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CFD Used to Develop Impulse Ventilation for Car Parks
Posted Thu March 29, 2007 @08:51AM
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Application Ventilation of covered car parks is essential for removing harmful vehicle exhaust fumes and providing assistance to fire fighters by clearing smoke in the event of a fire. Ventilation using impulse fans has been in use in Europe for more than 10 years and is becoming increasingly popular in the UK.

CFD (Computational Fluid Dynamics) is now being increasingly used in the design of car parks for modelling the carbon monoxide distribution and concentration as well as the thermal gradients and smoke densities in the event of a fire.

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The system of impulse ventilation is based on a number of small high velocity fans in place of the distribution ductwork. Impulse fans operate on well proven tunnel ventilation principles, producing a high velocity jet which thrusts against the air in front of the fan imparting momentum to all the surrounding air through entrainment as it diffuses. The volume of entrained air is significantly greater than the air quantity passing through the fan. The impulse fans are carefully positioned to direct the airflow towards the main extract fan intake points. The main extract fans are sized to provide the required flow rates, but given the reduced need (or complete elimination) of intake-scavenge ducting within the car park area, do not have to overcome the system resistance inherent in a fully ducted ventilation system.

Impulse fan performance is rated in terms of the thrust developed by the fan, which is the product of the mass flow rate times the change in velocity, ie. volume flow rate times the air density times the fan outlet velocity, and is measured in Newtons (N).

The major differences between traditional mechanical and impulse ventilation systems are:

  • The distribution ducting used in traditional systems is replaced by a number of small impulse fans to direct the airflow across the car park.
  • Without the distribution duct resistance, smaller exhaust and supply fans and/or motors can be used.
  • For impulse systems with smoke control design objectives, the airflow rates are calculated based on a design fire size rather than the prescribed number of air changes per hour.

Commercially available design packages facilitate detailed analysis of airflow in car parks, taking into account the often complex geometry of awkwardly shaped individual buildings as well as variations in vehicle movements and local weather conditions.

The use of Impulse Ventilation for Car Parks has a proven track record in Europe for providing benefits in terms of installation and running costs as well as providing protection to life and property in the event of a fire. The inclusion of impulse fans in the new Draft British Standard BS7346-7 signifies that Impulse Ventilation is becoming the norm for Car Parks in the UK and other countries.

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