The criterion for building smoke control is not the complete removal of smoke. It is that a tenable layer is preserved from floor level up to a defined height for the duration of the evacuation: visibility, temperature and gas concentration limits.
In an atrium: preserve the stratification
The plume rising from the fire entrains surrounding air as it grows. As the smoke layer at the ceiling thickens, its lower boundary descends.
The job of the extraction system is to keep that lower boundary above the critical height. There is a counter-intuitive trap here: increasing the exhaust flow beyond what is needed does harm. Excessive extraction draws clean air from beneath the smoke layer (plugholing) and clean air is exhausted instead of smoke; the layer does not thin, it mixes.
The number of extract points, their spacing and the flow through each are therefore calculated — one large opening is worse than several distributed ones.
In a car park: a different problem
Ceilings in car parks are low; preserving a stratified layer is usually not possible. The objective changes: drive the smoke in a defined direction and keep the fire service approach route clear.
This is an entrainment problem solved with jet fans and belongs to the same family as critical velocity in tunnel fires: too little velocity and the smoke backlayers, too much and the stratification is destroyed altogether.
Modelling
The fire growth curve is an input. How the heat release rate rises with time is defined by the scenario and sets the result directly.
Buoyancy dominates, Boussinesq is invalid. The temperature difference is hundreds of degrees. See the natural convection note.
Radiation and soot. The visibility criterion depends on soot concentration, and radiation sets how fast the smoke layer cools. See the radiation note.
Do not forget the make-up air. Unless it is defined where the extracted air comes from, the solution is not physical; the fresh air path belongs in the model.
References
- Wu, Y. & Bakar, M. Z. A. (2000). Control of smoke flow in tunnel fires using longitudinal ventilation systems — a study of the critical velocity. Fire Safety Journal, 35(4), 363–390. DOI: 10.1016/S0379-7112(00)00031-X
- National Fire Protection Association. NFPA 502 — Standard for Road Tunnels, Bridges, and Other Limited Access Highways. NFPA, Quincy, MA.