|Author:||Chan, Kaai Yip|
|Title:||Experimental study of water mist on rack fire|
Hong Kong Polytechnic University -- Dissertations
|Department:||Faculty of Construction and Environment|
|Pages:||xi, 72,  leaves : ill. ; 30 cm.|
|Abstract:||Early studies on water mist focused on the extinguishing mechanism of water mist and the optimum droplet parameters for efficient fire suppression. Water mist had been considered as an alternative to gas agent like Halon. Unlike extinguishing gas, fine water mist droplets may encounter difficulties in penetrating to shielded area. Experimental data for water mist in shielded fire is not currently available and international standard for the particular water mist application is not comprehensive. Therefore the application of water mist system in such a case is limited. For these reasons the performance of water mist application in horizontal spray under shielded environment was examined under this dissertation. Total 81 sets of experiments were conducted, including tests on different nozzle orientation (vertical spray against horizontal spray); discharge pressure (12 bar against 16 bar); nozzle distance away from flame and different fire sizes. Experimental results revealed horizontal spray is more effective in extinguishing fire on rack, providing the measured mist concentration (which was measured without fire) is significantly higher. The averaged Fire Stop Time (FST, which is defined as the extinguishing time of fire by water mist or the burn out time of fuel if the fire cannot be extinguished by water mist) is 193% faster for horizontal spray than vertical spray under the same experiment condition. While it was noted mist concentration in horizontal spray was obviously and consistently higher than vertical spray, resulting in better fire extinguishing performance. Similar observations were recorded for different fire sizes. The corresponding mist concentration is 274% higher for horizontal spray than vertical spray. Furthermore increasing the discharging pressure (from 12 bar to 16 bar) under vertical spray was not able to boost up the performance significantly while reduction of vertical distance by 900mm (the distance between nozzle and flame) had an adverse effect (increment of Fire Stop Time by 166%) as mist momentum was reduced by the 'rebound' action from shield. The spread out of mist was restricted because of the reduced distance. In contrast, horizontal spray fire extinguishing performance is observed to be the highest at certain horizontal distance and pendent distance between the nozzle and fuel providing the spray had not been blocked by the shielded compartment. This is supported by the measured mist concentration where the mist concentration was also measured to be the highest at that condition with fastest fire extinguishment.|
It could be deduced from the experiment result that the application of water mist for vertical spray is not as effective as horizontal spray for extinguishing shielded fire on rack. During the vertical spray, mist momentum and quantity delivered to fuel source was diminished by the shield (rack compartment). In view of this an extinguishing model was developed to correlating the performance of fire extinguishment (extinguishing time) with mist concentration. It is an energy extraction model using mist concentration as the parameter where total heat extraction rate is the sum of rate of water mist evaporation and the radiation attenuated by mist. When Qext < Qfire, the fire can be extinguished by the water mist. Higher mist concentration is resulting in faster fire extinguishment (shorter extinguishing time) as the rate of heat extraction is also faster. The combined experimental data for vertical spray and horizontal spray under shielded environment well fit the above theory. It could be seen the Fire Stop Time decreases with the increase of mist concentration from the plot. Base from the study, mist concentration is an important indicator to fire extinguishing performance for liquid pool fire in shielded environment. Therefore it is suggested mist concentration could be considered as a supplement or alternative to the prescribed spacing requirements for nozzle installation, especially for complex room or compartment protection.
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