Underground Garage: Ventilation and Fire Study


PHOENICS is increasingly being used for the study of ventilation and fire hazard in garages and car parks. In this case the simulations were carried out by Van Hooft Adviesburo B.V. fire & life safety engineers, an engineering consulting company in the Netherlands.

The aim was to investigate the movement of air under normal circumstances and the movement of heat and smoke in the case of a car fire.

Garage layout - click for larger image The garage, which is situated in Utrecht, is single-storey and almost rectangular in shape; its size is 132m ´ 56m ´ 2.5m. The ventilation system is mechanical. Two ventilation shafts are used, both at the opposite end of the garage from the open entrance. Under normal circumstances only one shaft is used, but both are used during a fire emergency. Additional control over the air flow is provided by a number of jet fans near the roof. Under regular conditions the air is extracted via the shafts and is drawn in through the garage entrance, but all elements of the system are bi-directional: this means that the flow direction for emergency ventilation can be selected to maximise the time for evacuation and fire fighting, and to minimise the risk of the fire spreading to the rest of the building.

Streamlines under normal ventilation - click for larger image The basic purpose of the ventilation system is that it should avoid the build-up of excessive exhaust fumes. This requires an adequate air flow in all parts of the space - dead zones are to be avoided. The regular ventilation system draws air through the garage, but some of the jet fans near the centre are operating to force flow back towards the entrance. This is intended to maintain air movement and mixing. The figure shows streamlines, coloured by time; it is apparent, at least qualitatively, that the air reaches almost all of the garage in a satisfactory manner.

Smoke concentration under maximum ventilation - click for larger image During a fire the requirements are rather different: the garage must be kept free of smoke in such a way that suppression by the fire brigade is possible. The simulations assumed a heat source of 5MW, typical of the burning of one large car or two small ones. The worst case scenario was considered to be when the garage was free of other parked cars, because in that case the ventilation system has to handle a larger air volume. In view of the location of the fire the ventilation direction was reversed, with air entering through the ventilation shafts and leaving by the garage entrance. The figure shows contours of smoke concentration at a height of 1.9m. The scale is relative to the smoke level at the fire location and it is clear that the conditions in the garage are under reasonable control. The only region of concern is the space between the fire and the side wall, but that does not seriously prohibit movement of people.

The PHOENICS simulations enabled a good understanding of the air flow in the garage to be obtained, under a number of different circumstances. It was shown that the ventilation system had the capability to deal well with the required range of conditions.