On the mechanisms of extinction of fire by the application of water sprays
An investigation has been conducted into the mechanisms by which water sprays
interact with solid fuels undergoing thermal decomposition.
A cone heater was used to provide one-dimensional radiative heating to an oven-dried
wood sample with an exposed face measuring 100 by 100mm. A continuous scanning
gamma ray densitometer was designed and built to provide a density profile of the
sample as it undergoes decomposition. This non-destructive analysis technique
137Csto produce a narrow, collimated beam of gamma ray radiation which is
passed through the sample. Analysis of the transmitted intensity of this beam is then
related to the local density of the sample.
Thermocouples, imbedded within the sample are used simultaneously, to provide a
temperature profile of the sample and a load cell is used to provide thermogravimetric
data as thermal decomposition proceeds. Together, these measurements serve to
characterise the progress of the decomposition zone and the development of a char
layer on the sample.
A simple and easy to replicate, external-mix, air-assist water spray nozzle was
developed to produce water sprays with a wide variety of characteristics. Laser
diffraction particle sizing and laser Doppler velocimetry were used to characterise the
water sprays produced. Droplet sizes from 8 μm up to 150 μm can be produced, with
a wide variety of velocities and application rates.
With spray applied to the decomposing solid, the instrumentation was employed to
study the progress of water through the char layer. With experiments looking
separately at pooled water, large droplets and fine droplets applied to the surface of
the solid, the water could be directly measured as it is adsorbed into the sample
surface. The temperature effects within the sample were also measured.
It was found that with water pooled on the surface, there was penetration of this water
to the upper levels of the decomposition zone. With large droplets, at higher
application rates, water soaked deep into the char layer to the decomposition zone.
Temperature profiles suggested that movement of water was probably through the
cracks and fissures in the char layer rather than soaking through the char itself. At
lower application rates, penetration was lower but water deep in the sample took some
time to evaporate after the spray was switched off, providing a barrier to externally
Evaporation of water which has penetrated into the sample is seen to occur largely
from the layers nearest the surface at first, but some water remains in these layers
until considerable evaporation has occurred from lower layers. This suggests a
cooling mechanism from the mass transfer of steam rising through the sample.
A computational model has been developed, based on fundamental heat transfer
mechanisms which predicts with reasonable accuracy the temperature, and density
profiles found by experiment as well as the thermogravimetric results. The model has
been extended to account for water which has penetrated into the char layer and
approached the decomposition zone