Analysis on the heat transfer in a square cavity with a semitransparent wall: Effect of the roof materials

A theoretical study on conjugated heat transfer (natural convection, radiation and conduction) in a square cavity with turbulent flow is presented. The cavity is a representation of a room, where the left wall is isothermal, the right wall is semitransparent (glass), the lower wall is considered as insulated and on the upper opaque wall heat conduction is present. Both conductive walls (opaque and semitransparent) interact with the ambient. The semitransparent wall is subject to a constant heat flux (G2 = 736 W/m2) whereas on the opaque wall a constant heat flux (G1 = 875 W/m2) falls perpendicularly. The sizes of the cavity under study were 5.0, 4.0, 3.0 and 2.0 m. The upper opaque wall was considered as a mixture of concrete and a composite material (concrete-expanded polystyrene) with different thicknesses and diverse types of water-repellent coatings on top of it. From the results, it was found that the white coating on top of the opaque wall significantly reduces the amount of energy towards the inside of the cavity. It was also determined that the opaque wall with a 20 cm thickness shows the best thermal performance and it is the most adequate to reduce thermal gains inside the cavity. Correlations for the total heat transfer as a function of the cavity size, the type of coating and material of the opaque upper wall are proposed.