Study On Flow Field Characteristics And Fire Suppression Effectiveness Of Water Mist In Aircraft Cargo Compartment

Civil aviation safety is indispensable during the transition from being a civil aviation power to being strong in civil aviation.As the types of cargo carried in aircraft cargo compartment become more and more complex,the fire safety of civil aircraft cargo compartment must be taken seriously.Halon potential alternatives have been constantly developed in the countries all over the world,because halon,as the traditional airborne fire extinguishing agent,depletes the stratospheric ozone layer.Water mist has become an ideal Halon alternative in cargo compartment,due to its fast fire extinguishing and cleaning characteristics.In this paper,the flow field characteristics and fire suppression effectiveness of water mist in aircraft cargo compartment are studied.The research results will provide foundation for the airborne application of water mist fire suppression technology and the development for next generation of fire suppression prototype system used in aircraft cargo compartment.According to the requirements of the Minimum Performance Standard for Aircraft Cargo Compartment Halon Replacement Fire Suppression Systems?MPS?,an experimental platform was designed and built to investigate on the flow field characteristics and fire suppression effectiveness of water mist.The self-designed supersonic resonance twin-fluid water mist nozzles were selected in this study,four of which were arranged in the experimental platform.In the experiments,total gas flow rates were set as 1000,1100,1200,1300,1400 lpm,respectively,while total liquid flow rates were set as 2,3,4,5,6 lpm,respectively.In this paper,the parameters for analysing the flow field characteristics such as the water mist volume flux,the the water mist particle size distribution,and the droplet velocity were measured with the water mist collected,the laser particle size analyzer and the three dimentional particle image velocity?3D-PIV?,respectively.In order to analyse and validate the fire suppression effectiveness of water mist,several instruments were utilized for measuring the typical parameters such as the flume temperature,the ceiling temperature,the radiation heat flux,the weight loss,the smoke concentration,et al,while the video images were recorded by the monitoring camera for revealing the fire suppression process and obtaining the extinguishing time.During the experiments for the flow field characteristics of water mist,it is found that the flow field characteristics are mainly determined by gas flow rates and liquid flow rates.There is a positive correlation between the gas flow rate and the water mist particle size,while a negative correlation between the liquid flow rate and the water mist particle size.The D_V90 is less than 200?m under all experimental conditions,indicating a good atomization effect.The water mist volume flux in the center of spray is large,and the volume flux at the edge is small.The distribution of water mist volume flux is mainly affected by gas flow.The droplet velocity decreases with the increase of the distance from the nozzle.There is a positive correlation between the gas flow rate and the droplet velocity,while a negative correlation between the liquid flow rate and the droplet velocity.Based on the water mist fire suppression effectiveness experiments,it is found that there are different combustion characteristics for the surface-burning fire and the containerized-load fire.The containerized-load fire can be divided into three stages:internal combustion,outward spread and downward spread.By analyzing the variation of the flume temperature,the ceiling temperature,the radiation heat flux,the weight loss,the smoke concentration,et al,according to the requirements in the MPS,the results show that a good fire suppression performance of the twin-fluid water mist in the scenarios of surface-burning fire and containerized-load fire.The time-temperature area is computed after the activation of the suppression system for quantitative analysis on the cooling effect of the twin-fluid water mist in fire suppression.Through the comparison between the actual water mist particle size and the theoretical minimum particle size that can pass through the flame,it is found that the flame cooling is the dominant mechanism for the twin-fluid water mist in suppressing the surface-burning fire,while the fuel cooling is the dominant mechanism in the containerized-load fire suppression.The influence of water mist on smoke during fire suppression were also studied,and the results show that there is a good dilution effect of the twin-fluid water mist on toxic and harmful gases.