Car park ventilation – searching for stagnant areas with CFD

For car parks BS7346-7 requires for both Vehicle exhaust pollution control (clause 6) and Impulse ventilation to achieve smoke clearance (clause 9) that where a mechanical means of ventilation is used, the system should be designed such that there are no stagnant areas in daily ventilation or smoke ventilation operational mode. This is often verified using Computational Fluid Dynamics (CFD) simulations. However, the definition of a stagnant area is very clear in the standard, but we found a solution!

BS7346 defines a stagnant area as an area in which there is little or no air movement resulting in an undesirable build up of contaminated air. It suggests that there is a relation between air movement and the build up of containments, but in certain cases the two do not mean the same. A low speed area does not always mean that containments would build up. Vice-versa is also true where a high speed does not always mean no containments build up, see the examples below. The build up of containment can happen when a source of pollutants is present and there is little air replacement. Air replacement therefore is the key term related to build up of containment and not air movement or velocity, which can be misleading as demonstrated below. We therefore devised a new parameter called Age of Air (AoA) or air residence time which takes into account air replacement by means of CFD and gives a much better picture of stagnant areas.

What is AoA or air residence time and why is it meaningful for fire safety?

To verify the absence of stagnant areas with our simulations we track the residence time of the air inside the car park. Every second that air is present in the car park it ages a second. This allows us to track the fresh air distribution throughout the car park. When fresh (young of age) air mixes with older air, the fresh air ages faster (gets polluted) and the aged air gets diluted (gets less polluted). Therefore, air residence time or age of air gives a much better understanding of stagnant areas as it virtually tracks the pollutants distribution and movement. Hence, for verification of the absence of stagnant areas, air residence time gives a more meaningful oversight of the containments build up compared to merely using air speeds.
However, question remains, what is the threshold of an acceptable local air residence time to avoid stagnant areas? To answer this question we have to take a look at the guidelines in BS7346-7 again.

What does the code say about (local) air residence time?

When designing a car park you have several approaches available to design a vehicle exhaust pollution control system. One option is a mechanical and natural ventilation system using permanent natural ventilation openings with an aggregate equivalent area of at least 2.5% of the floor area. This should be combined with a mechanical ventilation system capable of at least three air changes per hour. A second option is to use a full mechanical system that provides at least six air changes per hour. For the full mechanical option there are no further requirements regarding ventilation openings.
Both design approaches can be used for the same car park with the same production of pollutants. However, the air change rate per hour is lower for the mechanical and natural system (3 ACH) opposed to the full mechanical system (6 ACH). This is explained by the extra requirement regarding the natural ventilation openings which make it more likely that the fresh air is well distributed. The extra 3 air changes required for a full mechanical system is a compensation for the expected loss of efficiency due to the less well distribution of fresh air. Probably the standard expects a full mechanical system to operate at 50% efficiency compared to a mechanical and natural ventilation system hence the global number of air changes per hour is doubled.
So with a good air distribution 3 air changes per hour is sufficient to keep pollutant levels in a car park within acceptable range. This means that every hour the air should be replaced three times. An hour has 3,600s so every 1,200s air should be replaced, hence an air residence time of 1200 sec for the air leaving the car park is considered acceptable.

How can this be translated into local air residence time or age of air (AoA)?

The air residence time of 1,200 s derived from the standard is an average for the air leaving the car park (mostly at a mechanical extract), but the local age of air inside the car park is likely to deviate in time and location. In an ideal replacement ventilation system design air ages gradually from 0 to 1,200s towards the exhaust. However, car parks are not designed as such. A car park is designed to mix as much air as possible using thrust fans. In this case the age of air is still 1,200s at the exhaust as dictated by the global air change rate per hour at which the system operates. However, only in an ideally designed system (ideally mixed) the age of air is 1,200s at all locations in the car park. In practice you’ll find locations with short circuiting which likely have a lower age of air and you’ll find locations with an older age of air. Hence, it is not realistic to expect the age of air to be at 1,200 s throughout the entire car park, but there will be locations with a higher air residence time.
A higher value of local air residence time could mean a stagnant area, but how high is still acceptable? Since there in no specific requirement in the code regarding local stagnant areas we call an area stagnant when the local air residence in that area is more than 1.5 times the maximum allowable residence time of 1,200 s for air leaving the car park. Hence, a maximum of 1800 s local residence time is the maximum allowed to avoid having a stagnant area. Note that this accounts for an efficiency of 67% which is more than the efficiency expectation of 50% that the code uses for a mechanical vs mechanical and natural system.
To conclude, for a carpark to provide acceptable performance, the maximum local air residence time should be below 1800 s to avoid any stagnant area.