Characterization of boundary layers in solar chimneys from numerical simulations with CFD techniques.

Abstract

Solar chimneys use solar energy to heat the air inside them and thus produce a density gradient there, which due to buoyancy effects results in a natural circulation of this air. They have various applications, such as thermal conditioning and passive ventilation of buildings. They can contribute to the energy efficiency of buildings, and are an interesting alternative due to their possible economic advantages, so it is important to have models that allow estimating the air flows that are achieved (and air outlet temperatures, for heating applications) based on knowing the available solar radiation. In the present work, different ways of modeling the air flow through a solar chimney are evaluated from numerical simulations with CFD techniques. Using the OpenFOAM 8 software, chimneys from an experimental work were simulated, work in which the authors built a laboratory model of a solar chimney with uniform heat flux in one of its walls, which they achieved from electrical heating. Taking only the chimney as the working domain for the simulations, different options for the boundary conditions were considered, both in the inlet and outlet sections, and in the solid walls (absorber plate and transparent cover). Some difficulties were encountered with the boundary conditions in the inlet and outlet sections, linked to the way buoyancy effects are implemented in the buoyantPimpleFoam solver in OpenFOAM version 8, which works with variable density instead of using the Bousinessq approximation. This work shows how these difficulties were overcome in this particular case, which has horizontal inlet and outlet sections. Regarding the thermal boundary conditions on the walls, it can be seen that their choice in the transparent cover strongly impacts the type of air flow obtained in the chimney and the numerical values of mass air flow rate that are reached once the simulation arrives the steady state.