The formation and evolution of stratification during transient mixing ventilation

Abstract

We investigate the buoyancy-driven ventilation of an enclosed volume of buoyant fluid, which is connected to the exterior through two openings at the top of the enclosure. An exchange flow becomes established, with outflow through one opening being matched by an equal and opposite inflow through the other vent. The inflowing flux of dense exterior fluid develops a turbulent buoyant plume, which mixes with the interior fluid as it cascades to the base of the fluid volume. An upward return flow gradually stratifies the confined volume of fluid, with a first front of relatively dense plume fluid advancing to the top of the space. Initially, the exchange flow is steady, but as the first front rises above the inflow opening, the flow rate wanes. The initial development of the exchange flow and interior stratification follows the classical filling box work of Baines & Turner (J. Fluid Mech., vol. 37, 1969, p. 51), with a plume of constant buoyancy flux. Once the first front exits the space, the volume flux rapidly asymptotes to the form Q = Q0(τ/(τ+t)) from the initial value Q0, where τ is the buoyancy-driven draining time, based on the initial density contrast with the environment. The vertical structure of the interior density stratification asymptotes to a profile of constant shape whose amplitude decays in time as (τ/(τ+t))2, and we identify conditions under which the vertical variation in density is comparable to the difference between mean interior density and the exterior. This generalises the analysis presented by Linden, Lane-Serff & Smeed (J. Fluid Mech., vol. 212, 1990, p. 309), who assumed that the interior fluid is well mixed. New laboratory experiments of the process are shown to be consistent with our predictions of the evolution of the flow, the interior stratification and the migration of contaminants. We also develop our analysis for situations where there are multiple stacks and show how this improves the mixing efficiency for a given ventilation flow. The model has relevance for the design of transient mixing ventilation in a building, especially when the effect of vertical stratification is important for ensuring thermal comfort.

On the transport of heavy particles through a downward displacement-ventilated space

“…We measure the speed at which the dyed fluid rises as a result of the return flow in the lower part of the tank and use (2.3) to infer the plume flow rate Q p at different levels in the tank. We then use (2.2) to estimate that in our experiments the particle-laden plume has a coefficient λ = 0.17 ± 0.01, which is consistent with the estimates of Bower et al (2008) and Kuesters & Woods (2011). The same equations are also used to estimate that the virtual origin of the plume is located at a distance z 0 = 7.9 ± 2.0 mm above the tip of the glass nozzle through which the particle-laden fluid is pumped into the tank (vent A in figure 2).…”

Effect of vertical density stratification on the buoyancy-induced flow at a doorway opening

“…Subsequently, many studies for ventilation applications were undertaken. Caulfield and Woods 6 and Kuesters and Woods 7 studied transient stratification in one room with an opening at the ground of the enclosure. Conroy et al 8 included the consideration of a reacting plume, Sandbach and Lane Serff 9 considered a doorway opening, and Bolster and Linden 10 considered existing contaminant layers initially in the enclosure.…”

Quasi-steady states in natural displacement ventilation driven by periodic gusting of wind

“…Much of this work has focused on the investigation of continuous sources of buoyancy and continuous flows through openings on the boundary of the flow domain. Additional work has focused on transient ventilation flows (Germeles 1975;Baines 1983;Kuesters & Woods 2011;Mott & Woods 2011), the transient adjustment to steady state (Kaye & Hunt 2004;Fitzgerald & Woods 2007;Bower et al 2008), the effects of steady or slowly varying wind to modify the flow regime (Hunt & Linden 2001;Li & Delsante 2001;Jiang et al 2003;Economidou & Hunt 2010) and situations in which there are multiple flow regimes (Hunt & Linden 2005; (Linden et al 1990). (b) Natural displacement ventilation between a space (A), and a warm connected space (B), due to cold gusts of exterior air entering space A at high level.…”

“…Other work has explored the fluid dynamics of mixing ventilation (Woods, Caulfield & Phillips 2003;Kuesters & Woods 2011) and models of contaminant transport in natural ventilation (Hunt & Kaye 2006;Bolster & Linden 2007). However, many naturally ventilated buildings are subject to fluctuations in the wind and this can lead to discrete gusts of wind entering a building, producing a qualitatively different unsteady flow associated with the exchange between the interior and exterior (Awbi 2003;Mott & Woods 2011).…”