Following an earlier report which dealt with measurements of the rate of
smoke spread and the depth of the smoke layer in experimental fires in a disused
railway tunnel representing a pedestrian mall, data are now given for the
temperature and opacity of the smoke layer.
The fall of temperature of the smoke layer as it passed along the tunnel
could be accounted for entirely by convection and radiation transfer to the
walls and floor of the tunnel so that there seems to have been little mixing of
the layer with the cold air underneath, a factor which could also lead to a
fall in temperature along the tunnel.
One smoke test was made outside the tunnel under a canopy. This emphasised
the dependence of smoke spread on the wind conditions and showed that smoke
logging to a low level could occur even when the smoke was not completely
confined.
Values can be derived for the optical density of the smoke produced by
burning a given weight of kerosine in a given volume, enabling an estimate of the
visibility to be derived in other comparable situations where the burning rate
and mixing conditions are known. The values obtained exhibit substantial variation
so that the precision of an estimate of visibility is low, though sufficient for
the present purpose. Such variation has a bearing on the problem of relating
the behaviour of materials in a smoke test to some assumed real fire situation,
a problem which itself falls outside the scope of this report.
