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By A.K. Ahmed, Fluid Codes Ltd. (Fluent Distributor for the Middle East), Greenford, Middlesex, UK
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People living in countries with a hot climate and high humidity during
the summer have, for centuries, adopted natural ventilation systems
for their traditional houses. One such approach, used along both
sides of the Gulf coast, is the wind tower house. These houses make use
of one or more towers to cool the internal premises.
A typical wind tower house in Dubai
Photograph © 2004, www.datadubai.com
To better understand the cooling process, the Al-Bastakia tower house
in the old section of Dubai was studied. The house contains a rectangular
bedroom (3 x 10 x 4 m3) and a square cross-section wind tower (3 x 3 x
15 m3), located at the east side of the room. One of the tower walls extends
1m down from the ceiling to create a partition between the tower and bedroom.
The room has three horizontal windows halfway up the west side
wall of the room.
The cooling performance of a tower house is affected by two main factors:
the tower design, and the strength and direction of the wind. The
tower design details, such as the height, cross sectional area, location, and
orientation have a significant effect on its performance. The direction of
the wind outside the house governs the general flow pattern inside the bedroom.
The main stream blowing in through the tower, which is higher in
both flow rate and temperature than that blowing in through the window,
mixes with the weaker flow inside the room to govern the cooling process.
Full-scale 3D models of the tower and its attached bedroom were simulated
using FLUENT. Four scenarios of wind blowing from the north, south,
east, and west were solved to determine the steady-state internal flow for
each. Wind speeds of 10 m/s and a temperature of 311K were used at the
tower inlet on the windward side, with pressure outlets on the three remaining
sides. Speeds of 7 m/s with a temperature of 301K were assigned to
the open windows. The doors to the room were assumed closed.
Pathlines colored by
temperature illustrate
recirculation zones in
the tower and room
when the wind blows
from the west
Iso-surfaces of temperature
show that for a westerly
wind, cooler air collects
below the windows and
circulates in the room
For the west wind scenario, two main flow circulation loops were observed.
A primary circulation loop was established between the west and east channels
of the tower. A secondary loop was established inside the bedroom.
Beneath the wall partition, a one-dimensional downward flow was generated.
This air curtain played a significant role in the entrainment and mixing
of the two circulating streams. The air curtain helped force cold air from
the windows downward, strengthening the circulation pattern in the room.
The height of the wall partition was found to play a significant role on the
strength of the air curtain, so modifications to the partition could provide
an easy and low cost solution for improving the cooling performance. While
the west wind was found to represent the best scenario for cooling, the
south wind represented the worst. No air curtain developed, so a single
circulation loop became established, with the coolest air pooling above the
windows.
The CFD results have shown that the placement of the windows and
tower can be chosen so that the cooling of these houses best utilizes the
renewable wind energy in an efficient manner. The tower house concept
has been used for many types of buildings, including markets, mosques,
and traditional cafes. Hopefully, modern houses and buildings in the Middle
East will continue to adopt this green architecture approach that has played
such an important role in its heritage.
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