The analysed system is a typical stairwell of an in-line multifamily building divided into five storeys and with two dwellings per floor (see figures 1,2).

The main components of the ventilation system for each apartment are:

• wind-sheltered inlets located near windows or heating devices (for example, a grille mounted into an external wall or windowsill, equipped with adjustable deflectors);

• air-transfer device (for example, grilles mounted into the internal doors);

• operable louvers mounted at the top of the entrance unit;

• semi-automatic control system (each occupant can manually adjust the air-inlets of some or all the rooms to suit personal requirements).

The natural ventilation system has been designed in order to extract the exhausted air from each apartment and each unit is sealed off from the others. These are important characteristics of the ventilation system in order to avoid a short-circuit and guarantee adequate indoor air quality (IAQ) in each unit.

The system was verified in stationary winter climatic conditions (the hypothesised external temperature is 0°, a typical design value in a temperate climate), all dwellings are heated at 20 °C and the stairwell is unconditioned.

Conductive loss from interiors warms air in the stairwell, causing it to rise toward the roof. This warmed air is extracted at the top of the stairwell and, consequently, fresh air is supplied to each dwelling. The major driving forces causing air movement are the above illustrated mechanism and the stack effect; the wind effects were been ignored. The behaviour of this natural ventilation system is mainly related to the heating needs of buildings; so, this mechanism may be more affordable and continuous than wind-induced ventilation, especially in urbanized areas.

The model used in CFD simulations was simplified in order to optimise the computational time. The pressure losses in the apartment path were collapsed into a single resistance. Therefore, only the stairwell was modelled. The stairwell geometry was simplified and reduced to the essential components (flights of stairs, landings, steps). The openings were modelled as simple resistance or as simple sloping planes. The walls were 30 cm thick and simplified as a single layer. A three-dimensional view of the geometric model is presented in fig. 3.

Concerning the temperature boundary conditions, the external air temperature was set at 0°C near the external surfaces of external walls and ceiling of the stairwell; the internal air temperature (20°C) was considered near the internal surfaces of internal walls (dwellings all heated).

Fig. 3: CFD computational model and temperature boundary conditions.

The design process follows an evolutionary approach, by single or small changes to the geometry and boundary conditions [4].

The locations, the sizes and the pressure losses of openings were modified in order to design and/or verify the behaviour of the natural ventilation system. This data is specified in the subsequent chapter.