Study On Gas-solid Two-phase Flow For Dry Powder Fire Extinguishers

The research of supersonic particulate two-phase flow is widely used in various fields such as aerospace,coal chemical industry,surface forming,fire protection engineering and so on.Dry powder fire extinguishers occupy a dominant position in the field of fire fighting due to their low economic cost,simple operation,high fire extinguishing efficiency,and wide application range.The flow process of supersonic particulate two-phase flow gas phase and particle interaction is more complicated.With the addition of solid particles,Mach number distribution,and pressure distribution of the two-phase flow field also change.The movement characteristics of solid particles are also different,thereby affecting the fire extinguishing efficiency of dry powder fire extinguishers.In this paper,based on computational fluid dynamics(CFD),a numerical study of supersonic particulate two-phase flow used in dry powder fire extinguishers is carried out.Taking the common dry powder fire extinguisher hose as a prototype,a geometric model is constructed,the SST k-?model is used to calculate the turbulent continuous phase of the flow field through the dry powder fire extinguisher hose,and the DPM model is combined to calculate the discrete coherent powder particles.The two-phase flow field characteristics and particle movement characteristics of the supersonic gas-solid two-phase flow of different particle sizes(1,10,30,50?m),different nozzle half-diffusion angles(a₂=3°,4.5°,6°,8°,10°),and different nozzle cross-sectional shapes(rectangular cross section,round cross section)are analyzed.The main research conclusions of this paper are as follows:(1)With the particle size increases,the Mach number contour moves downstream along the nozzle axis,and the shock wave position moves downstream accordingly,the shock wave effect increases as the particle size increases.From the analysis of particle movement characteristics,it is found that based on the comprehensive consideration of the concentration of dry powder particles and the injection speed,dry powder particles with a particle size of 10?m can achieve a better fire extinguishing effect.(2)With the increase of the half-diffusion angle,the shock wave intensity of the external jet flow field increases continuously,and the shock wave position gradually moves upstream and close to the nozzle outlet position;when ?₂=4.5°,the jet core area is the longest.The jet core area has the highest velocity,and the two-phase flow velocity can still maintain a high level at a longer jet distance.From the particle velocity distribution analysis found that at the best distance from the usual fire extinguishing source target,combination of dry powder particle jet velocity and concentration,considering that when the nozzle half-divergence angle is 4.5°,a better fire extinguishing effect can be achieved.(3)For the three-dimensional rectangular cross-section nozzle,the particle velocity in the core area of the gas-solid two-phase jet is higher,and the surrounding particle velocity gradually decreases,and the particle velocity distribution appears obvious stratification.The particle velocity above the peripheral jet is higher than that on the side,and the particle distribution at the exit boundary of the external flow field is vertically elliptical.The contour of the Mach number in the nozzle with rectangular cross-section is nearly linear distributed,while the contour of the Mach number in the nozzle with circular cross-section is obviously parabolic.The jet axial velocity of the rectangular cross-section nozzle is higher than that of the circular cross-section nozzle,and the acceleration effect on the central area of the two-phase flow is better.Compared with the circular cross-section nozzle,the rectangular cross-section nozzle has a poorer particle flow uniformity,but the rectangular cross-section nozzle has a better acceleration effect on the central jet particles.The research work and results of this article will provide some theoretical support for the promotion and application of ultra-fine dry powder fire extinguishers.Meanwhile,laying a certain foundation for further research work.