Abstracts - KTH Mechanics

180
Modelling buoyancy driven displacement ventilation
S. D. Sandbach a
Displacement ventilation driven by buoyancy is investigated using full-scale and smallscale
laboratory models. A new transient displacement ventilation model (modelling
buoyancy evolution over time) is also presented. The convective flow rising from the heat
source is modelled using plume equations 1. Once the hot air reaches the ceiling it spreads
out and begins to fill the room. This process is modelled using an approach similar to
Germeles 2 modified for displacement flow 3. Conductive and convective heat transfer
close to the surfaces is modelled in the normal way and radiative heat transfer is calculated
using Gebharts 4 modified view factor. The model was used to investigate the
configurations given in figure 1, these results were then compared with results from
existing simplified mathematical models and experimental work. Full-scale results were
obtained in an environmental chamber measuring 7.45 m by 5.5 m by 2.78 m high. It is
located in a laboratory that is 7.5 volumes larger and of relatively constant temperature.
There are two standard size doorway openings to the room (0.83 m by 2.05 m each) as
well as ventilation openings. The chamber is equipped with 59 thermocouples located
strategically around the chamber and heat sources (1.2 and 2.4 KW) at floor level. Small–
scale results were obtained using a 1/15 th scale replica of the chamber, modelling the heat
input and buoyancy with dyed saline solution. The scaled model is housed in a larger tank
of fresh water with transparent sides and a capacity 20 times that of the scaled model.
Video footage of the ensuing flow was captured and analyzed using flow visualization
software.
(a) (b)
Figure 1: (a) Displacement flow. (b) Doorway Flow. (c) Doorway and vent
a
School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, UK.
1
Morton et al, Proc.of Royal Soc 234, 1 (1956).
2 Germeles, J. Fluid Mech. 71, 601 (1975).
3 Linden et al, J. Fluid Mech. 212, 309 (1990).
4 Gebhart, ASHRAE Trans 65 321 (1959)
(c)