ENCLOSURE FIRE DYNAMICS

1. Suggested solution:
Heat release rate is given as the product of burning surface and the heat release rate per
unit surface area (Hurley, Morgan and Quiter, 2003, p. 45):
Q  q0 Am  As 
A m stand for the melting area, while As stand for the stacked area.
Q  (4.2×6.2) (1.8×0.65)
Q  26.04 + 1.17
Q = 27.21
There are formulas of deriving heat release rate which were developed by McCaffrey and
Heskestad.
Limitations of the two methods:
They do not produce exactly the same figures. They are used to bind a problem since they
give a range of possibilities in comparison to scientifically proven results (Hurley,
Morgan and Quiter, 2003, p.66).
2. Suggested solution:
At 20°C k = Ae Ea/RT

k = 2.5 × 10 6 × 1.21 × 10 -9
k = 3.02 × 10 -3 s -1
At 30°C k = Ae Ea/RT
k = 2.5 × 10 6 × 2.38 × 10 -9
k = 5.94 × 10 -3 s -1

k at 30°C is (5.94 × 10 -3 s -1 ) / (3.02 × 10 -3 s - 1) 1.97 times greater that is 1.97 times greater,
hence the reaction is 1.97 times faster.

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3. Suggested solution
Using equation 3.3 we get (Heskestad, 2016):
Q=3g/s.25kJ/g.0.7 =52.kW
From table 3.2 we find that PMMA has an average burning rate per unit area of 30g/m 2 s),
while table 3.3 gives 20g/ (m 2 s). This is partly due to experimental differences. Assuming
an average value of 25g/ (m 2 s), the size of the burning slab is roughly 3/25= 0.12m 2 .
4. Suggested solution
Calorific power of the flame
H f = 0.235 Q2/5 - 1.02 D
Where
H f = pool fire flame height (m)
Q = pool fire heat release rate (kW)
D = pool fire diameter (m)
H f = 0. 235 (28.6)2/5) - 1.04(1)
H f = 6.38- 1.04
H f =5.34 m
5. Suggested solution
When gases are mixed in advance, the laminar burning velocity will experience a change
in accordance with the range of flammability where the mixture falls. The speed of
unburnt gases inside the flame is referred as the laminar burning velocity. In case the
mixture is situated in the outer flammability ends the velocity will burn slowly. On the
other hand, in case the mixture is situated to the stoichiometric point the burning velocity
will be more quickly (Heskestad, 2016, p.33).
According to Karlsson and Quintiere (2000), the burning velocity is expressed as Mach
number equals speed of flame divide by the speed of sound in a stable fluid that is ahead
of the flame. In case the value of Mach is greater than one, it produces supersonic speed.
But when it is less than one it is referred as subsonic.
A pressure of 9.45 the laminar velocity is supersonic.
6. Comparison of toxic potency of wool and rigid polyurethane

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According to Karlsson and Quintiere (2000), toxic product yields are affected by two
factors which are chemical properties of materials used and ventilation of the fire. In
most cases fires taking place in buildings experience little oxygen despite a lot of heat
within rooms that yield too much heat that is very high compared to an open space. Fire
potency and toxicity of material is greatly affected by materials used. Toxicity of fire in
insulating materials has a big role in safety of fire when constructing a modern house.
Materials like rigid polyurethanes have the potential of creating Hydrogen Cyanide gas
can release a potent carcinogen called Benzene. Mineral wool and glass wool are not
immune as they are made from materials composed of fibres which produces toxic gases
although at minimal levels. In burning regimes these insulating products has the potential
to release irritant gases such as Hydrobromic, Hydrochloric and Hydrofluoric acids which
also include various NOx gases and Acrolein and Formaldehyde. In higher concentration
these may lead to fatalities after long exposure as a result of lung inflammation.
7. Suggested solution
Estimate the extinction coefficient of the smoke produced by flaming combustion of
0.2kg of polystyrene-foam in a square 6m room of 2.5m height
Absorbance = Concentration × Extinction coefficient Ɛ × path length
Extinction coefficient = (Absorbance)/ (Concentration × path length)
Extinction coefficient = 200g/250cm
Extinction coefficient =0.8g/cm
8. Suggested solution
Mean flame height
Pool diameter = 2.5m
Heat release rate = 500kW/m2
L f = 0.235 Q 2/5 – 1.02D
L f = 0.235(500) 2/5 – 1.02 (2.5)

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L f = 2.82 – 2.55
L f = 0.25 m
9. Suggested solution
Safe levels of CO, CO2, HCN


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