제트확산화염에서 불활성 가스 첨가에 따른 소화농도 및 매연특성에 관한 연구

Abstract

In the event of enclosure fires such as compartment, the flame structure and the combustion products are different from those of natural burning because the inert gases are generated and sometimes imposed during the fire suppression tactics. Therefore, various fire behaviors in the inerting oxidizer have to be examined. This study was focussed on both the flame extinction related to the minimum extinguishing concentration (MEC) and the soot formation in jet diffusion flame diluted by inert gases. For the flame extinction, the coflow velocity effect was investigated experimentally in the jet diffusion flames. The flame extinguishing behavior in both nitrogen and carbon dioxide dilution was classified into three modes according to the coflow velocity. In the relatively low and high velocities, jet flames were lifted off before blowout and the flame bases were appeared to lifted triple flames and fully turbulent flames, respectively. At moderate velocity, jet flames were extinguished by blowoff mode without liftoff behavior. The PSR model was introduced to predict the MEC of the blowoff modes and the residence time of PSR was defined to the ratio of a convective time to the radial diffusion time. The numerical prediction with modified PSR was in good agreement with an experiment diluted by CO2 but overestimates a N2 dilution measurement. The experimental and numerical methods could be used for the assessment of various flame suppression agents. Also, an experimental study on the unsteady effect of the extinction limit was performed in ethene jet diffusion flames. To impose the unsteadiness on jet flames, the amplitude and frequency of a co-flow velocity was varied, and the two inert gases, N2 and CO2, were used to dilute the oxidizer for extinguishing concentration. The experimental results shows that large amplitude of velocity induces a low extinguishing concentration, which implies that flow variation affects on the blow out mechanism. Also, the flow oscillation effects under high frequency attenuates the flame extinction. These results means that flow unsteadiness extends the extinction limit and finally minimum extinction concentration by inert gases. When the Stoke's 2nd Problem is introduced to explain the flow unsteadiness on extinction concentration, the solution predicts the effect of amplitude and frequency of velocity well, and hence it is concluded the effect of low frequency velocity excitation was attributed only to flow effect. For combustion product, the effect of carbon dioxide addition on soot formation was investigated in jet diffusion flames. Laser extinction method was introduced for the relative soot density and the plume temperature was measured. Also C/H ratio, TEM analysis, gas analyzer were used to investigate the soot formation with products quantitatively. The results show that the relative soot density is lower when carbon dioxide is added in oxidizer stream and oxidizer velocity increases because of reduction of flame temperature and residence time for soot growth. Also, carbon dioxide addition enhanced the instability of jet flames, so plume temperature was increased according to add of CO2 cause of extended reaction zone.