Radiation Effect on Transient Natural Convection in Ventilated Roofs

This paper illustrates a numerical investigation on a prototypal ventilated roof for residential use, under summer and winter conditions. The roof is modeled as a single flap, due to its geometric and thermal symmetry, and it is analyzed as two-dimensional, in air flow, thanks to the commercial code Ansys-Fluent. The governing equations are given in terms of k- turbulence model taking into account the radiation effect inside the channel. The analysis is performed in order to evaluate thermofluidodynamic behaviours of the ventilated roof, in transient regime with radiative heat transfer presence, as a function of the solar radiation applied on the top wall of the ventilated roof. The net radiative heat flux from the surface is computed as the sum of the reflected fraction of the incident and emitted heat fluxes. The discrete transfer radiation model (DTRM) is chosen. All surfaces are assumed diffuse. Moreover, the scattering effect is not taken into account and all the walls are assumed to be grey. Typical summer and winter conditions with heat transfer from the channel top wall toward the external ambient are examined. The bottom wall of the ventilated channel is simulated as isothermal. In summer conditions, the bottom wall temperature is assigned equal to 298 K and it is considered as optimal temperature value for the internal ambient in summer regime. In winter conditions, the operative temperature is assumed equal to 293 K which is an optimal internal ambient in winter regime. Results are given in terms of temperature and pressure distributions, air velocity and temperature profiles along longitudinal and cross sections of the ventilated layer, in order to estimate the differences between the various conditions. Ventilated roof configuration results significant to reach optimal thermal and fluid dynamic conditions in summer and winter regimes.