This paper reports the computational results of fluid flow and thermal characteristics in an isosceles trapezoidal solar air heater (SAH). By varying the base angle of the trapezoidal duct from 90 deg to 45 deg, six different models of solar air heater ducts are obtained. The six geometries of SAH ducts have cross sections of rectangular (90 deg), triangular (45 deg), and four isosceles trapezoidal (having base angles of 50 deg, 60 deg, 70 deg, and 80 deg) shapes. The solar radiation absorber plate width and the duct heights are maintained constant for all six models of SAH, i.e., 160 mm and 80 mm, respectively. The SAH is subjected to a constant and uniform heat flux value of 1000 W/m2 and Reynolds numbers varied from 5000 to 28,000. In all six cases, the size of the air heater is the same and having dimensions of 0.16 m width and 1 m length. For this investigation, a three-dimensional computational model has been developed and simulations are carried out using commercially available ansys fluent software. The numerical results are validated with the standard correlations and literature data, and a suitable model has been identified for the turbulence closure. A detailed analysis of the Nusselt number, temperature distribution over the SAH, and friction factor across the SAH duct is done. Empirical correlations for the estimation of heat transfer and friction factor have been developed as functions of the base angle of the duct and Reynolds number. An overall performance factor (λ) is adopted to get the combined effect of friction factor and Nusselt number with an intention to arrive at the optimum base angle of the SAH duct, and optimum geometry is identified. Based on the value of λ, it is concluded that the SAH duct with the highest base angle (90 deg) in this investigation, i.e., the rectangular duct, is the optimum among all the ducts considered in the study.