The distributions of the velocity and temperature for the four different cases including the zero-slope collector roof (H2 = H1 = 1.85 m), H2 = 3 m, H2 = 4 m, and H2 = 5 m are illustrated in Fig. 2 and Fig. 3, respectively. The effect of the collector roof inclination on the velocity distribution through the system is evaluated in Fig. 2. It is observed that the inclination causes the mass flow rate through the system to increase. One reason is that when natural convection inside a closed domain is modeled, the resulting solution depends on the mass inside the domain [25], and the mass flow rate therefore increases as the mass inside the domain increases. The results show that the mean velocity at the chimney base reaches 9.7 m/s, while it reaches 9.1 m/s in the zero-slope collector condition. Therefore, in the inlet of the inclined collector, the velocity of the airflow is higher, while the air that flows through the collector velocity is less at the same radius compared with the zero-slope collector. The main reason for this is that the inclined collector has a larger cross-sectional area at the same radius compared to the zero-slope collector with the same inlet height. Hence, the velocity in the inclined collector at the same collector radius is lower while the airflow moves toward the chimney.