Sensitivity Analysis of Constructional Specifications on Thermal Performance of Naturally Ventilated Box Window Double Skin Facade in Hot Arid Climate of Iran (Tehran)

The function of the double-skin facade with natural ventilation is based on the thermal performance and airflow of the cavity. Structural features of a naturally ventilated double-skin façade that affect its thermal performance and fluid dynamics include geometric features such as cavity dimensions, airflow path, air inlet, and outlet areas, shading devices material and their location in the cavity, and material properties that are optical and thermal characteristics of transparent skin. In this research, Fluent Software was used to simulate the computational fluid dynamics, and by sensitivity analysis, the effect of changes in structural properties on thermal performance was evaluated. For this aim, 144 simulation scenarios with cavity depths of 30, 60, 90, and 120 cm, inlet and outlet cross-sections of 0.2, 0.4, and 0.8 m2, louver shadings with 0, 30, 45, 60, and 90 degree angles located at one third distance close to the exterior wall of the façade, the model without shading, exterior glass with regular and low emission layer were investigated, and the following results were obtained:- Cavity Depth: It was observed that as the depth increases, the velocity of airflow in the cavity decreases. Changing the width of the hole changes the surface temperature of the inner glass up to a maximum of 0.80 ° C (up to 2%). As the depth of the cavity increases, the heat flux transmitted through the inner glass surface decreases in most cases (up to 6.5%).- Cross-section: As the cross-section increases, the air velocity in the cavity increases the temperature of the inner glass surface changes irregularly but, in most cases, decreases. In addition, the heat flux passing through the inner glass surface often increases (up to a maximum of 16.3%).- Shading: In the case without shading, the air velocity in the cavity is 0.30 to 0.70 m/s, and in the case with 45 degrees of shading, the air velocity in the cavity is between 0.30 and 0.92 m/s. In the case without shading, the temperature of the inner glass surface changes from 39.3 to 41.0 ° C, and in the case with 45 degrees, changes are between 38.65 to 39.5 ° C. In the case without shading, the heat flux transmitted through the inner glass surface changes from 78.0 to 120.0 W/m2, and in the case with 45 degrees shading, differences between 43.0 to 48.2 W/m2. The sensitivity of the heat flux transmitted through the inner glass surface to the presence or absence of shading can lead to a reduction of the heat flux transmittance through the inner glass surface by up to 61.0%.- Exterior glass type of DSF: In the case without shading, the reduction rate of heat flux from the inner glass surface is 19.44% to 29.78%, and in the case with 45-degree shading is from 17.71% to 24.44%. By changing the exterior glass's material (low emission or regular), the heat flux transmitted through the inner glass surface changes by 32.2 and 120 W/m2.