Kodjo Kpode, Yawovi Nougbléga, Hodo-abalo SAMAH, Batanta Bataka

Abstract: Natural convection from roofs subjected to directional solar irradiation is numerically studied in a typical habitat. The roof consists of two slopes that do not receive the same amount of solar irradiation at any given time. The slopes would act as a thermal screen for each other, resulting in reduced heating through the roof. To achieve this, a cavity with a straight pentagonal cross-section is subjected to a heat flux model whose intensity on each slope depends on the apparent position of the sun relative to the roof. The numerical approach is based on the finite volume method. The resolution of the unsteady natural convection equations has enabled us to determine the heat transfer and air flow which are highly dependent on the apparent position of the sun and Rayleigh number. The east-west orientation and the apparent movement of the sun cause the right slope to receive solar irradiation first. It acts as a thermal screen for the opposite slope, which remains in the shadow until it reaches its peak solar irradiation. The average Nusselt numbers show that, overall, the heat exchange between the fluid and the slope that first received the solar irradiation remains high. Unlike many previous studies in which the slopes are subjected to the same heat flux profile (constant or variable over time), the flow exhibits no symmetry and is highly unstable, with the formation and disappearance of flow cells. The thermal field and system dynamics indicate that, for optimal ventilation driven by thermal buoyancy forces, it is essential to position the ventilation systems at the center of the slopes walls, especially the one that first receives the first solar radiation.

Keywords: Thermal screen, roof, solar radiation, natural convection, asymmetric flow, unstable flow.

Date Published: April 15, 2025
DOI: 10.11159/jffhmt.2025.014

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