type 2 (with a 4:1 convergence ratio). CFD analysis is performed by using ANSYS software for both designs with 3, 4, 5, 6 and 7 rows of fins, at Reynolds numbers 4000, 8000, 12,000 and 16,000. The efficiency of the fin is quantified by using dimensionless parameter thermo-hydraulic performance parameter (THPP). The results show that the converging and diverging passages induce swirl in the fluid, which helps in enhancing heat transfer with little increase in friction loses. The swirl flow sustains longer for Type 2 design with higher convergence ratio, thus requiring lesser number of rows of fins compared with Type 1 for the better performance. Also, it is observed that overall the Type 2 arrangement gives higher THPP compared to the Type 1 arrangement with the highest THPP of 1.588 for 5 rows of fins at Reynolds number 8000.