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forcedDraught.htm
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External Window Fires - Forced Draught Fires
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The force draught conditions are assumed when
- there are windows on opposite sides of the fire compartment
or
- additional air is being fed to the fire from another source
(other than windows)
The thermal actions for external members are determined as follows.
Fire Compartment
The rate of heat release Q [MW] or the rate of
burning is given by:
 |
(1) |
where
| Af |
is the floor area of the fire compartment [m2]; |
| qf,d |
is the design fire load density related to the floor area Af [MJ/m2]; |
| τF |
is the free burning duration [=1200 sec]. |
The temperature Tf of the
fire compartment is given by:
 |
(2) |
| with |
 |
where
| T0 |
is the initial temperature [= 20°C]; |
| At |
is the total area of enclosure (walls, ceiling and floor,
including openings) [m2]; |
| Av |
is the total area of vertical openings on all walls [m2]. |
Window Flame
Unlike for the conditions of no forced draught,
the dimensions and trajectory of flame through window of a fire
compartment under forced draught conditions is assumed to be
independent on whether there is wall above the fire compartment
and the ratio of the window height and width.
Figure
1 summarises the design equations for calculating the trajectory,
temperatures, emissivity and convective heat transfer coefficient
of a window flame under forced draught conditions.
Effects of Awning or Balcony
If an awning or balcony is located at the level
of the window top, the flame trajectory is assumed to be deflected
outwards by the depth of the awning, but with the flame length
Lf remaining unchanged, as illustrated in Figure 2. The rest
of flame dimensions should be modified according to the geometric
change.
Figure 2 Deflection of flame by awning
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