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Fire Plume Model Comparisons

The document discusses several formulas for calculating the mass flow rate of smoke and hot gases (M) from a fire. It compares the Heskestad, Zukoski et al, large fire plume model, and CIBSE methods. The Heskestad method consistently predicted a higher M than the Zukoski et al method. The CIBSE method became less accurate for fires with lower heat release rates per unit area and shorter heights compared to the large fire plume model. Differences between the methods were generally within 20% but could be up to a factor of 2 for some conditions.

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Ramyaa Lakshmi
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55 views1 page

Fire Plume Model Comparisons

The document discusses several formulas for calculating the mass flow rate of smoke and hot gases (M) from a fire. It compares the Heskestad, Zukoski et al, large fire plume model, and CIBSE methods. The Heskestad method consistently predicted a higher M than the Zukoski et al method. The CIBSE method became less accurate for fires with lower heat release rates per unit area and shorter heights compared to the large fire plume model. Differences between the methods were generally within 20% but could be up to a factor of 2 for some conditions.

Uploaded by

Ramyaa Lakshmi
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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be small and for design purposes it may be taken as zero, that is, the source is at the base

of the fire.’
By assuming that z 0 = 0, Morgan and Mason [14] treated the CIBSE [13] approach as a
separate formula for comparison purposes.
Heskestad [16] developed an alternative design formula by re-analysing the data given by
Zukoski et al [15], given by Equation 3.

M = 0.071Qc0.33 (z − z 0 )
1.67
[1 + 0.026Q 0.67
c (z − z 0 )−1.67 ] (3)

In this case, both z and z 0 are measured from the top of the fuel stack, rather than at the
base.
The final design formula used for comparison purposes was developed by Thomas et al
[17]. This is more commonly known as the ‘large fire plume model’ and was simplified
by Hinkley [18] to become

M = 0.188Pz 1.5 (4)


where
P = the horizontal perimeter of the fire (m)
It was originally thought that Equation 4 was only applicable for low heights of rise of
plume, however, Hinkley [18] and Poreh and Morgan [19] have shown that Equation 4
applies a much greater range of heights of rise of plume (e.g. up to 10 times the value of
D ).
Morgan and Mason [14] carried out a range comparisons by varying z and the
convective heat output per unit area of the fire source. Comparisons gave rise to the
following conclusions:
• The Heskestad method consistently predicted a higher mass flow rate compared to the
Zukoski et al method. Since the Heskestad method defines z from the top of the stack
of fuel, whereas, the comparisons assumed a liquid fuel fire at floor level, Morgan
and Mason raised the possibility that this method was being used outside its range of
application. However, the Heskestad model gave reasonably close agreement with the
large fire plume model, particularly at higher heights of rise.
• The Zukoski et el method usually predicts a lower entrainment compared to the large
fire plume model, however, the difference was within 20% for the range conditions
studied.
• The effect of assuming that z 0 = 0 (i.e. using the CIBSE method) was relatively small
for fires with large convective heat release rates per unit area (i.e. approx 750
kW/m2), but becomes very significant for fires with smaller heat release rates (i.e.
approx 250 kW/m2). These discrepancies were worse for shorter heights of rise, with
differences up to a factor of 2 for the range of conditions studied (i.e. for heights of
rise up to 4 m).

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