| In recent years,the fire accidents aroused frequently of electric vehicle,due to thermal runaway of battery,that threaten human life and property.Therefore,it is particularly important to explore the technology that can extinguish fires effectively to ensure the fire safety of electric vehicle.The fire model of lithium-ion battery box was established,in which the water mist was added in order to investigate the influence of featured parameters and layout schemes of nozzle on the fire extinguishing.Firstly,fire models of vehicle lithium-ion battery cell and module were established and simulated,compared with the temperature variation in previous experiment to ensure the suitability of the fire models,that the revised battery module model of fire dynamics was obtained after continuous optimization.Secondly,the fire model of the battery box was established based on the technical and regulatory requirements,in which the water mist was added,and the influence of featured parameters was discussed,including:spray angle,spray intensity,droplet diameter and initial spray speed,that controlling variable was adopted,different effects of fire extinguishing were analyzed.The results showed that the rise rate of average ambient temperature was significantly lower when the water mist was applied,due to the combustible material was cooled rapidly.The protective area was related to the spray angle,the change of the spray angle showed minimal influence on the fire extinguishing effect of the battery box which was a limited and dense space.The filled rate of the protective area and the achieved concentration of droplets were both determined by the spray intensity,that the effectiveness of the water mist will be improved with the increase of spray intensity,but when the spray intensity was greater than 3 L/min,the effectiveness was not enhanced significantly.The ability to absorb heat through gasification and specific surface area increased as droplet diameter was reduced,while the effect showed no obvious improvement when the diameter was reduced to 200 μm.Higher relative initial spray speeds resulted in more loss of kinetic energy,which had an impact on the ability to scour the flame,but the effect of fire extinguishing was not improved significantly with the increase of initial spray speed.The optimal combination of featured parameters of the water mist was obtained after extinguishing efficiency and monetary cost were taken into account,with 60° spray angle,3 L/min spray intensity,200 μm droplet diameter and 30 m/s initial spray speed.Finally,the optimal layout scheme of nozzle was also discussed.Therein,three different installation schemes:the single nozzle,double nozzles on the top left and right,double nozzles on the top diagonal were designed respectively in the fire model,the process of fire extinguishing with the initial fire source of the bottom center,the top center,and the bottom corner in the battery box was simulated and analyzed.The results showed that the effect of the single nozzle was the worst when the initial source of fire was located in the bottom corner.Double nozzles on the top diagonal and on the top left and right both showed the similar effect for the fire of bottom corner and center,the former demonstrated the best effect of fire extinguishing when the initial source of fire was in the top center.Setting up double nozzles on the top diagonal was the optimal scheme of the three after comprehensive analysis.The fire models of lithium-ion battery cell and module were obtained,and the change of temperature can be simulated realistically using software,the optimal combination of featured parameters and the optimal layout scheme of nozzle,which were obtained can not only provide ideas for safety design of the battery box to extinguish the early flames timely,but also provide important references for improving the safety of electric vehicle. |
