264
7 The Role of Ventilation in Fires and Explosions
(a) Downgrade (negative): The entrance of fresh air is at a higher elevation than the
fire source. In this situation, the flowrate must be sufficiently high to overcome
the draught caused by the fire. Calculations indicate that V c will be greatest in
this kind of situation.
(b) Upgrade (positive): Clean air flows from bottom to top coinciding with the
draught caused by the fire.
In the case of downgrade slopes, an alternative expression is frequently used to
describe variation in K g (Eq. 7.14):
K g = 1 + 0.04[−RG(%)]
0.8
(7.14)
Exercise 7.2 A fire with Heat Release Rate of 6 MW has started in a 3.5 m high
gallery with a free passage cross section of 10 m
2 . The gallery is on a downhill slope
of 4%. Calculate the critical speed at which the ventilation air must be supplied to
ensure that there is no backlayering. The average temperature of the ambient air is
26 °C.
Solution
For the first iteration:
T f (i=1) = 273.15 + 26 = 299.15K
V c = K 1 K g
g H Q
ρ a C p A x T f
1
3
V c (i = 1) = 0.87 · 1.1188
9.81 · 3.5 · 6000
1.204 · 1.005 · 10 · (273.15 + 26)
1
3 = 3.76945
T f =
Q
ρ a C p A x V c
+ T a
For the second iteration, we have that:
T f (i = 2) =
6000
1.204 · 1.005 · 10 · 3.76945
+ (273.16 + 26) = 160.24
The following table shows the results of iterations 1–8:
Iteration (i)
T f (°C)
V c (m s −1 )
1
26.00
3.76945
2
160.24
3.33131
3
177.89
3.28727
(continued)
7 The Role of Ventilation in Fires and Explosions
(a) Downgrade (negative): The entrance of fresh air is at a higher elevation than the
fire source. In this situation, the flowrate must be sufficiently high to overcome
the draught caused by the fire. Calculations indicate that V c will be greatest in
this kind of situation.
(b) Upgrade (positive): Clean air flows from bottom to top coinciding with the
draught caused by the fire.
In the case of downgrade slopes, an alternative expression is frequently used to
describe variation in K g (Eq. 7.14):
K g = 1 + 0.04[−RG(%)]
0.8
(7.14)
Exercise 7.2 A fire with Heat Release Rate of 6 MW has started in a 3.5 m high
gallery with a free passage cross section of 10 m
2 . The gallery is on a downhill slope
of 4%. Calculate the critical speed at which the ventilation air must be supplied to
ensure that there is no backlayering. The average temperature of the ambient air is
26 °C.
Solution
For the first iteration:
T f (i=1) = 273.15 + 26 = 299.15K
V c = K 1 K g
g H Q
ρ a C p A x T f
1
3
V c (i = 1) = 0.87 · 1.1188
9.81 · 3.5 · 6000
1.204 · 1.005 · 10 · (273.15 + 26)
1
3 = 3.76945
T f =
Q
ρ a C p A x V c
+ T a
For the second iteration, we have that:
T f (i = 2) =
6000
1.204 · 1.005 · 10 · 3.76945
+ (273.16 + 26) = 160.24
The following table shows the results of iterations 1–8:
Iteration (i)
T f (°C)
V c (m s −1 )
1
26.00
3.76945
2
160.24
3.33131
3
177.89
3.28727
(continued)