  Displacement ventilation, where cool external air enters a room through
low-level vents and warmer air leaves through high-level vents, is
characterised by vertical gradients in pressure arising from the warmer indoor
temperatures. Models usually assume that horizontal variations of temperature
difference are small in comparison and are, therefore, unimportant. Small-scale
laboratory experiments and computational fluid dynamics were used to examine
these flows, driven by a uniformly heated floor. These experiments and
simulations show that the horizontal variations of temperature difference can
be neglected for predictions of the bulk ventilation rate; however, they also
evidence that these horizontal variations can be significant and play a
critical role in establishing the pattern of flow within the room -- this
renders the horizontal position of the low- and high-level vents (relative to
one another) important. We consider two cases: single-ended (where inlet and
outlet are at the same end of the room) and opposite-ended. In both cases the
ventilation flow rate is the same. However, in the opposite-ended case a dead
zone is established in the upper part of the room which results in significant
horizontal variations. We consider the formation of this dead zone by examining
the streamline patterns and the age of air within the room. We discuss the
implications for occupant exposure to pollutants and airborne disease.
