Thermal and fluid dynamic behaviors of a slightly horizontal ventilated roof under variable climatic condition

One of the European Directive priorities is represented by the improvement of “building performance requirements” and the development of new strategies for “very low energy buildings”. The goal is the reduction of the energy consumptions due to the heat flux transmitted through the envelope of residential and commercial buildings. In regions, like Mediterranean region object of this study, with high level of solar radiation ventilation allows to the cooling load during summer period and contributes to the reduction of the energy needs of buildings. The most important advantage is the reduction of the heat fluxes transmitted by the structures exposed to the solar radiation, thanks to the combined effect of the surfaces shading and of the heat removed by the air flow rate within the ventilated air gap. This paper illustrates a numerical investigation on the effect of a slight inclination angle of a prototypal ventilated roof for residential use site in Aversa (Italy). The system is studied considering a roof with a length equal to 6.0 m and three different inclination with respect to the horizontal plane (0°, 2°, 5°). The ventilated channel, under the roof, has a height of 10 cm. The analysis is carried out on a two-dimensional model in air flow and the governing equations are given in terms of k-ε turbulence model. The investigation is performed to evaluate thermal and fluid-dynamic behaviors of the ventilated roof as a function of the different conditions applied on the top wall and the bottom wall of the roof in summer and winter regimes. Different values of heat fluxes are applied on the top wall of the ventilated cavity, whereas through the bottom wall there is a heat transfer toward an ambient by means a thermal conductance. Moreover, the upper surface of the ventilated roof exchange heat with the external ambient. The problem is solved by means of the commercial code Ansys-Fluent. Results are given in terms of temperature and pressure distributions, air velocity and temperature profiles and show a better performance of the system when increase the inclination of the pitched roof, especially in velocity values with an increase of 0.40 m/s.