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Title: On the mechanisms of extinction of fire by the application of water sprays
Author: Price, Edward Michael Francis.
ISNI:       0000 0001 3500 1044
Awarding Body: London South Bank University
Current Institution: London South Bank University
Date of Award: 2005
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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 utilised 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 applied heat. 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
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available