Enclosure fires

Heat release rate required for a fl ashover to occur
Data has been correlated from over 100 experimental fi res for varying room
sizes. 21 These experiments have formed the basis for equation 18 below. This
equation can be used to determine the amount of fuel burnt up when the fi re
progresses to fl ashover. The heat release rate required for a fl ashover to occur
can be described using the following formula:
Q fo = 610(h kA TA W√H) 0.5
where Q fo – heat release rate required to trigger a fl ashover (kW)
h k – heat transfer coeffi cient (kW/m 2 K), which specifi es the amount of
heat absorbed by the enclosure surfaces
A T – internal enclosure area in room (m 2 )
A W – ventilation opening’s area (m 2 )
H – ventilation opening’s height (m)
Equation 18 has been produced for a normal-size room. But it has also proved
to be valid for signifi cantly larger rooms. Many different types of fuel have been
tested, including wood, plastic and various gases. Both windows and doors have
been used as openings. The material which the walls and ceiling are made of
have had a wide variety of properties.
However, this equation does have certain limitations. It only applies to fuelcontrolled
fi res and when the source of the fi re is not located close to any wall.
If it was located close to a wall the coeffi cients in the equation would be different.
The equation applies to temperatures between around 20 and 700 °C 7 .
There is a sample calculation on page 181, which shows the scope of application
for this equation. The equation is often used for scoping purposes.
The fully developed fi re is ventilation controlled, which means that equation
18 cannot be applied at this stage. When the fi re is ventilation controlled the
heat release rate, in particular, is limited in the room by the amount of air
fl owing in through the opening. This amount is calculated using equation 19.
When a fl ashover has defi nitely taken place we can use equation 19 to estimate
the actual heat release rate.
In actual fact, all the air which fl ows into the room is not used for combustion.
Consequently, the amount of energy released is not as much as equation
19 gives, but the approximation must be regarded as being adequate for our
purpose.
m a = 0.5 A w √H
Equation 18
Equation 19
where m a (kg/s) is the mass fl ow of the incoming air, A w (m 2 ) is the opening’s
area and H (m) is the opening’s height. (cont. on next page P)