U p w a r d – A i r f l o w V e n t i l a t i o n S y s t e m
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5.1. Coupling the upward-airflow with cross natural ventilation
As observed before, cross natural ventilation has a practical limit in between the inlet-
outlet opening distance. Inlet-outlet opening distances greater than 6 to 10m are critical
for cross ventilation efficiency. In case A, which is an upward cross ventilation, this
practical limit can be higher if we consider that the stream airflow goes from the inlet to
the outlet opening, at the top of the sawtooth roof, without going through the deeper part
of the room. As a result, stagnation zones can be created at the bottom room. For
instance, the simulation of the model A1 with upward cross ventilation (Case A - without
the on-floor air diffuser) presented a stagnation zone at the room bottom (Fig. 17).
Model A1 - under a wind velocity of 1m/s.
Figure 17. Case A: Air
distribution
for
cross
ventilation. The internal air
velocity at point S2 was
0.18m/s.
Figure 18. Case B: Air
distribution (with opening at
floor level by naturally
pressurized plenum). The
internal air velocity at point
S2 was 0.18m/s.
Figure 19. Case A + B: Air
distribution for both (Case A +
Case B).
However, the upward air stream flow by naturally pressurised plenum (Case B) is an
option for facing the potential problem limitation of cross ventilation. The incorporation of
the on-floor air diffuser by naturally pressurised plenum increased the internal air velocity
at the stagnation zone. For instance, in Model A1, when the wind velocity was 1m/s, the
internal air velocity at point S2 was 0.18m/s (Fig. 18). With this configuration (Case B),
model A1 had a slightly better performance. The main features of Case A and B for
Model A1 are summarized in table 6.
Table 6: Model A1 – Case B
Model A1
Wind
Velocity
Average Pressure
(Pa)
Internal mean speed (m/s)
S1
S2
*Point d
Case
A
at
internal
space
1
-
0.65
0.08
-
Case
B
at
internal
space
1
-0.22
0.08
0.18
1
3
-1.50
0.22
0.37
3
5
-4.35
0.44
0.70
5
10
-17
0.88
1.45
10.10
at
Plenum
entrance bottom
1
0.41
0.46
3
3.60
4.10
5
10.30
11.40
10
41
45
*Point d = velocity at the on-floor diffusser
