Roof-Mounted
Ventilation Towers – Design Criteria for Enhanced Buoyancy-Driven
Ventilation
G.R.
Hunt and K. Syrios
Department
of Civil and Environmental Engineering,
Imperial College London, London SW7 2AZ, U.K.
Abstract
Ventilation towers are often incorporated into the design of
naturally-ventilated buildings. These towers increase the physical height
of the building and thereby potentially enhance the buoyancy-induced air
velocity. However, acoustic baffles, insect meshes, etc., placed within
the towers result in pressure losses that effectively reduce the area of
the flow path, thereby restricting the rate of airflow. These competing
effects (enhanced velocity and reduced area) mean that without careful
attention to design, a building with a tower may yield lower airflow rates
than a building without a tower. Industry has sought guidance on the
effect of these towers on the ventilation of the adjoining building and we
derive conditions for a ventilation tower to have a beneficial effect on
the airflow rate. Conditions are developed for passive-stack ventilation
driven by distributed and localised internal heat sources and then
extended to take into account solar heat gains. Based on these conditions,
design curves are presented together with a methodology for estimating the
minimum tower height that will not be detrimental to the ventilation flow
rate. An example is given illustrating the use of this technique.
Key words: ventilation
tower, natural ventilation, stack effect, enhanced ventilation, pressure
losses.
References
Andersen KT: (1995)
“Theoretical considerations on natural ventilation by thermal buoyancy”. Trans.
ASHRAE, 101, pp1103-1117.
Aynsley RM, Melbourne W and Vickery BJ: (1977) Architectural
aerodynamics, chap. 6: Natural ventilation.
London
: Applied Science Publishers Ltd.
Bansal NK, Mathur R and Bhandari MS: (1993) “Solar chimney for
enhanced stack ventilation”. Build. Environ. 28,
pp373-377.
Bansal NK, Mathur R and Bhandari MS: (1994) “A study of solar chimney
assisted wind tower system for natural ventilation in buildings”. Build.
Environ. 29, pp495-500.
Batchelor GK: (1954) “Heat convection and buoyancy effects in
fluids”. Q. J. Roy. Meteor. Soc. 80,
pp339-358.
Batchelor GK: (1967) An introduction to fluid dynamics.
Cambridge
,
UK
:
Cambridge
University
Press.
Flourentzou F, Mass, J van der and
Roulet
,
CA
: (1996) “Experiments in natural ventilation for passive cooling”. Proc.
17th AIVC Conf. (ed. M. Orme), pp121-134.
Gothenburg
,
Sweden
: Air Infiltration and Ventilation
Centre
,
U.K.
Gladstone C and
Woods AW: (2001) “On buoyancy-driven natural ventilation of a room with a
heated floor”. J. Fluid Mech. 441,
pp293-314.
Heiselberg P, Svidt
K and Nielsen PV: (2000) “Windows – measurements of air flow capacity”. Proc.
ROOMVENT 2000, 7th Intl Conf. on Air Distribution in Rooms (ed. H. B.
Awbi), pp749-754.
Reading
,
U.K.
: Elsevier Science Ltd.
Holford JM and Hunt
GR: (2000) “Multiple steady states in natural ventilation”. Proc. 5th
Symp. on Stratified Flows (ed. G. A. Lawrence, R. Pieters & N. Yonemitsu),
pp661-666.
University
of
British Columbia
, Vancouver.
Holford JM and Hunt
GR: (2001) “The dependence of the discharge coefficient on density contrast
– experimental measurements”. Proc. 14th Australasian Fluid Mech. Conf. (ed.
B. B. Dally), pp123-126.
Adelaide
University
,
Adelaide
,
Australia
.
Holford JM and Hunt
GR: (2003) Fundamental atrium design for natural ventilation. Build. Environ.
38, pp409-426.
Holford JM, Hunt,
GR and Linden PF: (2002) “Competition between heat sources in a ventilated
space.” Proc. ROOMVENT 2002, 8th Intl Conf. on Air Distribution in Rooms (ed.
P. Nielson & A. Melikov), pp577-580.
Copenhagen
,
Denmark
:
Technical
University
of
Denmark
and Danvak.
Hunt GR and Holford
JM: (2000) “The discharge coefficient – experimental measurement of a
dependence on density contrast.” Proc. 21st AIVC Conf., pp197-208.
The Hague
,
Netherlands
.
Hunt GR and Kaye
NG: (2001) “Virtual origin correction for lazy turbulent plumes.” J.
Fluid Mech. 435, pp377-396.
Hunt GR and Linden
PF: (2001) “Steady-state flows in an enclosure ventilated by buoyancy forces
assisted by wind.” J. Fluid Mech. 426,
pp355-386.
Idelchik IE: (1994)
Handbook of hydraulic resistance, 3rd edn. Boca Raton, Florida: CRC
Press.
Linden PF: (1999)
“The fluid mechanics of natural ventilation.” Annu. Rev. Fluid Mech.
31, pp201-238.
Linden PF, Lane-Serff
GF and Smeed DA: (1990) “Emptying filling boxes: the fluid mechanics of
natural ventilation.” J. Fluid Mech. 212, pp309-335.
Morton BR: (1959)
“Forced plumes.” J. Fluid Mech. 5,
pp151-163.
Morton BR, Taylor
GI and Turner JS: (1956) “Turbulent gravitational convection from maintained
and instantaneous sources.” Proc. Roy. Soc. Lond. A. 234, pp1-23.
Syrios K and Hunt
GR: (2004a) “Design
considerations for roof-mounted ventilation systems”. Intl J.
Ventilation 3 (2), pp89-104.
Syrios K and Hunt
GR: (2004b) “Ventilation
towers in passive-stack ventilation – determining pressure losses and airflow
rates”. Proc. CIBSE Nat. Conf. (ed. G. Manly). Docklands, London.
Turner, JS: (1986)
“Turbulent entrainment: the development of the entrainment assumption, and its
application to geophysical flows”. J. Fluid Mech. 173, pp431-471.
Ward-Smith AJ: (1980) Internal fluid flow: the fluid dynamics of flow
in pipes and ducts. Oxford: Clarendon Press.
