Experimental study of backdraught using solid fuels
Item statusRestricted Access
Embargo end date03/07/2020
Wu, Chia Lung
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Backdraught is one of fire phenomena that is yet to be fully understood. Previous researchers have tried to use gaseous fuel to investigate the mechanism of backdraught. Some results for this phenomenon have been proposed, including a minimum concentration value required to trigger backdraught. However, the value may not be able to widely be used as it was based on methane, which can be ignited at ambient temperature, as the fuel. Backdraught can be deemed as an expression of a fire, it requires heat, oxygen, and fuel itself. Unlike a normal compartment fire, a starting point of this phenomenon must make the fire compartment become under-ventilated (less oxygen supply, even no oxygen supply). Then a sudden door/ window opening occurs, the incoming fresh air support the fire reignition. The opening introduces oxygen to allow backdraught to occur, which reflects the critical value of unburned smoke, as the fresh air will dilute the smoke; as to the effect of heat, there is still a lack of research about the role of the temperature inside fire compartments for the occurrence of backdraught. In previous research, electric sparks were commonly used as an ignition source, therefore, the heat required to trigger backdraught may not be an issue in their backdraught studies, but it is possible to generate backdraught without any ignition source present, in demonstrations of most firefighting training centres. Hence, the characteristics of backdraught may be different. A series of experiments was conducted to investigate factors that influence the occurrence of backdraught and its behaviour. A new finding is that a ‘a critical temperature exists’ before a backdraught can be triggered. Also, it was found that any temperature higher than the critical temperature can always trigger backdraught if the experimental fire experienced flashover before the door was closed. But this critical value varies with fuels used. For polypropylene (PP) and high-density polyethylene (HDPE), the critical temperatures are 350°C and 320°C, respectively. However, even with a temperature below the critical temperatures, a pilot spark still can trigger a backdraught, but generating a backdraught almost immediately. Also, CO/CO2 value and CO concentration are other factors governing the occurrence of backdraught, but the values change with temperature, and also if a pilot is present. If there is no extra ignition source, a CO concentration of at least 0.6% in the compartment, before the door is opened, is necessary for backdraft to occur without a pilot spark. When at temperatures slightly below the critical temperature for non-piloted backdraught, a CO concentration of above 2.8% and a pilot source are essential conditions for the occurrence of backdraft. Backdraught always requires a time of the delay after the change of ventilation, it makes firefighters hard to know when a backdraught may occur in fire scenes. As known that the characteristics / behaviours of backdraught are not identical when the ignition sources are different. Therefore, the prediction of the delay time on the basis of the ignition sources is crucial as it affects the life safety of firefighters. There is no guarantee any type of ignition may be in fire scenes, hence, both of non-piloted and piloted ignition source should be considered. Result shows that the delay time of backdraught can be predicted. For non-piloted ignition experiments, the stability of the hot gases in the fire compartment directly affects the time of the ignition of backdraught. In addition, a suitable condition is required for non-ignition backdraught, when the opening size and the volume of the compartment is known, the longest delay time of backdraught may be predicted. In general, when a pilot is present, a backdraught event will occur much faster. This work demonstrates that when using solid fuel to generate backdraught, the result provides some new findings compared with the previous research. The experimental design was used to create a realistic fire condition that may suitable for backdraught to occur without any ignition apart from the hot smoke, although the study was limited to PP and HDPE fuels only. But, at least, it was found that both temperature and unburned smoke are essential for backdraught. It makes the future work on backdraught can have different views, which is that if the regime of the critical temperature diagram can be mapped out by investigating different fuels (or fuel combinations) by continuing the experimental procedure in the thesis. In the future, a safe working environment for firefighters when tackling a possible backdraught event may be able to be achieved.