Numerical And Experimental Study Of Fire-Extinguishing Performance Of Ultra-fine Water Mist

Fire protection for electrical equipment space is a special problem in fire engineering. In the past, Gas fire-extinguishing systems (GFES), such as halons, carbon dioxide, etc. were widely used to protect these kinds of space. However, these systems have some deficiencies:false actuation of GFES in enclosed space may threaten the safety of occupants such as, emergency fire-fighting need evacuation. Many studies focus on the extinguishing efficiency of water mist in these kinds of space. Conventional water mist system has limitations:1) Difficulty to extinguish small fires;2) Difficulty to extinguish shielded or obstructed fires;3) The increases of flow rate of water mist had little, if any, effect on fire extinguishment capabilities of water mist system against shielded or obstructed fires;4) Cause water damage to equipment, particularly in electronics spaces or data center sub-floors. These limitation is mainly associated with high droplets-fallout rates due to gravity that tend to significantly decrease the mist concentration especially in regions away from the nozzle spray patterns. Hence, new atomization technologies which can generate water mist with gas-like transport characteristics are needed. Previous studies discovered that ultra-sonic atomization could generate gas-like water mist (<10μum) which has the ability to transport around obstructions without great loss of water mist. Therefore, the present study focus on the fire-extinguishing performance of ultra-fine water mist (UFM), the method of improving its fire-extinguishing performance and the flow behaviour of UFM.The work and contributions of this study can be summarised as follows:1) On minimum extinguishing concentration of ultra-fine water mist:The minimum extinguishing concentration of ultra-fine water mist was modeled based on limiting oxygen concentration and combustion limit temperature, respectively. By analyzing minimum extinguishing concentration from the two models, it is concluded that heat absorption is a way more potential than oxygen dilution in extinguishing fire for ultra-fine water mist. Fire extinguishment experiment was then carried out in a modified cupburner. Tests using the same scenario were repeated many times to record the average and the standard deviation of extinguishing times. For minimum extinguishing concentration, experimental results agree well with the model based on limiting oxygen concentration, and disagree with the model based on combustion limit temperature, the reason of which is flow behavior of ultra fine water mist:a) Ultra-fine water mist totally evaporate around flame and hardly enter flame core; b) And only water vapor generated near flame followed the entraining flow and interacted with the lame. The mist concentration should be higher than a critical value to be able to extinguish the fire with a stable fire extinguishing time. However, there is no need to a mist concentration over certain quantity because it will not improve the efficiency of fire extinguishing system. Because the more potential mechanism cannot be activated, the fire extinguishing performance of ultra-fine water mist need improve. Reasonably increasing droplets size may improve the residence time of ultra-fine water mist in flame. Adding chemical additives can lead to the generation of sub-particles after total evaporation, and the sub-particles may penetrate into flame to improve the performance of ultra-fine water mist.2) On fire extinguishing performance of ultra fine water mist system:with additives Based on the analysis of droplets size distribution of ultrasonic atomization and the drag force of a droplet in airflow, effective mass fraction of ultra-fine water mist was modeled. The model indicated that the effective mass fraction of ultra-fine water mist would decrease with the increase of solution surface tension, and that the increase of water temperature can increase fire extinguishing effectiveness of ultra-fine water mist. A simple test was conducted to measure the change of effective mass fraction of ultra-fine water mist. Experiment agrees well with the theoretical analysis, which showed that the increase of the concentration of metal salt will decrease the mass of ultra-fine water mist, and that the adding of surfactant can increase the mass of ultra-fine water mist. Fire extinguishment experiment was then carried out. Experiment showed the increase of water temperature can increase fire extinguishing performance of ultra-fine water mist system, which agrees with theoretical analysis. The adding a small quantity of certain saline to the ultrafine water mist could significantly improve the fire extinguishing performance of ultra-fine water mist system. However, the increase of the fire extinguishing efficiency by the further increase of saline would not so obvious, which result from ultrafine water mist mass was decreased by the increase of surface tension of water solution. Based on the number of ultrasonic atomizer needed, on a mass fraction basis, the following order of effectiveness is:K2C2O4> K2CO3> KCl> KHCO3> NaCl> CH3COONa>KH2PO3> Urea. The adding of urea will lead to combustion enhancement. a multi-component method was proposed to improve the fire-extinguishing performance of ultra-fine water mist. The method is adding both a type of metal salt and a surfactant.3) Numerical study on the interaction diffusion flame with ultra-fine water mist: The empirical model for predicting local extinction based on the oxygen concentration in FDS, is unable to determine fire extinguishing in the study. The reasons of that are lack of model for simulating droplets transportation and evaporation around flame, and lack of model for fire extinction. The EDC-modified model in SIMTEC can capture the extinguishment of flame. But the model cannot predict the minimum extinguishing concentration of ultra-fine water mist. Finite-rate model with detailed chemical reaction in FLUENT, can simulate flame extinguishment, and predict the minimum extinguishing concentration of ultra-fine water mist. The primary mechanism of fire extinction in the simulation using FLUENT is oxygen displacement, which agree with experiment. Fire intensification was observed in the simulation using FLUENT. The mechanism of fire intensification by NH3and HNCO is a) NH3+H=NH2+H2, b) NH3+O=NH2+OH; and that the primary mechanism of fire intensification by HNCO is a) HNCO+O=NCO+OH, b) HNCO+H=NH2+CO, c) HNCO+OH=NCo+H2o.4)CFD simulation and experiment study on flow behavior of ultra-fine water mist: The CFD simulations using DPM cannot simulate the transport and flow behavior of the low momentum UFM. The dense gas model showed a significant improvement in predicting the UFM transportation and flow behavior. Larger size of fire is easier to extinguish in compartment space, which can promote the transportation of UFM in compartment. However, larger size of fire is more difficult to extinguish in tunnel space, which prevent UFM transport to the other side of fire. The effect of obstruction in extinguishing efficiency depends on the location of obstruction. If obstruction was located between fire source and mist source, the obstruction would decrease the extinguishing efficiency of UFM. If obstruction was located behind fire source, the obstruction would improve the extinguishing efficiency of UFM.