On the control of transients in mechanical displacement ventilation

We investigate transient ventilation flows following variations in the heat load and ventilation rate in a building with upward mechanical displacement ventilation. In steady state, thermal plumes that rise from localised heat sources interact with the upward ventilation flow to produce a two-layer stratification. Through new laboratory experiments, we examine the transient responses to changes in the strength of the heat source or modifications to the ventilation volume flux. When only one of these variables is altered, the evolution of the system is similar to the behaviour observed in the experiments of Bower et al. (2008) J. fluid Mech. 614, 15–37, proceeding through either an intruding or a filling flow. The evolution pathway then depends on gamma equals g prime Subscript p Baseline divided by g prime Subscript u γ = g p ′ / g u ′ γ = g'ₚ/g'ᵤ , the ratio between the evolving buoyancy of the plume, g prime Subscript p g p ′ g'ₚ , at the interface between the warm upper and cool lower layers and the buoyancy of the upper layer, g prime Subscript u g u ′ g'ᵤ . When gamma less than 1 γ < 1 γ < 1 , the plume is dense and intrudes at the interface; when gamma greater than 1 γ > 1 γ > 1 , the plume is buoyant, rises to the ceiling and forms a new stratified layer. Changes to the heat source immediately affect the plume buoyancy, while changes to the ventilation lead to a more gradual evolution of the system. This difference in time scales results in a range of transient flows that can switch between filling and intruding modes for different values of gamma γ γ . We develop a model for these transient flows that is consistent with our experimental observations. The insights from our modelling of these transients are key for developing control strategies which maintain thermal comfort in buildings subject to frequent changes in heat load.