**Abstract:**
A numerical investigation of fluid flows through a solar air heater with semi-circular sectioned transverse rib roughness has been executed. The physical problem is represented mathematically by a set of governing equations, and the transport equations are solved using the finite element method. The effect of relative roughness pitch on average Nusselt number, average friction factor, and thermohydraulic performance parameter (THPP) has been investigated. This investigation is taken for relative roughness pitch of 7.5 and relevant Reynolds numbers in the range of 3800 = Re = 18,000. The maximum thermohydraulic performance parameter is 1.616 and it is occurred for the relative roughness height of 0.06 and relative roughness pitch of 7.5.

**Keywords:**
Solar air heater, Solar energy, CFD, THPP, Nu, Re.