A new design of Solar Air Heater (SAH) with triangle cross-section is numerically studied. The thermal performance of SAH is studied at various mass flow rates, inlet air temperatures and solar irradiation intensities. The CFD model is developed using the software "ANSYS Fluent" to study the fluid flow and heat transfer in the solar air heater. 3D discretization is applied to study the thermal performance of solar collector with triangle cross section. Mesh independence is performed in order to choose the adequate mesh. The discrete ordinate radiation model (DOM) and the RNG k-ε turbulence model are used to study the radiative heat transfer and the turbulent flow inside the SAH. Particularly, effects of different internal peak angles (145°,126°, 100°, 80° and 67.5°) under different solar irradiation intensities (from 620W/m^2 to 1081W/m^2) are studied to improve the thermal performance of the SAH. The results show a good agreement between the numerical model and the experimental data with an average error of 6%. The maximum outlet air temperature of the SAH reached 72°C for the geometries with 12 and 16 channels (Internal peak angles of 80° and 67.5°, respectively) under mass flow rate of 0.0264kg/s. The thermal performances of the SAH with 16 and 12 channels are 24.2% higher than standard geometry, respectively for solar irradiation intensity of 1081 W/m^2. The configuration with internal peak angle of 80° and 12 channels is selected as the optimal with a thermal efficiency of 79%, a low pressure drops compared to geometry with 16 channels and lower costs.