Fires in road or rail tunnels could
increase in size dramatically as a result of current recommendations for
ventilating tunnels to mitigate the effects of smoke, according to
British research engineers.
The researchers, funded by the Engineering and Physical Sciences
Research Council have used mathematical modelling and statistical
techniques to determine the effect of ventilation rates on different
types of fire. Their results suggest that at currently recommended
ventilation regimes, some fires could be increased in size six-or
seven-fold. Rather than having a single rate of ventilation applied in
all cases, as international guidelines currently propose, each fire
should be considered separately, the researchers say.
The work, the first of its kind in the world, has been led by Dr Alan
Beard, of the Department of Civil and Offshore Engineering at Heriot-Watt
University. ‘If there is a tunnel fire, a lot of smoke is generated so
operators switch on fans to blow air down the tunnel – a process
called longitudinal ventilation – to help people escape and allow
access by the fire service,’ says Dr Beard. ‘But there is a problem
with this: how strong does the air flow have to be to blow the smoke
away, and might it in fact help to fan the flames and make the fire
bigger or spread it to other vehicles?’
Using mathematical techniques the researchers have developed a model
which examines different types of fire. For example an HGV containing
wooden furniture will burn differently to a small car or a pool of
diesel oil.
The model can calculate the rate at which heat would need to be released
from a given type of fire to enable the flames to jump to the adjacent
vehicle. The rate at which the heat is released can in turn be
correlated with the speed of the ventilation wind applied.
The team has now turned its attention to the effect of ventilation on
the size of the initial fire. To do this they have used a technique
called Bayesian statistics. This method relies on two types of
information: experimental data on the one hand and the judgement of
experts on the other.
‘This technique is useful where there are little actual data,’ says
Dr Beard. ‘Tests in tunnels are very expensive and over the last 30
years there have been only a scattering of tests, so there is a lack of
solid experimental data on the effect of ventilation on fires.’
The technique works by presenting around six or eight experts on
combustion with a given scenario: the size of a tunnel and the type of
fire. The experts estimate how different rates of ventilation would
affect the fire.
‘Having obtained the expert opinion you then feed in what experimental
data you have one piece at a time, which adjusts the output
accordingly’ says Dr Beard. ‘The beauty of it is that as the years
go by and you get more and more hard information you can feed it in and
continually refine the model. The opinions of the experts become
increasingly less important.’
The team intends to produce software for tunnel operators and designers
that presents information in the form of charts and tables, to enable
users to estimate the effect of different ventilation rates on a
particular type of fire.
‘There are current proposals that recommend a ventilation rate of
three metres per second regardless of the type of fire. But we have
examined cases where if this rate was applied it would increase the size
of the fire seven fold,’ says Dr Beard. ‘I believe that there is no
single ventilation velocity that will apply to all cases – you might
push the smoke back but if you are increasing the size of the fire
ten-fold you are generating an even bigger problem.’
Mr Geoffrey Moore
Engineering and Physical Sciences Research Council
01793 444241 geoffrey.moore@epsrc.ac.uk
