Correct FDS Pressure Simulation
The heat emission from a fire source leads to an increase in temperature within the room. If the room is airtight, the pressure inside must increase. The operation of supply or exhaust ventilation increases or, respectively, decreases the mass of air in the airtight room. Consequently, the pressure in the room must also rise or fall.
Below is an example: there is a room with a fire source with the following characteristics:
Room volume = 48 m³.
Fire source material: Gasoline.
Fire source area = 0.25 m².
Fire development time = 10 s.
As shown in the picture, the pressure increase is approximately 20 kPa. That is, 0.2 times the atmospheric pressure. This pressure is equivalent to a load of 2 tons pressing on each square meter of the floor, walls, ceiling, doors, and windows.
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The walls of most civil and industrial premisses are not able to withstand such a pressure load. The building would burst like an overinflated balloon. Window glass cannot withstand a pressure difference of more than 0.03 atmospheres.
In reality, buildings do not burst from fires or the operation of smoke control ventilation systems. Therefore, in real fires, such a significant increase in pressure does not occur. This means that the large pressure increase obtained in fire simulation is incorrect. Thus, some factors affecting fire simulation results, including the spread of fire hazards, have not been accounted for.
Walls, doors, and windows, defined in the FDS file via the OBST parameter group, are assumed to be airtight by default.
In most cases, real rooms are not airtight enough to create such a significant pressure difference, so structural failures do not occur. Fire simulations are usually conducted taking into account that such a large pressure change in the room is not possible. Next, we review what is necessary to account for the non-airtightness of rooms when simulating fire.
Pressure Zones
For accurate pressure simulating, FDS splits the entire computational domain into pressure zones. A pressure zone is a part of the computational domain that is airtight in relaation to other parts. For example, a pressure zone can be a room with all doors and windows closed, or a corridor along with all rooms where doors are open to this corridor. Next, we refer to “pressure zones” simply as “zones.”
At any time, the pressure inside a zone is the same at every point.
FDS automatically splits the computational domain into zones and assigns each zone a number. The number zero is assigned to all space outside the computational domain (the surrounding open space) and to the part of the computational domain that is not hermetically separated from the surrounding open space.
You can assign zone numbers manually. To do this, specify a point within the zone using the ZONE parameter group in the FDS file.
&ZONE XYZ=2, 3.5, 0.5/
The zones specified in the FDS file are numbered in the order they are mentioned, starting from 1. All zones not specified in the file are numbered automatically, but these numbers are unknown and cannot be referenced in other parameter groups. Points within the same zone must not be specified more than once; otherwise, a calculation error occurs.
Zone numbers are important for simulating leaks.