The H-type cap is one of the most widely used chimney caps in heating and industrial systems. Despite its extensive application, no specific standard has yet been established for determining the optimal dimensions of this type of cap. In the present study, the effect of the H-cap and its geometric dimensions on the performance of a chimney with a diameter of 20 cm and a height of 3 m is investigated. To generate a buoyancy-driven flow of hot air, a heat source with a constant temperature of 1800 K was placed at the bottom section of the chimney. The flow is driven solely by buoyancy forces resulting from the elevated temperature of the air surrounding the heat source, which, together with the chimney draft, induces a turbulent upward flow. This turbulent flow is modeled using the standard k-ε turbulence model. Simulations were carried out for the chimney without a cap, as well as for chimneys equipped with H-caps having various diameters and brim lengths (ranging from 15 to 25 cm). The average velocity and total pressure at the chimney inlet were evaluated for all cases. The results indicate that adding an H-cap with a diameter and brim of 20 cm reduces the average velocity by approximately 2.6%. This reduction is attributed to energy losses caused by the presence of the cap along the flow path. A cap with a diameter of 15 cm and a brim of 20 cm results in a significant reduction of 26% in velocity, whereas a cap with a diameter of 25 cm and a brim of 20 cm increases the velocity by 7%. The maximum velocity was observed for the cap with both diameter and brim equal to 25 cm, while the minimum velocity corresponded to the cap with a diameter of 15 cm and a brim of 20 cm. The findings also demonstrate that, among the various geometrical parameters, the cap diameter has a more dominant influence on chimney draft performance. Furthermore, the optimal performance is achieved when the diameter and brim of the cap are equal. These results are of considerable importance for optimal design, safety improvement, and energy efficiency enhancement.
