Experimental Evaluations of Portable and Fixed Water Mist Systems

• Main Authors: Chang, Wen-Yao, 張文耀
• Other Authors: Chen, Chiun-Hsun
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/21391660227209015690

博士 === 國立交通大學 === 機械工程學系 === 97 === This thesis consists of two parts. In the first part, a series of tests subjected to various discharge methodologies and fire scenarios were carried out based on a portable water mist fire extinguishing system with additive on pool fires. Different fuel types, nozzle discharge angles, additive solution volumes, amount of fuels and cross-section area of pans were selected as the major experimental parameters. The fuels used were heptane, gasoline, and diesel, the nozzle discharge angles are 30°, 45°, and 60° with respect to the horizon, and the additive solution volumes were 0%, 3%, 6% and 10%. The dominant mechanisms of restraining fire in the higher nozzle discharge angle regime (>45?a) are flame cooling and oxygen-displacement, and in the lower one (<45?a) are fuel vapors blocking and dilution. The portable water mist fire extinguishing system used has a good ability for radiation attenuation and temperature reduction that can provide a good protection for the operators. By using water mist with additive, the fire extinguishing efficiencies are significantly improved. However, if too much additive is provided, the fire extinguishment efficiency will decrease. The tendencies of the fire extinction times for different amount of fuel in a size-fixed pan are similar. Although the situation of non-uniform fuel surface resulted from water mist impingement slightly reduces the burning rate, it can be ameliorated as the height of liquid fuel attains at 1cm. The fire extinguishing efficiency is not only influenced by mist effects but also by additive ones. Therefore, there must be an optimal mixing ration between the mist and additive for fire suppression. In the second part of the thesis for assessing the fire protection performance in wet bench fires, several field tests were performed using a water mist system installed in the wet bench. The test parameters were operational pressure, pan size, nozzle location, cylinder obstruction and degree of door closure. An appropriate design for operating pressure and the location of water mist nozzles extinguished wet bench fires effectively in the early fire stages. The nozzles are suggested to be fixed above or on the each side of the pan, ensuring that mist can completely cover a pan surface with sufficient momentum. With this suggested design, fires can be extinguished in the pan and do not spread over the wet bench. In the particle image processing, SMD and water mist density distribution were investigated with three parameters, discharge pressure, volumetric additive concentration and nozzle type. By using PIV, all experimental pictures were caught in three acquisition locations, 23 cm, 40cm and 52cm away from the nozzle respectively. The study indicated that SMD shrunk as discharged pressure increased. With different concentration of additive, the “W” shape curve for SMD variation gave explanations for the extinction time mentioned in the previous thesis. The result showed that the major factor for the performance of fire extinguishing was SMD and the minor one was chemical reaction by organic metal compound.