Numerical studies on laminar, transitional and turbulent convective airflows in channels with generalised geometry, including applications to thermal-ventilation passive systems

Abstract

The flows induced by natural convection appear in many engineering problems. Configurations formed by heated plates where these processes occur, were the topic of intense study in past years. However, new systems with a different constructive disposition and applications in several fields (bioclimatic architecture, electronic cooling, nuclear energy cooling systems) are at the moment of great interest. Since typical applications of buoyancy-driven airflows in smooth vertical channels are usually in small-scale devices (i.e. in electronic cooling equipment), most investigations were carried out for laminar flows. As a consequence of the great scale of certain passive ventilation systems, as solar chimneys, Trombe walls, or roof collectors, the flow established becomes transitional or even fully turbulent. The regarded geometries frequently involve structures based on converging and sloped channels formed by heated plates, where buoyancy-driven flows take place. Therefore, the study of problems such as the transition to turbulent regime or flows with walled-channel geometries including sloped and converging walls are key to assertively find out the real heating transmission existing in these new systems in a more realistic manner. Nowadays, the need to achieve human comfort by passive heating and ventilation techniques is greater as is the requirement for energy saving. Passive solar systems are the basic elements of bioclimatic design and they do not involve the use of mechanical or electrical devices. The Trombe Wall is the primary example of the technique called indirect gain, whose typical configuration is usually formed by a thick, darkened, masonry wall and a glazed wall. In ventilation applications, other passive systems, called thermosyphons, heat syphons or solar chimneys, can yield natural motions of air due to the induced temperature differences by solar heating. Although several reported works provided useful results for the analysis and design of passive solar devices in buildings, these works cannot determine the necessary details of convection for numerical simulations. Furthermore, as an important lack of design correlations is detected, it is necessary to carry out a systematic study that supplied the heat transfer coefficient and the mass-flow rate as a function of relevant parameters, for several configurations.