Simulation Study On The Release Characteristics Of Expansion Nozzles Of Typical Gas Fire Extinguishing System

Fire extinguishing system is an important part of aircraft,and it plays a vital role in aircraft safety.Because the aircraft has very strict requirements for safety and economy,the requirements for the reliability and economy of the fire suppression system are also very high.As one of the important components of aircraft fire extinguishing systems,fire extinguishing nozzles seriously affect the release rate of fire extinguishing agents,and then have a great impact on the fire extinguishing efficiency of the entire system.For this reason,this paper focuses on the release characteristics of the expanding gas fire extinguishing agent nozzle.Through numerical simulation experiments,the influencing law of the key parameters such as inlet/outlet area ratio,aspect ratio,roughness,and cross section expansion angle on the nozzle release characteristics is studied in detail.The mechanism of various factors affecting the nozzle release characteristics is studied.Firstly,the inlet boundary conditions of the model in the steady-state flow of fire extinguishing agent were determined experimentally.Taking halon 1301 as an example,the gasification phase transition model of the liquid fire extinguishing agent was implanted based on previous experience.By comparing two density change models,more accurate simulation results were obtained.Comparison of simulation results with experimental values and literature reports shows that the model has good accuracy and effectivenessSecondly,based on the FLUENT simulation software,a three-dimensional simulation model of the release characteristics of the fire extinguishing agent nozzle in steady-state conditions was constructed,and the influence of inlet/outlet area ratio,aspect ratio,roughness,and expansion angle of two types of expansion nozzles,sudden expansion and divergent nozzle,on the release characteristics such as pressure and velocity at the nozzle outlet were explored.The results show that roughness basically does not affect the velocity and pressure change of the nozzle outlet;as the area ratio increases,the average pressure at the outlet of the sudden expansion nozzle gradually decreases and becomes flat,the average velocity shows a segmental change.The outlet pressure of divergent nozzle drops,then rises after reaching a certain value,and the outlet velocity gradually increases.The outlet velocity turns to maximum when the area ratio is 2,and then decreases;as the aspect ratio increases,the nozzle outlet velocity and pressure change in stages;when the expansion angle is small,the influence on the nozzle release characteristics is large.Then,based on the aforementioned simulation results,the characteristics of the flow velocity and pressure distribution of the fire extinguishing agent inside the nozzle,the fluid velocity and pressure distribution on the nozzle’s central axis were compared and analyzed,and the internal flow fields of the sudden expansion and divergent nozzles were compared.The formation law of the flow field inside the nozzle and the influence mechanism of various factors on the release characteristics of the nozzle were studied.The results show that the influence of nozzle roughness on the structure of the flow field in the nozzle is small,and the inlet/outlet area ratio,aspect ratio,and expansion angle have a greater effect on the structure of the internal flow field of sudden expansion and divergent nozzles.Finally,based on the experimental and literature data,a three-dimensional simulation model of the extinguishing agent flow in unsteady-state conditions was constructed.Through the analysis of the structure characteristics of the flow field inside the nozzle,the influence law of the changing inlet conditions to the pressure and fluid velocity of the sudden expansion nozzle was initially explored.The results show that the pressure and velocity of the nozzle increase extremely fast in the initial stage of the fire extinguishing agent releases.During the stable release stage,the flow field structure in the nozzle is consistent with the research results in steady state conditions.After the fire extinguishing agent is completely released,the pressure in the nozzle continues to decrease.The speed increased sharply and then decreased slowly.