Transient buoyancy-driven ventilation: Part 1. Modelling advection

The unsteady development of the vertical temperature profile in a ventilated space containing a heat source is modelled. The buoyant fluid released from the heat source is modelled as a turbulent buoyant plume, using a standard integral plume model with a fixed entrainment coefficient. Two types of natural ventilation flow are considered, with the flow driven entirely by the density contrast between the fluid inside and outside the space (stack effect). The ventilation types are (a) classic displacement ventilation, with outflow of warm air through upper openings and inflow of cool air through lower openings; and (b) doorway ventilation, with an exchange flow through the doorway. An improved version of the doorway exchange flow model is given as compared to previous studies. The boundaries of the space are considered to be perfectly insulating, so that heat is transported entirely by the fluid motion. The temporal stratification that develops within the space (outside the plume) is calculated using a modified filling-box model, with successive layers added to the top of the space over time. Laboratory experiments giving reduced-scale simulations of the flows were also conducted, where saline solution and fresh water are used to model fluid of different density. The developing density profiles in the laboratory experiments compare very well with the model predictions. The use of this type of model, capturing the main physical flow features, allows rapid and accurate calculations of transient stratification in ventilated spaces.