The formation of multi-steady-states of buoyancy ventilation in underground building

Abstract

The study of buoyancy ventilation in underground buildings is crucial for the purpose of temperature control and smoke extraction. Some underground spaces have unique properties such as their deep location, with installed vertical channels of several hundred meters in length, raising the potential to rely on buoyancy driven ventilation only. This paper studies a particular phenomenon in such deep spaces with high intensity heat sources, e.g. power generating turbines. The phenomenon studied in this paper concerns the way that different steady states of flow and temperature can establish themselves. They are studied through an experimental set up and numerical simulations. Multiple factors, like the initial condition, the intensity and location of the heat source, are investigated, in particular to find how the final steady states in different situations are reached. It is well understood that the dominant flow tends to follow the expansion of the local thermal plume. An initial velocity may however alter this trend and an alternative steady state can be reached but this depends on the magnitude of the initial velocity. The critical initial velocity for the transition among these steady states is therefore studied in detail. However, once the steady solution is reached, this is not dependent on the initial velocity or other conditions. Thermal buoyancy pressure of fluid flow is generated by gravity forces due to density difference. Buoyancy forces work together with other forces (such as mechanical fans which have a given location and direction, and wind pressures over openings) to result in natural ventilation flows in spaces with openings that connect with outside air. The multi-steady-states of fluid flow in an underground space is determined by the fundamental characteristic of thermal buoyancy pressure and its understanding and predictability is crucial to be able to predict temperatures and natural ventilation flows correctly.