|Title:||Experimental performance evaluation of water mist fire suppression system|
|Subject:||Hong Kong Polytechnic University -- Dissertations.|
|Department:||Department of Building Services Engineering|
|Pages:||144, 19, 32, 75, 42 leaves : ill. ; 30 cm.|
|Abstract:||The present thesis attempts to understand the performance of the water mist fire suppression system, including the water mist nozzles and the whole system operation under different conditions through experiments and theoretical analysis. The impacts of the water mist nozzle, different fire parameters on the performance of the fire as well as the quantitative correlations between the fire size, i.e. the heat release rate of the fire and the room temperature change with respect to the extinguishing time were studied. A comprehensive literature review of the studies of the water mist system conducted by different researchers and organizations in different countries has been carried out. The findings from the study were presented in the thesis. It was found that under the ideal situation, water mist discharged is assumed to evaporate completely after coming into contact with the flame. With a proper design, the WMFSS can be used to extinguish electrical fires without danger. The WMFSS could be applied in different forms including total flooding and local applications. Apart from the many advantages, on the other hand, the WMFSS was also found to have some limitations under different fire scenarios. Its extinguishing performance is case-dependent and highly relates to the fire scenarios concerned. In spite of design protocols from four big international organizations from different countries being provided, no "rule of thumb"- type design standard is available for the system. It was also found that three common types of water mist nozzles are widely applied including the impinging type, pressure-jet type and twin-fluid air-atomizing type, and different water mist nozzles have different characteristics and application areas. 30 full-scale experiments have been conducted in Mainland China. The full-scale burning tests were carried out in a room calorimeter at Lanxi, Harbin, Heilongjiang, China. In the full-scale tests, different ventilation conditions, fire sizes, discharging methods of water mist, pre-burn time of the fire and the fire types were chosen as the testing parameters. Water mist discharged from a low-pressure nozzle was the study target. Two ventilation conditions were used in the tests such as limited ventilation with one third of a door sealed and full ventilation with the door open. Two discharging modes including continuous discharging mode and cycling discharging mode were tested. Pre-burn time of 90s, 120s and 180s were allowed to set up different fire scenarios. Liquid pool fires using gasoline and diesel as the fuel source and solid fuel fires with PMMA cribs as the fuel source were set up. During the experiments, heat release rate, temperature and oxygen concentration were recorded for detailed analysis. The experimental results revealed that under the same operation condition of pressure and flow rate, factors like the fire size, type of fuel, pre-burn time and discharging method affected the performance of the WMFSS in different ways. Through results analysis, it was found that the extinguishing time is proportional to the heat release rate for liquid fires, which is consistent with the predicted results by a mathematical model. The steady state room air temperature after extinguishment also increased with the heat release rate. The peak temperature difference was directly proportional to the extinguishing time. Some experimental findings agreed with the literature results. The study results are helpful to understand the effectiveness of the water mist system when facing different fire challenges.|
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