Experimental Study On The Fire Suppression Effectiveness Of Water Mist In Low Pressure Environment

In order to find an alternative fire extinguishing agent for Halon and satisfy the requirements of fire re-ignition suppression technology in large aircraft cabins,experimental study on the fire suppression effectiveness of water mist in low pressure environment were carried out in the support of the National Natural Science Foundation of China,‘Key technology and experimental verification of water mist fire suppression in aircraft cargo compartment'.Water mist fire extinguishing technology has been favored by people in recent years due to its advantages such as high fire extinguishing efficiency,environmental compatibility and no toxicity.Large numbers of studies and applications of water mist in normal altitude areas have been carried out.Especially,owing to the above advantages,studies on fire suppression in low pressure environment with water mist were also performed recently.Fire suppression effectiveness of water mist will change with altitude and ambient pressure conditions.However,few studies on fire suppression effectiveness and mechanism in low ambient pressure conditions have been done.Systemic experimental research on fire suppression effectiveness of water mist under lower ambient pressure conditions were carried out in this paper.A small-scale closed experimental platform was built to conduct experiments.Gas flow rates were set as 120,150,180,210,240 and 270lpm and liquid flow rates were set as0.2,0.4,0.6lpm and droplet size,fire extinguishing time,smoke concentration,flame temperature and combustion rate were measured.The experimental results show that the average droplet size in low pressure environment was about 27.1%smaller than that in normal atmospheric environment and the average fire extinguishing time was 36.1%shorter.In lower ambient pressure conditions,the maximum temperature of the flame and burning rate of the flame are lower than that in the normal pressure.Concentrations of oxygen,carbon dioxide and carbon monoxide show different trends under normal and low pressures.To reveal the extinguishing mechanism of water mist under low pressure environment,a mathematical model which was used to analyze the size change of droplet before and after entering flame region was established.Based on the law of quadratic square on evaporation of single droplet,basic model was built combined with force analysis on droplets.The final model,(9²+_?92?⁶2?₀^?6)???(9?=(9₀²,was obtained by improving the model after layering the flame area.The critical particle sizes which were the initial particle sizes which the droplets just passed through the flame region and reached the oil surface were calculated as 96.98?m under normal pressure and 97.26?m under low pressure.Because they were larger than the optimum sizes of fire extinguishing under both conditions as 88?m and 68?m,it can be considered that the fire extinguishing under experimental conditions was the result of the combined action of flame cooling and fuel surface cooling.According to the experimental results,the recommended particle size ranges for fire extinguishing is 80?m-100?m under normal pressure and 60?m-80?m under low pressure.This model can be used to determine whether a certain size of water mist can pass through the flame zone and the particle size after that under the experimental conditions and qualitatively judge what kind of fire extinguishing mechanism plays a main role at this time.And it provides a foundation for quantitative analysis.